Help With Economical Semi-Planing Designs

[SAQuestor]

Folks,

Economy – for the purposes of this discussion let’s define it as nautical miles per gallon of fuel.

While using the above definition we may run into some variance between my definition of economical and your definition of economical. For someone that has more time than money, economical may mean 10+ NMPG. For someone that has more money than time, economical may mean anything less than 10 gallons per hour – damn the torpedoes and full speed ahead!

It is easy for me to grasp how a full displacement hull moves through the water in an economical manner. I can visualize the hull lines that are needed to minimize resistance. The preferred ratios of a displacement hull – length to beam – prismatic coefficient – speed to length – are mostly easily comprehensible to me.

I can also see how a planing hull works. I can visualize the various hull characteristics that are necessary for economical operation – and why certain design characteristics are necessary for sea conditions that the boat is designed for, deep vee is but one example. There are factors – beam to length (aspect) ratio, bottom loading numbers, why the chines need to be immersed for stability, what design elements help reduce the tendency broach, that are easy to understand – and how they affect the economical performance of any particular boat design.

My ‘problem’ is with semi hull forms. It matters not whether they are called semi-displacement or semi-planing. I have yet to be able to comprehend how these hull forms can be economical in that in-between speed range that exists between pure displacement speeds and minimum planing speeds – regardless of the exact numbers.

I see displacement boats with 10 or 20 or 30 horsepower engines that move along at very economically at 1.1x to 1.2x of the v‾ LWL.

Using Tom Lathrop’s Bluejacket designs as a basis, I see a planing hull that uses ~50 horsepower to attain a low-speed planing condition, 12-14 knots.

I recognize that a displacement hull has the potential to go much further on a gallon of fuel, but at the expense of time.

I also recognize that an efficient planing hull that can go some x lesser distance on a gallon of fuel – albeit still within the definition of economical for a planing hull – with the resultant gain of time.

Putting some concrete numbers into the equation – A weekend trip of 100 nautical miles – 50 out and 50 back.

A full displacement boat traveling at 7 knots would take ~7 1/8 hours to cover 50 miles. For purposes of this exercise, let’s say that the boat uses 6 gallons of fuel (.84 GPH) to accomplish this trip for a NMPG of 8.33.

A boat similar to a Bluejacket would be on plane at 14 knots and take ~ 3 9/16 hours to cover 50 miles. For purposes of this exercise, let’s say that the boat uses 8 (2.25 GPH) gallons of fuel to accomplish this trip for a NMPG of 6.25.

To my mind this essentially means that we traded 3.5 hours of time for the cost of 2 gallons of fuel - $6 to $8 at today’s USA marina prices. But this also means that in the height of summer I can (probably) make the anchorage before dark if I leave the launch site by 4 pm on Friday afternoon. It also means that I can leave the anchorage later on Sunday and still make it back in plenty of time to make it home before dark Sunday evening. Plenty of time to clean up the boat and put it away properly.

Guillermo and several others have made this point numerous times in the coastal cruising thread.

Now, on to semi boats. Without linking to the various manufacturer sites – you can google-‘em up yourself if you are really interested – most of the boats that I see described as semi-displacement or semi-planing have engine options that range from the low 100’s to 350 horsepower or more. All of this to achieve a typical cruise speed (with some variation on the upper end) in the 14 to 20 knot range. And the advertising copy often has a blurb along the same lines as mine above – “get to that anchorage and back in a hurry.”

Perhaps these fiberglass leviathans that I’ve seen are not a fair comparison to an efficient displacement hull and a lightweight planing hull. But other than Whio, I have yet to see a lightweight semi-displacement/semi-planing boat that specs out an engine of less than 100 horsepower to achieve the same cruising speed as a Bluejacket, 14 knots.

All of that to get to these two questions.

1) Can anyone point to a web site, a book, a section of a book, a magazine article, a SNAME paper, anything that clearly and unambiguously defines and illustrates the hull design features and lines of an efficient semi-displacement/semi-planing boat?

2) Can anyone provide any examples (other than Whio) of low power semi-displacement/semi-planing boats that can cruise at the same 12-15 knots (as mentioned above) and achieve at least 6 NMPG while doing that speed?

All of this boils down to my inability to understand how a semi-displacement/semi-planing boat can fit the more time than money definition of economical? Can someone help?

Thanks,

Leo

[Pericles]

This paper should help you. See page 4. http://www.sname.org/newsletter/Savitskyreport.pdf

Pericles

[RonW]

Leo, the atkin website is filled with semi- hulls that where designed in the 40's-50's & 60's, before planning hulls became popular. And in the same size boat as tom's bluejacket they use 25 horses, not the 100 to 350 as you list.
And most run in the same speed range you disscuss, 14 knots or 3 times the square root of the waterline. And I have now seen 2 examples of a semi- hull running 4 times the square root of the waterline. Both of these examples are herreshoff steam launches.
Check your math on consumption and time, a knot is 1.15 m.p.h.
A semi-hull stays in the water slicing through it, not on top of it, exsposing the bottom of the boat to the pounding of the waves.
More examples, bateau.com has the nina, a off copy of john atlkin's ninigret, then doug hylan has the tophat, and harry bryan has the handy billy.
Tom's blue jacket as set up is a low speed planning hull designed to run economically in the low 20's with a 50 horse, but is still a planning hull and will behave as one when up on plane.And when not on plane is a shallow vee with a straight run aft, and not the deep forefoot and steeper veed angles of a semi- hull for handling rougher water and choppy water.

[PAR]

The box keel designs that have gathered so much interest of late, will satisfy your needs, likely consuming less fuel then you would imagine.

Most of the manufacture's offerings, in the semi plane arena, are just under powered planning hulls. These beasts are what you have described, fat, stern dragging, tubs that can't get up on full plane, but can pile enough water under them to provide some dynamic lift. In fact some of these, offered with a single engine are semi plane, but the same model, equipped with twins will manage full plane. Same hull, just under powered.

What size range are you interested in? I know the box keel hull form is available from small (16') to mid size cruiser (45') and they have amazing performance from small engine output, plus drink fuel gingerly. They have wonderful sea keeping abilities and inherent shoal draft. They don't back up worth a damn in larger sizes, but most folks will install a bow thruster anyway.

I've seen this hull form as a double ender, but most commonly is a transomed craft.

[longliner45]

look up JC boat of new england......longliner

[SAQuestor]

The box keel designs that have gathered so much interest of late, will satisfy your needs, likely consuming less fuel then you would imagine.

Hi Par! Yes, I have looked and I would tend to agree with you 100%. There's a lot of positives to some sort of box keel. Unfortunately, other than the Atkin Seabright designs and Bolger's square boats I haven't seen anything that appeals to my aesthetic eye.

Over in the Jersey Sea Skiff thread I posted this (http://boatdesign.net/forums/showpost.php?p=126057&postcount=7) treatise on what I'm leaning toward.

Life intervened and I didn't get the chance to go back and complete the thought I had - namely a hybrid design that utilizes something like Tom's BJ hull form as a basis and then adds a box keel. The question I wanted to get to over in that thread was whether or not adding a box keel form would increase the weight carrying capacity without sacrificing too much of the low speed economy that the original BJ series demonstrates.

[snip]
What size range are you interested in? I know the box keel hull form is available from small (16') to mid size cruiser (45') and they have amazing performance from small engine output, plus drink fuel gingerly. They have wonderful sea keeping abilities and inherent shoal draft. They don't back up worth a damn in larger sizes, but most folks will install a bow thruster anyway.
[snip]

Size? Something in the 9m to 9.5m range. Click on the Jersey Sea Skiff link above for more detailed specifics.

Best,

Leo

[SAQuestor]

look up JC boat of new england......longliner

Nice boat... but: (http://powerandmotoryacht.com/boat-tests/j-c/0800jc31/index2.aspx) "1/370-hp Volvo Penta P63 diesel inboard" "Optional power: owner’s choice of brands: single gasoline to 350 hp; twin gasoline to 520 total hp; single diesel to 460 hp; twin diesel to 600 total hp"

Best,

Leo

[longliner45]

we had twin 210s turbos ,,duoprop outdrives ,,and got 20 knots with a 6000lb payload,,at about 1 gallon per hr,,,,,,longliner...

[SAQuestor]

This paper should help you. See page 4. http://www.sname.org/newsletter/Savitskyreport.pdf

Pericles

Great paper! Thanks for the link.

I read it today at lunch and it seems to verify what I already knew about hull forms. But I still couldn't see how a semi-displacement hull form can be fuel efficient and carry any weight.

Whio (http://www.amateurboatbuilding.com/articles/design/whio/whio.html) has been mentioned on different threads and forums several times as a fine example of a low power, fuel efficient semi-displacement design. And it is. But if I recall the WoodenBoat magazine article correctly, Whio's extraordinary performance is primarily a result of an exacting program to shave every excess ounce out of the build.

And that's exactly like my understanding of successful low power planing designs - low weight equals fuel efficient performance.

So the question still seems to be whether or not it is possible to have a fuel efficient semi-planing design that isn't feather-light?

Best,

Leo

[SAQuestor]

we had twin 210s turbos ,,duoprop outdrives ,,and got 20 knots with a 6000lb payload,,at about 1 gallon per hr,,,,,,longliner...

Longliner - I don't mean to be skeptical - but if I read this correctly you had 420 installed horsepower, traveled at 20 knots with a 6000 pound payload (plus the weight of the boat, fuel and gear) and did that on only 1 gallon per hour?

Please tell me more on how this is possible.

Best,

Leo

[longliner45]

in the 80s ,,I was a commercial fisherman in the gulf of mexico,,we operated out of destin fla, we used 31ft jc boat ,,the engines were stearn mounted 4 cyl,volvos connected to duoprop outdrives ,,the boat had 13 airtight compartments blown with poly foam self bailing deck ,,self bailing engine compartment,,6000 lb payload mounted at the center of gravity of the boat ,,the boat was the broadbill ,,there was 2 others that I commanded ,,the broadbill 2 and the touchea 35 ftr, there was also 2 other jc hull that worked with us the, kantoo and miss lila,, the boats had their advantages,,,,very sea worthy easy to work ,,I was caught in several tropical depressions and 3 hurricans,,we didnt have but one satilight at the time,,was able to retreive sword fish gear in 70 knots gusting to 120 ,we highlinged for 3 years ,,,,,averaging 200.000lbs of fish per year ..we put 4000 hooks in the water in a 24 hr period,with every 4000 hooks we avaeraged 1000 lb of yellowfin grouper,,,national fisherman mag ran very many articals on us and these boats ,we set the boats up so 2 men could sleep while 2 men worked ,a 24 hr a day operartion,,the boat had tractor wheels and speed props ,,when rough weather aproached ,,we raised the outdrive ,and changed the props,,,most will find this hard to believe but if you can get old national fisherman mags or contact someone at jc boat they will verifiy,,,,if you cant do this ,go to destin fla,,,at east pass marina,,,,, and ask about the (vacume boats),,,my name is capt john sanborn,,,,,,,,aka longliner

[Frosty]

Rule of thumb deisel consumption 5galls per 100HP per hour

Therfore it is quite likely the fuel consumtion would be in the range of 20gall per hour.

[fcfc]

I have already posted this link:
http://www.nigelirens.demon.co.uk/images/NigelIrens.pdf

To make it short :

"3.1.a The first and most obvious conclusion that can be drawn from this chart is that high speed is an expensive option – no matter what hull-form is chosen."

"3.1.b Non-planing or semi-planing RANGEBOAT-type hulls are very dependent on waterline length if they are to produce really convincing performance figures."
14 kts -> 40 ft len. (LOA)
18 kts -> 56 ft len. (LWL)

"3.1.d The issue of fuel cost differentials does not appear to be a significant one."

[fcfc]

Although paper below is not directly related to hull form, it give cruise speed 24 kts (20 - 24 kts) with a lwl of 104 ft. (with a lot of expensive studies)

It is on par with N Irens S/L around 2.2 / 2.3.

For a 31 ft lwl, it should give 12-13 kts cruise. No more.

http://www.daviscoltd.com/public/Engineering/documents/PDFs%20Chris%20Barry/Hydrodynamics/Hydrodynamic%20Efficiency%20Improvements%20to%20the%20USCG%20110%20Ft%20WPB%20.pdf

[FAST FRED]

"The box keel designs that have gathered so much interest of late, will satisfy your needs, likely consuming less fuel then you would imagine."

I have found many references to this and am wondering if my guess as to why? might explain it.

On old jet aircraft the engine just tossed exhaust exhaust rearward at great velocity , similar to a boat with a single prop.

Today a tiny central jet core pumps huge amounts of "bypass air" (its outside the engine) rearward. Huge volume at a somewhat lesser speed.

Perhaps the box keel with negative dead rise aft (typical Sea Bright skiff) is moving all the water trapped under the hull aft , with a higher volume of water , and a lower overall prop wash speed but gaining efficiency.

The process is self limiting as the shape of the stern causes it to lift.

ALSO, Could we also be re claiming some of the energy in the water that was accelerated sliding along the hull , feeding the prop region?

The known efficiency HAS to have a reason!



FF

[fcfc]

The known efficiency HAS to have a reason!
FF

Yes. The real reason is that this efficiency has NEVER been measured. And never compared in scientific reproductible way.:P

I just guess most powerboats with the same weigth, same LWL, same BWL, same engine, same propeller, but normal aft form will have at least the same, and probably better efficiency since they have less wetted area. But they will probably have more draft.

There is no free lunch in box keels.

There are worldwide numerous model bassin, for over a century, that run daily model tests on every conceivable hull form, including self powered models if that is a problem for your specific hull form. And you have found one, plans available over half a century, that beats all in term of efficiency. :D

Just look the report I posted above. You will see that some people go to test models of 18 ft lengh, with accuracy checked better than 0.03 inches. Just to win 3.7% of fuel (flaps only). And they would ignore a magical hull form. Be serious.

[Tad]

What Fc is saying above is correct, their is no "known" efficiency gain coupled to a box keel or Sea Bright form. What is "known" is exactly the opposite.

Thomas C. Gillmer is a respected NA and he has tank tested several of the Sea Bright hull forms against a series of typical Eastern US fishing vessel types. In his book, A History of Working Watercraft of the Western World (1994), he states "it was most evident that this type of configuration suffered an added drag of approximately 20 to 25 percent over hulls of similar shape but without boxed keel forms." "The survival of Sea Bright skiffs for their fifty years or more is undoubtedly due to other qualities."

There are a number of reasons for using a "box keel" hull form, transport efficiency is not one of them.

Tad

[fcfc]

For production semi displacement "small" boats, you should look also at the nordhavn 35:
http://www.nordhavn.com/glance/not_production.php4

Retired from production after two dozen built.

One of the reason (but I can no longer find where in their site ) is that they were trying to sell a 12 kts cruise boat when the market (customers) where asking for a 18 kts cruise speed.

I am not aware of "small" (under 40 ft) currently in production displacement or semi displacement hulls with significant market share. Seems all are planning. (or sailboats ...).

[tom28571]

Thanks Tad, fcfc

Everyone seems to be looking for magic but there just ain't none to be had. The most "efficient" monohull powerboat I ever heard about was "Coyote" and its predecessor in Weston Farmers' "From My Old Boatshop". This was designed specifically for highest mpg over its speed range up to about 18mph. Had no superstructure, had a high length/beam ratio and was, according to Farmer, considerably more stable than a round log but it worked best if you parted your hair in the middle. The hull was somewhat like a round hull unballasted sailboat would be.

Rating efficiency between two boats might well be to compare their pocketbook values, whether we are talking about mpg, cost to build or whatever. Even that makes no sense unless the two boats are doing the same thing with the same accommodation, load, etc. Auto pubs make me gag when they talk about the great fuel efficiency of some car that costs $50K to $70K.

[SAQuestor]

So... If I read all of this correctly it's yet another case of TANSTAAFL (http://en.wikipedia.org/wiki/Tanstaafl).

Or is there some little thing that I've missed?

Best,

Leo

[FAST FRED]

TANSTAAFL.

Could be, as the main proponent of these boats was the designer Atkin, who claimed sea worthyness , load carrying and good speed with moderate sized motors for the 15 -18 mph range.

However the Power Keel folks in Austria did make many interesting claims,
which could only be more advertising hype or ???

FF

[fcfc]

However the Power Keel folks in Austria did make many interesting claims,
which could only be more advertising hype or ???

FF

From http://www.alsphere.at/alsphere/index.shtml

"The company was set up in February 2001 to design and market a hull concept for fast displacement vessels. The founders of ALSPHERE came from aluminium yacht building industry and started in the early 1990s to develop a new hull form, the so-called Displacement Glider ("DG"), ..."

We are in 2007 now. How many boats do use that hull form ? If it had any interest, there would be more by now. Even if it was only for other independant verification of the conceptor claims.

[Tad]

The Camano 31 and 41 use a box keel hull form which they claim is revolutionary. About 25% of displacement is in the box keel, the remainder of the bottom is very shallow and flat. http://www.camanomarine.com/index.htm

But the 31' requires a 200 HP Volvo to cruise at 14 knts with a maximum not much higher than that. I think a round bottomed Maine Lobsterboat Hull will do better than that at equal loading.

Tad

[FAST FRED]

The Shannon 38SRD claims a top of 20.4K and a cruise at 18.5K on 11gph with twin Yanmar 160 hp LH series diesels.

Sounds quicker than the camano boat.

Interestingly the designer a Walter Shultz , developed a "unique " underbody " combining sharp forward and deep midsections with inverted V aft sections", Shultz calls this reverse deadrise.

Kind of sounds like Atkin to me .

The usual claim "soft riding , fuel efficient , stable , fast with moderate Hp "also sounds like Atkin.

shannonsrd.com

They are working on a next model,same claims. 50% improvement in fuel use , smooth stable sea ride.

The program used for the design was:

http://www.aerohydro.com/

FF

[tom28571]

Shultz claims a patent on his design. It would be interesting to know just what is patentable in his SRD since I did not see any single feature that has not been used or suggested before. Maybe it's a "Hinkley" style patent:rolleyes:

[longliner45]

tad ,,look at the 31 jc hull and you will see it is what your decribing,

[fcfc]

The Shannon 38SRD claims a top of 20.4K and a cruise at 18.5K on 11gph with twin Yanmar 160 hp LH series diesels.

Sounds quicker than the camano boat.

FF

http://www.motorboating.com/motorboat/seatrials/article/0,12696,553628,00.html
19.4 kts, 2 * 100 hp. No fancy hull form. Figures independant from builder.

http://www.classicsolution.nl/frameset.html
35 kts, 1 * 250 hp. Cruise at 25 kts on 7 gph. No fancy hull form.

http://www.yacht-design.nl/bosgraaf/designs/motoryachts/hawk41.html
Yet another one with 1 *250 hp that should be 30+ kts. Still no fancy hull form.

Where is the efficiency of the Shannon :P I do not even speak of the Camano.

[fcfc]

Another one : http://www.parker-marine.com/com44page.htm
Still no fancy hull form.

[FAST FRED]

.

The Parker,
The vessel is light, narrow and easily driven, making her both fast (20 kts) and fuel-efficient.
POWER
Shown with twin Yanmar 4JH3-DTE turbo diesels (86hp).

The weight was given as 10,000lbs

Couldn't find a speed /power for the others , but there sure NICE looking!


Seems the main thing they have in common , 10,000lbs or 4.5T is good light weight construction.

SO far nothing seems to be close to the speed/fuel at 70% of "efficient" as published in PBB.

The Latitude 46 ,,"Once on plane, I found the boat had a couple of sweet spots on the acceleration curve: one at 1500 rpm (7.3 knots) and another at 3000 rpm (14.3 knots). According to Rodgers, the 40 gets 2 mpg at 15 knots for an approximate range of 264 nautical miles."

About 2nmpg is where the Shannon is , nothing special.

FF

[fcfc]

.
Shown with twin Yanmar 4JH3-DTE turbo diesels (86hp).

FF

They mistook Kw and hp. 4jhe DTE is 86 Kw 125 hp ( http://www.barrus.co.uk/pdfs/4jh3-dte.pdf )

The powering of the Andreyale is in the specs. (beware, the price is old. Seems the current one is double).

specs of the Van de Stadt design is in the contact menu.

Yes the key id the weigth.

18.5 / 11 make 1.7, not 2. And the andryale has NO special hull form.

[fcfc]

.

The Shannon 38SRD claims a top of 20.4K and a cruise at 18.5K on 11gph with twin Yanmar 160 hp LH series diesels.

.

The Latitude 46 ,,"Once on plane, I found the boat had a couple of sweet spots on the acceleration curve: one at 1500 rpm (7.3 knots) and another at 3000 rpm (14.3 knots). According to Rodgers, the 40 gets 2 mpg at 15 knots for an approximate range of 264 nautical miles."


.

About 2nmpg is where the Shannon is , nothing special.

FF

There are also stupid silly figures I do not understand.

For the Andreyale, when they claim 2nmpg at 15 kts, that 7.5 gph for both engines, 3.75 gph per engine. When I look in the test performance curve, I see 3000 rpm 14.3 kts, 3300 rpm, 16.3 kts. So A wild guess of 3150 rpm for 15 kts may not be too wrong. And when I look at yanmar consumption curve for 3150 rpm, http://www.yanmar.com.au/express/Dealer/Prodspec/ENGINE%20PDF/4JH3_HTE_TechData.pdf , the manufacturer quoted engine consumption is under 3.75 gph (around 3.5 gph). So all look realistic and match.

For the Shannon, 18.5 kts at 11 gph, I see from the motorboating test that for 2750, the speed is 16.9 kts and at 3000 rpm, 18.6 kts. So a wild guess of 2950 rpm for 18.5 kts may be true. But when I look at the manufacturer consumption curve http://www.yanmar.com.au/express/Dealer/Prodspec/ENGINE%20PDF/4LHA-HTP_TechnData.pdf , at 2950 rpm, the fuel burn is over 6.5 gph, not 5.5 gph as claimed. So I fear either the speed at 11 gph is under 16.9 kts, or the fuel burn at 18.5 gph is over 13 gph. Unless of course Yanmar claims that its engine burn more fuel than they actually burn.

18.5 at 2 nmpg is 9.25 gph. 13 gph is somewhat more than 30% higher.

Now, if someone could explaine me where is my error ...

The other speed computation.
From the test, at 18.5 kts, the Andreyale would have its engines at 3700 rpm. At this speed, the Yanmar consumption is around 5 gph per engine. So that would 10 - 10.5 gph overall.

At 15 kts, the Shannon 38 engines are at 2400 rpm. At that rpm, it is 4 gph. So 8 gph overall.

[FAST FRED]

Unless of course Yanmar claims that its engine burn more fuel than they actually burn.

The engine mfg give the fuel burn at max hp at each point on the rpm graph.

Since the boats are usually set (propped)to get peak hp at max Rated rpm , any pullback will lower the power the prop can pull at the lower rpm.

So the Yanmar may be capable of creating 6gph worth of hp , but its only being asked to produce 5gph by the prop requirements at that rpm.

FF

[fcfc]

Easy. Just read the consumption curve (4LHA) = 4 gph @ 2400 rpm. Now read the power curve @ 2400 rpm. 130 hp max output. 70 hp propeller curve. Now guess what power 4 gph refer to.


BTW, what is the budget of a economical semi planning design as per the initial post ?

Because bean counting the gph for a Andreyale 40 worth half a million $ used
http://www.yachtworld.com/core/listing/boatFullDetails.jsp?currency=USD&units=Feet&currencyid=100&boat_id=1600991&ybw=&units=Feet&access=Public&listing_id=1808&url= is a bit meaningless.
The owner of such a boat do not count 1, 10 or 100 gph ...

I have looked in the O1 thread http://boatdesign.net/forums/showthread.php?t=12157 what could be such a beast.
The only thing I have found:
1) there is no practical layout.
2) Budget over 45 000 $
Machinery 40-50 hp 20 000 $ minimum from air vent intake and cooling water intake to exhaust transom , propeller, shafting and controls.
Bare naked hull around 10 000 $
Micelleanous fittings 15 000 $ (deck, interior, minimal electronics).

[FAST FRED]

Its probably cheapest to marinize a truck diesel.

Most all the bell housings are SAE , so a huge choice of reconditioned marine trannys should keep the cost down.
100hp to 250hp is easy. in truck hp not Marine rating.

Instead of a heat exchanger , extra pump and costly wet exhaust , the commercial boat style keel cooler and dry stack is also inexpensive.

The hull will probably be 5-6000lbs , probably hard to bring in for $10,000 in anything but strip plank wood , Epoxy covered.

Ideas?

FF

[fcfc]

Instead of a heat exchanger , extra pump and costly wet exhaust , the commercial boat style keel cooler and dry stack is also inexpensive.

FF

Can you price a stainless steel dry muffler , ABYC compliant for a 50 hp engine. What size will it eat from accomodation ?
Same for the sound proofing / thermal insulation / ventilation of a dry system.
Same for a cupro nickel keel cooler.

The fact is, at least for france, and certainly for whole europe, that if you do not comply with marine safety and pollution regulations, you will have very big problems entering any marina and trying to get any insurance coverage.
The problem goes from fire hazard to exhaust smoke to noise to black water.

And the cooling method of an engine may have an influence on the price, but it does not touch items like engine mounts, gearbox and couplings, remote control and instrumentation, fuel tanks and fuel system (vents,hoses, decks fittings, jauges , filters ...) , propeller and shafting (shaft, stuffing box, stern tube, cutlass bearings, ...) , etc etc etc...

From my price estimation above, the basic industrial engine was around 5000 $. The marine engine derived from 11 000 $, and the whole thing that you start pressing a button from the helm station, read state from helm, fill fuel from the deck entry, can still speak while it is running, does not smoke black, and finally makes the boat go forward when you push the forward lever is 20 000$.

[fcfc]

The hull will probably be 5-6000lbs , probably hard to bring in for $10,000 in anything but strip plank wood , Epoxy covered.


FF

I was speaking of a 31 ft. empty equiped weigth 4800 lbs. But bare naked hull and deck should be in 1000 -1500 lbs only in sandwich.

[fcfc]

Here, you can see a previsional budget for a 40 ft, single 250 hp engine :
http://hawk41.awardspace.com/#toplist

The grand total (without additional electric engines) is over 115000 € = 150 000 $ at current echange rate.

[FAST FRED]

On most workboats the ABYC is not involved and the exhaust is simply routed outside , thru one or two std truck mufflers.
There is a requirement to heat shield the exhaust against people contact , but thats cheap screening.

These truck exhausts seem to last for a couple of engines , so SS may be required as eye candy , but not for durability.

The heat exchanger most often used is simple pipe.

Our 6-71N with injectors for 180hp only requires 2, 21ft lengths of 1 1/2 pipe , bolted under the hull.
We sometimes cruise in FL and Bahamas , with no cooling hassles.

The pipe lasts at least a decade ( ours is over 2 , but well painted), and replacement would cost less than the VAT on a copper keelcooler setup.

Some folks have used copper pipe , rather than water pipe , but I'm not sure that lasts any longer.

FF

[fcfc]

2) Can anyone provide any examples (other than Whio) of low power semi-displacement/semi-planing boats that can cruise at the same 12-15 knots (as mentioned above) and achieve at least 6 NMPG while doing that speed?

All of this boils down to my inability to understand how a semi-displacement/semi-planing boat can fit the more time than money definition of economical? Can someone help?

Thanks,

Leo

Andreyale 33 already listed in pocket cruiser thread.
http://www.yachtworld.com/core/listing/cache/searchResults.jsp?cit=true&ybw=&slim=quick&sm=3&is=&type=&man=andreyale&luom=126&fromLength=30&toLength=35&currencyid=100

More than 14 kts cruise with less than 100 hp.

Economical :D :?: Budget 175 000 to 200 000 $ for a 6 year old used boat.

[Pericles]

I do hope these will not send people into paroxysms of laughter. :D :D http://www.common-sense-boats.com/in.htm

See also http://www.common-sense-boats.com/idaho.htm


Pericles

[fcfc]

I do hope these will not send people into paroxysms of laughter. :D :D
Pericles

As a matter of taste and colour :P , i would prefer this one http://www.cmdboats.com/rw26boxkeel.htm

[SAQuestor]

Andreyale 33 already listed in pocket cruiser thread.
http://www.yachtworld.com/core/listing/cache/searchResults.jsp?cit=true&ybw=&slim=quick&sm=3&is=&type=&man=andreyale&luom=126&fromLength=30&toLength=35&currencyid=100

More than 14 kts cruise with less than 100 hp.

Economical :D :?: Budget 175 000 to 200 000 $ for a 6 year old used boat.

OK. Nice boat. Using my own labor over a 3 to 4 year build period, can I build one essentially equivalent in size and power for under $60K?

Leo

[SAQuestor]

Here are a few pertinent pages about semi-planing/semi-displacement hulls from the book Working Watercraft of the World.

Page 226 (http://farm1.static.flickr.com/241/455873418_d6b67d3430_b.jpg)

Page 227 (http://farm1.static.flickr.com/242/455873420_a958cbb0b1_b.jpg)

Page 228 (http://farm1.static.flickr.com/239/455873422_bab638653c_b.jpg)

Page 229 (http://farm1.static.flickr.com/208/455873424_b7771d1112_b.jpg)

Page 230 (http://farm1.static.flickr.com/244/455873426_5ae3a69122_b.jpg)

Page 231 (http://farm1.static.flickr.com/218/455873428_224f670e26_b.jpg)

My takeaways from these pages are:

Fine entry.
Flat buttock lines.
Light weight.

What other important ideas/features did your eyes pick out?

Best,

Leo

[tom28571]

Fine entry.
Flat buttock lines.
Light weight.
[/LIST]
What other important ideas/features did your eyes pick out?

Best,

Leo

Leo, Those criteria seem to put us back just where we started. :D

[FAST FRED]

"Using my own labor over a 3 to 4 year build period, can I build one essentially equivalent in size and power for under $60K?"


Probably , but the shown fit and finish will take years and loads more cash to get.

If like me , a workboat style , clean and painted , with wood trim will do , it should be simple enough.

If you need forests if endangered wood species to clad the interior and deck,and magnificent Joinery to show it all off, it would be cheaper just to buy a used one.

FF

[Pericles]

fcfc,

Thanks for the link to Chesapeake. I return the favour with another box keel boat, from S Oz.
http://www.duckflatwoodenboats.com/mainpages/mundoo.php#Published%20Articles

The solar powered Mundoo III is another in the line of lightweight cruisers that would do well on the River Thames. It's got me thinking!

Regards,

Pericles

[SAQuestor]

{snippage}
Probably , but the shown fit and finish will take years and loads more cash to get.

If like me , a workboat style , clean and painted , with wood trim will do , it should be simple enough.

Quality paint is my best friend. Varnished wood is used for accent trim only. Obviously cheaper and IMO much more appropriate for a "homemade" boat in today's "environmentally concerned" collective consciousness. As always, YMMV.

Best,

Leo

[Wayne Grabow]

"My takeaways from these pages are:
Fine entry. Flat buttock lines. Light weight.
What other important ideas/features did your eyes pick out?
Best, Leo"

I used to live near the Antique Boat Museum in Clayton, NY. Old powerboats, some speed record holders of the day, are featured in their exhibits. The early engines (prior to surplus aircraft engines from WWI becoming available) simply did not have the power-to-weight ratio to make planing a possibility. Semi-planing was the best they could do. So I went there and sat or laid on the floor, studying the hull shapes, trying to determine what features helped them maximize their speed. I agree with your list.

I am guessing that a small B/L ratio is also assumed to be true. The fine entry pushes water aside gently. (Relative) Light weight minimized the displaced mass. Flat buttocks minimized stern squat. The small B/L ratio allowed adequate displacement with minimal frontal area. Additionally a long, slender hull allowed a gentle transition from water being pushed aside by the bow to water flowing under the low-deadrise stern.

I seemed to also notice one other feature- a gentle but noticeable rocker confined approximately to the middle third of the hull with a straight keel forward and a straight run aft. I can't be sure because these were old boats which might simply be sagging with age. But such a rocker would make sense. Without rocker that stern would likely be dragging in the water; not good at lower speeds. Also, at increased speed, power is applied low under the stern and increased drag would tend to concentrate at the bow wave. Under these conditions, would not the boat tend to squat slightly, rotating about that central rocker, presenting a more cutaway bow profile and a more flat planing surface aft? Perhaps this profile helped the hull to slightly exceed the confines of displacement hull theory and enter the indistinct semi-planing realm. This is only my guess; I'll leave it to the more experienced to decide if it may be valid.

[fcfc]

OK. Nice boat. Using my own labor over a 3 to 4 year build period, can I build one essentially equivalent in size and power for under $60K?

Leo

I think probably. But do not expect the same finish, style and performance as the andreyale.

The other question is what else can you do with that budget and time ?
How does it compare to a homebuilt ?

http://www.yachtworld.com/core/listing/cache/searchResults.jsp?cit=true&ybw=&slim=quick&sm=3&is=&type=&man=grand+banks&luom=126&fromLength=30&toLength=36&currencyid=100&toPrice=45000

You have $15K left for maintenance. The speed will be lower. The fuel burn a bit higher. But there will be no comparison possible for resale value in future time.

[SAQuestor]

{snippage}
You have $15K left for maintenance. The speed will be lower. The fuel burn a bit higher. But there will be no comparison possible for resale value in future time.

Geez... How I hate it when someone rubs my face in reality. ;)

[fcfc]

OK. Nice boat. Using my own labor over a 3 to 4 year build period, can I build one essentially equivalent in size and power for under $60K?

Leo

Do this kind of light semi displacement boat exist for homebuilding ? I have found none. You can find plans for 65 ft sailboats, worth hundreds k$, but a moderate 30-33 ft powerboat : No.

[RonW]

Sadly but unfortunately, this thread has done very little to define what is a semi- hull, and what advantages or dissadvantages they have over a planning hull.
Wayne mentions the back 1/3 of the bottom of the hull as to rising, rather then the straight run aft of the planning hull, and the deeper forefootof the semi- hull.
But that seems to be about it as to hull shape. Leo posted pages with lobster boats that are semi hulls.Wonderfully performing boats.
And yes there are plans for semi hulls in all sizes reasonable available, if you know what you are looking for. One designer has been metioned in this thread with semi- hulls in the 30 to 33 ft. range.
Then again for a 33 fter plans, a wonderfull boat is the potluck by glen-l, a 33 ft. semi- hull. Lobster Boat. Excellent choice.

[fcfc]

Then again for a 33 fter plans, a wonderfull boat is the potluck by glen-l, a 33 ft. semi- hull. Lobster Boat. Excellent choice.

May be, but at 9250 lbs displacement and 100-120 hp, the potluck 33 will nearly cost twice to build and operate than a 5500 lbs 60 hp andreyale 33 clone.

[fcfc]

And yes there are plans for semi hulls in all sizes reasonable available, if you know what you are looking for. One designer has been metioned in this thread with semi- hulls in the 30 to 33 ft. range.


I am looking at len 31-33 ft, displacement < 5500 lbs,preferably < 5000 lbs beam < 8'4" , power < 60 hp, inboard diesel. Top Speed > 14 kts.
If you know any design that match (homebuilt or production), please give indications.

And if you think there are unreasonable dimensions above, can you tell which one and why?

[Pericles]

fcfc,

There are two Sam Devlin vessels that should suit your description, for which plans are available.

http://www.devlinboat.com/dctopknot.htm

http://www.devlinboat.com/dcbc29.htm

There are photographs at bottom of each page.

Pericles

[FAST FRED]

The advantage of the semi plaining is that the hull forward stays in contact with the water.This is not great for high speed but fine for 14K.

By selecting a proper entry the vessel will be far kinder in rough going than the full plaining boats to the occupants.

Take a look at Atkins River Belle , and contemplate JUST the hull, place whatever deck and house on it as you require.

Building in Airex foam core is a very simple way to allow a complex hull shape to be built fast , strong and light.But unfortunatly not cheaply.

FF

[fcfc]

I am looking at len 31-33 ft, displacement < 5500 lbs,preferably < 5000 lbs beam < 8'4" , power < 60 hp, inboard diesel. Top Speed > 14 kts.
If you know any design that match (homebuilt or production), please give indications.


fcfc,

There are two Sam Devlin vessels that should suit your description, for which plans are available.

http://www.devlinboat.com/dctopknot.htm


Displacement 10800lbs >> 5500 lbs.
Beam 10'2" >> 8'4"
Power 250 hp >> 60 hp.

Do not fit.


http://www.devlinboat.com/dcbc29.htm

There are photographs at bottom of each page.

Pericles

Len 29'2" < 31"
Beam 10' > 8'4"
Power 130-200hp >> 60 hp.
Displacement 8400 lbs > 5500 lbs.

Do not fit.

I have looked for nearly 1 year. And I have found nothing even close.

[RonW]

FcFc- Neat, I see where you are coming from, the old time launch. I looked at the andreyale, nice looking and proportioned boat.
I take it you are wanting to push the boat to towing length, when you add in the length of the trailer tongue, and you are right at the width without a permit. Weight issue for a heavy truck or such.
There are a few boats that come to mind, one that looks almost the same is ruel parker 36 ft. commuter, but it is 3 ft. longer. Same weight and it uses a 90 horse outboard.
http://www.parker-marine.com/commuterpage.htm
The architect mmd, can be found on the wooden boat board, drew plans for the same boat but in aluminum, You could get him to draw a set in wood, at a reduced length of 33 ft. There are very good reports on the performance of the boat as well as stability and smooth ride.
Another design that comes to mind is the whio- it was featured on wooden boat a while back. It is 30 ft. long and only 7 & 1/2 ft/. wide, 3,000 lbs, and used a 60 horse inboard. This is a nice boat too.
And of course there are a couple boats by bolger, the idaho and tennessee I believe, They fit your weight, power and length roughly, and one is a lot longer as I remember. But they are flat bottomed plywood boats, that look kinda boxy and in my opinion are no where near or even close to being in the class of the andreyale, parker's commuter or the whio.
There are a couple of old time atkin designs in the high 20 ft. range that can easily be stretched 3 or 4 ft. by simple respacing frames. Atkin had a habit of using 12 frames on anything between 15 ft and 40 ft. by just increasing the spacing and size of framing and planking thickness.
Your 60 horse and 14 knots or 16 m.p.h. is pushing the engine to max, and the speed will be iffy, and not a cruising speed, you may want more like a 90 horse, so as to not push the power to the max. just what the parker commuter uses.
If you are serious, I would say to look at mmd's version and see about having a set of plans drawn to your specs, they will be well worth the price for the boat you want.

[RonW]

Here is the link to the mmd version of the commuter.
http://www.abco.ca/marine-abco36-pleasure.html
It is a little longer, lighter, and higher speeds.


Here is also a suggestion on a atkin boat, that could easily be stretched 3 ft. and could be converted to a outboard, particularly by using a bracket or manual jackplate and keeping the transom whole and solid.
I think it would make a wonderfull boat, and a little shorter for easier launching.
http://www.boat-links.com/Atkinco/Cruisers/Hope.html

--In reviewing the previous pages, I see where the 44 ftr commuter was listed and one of the bolger designs. too many pages to keep track of everything.

[fcfc]

There have already been lengthy threads about the GRAAL.
http://www.boatdesign.net/forums/showthread.php?t=5073
http://www.boatdesign.net/forums/showthread.php?t=11670

I am not particulary found of outboards. And when you put a inboard in commuter 36, the layout become awkward.

The second issue is I doubt seriously about resale/real value of a custom designed custom built boat. Especially when the primary reason for custom design/building is that no other similar boat do exist. That simply means there is NO market.

[tom28571]

The second issue is I doubt seriously about resale/real value of a custom designed custom built boat. Especially when the primary reason for custom design/building is that no other similar boat do exist. That simply means there is NO market.

It occurs to me that if someone wants a boat that cannot be found in the mass market, that boat will necessarily not fit the usual profile of easy resale. If it did, then the search could be completed by a trip to local boat dealers. That is a deciding factor that all of us must face when trying to satisfy our personal tastes.

If the desire is to have a boat with good resale value then it must meet what the market expects and favors. I know of more than one designer who must design boats that they don't particularly favor in order to eat and send the kids to school.

Peace:)

[RonW]

First of, what is graal??
Interesting threads, will read.

fcfc says.--And when you put a inboard in commuter 36, the layout become awkward.
Of course if you only have around or less then a 7 ft. wide area and put a box 2ft wide by 3 or 4 ft. long in the middle of it, you layout kinda gets screwed up. That is why you consider other options such as stern drive or outboard, god forbid.

As for market and resale value, then you need to find a 2nd hand boat of your specifications that the owner is in a bind and you can steal it for a song and dance. Enjoy it for years and then resale it and make money. GOOD LUCK..

I think the saying is, if you want to play, then you gotta pay.
Now the big decision of deciding what you want and what you can actually afford. And as they say in the boating world, all boats are a compromise..

[FAST FRED]

" The second issue is I doubt seriously about resale/real value of a custom designed custom built boat. Especially when the primary reason for custom design/building is that no other similar boat do exist. That simply means there is NO market."

This would imply that,
1, everything is already known and there is no such thing as progress.

2, The market is so well informed that every buyer knows of the existance of every item for sale, worldwide.

The containerable motor sailor on the Motor Sailor board shows an idea that may not be new , but is certainly unique.

"Build it , they will come" is how the normal market forces work.

It will be interesting to see of in 10 years they are still in business , and how many (if any) competition spring up to create a broader market.

I'm interested in creating an outstanding cruising motor boat to be container shippable . Weather the world flocks to the designers door , or only one is ever built will depend on the market acceptance of the basic cruising concept.Cruise worldwide , with out ocean crossings.

That a boat is not mass produced , does not mean there is not yet a market for a specialized design.

FF

[RonW]

Good post fred, and the needs and wants of the marketplace is a constant changing situation due to various reasons.
On the pervious page the link that fcfc posted about pocket cruisers - with motors, not sail and under 30 ft. definitely trailerable has 21 pages of some interesting and very nice boats.
And these come from all over the world, so the world seems ready for a economical, trailerable cruiser that is smoother riding and has as much room and as good a layout for comfort as possible in such a small space.
Of course the consumer wants the best for the least amount possible.
The point that I keep seeing is they want the smooth ride and rough water capability, but somehow they are in between the semi- hulls and the planning hull blend, as to actual speed capabilities.
They want the cruise of 20, and are willing to drop the top end down to 25, but for a boat this size that still demands some form of planning hull.
Where as a straight semi- hull of this size will basically cruise at 15 and have a top end of about 20. So somehow they want to blend the two together as for the ride of the semi- but a little more speed of the planning hull.
Maybe some designer will be able to give the market place the best of both worlds and design a slightly altered, whatever we are going to call it.

[SAQuestor]

One of the positive attributes that Downeast semi-displacement hulls is their ability to go out into a seaway. See attached pics and the Yachtworld listing for this Duffy 31 Fisherman. (http://www.yachtworld.com/core/listing/pl_boat_full_detail.jsp?slim=quick&boat_id=1419188&ybw=&units=Feet&currency=USD&access=Public&listing_id=1852&url=)

But I can't help but question whether or not a pleasure cruiser would actually need this amount of seaworthyness? Would I go out in the conditions as pictured when on a pleasure cruise? Probably not. But... Is this sort of capability worth the extra cost and fuel burn if it's only a capability and not used except in an emergency?

Hmmm... Something to ponder. What are your thoughts on whether this sort of seaworthyness is needed - Please comment TAD, PAR, fcfc, Tom28571 and Fast Fred?

Best,

Leo

[SAQuestor]

For reference only.

The attached pic is from this Yachtworld listing. (http://www.yachtworld.com/core/listing/pl_boat_full_detail.jsp?slim=quick&boat_id=1626282&ybw=&units=Feet&currency=USD&access=Public&listing_id=1645&url=)

This picture exactly illustrates what I have always thought of as typical performance of a semi-displacement hull form.

The listing says that this boat cruises at 14/17 knots and has a 320 hp Cat engine to do so. Fuel burn? Likely gallons per mile instead of miles per gallon.

Now y'all know why I want to understand more about semi-planing/semi-displacement hull forms. And especially can they be efficient? As defined by a 12-14 knot cruise speed at a low fuel burn rate.

Best,

Leo

[longliner45]

ya might not go out into it intentionaly,,,,but sooner or later the odds will catch up to you,longliner

[RonW]

Your post is very missleading leo.

You fail to mention that your example boat above is 35 ft. long with a 12 ft. beam and dissplaces 7 tons. It also has a top speed of 23 knots or 26.45 m.p.h.

[SAQuestor]

Your post is very missleading leo.

You fail to mention that your example boat above is 35 ft. long with a 12 ft. beam and dissplaces 7 tons. It also has a top speed of 23 knots or 26.45 m.p.h.

Misleading how ron? I said:

For reference only. This picture exactly illustrates what I have always thought of as typical performance of a semi-displacement hull form.

What part of these two simple statements is misleading? I said it is for reference and it's what I think of when a semi- hull form is mentioned.

It is NOT misleading at all - the actual size and tonnage of the boat is irrelevant. Stop creating strawman arguments (http://en.wikipedia.org/wiki/Strawman) that are based on a misrepresentation of my statements.

[RonW]

It is simple leo, as we have disscussed before on the wooden boat forum, the merits of a semi- hull versus those of the planning hull. You took the position that the planning hull is more fuel efficent then the semi-hull and that is not so, as long as you compare apples to apples. Speed is the great consumer of fuel. You used the example of the horsepower, but neglected to specify the weight and the correct maximum speed that actually required that much horsepower. So the actual tonnage, size, speed and horsepower are all relevant in determing what is what.

leo says-The listing says that this boat cruises at 14/17 knots and has a 320 hp Cat engine to do so. Fuel burn? Likely gallons per mile instead of miles per gallon.

Now y'all know why I want to understand more about semi-planing/semi-displacement hull forms. And especially can they be efficient? As defined by a 12-14 knot cruise speed at a low fuel burn rate.


The actual mileage at the cruise rate of 14-17 should be about 1 & 1/2 miles per gallon, not gallons per mile.
I thought earlier of the lobster boat, probably the hull form that has already combined the merits of the semi-hull with a low speed planning hull. Maybe we need to look no further for the do it all hull..

[longliner45]

what is amasing to me is that ,,designers,and na/s not only have disregarded,the statements I have made of 31 ft jc boat ,,but not even looked into it ,,if that was my bread a butter,you can rest asured I would have at least looked into it,,,this tells me that this field will be controlled by others,,in other lands ,,,,,,longliner

[SAQuestor]

what is amasing to me is that ,,designers,and na/s not only have disregarded,the statements I have made of 31 ft jc boat {snip}

Please, provide some links of exactly what boat and design you are writing about. I've Googled, but all I saw were commercial boats. Are commerical fishing boats what you meant?

Best,

Leo

[Frosty]

A 320Hp cat This is the 3208 I should immagine will use 16 gallons per hour at max.

16Kts at 16gallper hour =1 gall per nautical mile.

[RonW]

Jack frost says--A 320Hp cat This is the 3208 I should immagine will use 16 gallons per hour at max.
16Kts at 16gallper hour =1 gall per nautical mile.--

Not to be argumental, but that is not correct. If you are going to try and compare performance, especially fuel economy, then you need to get the actual figures as close as possible for a fair comparison.
The example above of the lobster boat actually has a top speed of 23 knots or 26.45 m.p.h. So don't take a lower speed and then use the maximum fuel useage and try and make a comparison.
Besides I thought diesels where pretty much in the fuel useage range of using 1 gal per 16 horses, so max. fuel useage on a 320 horse diesel should be closer to 20 gals. But it is gonna take no where near that to cruise in the mid teens.

[longliner45]

the boat was a 31 ft jc hull we custom built for commercial longlining ,,,bottom and sword,,I did try national fisherman mag ,but they only go back 3 or 4 yrs,,and I tried to contact jc boat ,at least tried to find a web site..all you guys are more savvy at that than I,,but I will try to explain,,,,we took the 31 ftr,,placed the volvos on the stearn ,duoprpop outdrives,this does 2 things we felt were advantageous,,most lobster boats ,and other commercial fishingboats must mount the iceboxes (on )deck,,since we had no engine and shaft to contend with ,,we could sink the 6000lb icebox in the center of gravity,,allowing the boat to travel more stable and even,,,in calm seas we could get 20 knots at about a gallon a hr,,in rough seas we could get 8 or 10 knots depending on conditions,, we had 375 gallons of fuel and 150 gallon of water for drinking we estimated about a 900 mile fuel range,I know you guys arent calling me a liar ,,I m just a little frustrated that I cant find any meaningfull documation,,but I can give you names,,nikis seafood in orange beach alabama owned one(miss lila),paul pence and steve perdicaro of penco tackel destin fla ownd the broadbill,and broadbill2,, the touch` was owned by sanders mfg in destin fla,and pauls brother ron ownd the kantoo ,,thats the best I can do ya ,now if I were going to make a substantial investment for a boat ,,I would try to contact jc boat in newengland,,oh yea the boat hull itself wieght was 11.000lbs empty.longliner45

[tom28571]

the boat was a 31 ft jc hull ----,we took the 31 ftr,,placed the volvos on the stearn -------,we could sink the 6000lb icebox in the center of gravity,-------,in calm seas we could get 20 knots at about a gallon a hr,,------oh yea the boat hull itself wieght was 11.000lbs empty.longliner45

Looks like maybe 10 tons at 31 feet and 20 knots at one gal per hour. I'm impressed:eek:

Actually, I'm more like a caveman stepping out on the freeway at rush hour. What have all the other designers been doing?

[Frosty]

Ron --rule of thumb but pretty accurate


diesel 5galls per 100hp used
petrol 7 gall'' '' '' '' '''
2 stroke 10galls''' ''' ''' ''


It doesnt matter if the cat is in a generator or a truck at "full" 320HP it has to consume 16galls.

The trick here is HP 'used' not the size of the engine.

[Willallison]

My 2001'st post and I hate for it to be a negative one, but sorry Longliner, I'll take Tom's polite scepticism and raise him.... Unless this jc hull has rewritten history, then that just ain't possible. The only boat I can imagine getting 20nmpg is a sailboat with it's genset running to boil a cup of tea..... in which case it's more likely doing 2 knots, not 20.:(
I think you said the boat you were aboard had a pair of volvo's with dp legs. If we assume that they were the 6 cyl 200hp engines, then even if 20 knots only required 1/2 of their max output, combined they would be consuming about 10gph. Which gives us a more realistic 2nmpg. In order to provide the sort of economy you're suggesting, they would only be producing about 5hp each!

[SAQuestor]

A 320Hp cat This is the 3208 I should immagine will use 16 gallons per hour at max.

16Kts at 16gallper hour =1 gall per nautical mile.

Okie dokie - here's a repower (http://www.algroversmarine.com/jc.htm) from 3208 T Caterpillar Diesel with 320 horsepower to a 350 HP 6LYA-STE Yanmar.

"I was delighted at the final results. Kristen went from a 15 knot cruising speed at 12 GPH to 22 knot cruise at 14 GPH. The top speed went from 20 knots to 25 knots."

So with the 3208 Cat he cruised at 15 knots @ 12GPH. So that's 1 1/4 NMPG.

Doesn't look like you're too far wrong Jack.

Best,

Leo

[longliner45]

JC boat sales,,,,10 progress ave Nashua new hampshire USA,,tel,,603- 886- 0300,,my 32 ft sailboat weighs 10500lbs and its made of wood,,,longliner

[longliner45]

found this too ,,might have been a little hard ,,they moved part of thier operation to fla ,here goes,,,,,,,powerandmotoryacht,com look under 31 jc ( boat test)also says at 6 knots 1gph,,and at 20 knots 8gph,,but that is with conventional single screw drive,,,,not duoprops with twin turbo 4 cyl volvos,have fun ,longliner

[longliner45]

Hey ,,,,,were did ya go?????????dont ya just hate it when im right,,,,longliner;)

[Willallison]

:?: :?:

Ok - lets put this JC31 thing to bed. I'm not sure where you're getting your numbers from Longliner, but here's the specs on the JC31 as published in Power and Motoryacht

They clearly show that this boat is no more economical than any other of the type. Indeed, whilst it might be a fabulous boat, I'd consider it to be pretty bloody thirsty! At the twenty knots you refer to, powered by a single 370hp shaft drive diesel, the engine is consuming 19.5 usgph - equating to 1.03 nmpg. Even allowing for the greater efficiency of the dp sterndives (which would be offset somewhat by having twins), there's absolutely NO WAY that it could use almost 20 times less fuel at the same speed........

[Frosty]

Every one has told you your fuel consumption figure are unrealistic. explaining with formula.

You refuse this-- Whats left to say?

[longliner45]

yes will you can ,,with the twin model210 turbo charged diesles ,,and the counter rotating duoprops ,,with 0 cavitation buy the way ,,it can be done .and was done,,remember none of you belived the 31 ft boat could weigh 11000 lb,, one said .it was more like 11 tons,,we also ran the engines at 3300 rpm.our engines were pretty light also ,,2 men could pick one up ,,im not going to defend myself any further ,,I know what I did in my life ,,on these boats,,if you refuse to believe ,no problem ,,if you want to learn more ,you have JCs corporate address,,,and please tell me how I could fish 100 miles offshore for 9 or 10 days,,,with 375 gallons of fuel,,, and jack you say 5 gallons per 100 hp.jc is showing on the above chart 1 gph with 370 hp engine 6 knots ,,not shabby ?longliner

[Willallison]

Look - I'm sorry - I'm not trying to have a go at you.
Ok - so these were 4 cyl, 210hp a piece, running at 3300rpm. According to the Volvo website, they never made such an engine: their largest 4 cyl being the fairly recent 170hp KAD32P.
But if we look at one of their current engines, the 225hp D4, at 3300 rpm the engine is producing 180hp and consuming 10 usgph. 2 such engines would of course be using 20 gph, so your JC31, if it was doing 20 knots would be getting 1 nmpg. Now it's possible - though I would contend somewhat unlikely - that Volvo have regressed somewhat and their new engines use 20 times as much fuel as their old ones.....;)

[Frosty]

5 gall per 100HP used not the size of engine.

It is quite likely your 2 Volvo 210Hp engines would use almost 1 gall per hour just standing stll in neutral sat in the harbour.

[Willallison]

5 gall per 100HP used not the size of engine.



Yes - I use 20hp per gallon per hour - so the same thing. It is of course only a ballpark figure. The D4's that I quoted, which are one of their latest and greatest, are only managing 180 hp/gal/hr (taken from their prop curve)

[RonW]

Hey Longliner-- I think these guys are missunderstanding what you are saying. I take it that you are saying that if you where out fishing for 8 days or 200 hours, you would average about 200 gallons of fuel useage.
Or about a gal a hour.

They think you are saying that you could run 20 knots a hour and use one gal a hour to do it. Thats impossible.

This duo prop counter rotating props with 0 cavitation is interesting.
As I recall a outboard has around 20 to 40 % slip.

[longliner45]

willalisson anjack and all others ,,,I was wrong ,we got a gallon per hr with a 6000 lb payload ,,,,,but the controversy that stuck in my mind was that at the time fishing boats only went 6 or 8 knots,and burn allot of fuel ,,,but our boats could go 20 knots 26 at top,,,,,,,,,,I was wrong you were right ,,I am a firm beliver of truth,we were also told by volvo that no matter what the boats could only do 40 knots because of phyics,,that was proven wrong by someone one this site that set a speed record ,,with one duoprop ,,the picture shows the boat flying with only the prop in the water,,,,,,Ill get you all back later,,,,,lol.....longliner,,,,,dont ya just hate it when jack is right?:)and yes ron the tecknology is from ww2 topedoes,so they would go staight, and behind the boat that is what it looked like...

[charmc]

I was wrong ,we got a gallon per hr with a 6000 lb payload ,,,,,but the controversy that stuck in my mind was that at the time fishing boats only went 6 or 8 knots,and burn allot of fuel ,,,but our boats could go 20 knots 26 at top,,,,,,,,,,I was wrong you were right

Longliner, I'm proud of you, my friend. On another thread I questioned your 1 gph @ 20 knots (that's knots, Ron, not "knots per hour") claim for the JC hull. But the overall performance ... being able haul 6000 lb loads at displacement speeds at only 1 gph, get over 20 knots when needed, and still get a week or more fishing offshore on 300 gal of fuel.... no wonder JC never had to advertise, the word of mouth in the commercial fishing community had them able to sell more boats than they could build. Obviously, that's one efficient hull.

[charmc]

OK, here's another modern application of a box keel in a semi-displacement hull, if the claimed performance is accurate, this is very efficient:

http://www.alsphere.at/dg/index.shtml

[longliner45]

let me be more clear,,1 gallon per hr ,,,at 6 or 8 knots,,,longliner

[u4ea32]

So the question still seems to be whether or not it is possible to have a fuel efficient semi-planing design that isn't feather-light?


No. Its light weight that makes any boat fuel efficient.

I've been looking into exactly this same question for years, and the experimental, observable answer is always the same: Same weight planing boats (constant deadrise) are always, at all speeds, more fuel efficient that semi-planing boats (variable deadrise, or round bottom).

The full planing boats do burn more gal/hr when they are going faster, but they burn LESS gal/hr when they are going the same speed as the semi planing hulls (except, perhaps at 3 knots, but at that speed, who cares what the fuel burn is -- its trivial in any boat). And of course as soon as you start talking about mpg, then the planing boat wins hands down.

The reason is obvious, and well supported by tank tests for a century: once its making a bow wake, the planing boat has (a) less wetted surface and (b) more weight of the boat carried by dynamic lift. There is a reason there are zero commerical airlines using blimps.

You can simulate a semi-planing boat with a planing boat very easily, if you have big enough trim tabs. Push those tabs down, and the speed drops and the fuel flow stays the same. Yup, you are pushing the lift way aft like having a warped bottom, and getting the forefoot in the water. Smooth, yes. Lots of wetted surface, lousy dynamic lift, and you've got a semi-planing boat, and lots of fuel burn.

Its weight, pure and simple.

That said, there is an advantage to semi-planing hulls, but its not efficiency: its smooth ride in a seaway.

But even a smooth ride is trivially easy to achieve with a planing hull: make the deep vee constant deadrise (better, stepped like a Fountain) hull looooooong. Like 8:1. You'll get all the benefits of a very low hump, low vertical accelerations, but you will still be able to go really fast if you feel like it.

[Wayne Grabow]

"Same weight planing boats (constant deadrise) are always, at all speeds, more fuel efficient that semi-planing boats"

At all speeds.... even when not planing, with the planing hull dragging its transom underwater creating form turbulence and having greater immersed surface area? At low speeds, the semi-planing hull acts similar to a displacement hull; is the planing hull then superior to the displacement hull also (at speeds from, say, 3 knots up to hull speed)? If so, then should we start thinking about more full-planing daysailer designs? A square-sterned (planing) racing canoe? Lighter is certainly better. Flat bottomed boats plane easiest. So, is the jon boat the ultimate design for fuel efficiency? I appreciate much of what you are saying, but I can't yet accept the idea that a constant deadrise transom-sterned planing hull trumps all other shapes at all significant speeds. Help me out with some more details. How does rocker figure into this situation?

[charmc]

"Same weight planing boats (constant deadrise) are always, at all speeds, more fuel efficient that semi-planing boats"

At all speeds.... even when not planing, with the planing hull dragging its transom underwater creating form turbulence and having greater immersed surface area?


I'm not an NA or professional designer (not a rocket scientist, either :) ), so I'm not quoting charts or curves. I question the "always, at all speeds" claim for constant deadrise planing hulls, however. The speed range Wayne seems to be describing is the one at which the planing hull is at its worst. On the hump, beyond hull speed but not yet on plane ... bow high, stern buried, unable to get out of the hole and climb over the bow wave, dragging a huge stern wave ... it's ugly and I'm convinced it's not at all fuel efficient. True, no operator with any skill at all would remain in this speed range for long, but I believe, as Wayne said, the planing hull in this admittedly narrow speed range is not more efficient.

[Willallison]

Regardless of type, I'd agree that for a given vessel lighter is always going to be more fuel efficient. As for the rest of it - constant deadrise boats being more efficient at all speeds etc - well, without wanting to be rude....that's just bollocks.;)

[Willallison]

Another perspective on the same issue

"In general, a hull should be shaped for the cruising speed the boat is locked into by the power installed. If a hull is powered such that it will never reach planing speeds, it should not have a planing (hard-chine) bottom shape. (Ever see a duck with chines?) Water does not like hard corners unless true planing occurs; that is, when the water cleanly separates from the transom and chines at speed. Otherwise, a semiplaning (or penetrating) hull form will be more efficient. There are far too many boats designed with planing bottoms that never have a prayer of planing—except, maybe on the face of a 50-foot wave."

Tom Fexas, "The Spectator", PMY, Nov 2001

[tom28571]

I am not sure just what David is getting at. Will's comments seem spot on about the "at all speeds other than 3mph" statement.

In the first place, it would be very difficult to compare planing and semi planing hulls of the same weight. By it's nature the semi-planing hull would have convex bottom surfaces. This means it would have shorter waterline length, narrower beam or both. If both hulls are to have the same displacement, the above must be true. If one of the boats is designed optimally and both are the same weight and general dimensions, the other one will be certainly not be optimally designed for its use.

This is actually not a good area of argument. Both hulls have their proper place but neither is altogether happy operating in the other's "best" speed range, especially when the measure of "best" is fuel use.

[Frosty]

Bollocks???

Probably but when you talk semi displacement catamarans then they are the most economical craft on the planet.

At the speed they were designed to do.

[u4ea32]

Hmmm. Let me use less hyperbole.

And please, first restrict your consideration to very light displacement boats. Very light. If they are not very light, efficiency is already lost.
And if they are not long, they are not efficient either. I don't care about piggy boats that don't fit both of these criteria.

If you are worrying about transom drag being a significant portion of drag at any speed in a long thin light displacement vessel, you are thinking too much and you are not rowing or sailing enough. Try it, and you'll see that everything else is more important than how much transom is immersed.

Shape is only involved at low speed if the shape leads to less wetted surface. Semi-planing hulls have a far higher wetted surface shape than a deep vee hull form. Therefore, at very low speed (when fuel burn simply does not matter at all) the advantage should go to the fixed deadrise hull.

As we go from 1 to 2 times the sqrt WL, wave drag rapidly overtakes wetted surface. The non-planining shape starts to sink into the water due to the negative pressure underneath, and therefore displaces MORE water, and its wave drag increases even more steeply than it should.

A planing boat starts to develop dynamic pressure under the hull, reducing its displacement, and therefore wave drag. A long thin light and especially stepped planing hull exhibits very little to no bow rise. And more importantly, with a stepped hull the wave length is quickly halved or less, thereby GREATLY decreasing wave drag. If you don't understand this, you don't understand planing boats and why their wave trains disappear at speed.

Therefore, it does make sense that light planing boats always beat light semi-planing boats.

But before arguing some more, here is some data you can explore yourself and come to the same conclusion. Compare the two 42 foot power offerings by Beneteau. The planing boat always beats the semi planing boat in nmpg.

[SAQuestor]

Hmmm. Let me use less hyperbole.

Indeed.

Now, you've made some claims but I've seen no references cited. I've seen no links to online sources. When one wants to make a point that is - shall we say - thought provoking, one normally cites references so those that actually care to verify the data can do so with ease.

Without citing sources or providing online links, then discussions like this become simple he said-she said arguments when one person expresses opinion and another expresses opinion. Nothing gets resolved and little if any actual learning takes place.

And you know about opinions right? They're like a-holes, everybody's got one and they all stink. Mine, yours, everyone's. So let's get away from expressing opinion by your providing citations and/or links to data that back up your recent posts.

Best,

Leo

[Willallison]

To be fair, Leo, I think David intended to post some examples from Beneteau...probably forgot to add the links..?

[SAQuestor]

To be fair, Leo, I think David intended to post some examples from Beneteau...probably forgot to add the links..?

OK Will - Benefit of the doubt in that case. Regardless, there have some statements that could stand some supporting documentation. As ol' Ronny Ray-Gun once said, "Trust, but verify."

[tom28571]

If a boat is on motion on the water, it must be supported by either buoyancy, hydrodynamic lift, aero forces or a combination of these. Waves do not disappear at high speed. Once a boat is planing and almost all support is from hydrodynamic force, they do get smaller the faster the boat goes but the energy in them is nearly constant relative to time. That is, the work done by the water in supporting the boat must be constant, disregarding aero forces. Work done has a time element so the waves are spread out over a greater area of water due to the speed of the boat, thus appearing to be of lesser importance. Unless we can somehow get rid of Newton, this must be true.

Dynamic lift and momentum imparted to the water are exactly equal and opposite vectors. Doesn't matter whether it's a regular planing monohull or a stepped hull. Momentary imbalance of these forces do occur but the average must be equal to the weight of the boat or the boat will either sink or fly. There are other minor considerations but the above covers the basics.

[Excalibur]

Can I get back a page or two and return to seakeeping? The question was asked if real seakeeping abilities are needed. I'm not certain if the need to brest 25 foot waves and live is there, but there can be consideration given to how the boat will react when it's just a little snotty. I'll go fish the Chesapeake in 2-3 feet (real feet, not tall tale feet) if the boat I'm in has a comfortable motion in a chop, but it will stay at the dock if it snap rolls me right out of my seat (been there, done that). Is either the Devlin or the Atkins Seabright known for it's easy rolling motion while trolling in a chop? Would either boat be able to maintain cruise speed in 2-3 foot waves?

[SAQuestor]

Can I get back a page or two and return to seakeeping?
Please!

The question was asked if real seakeeping abilities are needed. I'm not certain if the need to brest 25 foot waves and live is there, but there can be consideration given to how the boat will react when it's just a little snotty. I'll go fish the Chesapeake in 2-3 feet (real feet, not tall tale feet) if the boat I'm in has a comfortable motion in a chop, but it will stay at the dock if it snap rolls me right out of my seat (been there, done that). Is either the Devlin or the Atkins Seabright known for it's easy rolling motion while trolling in a chop? Would either boat be able to maintain cruise speed in 2-3 foot waves?

A bit of clarification if you please. A 3'er is about waist high on me. (Chest high on my buddy :eek: ) Depending on the period of the wave train, waves that high could be a nuisance or more serious. So in the areas you frequent, would you say the wave period is very short, i.e., close together from crest to crest or what exactly?

I've never been aboard a boat on the Chesapeake, but I must assume that there are no swells underlying these 3'ers. Right?

Next, what's your definition of reasonable cruise speed in the conditions you frequent for a boat that's in the mid-20 foot range like some Devlin and Atkin designs? 15 knots? 12? 10? 8?

Thanks for clarifying the conditions you're asking about.

Leo

[u4ea32]

Sorry, got distracted (dinner was ready) and forgot the links and the total data.

The conclusion: the semi planing boat is about as good in low planing speeds, and much worse the rest of the time.

The two boats:
Beneteau Swift Trawler 42, 33000 lbs displacement
Beneteau Antares 13.80, 33960 lbs displacement

8kts:
8kts: ST42 burns 5.3 gph
8kts: AT13 burns 4.2 gph -- 25% better mileage

10.3kts: ST42 burns 12.7 gph = 0.81 nmpg -- 6% better mileage
11.0 kts: AT13 burns 14.3 gph = 0.77 nmpg

15.5 kts: ST42 burns 21.5 gph = 0.72 nmpg
14.3 kts: AT13 burns 19.3 gph = 0.74 nmpg -- 3% better mileage

23.3 kts: ST42 burns 37 gph = 0.63 nmpg
24.0 kts: AT13 burns 31.7 gph = 0.75 nmpg -- 20% better mileage

28.5 kts: AT13 burns 39.1 gph = 0.73 nmpg

Full reports:

http://www.beneteauusa.com/wps/wcm/resources/file/eb695e0c1d6f113/SEA%20TRIAL%20TEST%20REPORT-Antares%2013.pdf

http://www.beneteauusa.com/wps/wcm/resources/file/eb39584a0a2fdf9/SEA%20TRIAL%20TEST%20REPORT%20-%20ST%2042.pdf

[Willallison]

1st conclusion - both are pretty thirsty! (But typical)

[u4ea32]

Hi Tom,

I've read your posts many times over the years. This is a very good argument:

If a boat is on motion on the water, it must be supported by either buoyancy, hydrodynamic lift, aero forces or a combination of these.

Waves do not disappear at high speed. Once a boat is planing and almost all support is from hydrodynamic force, they do get smaller the faster the boat goes but the energy in them is nearly constant relative to time. That is, the work done by the water in supporting the boat must be constant, disregarding aero forces. Work done has a time element so the waves are spread out over a greater area of water due to the speed of the boat, thus appearing to be of lesser importance. Unless we can somehow get rid of Newton, this must be true.

Dynamic lift and momentum imparted to the water are exactly equal and opposite vectors. Doesn't matter whether it's a regular planing monohull or a stepped hull. Momentary imbalance of these forces do occur but the average must be equal to the weight of the boat or the boat will either sink or fly. There are other minor considerations but the above covers the basics.

The thing about waves is that the longer the wave, the more energy they consume.

Read this... Dang! I hate losing my references, I'll have to find it. The paper I am looking for is by Fox Associates in Seattle, Washington on the All American built Condor Express, launched in February 2002. They studied the wave energy of the ferry as it passed through a narrow strait (Rich Passage) at various speeds. The interesting thing, to me, was that as the boat hit a certain speed, their wave train wave length was halved (I can't remember why, had something to do with bottom configuration) and the wave energy at that point plunged: a big discontinuity. The reason: the volume of a wave is the square of the wave length, as both the length and height drop. The volume is where the energy goes, so it is proportional to the wave drag.

Now this ferry was using a foil for hydrodynamic lift, but a lifting bottom is exactly a supercavitating foil. In fact, the ferry drag dropped when the foil started supercavitating.

So don't let the foil stuff confuse the issue.

A stepped hull is so much more efficient than a non-stepped planing hull for exactly this reason: two little waves instead of one big wave.

Also, each planing surface becomes higher aspect, and on a lifting surface (like the supercavitating foil that is the bottom of a planing boat) increasing the aspect ratio increases lift to drag. For the same reason, by the way: a shorter wave.

So don't think that all waves that provide the same amount of hydrodynamic lift are the same. They are not.

[u4ea32]

1st conclusion - both are pretty thirsty! (But typical)

Right! They are both HEAVY!!!

Gotta be light to be efficient.

[u4ea32]

Seakeeping: Long and thin makes slower accelerations.

Slow speed through the water helps too.

A semi-planing hull is smoother than a typical equivalent planing hull because more of its length stays in the water, and because it goes slower.

You can make a planing hull act like a semi-planing hull simply by having sufficiently large trim tabs: it will be smooth, slow, and burn lotsa fuel.

[u4ea32]

Please!

Next, what's your definition of reasonable cruise speed in the conditions you frequent for a boat that's in the mid-20 foot range like some Devlin and Atkin designs? 15 knots? 12? 10? 8?

Thanks for clarifying the conditions you're asking about.

Leo

Out here near Los Angeles, the typical weekend has a lot of boat wakes, and the wind kicks up in the afternoon from about 8 knots at noon to a peak of about 15 knots at sunset, pretty much every day (thermals from cold ocean to warm land). The chop build up to about 3 feet trough to peak. The swells are long, and so don't come into play under 15 knots.

I find that a very fine entry boat from 20 to 40 feet long can easily make a very smooth 12-15 knots. No real improvement going slower.

[tom28571]

A stepped hull is so much more efficient than a non-stepped planing hull for exactly this reason: two little waves instead of one big wave.

Also, each planing surface becomes higher aspect, and on a lifting surface (like the supercavitating foil that is the bottom of a planing boat) increasing the aspect ratio increases lift to drag. For the same reason, by the way: a shorter wave.

So don't think that all waves that provide the same amount of hydrodynamic lift are the same. They are not.

Yes, there are a lot of factors that can change the effect of wave interaction with the boat. The bulbous bow comes to mind. Newton is not contradicted, just fooled a bit. Wave collector chines and the Hickman sled are two more. Some power trimarans also use waves created by the central hull to add lift to the amas and thus reduce wave size away from the boat.

A stepped hull has a huge amount of longitudinal stability which allows both planing surfaces (single step, of course) to have two good features. They are a constant and optimum angle of incidence and, as you point out, a high beam length ratio of the planing surface. I know the latter runs counter so the beliefs of many, but a high aspect ratio is just as important to a planing boat as it is in a boat sail or a sailplane wing. And yes, the negative effects of wake from two smaller waves is less than that of a single wave but the total energy to support the boat must remain the same.

[u4ea32]

And yes, the negative effects of wake from two smaller waves is less than that of a single wave but the total energy to support the boat must remain the same.

Exactly.

[Excalibur]

Please!



A bit of clarification if you please. A 3'er is about waist high on me. (Chest high on my buddy :eek: ) Depending on the period of the wave train, waves that high could be a nuisance or more serious. So in the areas you frequent, would you say the wave period is very short, i.e., close together from crest to crest or what exactly?

I've never been aboard a boat on the Chesapeake, but I must assume that there are no swells underlying these 3'ers. Right?

Next, what's your definition of reasonable cruise speed in the conditions you frequent for a boat that's in the mid-20 foot range like some Devlin and Atkin designs? 15 knots? 12? 10? 8?

Thanks for clarifying the conditions you're asking about.

Leo


The Chesapeake is broad and shallow. It's 4 miles wide at the top, 30 miles wide at the bottom, and about 200 miles long. The average depth is only about 20 to 30 feet. With 20 knots of wind from the (worst case) NW, it builds a very vertical chop that measures about 2-3 feet from trough to crest with a short period. I don't think I've ever been foolish enough to be in the Bay with any conditions that would build swells. The chop though, gets up very fast, and because it is very vertical, can cause pounding easier than you might think. My (current) 38' x 12' 10.5 ton semi planing hull bridges that chop very nicely, and my speed is not regulated by it. I expect 32 feet would do so as well. Cruise on my boat is 12.8 kts @ about .9 gph. (should read .9nmpg edit). I'm satisfied with that cruise speed (although like most, more would be better), but I hate feeding the twin 454 monsters that drive it. My dreamboat for the bay has minimal accomodations (porta potti, canvas bunk, bait prep sink, a place to keep dry), has a slow even roll in the above conditions, is diesel inboard, and can carry enough gear to make 4 recreational fishermen happy. It cruises at 12.5 knts or better, and gets at least 3nmpg at that speed. The classic work boat for the Bay (the Chesapeake Deadrise) is about 35 to 45 feet long with a beam of nine to twelve feet. It has a fine entry and a shallow V at the transom. It is a fine boat, but with a gross tonnage of 10 to 15 tons it's not much more efficient that what I have. I've been looking at some of the largest Atkins designs in the 28-29 foot range. The Devlin I've been admiring is 32' 10". Of the two, the Devlin would certainly be easier to build, but it has a very conventional semi displacement hull and I am wary of Devlins claims of 18 knts and 4.5 nmpg at that rate.

[tom28571]

Cruise on my boat is 12.8 kts @ about .9 gph. I'm satisfied with that cruise speed (although like most, more would be better), but I hate feeding the twin 454 monsters that drive it. My dreamboat for the bay has minimal accomodations (porta potti, canvas bunk, bait prep sink, a place to keep dry), has a slow even roll in the above conditions, is diesel inboard, and can carry enough gear to make 4 recreational fishermen happy. It cruises at 12.5 knts or better, and gets at least 3nmpg at that speed. The classic work boat for the Bay (the Chesapeake Deadrise) is about 35 to 45 feet long with a beam of nine to twelve feet. It has a fine entry and a shallow V at the transom. It is a fine boat, but with a gross tonnage of 10 to 15 tons it's not much more efficient that what I have. I've been looking at some of the largest Atkins designs in the 28-29 foot range. The Devlin I've been admiring is 32' 10". Of the two, the Devlin would certainly be easier to build, but it has a very conventional semi displacement hull and I am wary of Devlins claims of 18 knts and 4.5 nmpg at that rate.

Surely that is a typo and you mean 9gph not .9gph.

I also am fond of Devlin's Topknot. It would be serious build project though and those numbers do sound pretty generous. Maybe he got them from a client.

[moTthediesel]

Surely that is a typo and you mean 9gph not .9gph.

Aw shucks, I was gona ask him about his special carburetors that give that kind of economy from a pair of BBC's.

Perhaps the new "Loaves and Fishes" model that I've heard Holley is working on :D

moT

[Excalibur]

Yup, brain f**t. It should read .9 nmpg. I don't think you want my carbs :)

[kengrome]

Some power trimarans also use waves created by the central hull to add lift to the amas and thus reduce wave size away from the boat.

I think this is what Atkin tunnel-stern Seabright skiffs do, don't they?

[SAQuestor]

The Chesapeake is broad and shallow. It's 4 miles wide at the top, 30 miles wide at the bottom, and about 200 miles long. The average depth is only about 20 to 30 feet. [snips]

I ran across this aggregation site the other week and it seems to me that you might find some value here. (http://midcoastyacht.com/A-downeastboats.html)

Additionally, it appears to me that any one of these boats (http://www.yachtworld.com/privatelabel/listing/cache/pl_search_results.jsp?slim=pp253878&cit=true&sm=3&type=(Power)+Downeast&man=dyer&fromLength=&toLength=100&luom=126&fromYear=&toYear=&fromPrice=&toPrice=&currencyid=100&hmid=&ftid=&enid=&city=&spid=&r)might fit your needs to a tee. YMMV and all that.

Best,

Leo

[Excalibur]

Thanks, Leo. The Dyer does indeed seem to come very close to what I am looking for, except that I am looking for a boat to build, not buy. That's why I asked about the Atkins and Devlin. As far as what is different about the seabright skiff, it's my (limited) understanding that the box keel and reverse deadrise are thought to create lift at cruise speed, and delay the point that the hull begins to dig a hole in the water. The only thing I can see that is advantages about my semi planing hull is what I mentioned earlier; it maintains a smooth crusing speed as the seas get up better than a pure planing hull. I think it's partly because it's still got it's forefoot wet, and partly because it remains closer to it's optimum design speed (for that hull) in the lumps. My old Mako was a pretty nice boat and was reasonably efficient on plane, but it was hard to keep it on plane in a chop because of the pounding. It just needed too much speed to plane. I eventually put big tabs on it so I could run in between it's waterborne speed and it's planing speed. It was expensive, but it was a lot better running 42 miles home at 12 knots than it was at 5.5. Of couse I was fishing the gulf stream then and swells WERE a consideration there :)

[kengrome]

It's my (limited) understanding that the box keel and reverse deadrise are thought to create lift at cruise speed ...From everything I have read about tunnel-stern Seabright skiffs, I believe this is true. Robb White said his rendition of Rescue Minor lifted its aft end higher as more power was applied. This naturally pushes the bow down and insures that it stays in the water instead of rising up and allowing waves to hammer on the bottom of the boat in choppy conditions.

... and delay the point that the hull begins to dig a hole in the water.I think these boats have zero tendency to squat because they simply do not dig a hole in the water like many other boats ...

As more power is applied the aft end rises up because of the downward thrust of water created by the hook in the aft most section of the tunnel portion of the hull. Since most of the boat's weight is being carried by the long, slim, double-ended flat-bottom box keel anyways, the boat simply accelerates smoothly from displacement to planing speed with little or no pitch change.

Robb White said he thinks the reason these hulls are so fuel efficient and easily driven is because the aft tunnel section of the hull traps the energy created by the stern wave of the box keel, so the boat basically surfs on its own stern wave. Interesting theory ... as this would also explain the small wake created by these hulls.

[Willallison]

I'd still like to see some real data that suggests that the SB skiff is any more efficient than a comparable vessel of 'normal' form.
There's no doubt that the downturn in the tunnel creates upward pressure, but does it simply offset the loss of lift created by having what is in effect a giant prop tunnel? And the added wetted surface of the box keel must be of some detriment...

Hey - I like the boats, but some of the apparent efficiency must come from their light weight.

[kengrome]

I'd still like to see some real data that suggests that the SB skiff is any more efficient than a comparable vessel of 'normal' form.Me too, but even if tunnel-stern Seabrights are not any more efficient than comparable 'normal' vessels they still have other virtues that make them attractive:

1- Extreme shallow-draft that only a jet or airboat or "tunnel hull" (as opposed to tunnel-stern) can match while under power.

2- Easily beachable and sits upright and stable on its flat box keel bottom.

3- Stability that far exceeds the boat's size and weight while running -- this according to Robb White who had no reason to embellish the truth like the original designer might have.

4- Relatively flat running at all speeds -- they never have to climb out of a hole to get on plane, instead they perform consistently throughout their entire design speed range.

There's no doubt that the downturn in the tunnel creates upward pressure, but does it simply offset the loss of lift created by having what is in effect a giant prop tunnel?This is possible of course, but I don't think so. This is an inboard boat with the engine near the middle of the boat so it doesn't need the broad flat immersed bottom that most ouboard require to support all that weight aft while at rest.

Robb White said the hull sits a few inches high at the bow while the boat is standing still (with people in it I think, but maybe when empty too) ... and then as power is applied the stern gradually lifts, pushing the bow down and keeping it in the water. I suspect that this slightly bow-high attitude is partly the result of the aft portion of the boat not having a broad flat bottom that is already in the water until after the boat is moving, and partly where the people are located in the boat.

One thing Robb said is that when the boat is underway it doesn't matter where the people are located because the boat rides flat all the time and feels like it weights 10,000 pounds -- which I took to mean that it feels far more stable when underway than a boat this size should feel -- and that's a "good thing" in my opinion.

I think the prop is sucking water into the tunnel section from forward then pumping it out of the tunnel section fast enough to create a suction between the hull bottom and the surface of the water. A suction like this would probably be so strong that it would easily resist the movement of people in the boat -- and keep the hull vaccumed tightly to the surface as long as the tunnel is full of water -- which of course depends up the prop spinning.

And the added wetted surface of the box keel must be of some detriment...Some writer said something like this too, but I think just the opposite is true. I think the box keel acts like a super-efficient displacement hull (like a catamaran or kayak hull for example) that supports most of the weight of the boat and its contents. This portion of the hull would naturally be easily driven through the water.

I think the boat smoothly accelerates from a standstill all the way through the range that most people refer to as "semi-displacement speeds" without trying to bury its aft end because the box keel part of the hull is effectively still running in displacement mode until the boat is very near its top speed.

During this acceleration, long before the boat begins to close in on the box keel's "hull speed" and starts trying to squat (assuming it ever does try to squat), the water has filled the tunnel and is gradually contributing more and more support or 'lift' to the aft bottom section of the hull, thus counteracting any attempt to squat.

I also suspect that this is the reason why these hulls do not feel safe when you overpower them. I think the box keel section gets unstable when you push it too fast. This section of the hull can probably be pushed a bit beyond its theoretical hull speed just like any other displacement hull can ... but if you push it too far it will likely begin to behave badly, and then you're no longer running safely. Better to run these boats with only as much power as they need, after all they are not speed demons to begin with.


Hey - I like the boats, but some of the apparent efficiency must come from their light weight.The Atkin boats were never designed to be "light" but William Atkin claims they are efficient. Then again he's a designer and we all know that designers will brag about their boat's capabilities even though the boats themselves may not live up to the hype. Unfortunately the only recently built tunnel-stern Seabright skiff I'm aware of is the one built by Robb White, and with him gone we really have no other similar boat to look at until someone else finishes one.

I'm building one now, but it won't be done for a while, and even when it is completed and on the water I don't know how it might compare with other similar boats because I still have a problem defining what is 'similar' to these boats.

Note that I'm not building it primarily for the fuel efficiency either, although it would be nice to see the numbers some people have reported. My favorite feature of this boat is its extreme shallow draft combined with its reportedly excellent seakeeping ability, and the fact that the boat can be beached with no worries about prop or rudder damage. It should be able to run over things in the water more safely than an outboard too because nothing extends below the flat bottom box keel. To me this suggests excellent potential durability and reliability -- with fuel economy a big plus if it is actually there.

[fcfc]

From everything I have read about tunnel-stern Seabright skiffs, I believe this is true. Robb White said his rendition of Rescue Minor lifted its aft end higher as more power was applied. This naturally pushes the bow down and insures that it stays in the water instead of rising up and allowing waves to hammer on the bottom of the boat in choppy conditions.

%<...

.

There are other way fully studied ways to avoid bow up and associated resistance.
http://www.scielo.org.ar/pdf/laar/v34n4/v34n4a11.pdf
http://www.daviscoltd.com/public/Engineering/documents/PDFs%20Chris%20Barry/Hydrodynamics/Hydrodynamic%20Efficiency%20Improvements%20to%20the%20USCG%20110%20Ft%20WPB%20.pdf
http://www.amc.edu.au/system/files/Paper-1.pdf

And the best seems to integrate the stern lifting effect directly in the hull form (See page 28)
http://www.na.chalmers.se/~bathfiel/CYR/clementine_thesis.pdf

Note the drawback of bow down is wetness and some times instability. Spray rails seem to go with flaps.


On the design forum, Fast Fred says that Robb White boat is 550 lbs empty. That make a D/L of 20 empty and of 50 when 2 200 lbs people on board. I think this is certainly the key factor for performance.

[FAST FRED]

""I think the prop is sucking water into the tunnel section from forward then pumping it out of the tunnel section fast enough to create a suction between the hull bottom and the surface of the water."

I think the efficiency comes from the water already accelerated by the forward part of the boat, not suction. A prop is a pump, and all pumps push better than they suck.

If the water drug along by the hull is 3 or 4 inches deep, almost ALL the water at the prop cavity will contain energy , to be reused when more power is added by the prop.


"And the added wetted surface of the box keel must be of some detriment...
Some writer said something like this too, but I think just the opposite is true. I think the box keel acts like a super-efficient displacement hull (like a catamaran or kayak hull for example) that supports most of the weight of the boat and its contents. This portion of the hull would naturally be easily driven through the water."

I believe the submerged box keel will not generate surface waves , as a submerged submarine does not.

IF The 60% or 75% of the displacement in the box does'nt create waves (and loose energy) , all we are "paying for" is moving 25% of the hull weight .And I'm not sure the reversed V has much more wetted surface than a stock V boat.

"I think the boat smoothly accelerates from a standstill all the way through the range that most people refer to as "semi-displacement speeds" without trying to bury its aft end because the box keel part of the hull is effectively still running in displacement mode until the boat is very near its top speed.


During this acceleration, long before the boat begins to close in on the box keel's "hull speed" and starts trying to squat (assuming it ever does try to squat), the water has filled the tunnel and is gradually contributing more and more support or 'lift' to the aft bottom section of the hull, thus counteracting any attempt to squat.

I also suspect that this is the reason why these hulls do not feel safe when you overpower them. I think the box keel section gets unstable when you push it too fast. "

The two write ups on Atkin hulls that were overpowered said the steering got wonky at top speed .
Atkins cure was a simple lifting strake to allow the bow to rise to the hight of the stern.

MY cure would be to use OTS trim tabs to adjust the bow hight.

At "normal" speeds the tabs would be part down to allow the stern area to perform as the stabelizing lifting surface and carry weight at the proper angle.

At the highest speed the tabs might need to be thinned to reduce the aft lift ,stop depressing the bow , and allow the bow to run on the normal waterlines.

Yes, the Rob White boat was light , but 28nmpg at 10.5K is a claim I have never seen anywhere else.

The Atkins boats were claimed to be flat in turns at speed. Not sure if this is good or bad , but Rob White got to like it.

With tabs a small one sided trim could have the boat bank into a turn if that was considered desirable.

With a fancy joystick steering , it could be automatic.

FF

[Willallison]

FF-
I actually think the SBS has great merit for your application. As you say, there are many other attributes that make it a worthy contender. I would expect that the box keel configuration ought to suffer less from the sort of overloading that becomes so common on board cruising boats too. I know there are a number of people - myself included - that have at least toyed with the idea.

But, there's been a great deal made of its apparent fuel economy And as it was designed (and I certainly believe it was designed to be light, as fcfc has demonstrated) I'm just not convinced that it's any more efficient than a simple lightweight boat of 'normal' form would be.

I can't wait till you have yours in the water - then we'll all know!!:p

[SAQuestor]

Thanks, Leo. The Dyer does indeed seem to come very close to what I am looking for, except that I am looking for a boat to build, not buy.

You're welcome. Some of the links here (http://midcoastyacht.com/A-downeastboats.html) - don't know which ones now - are advertising bare hulls for sale. That might be an alternative to building the entire boat. Find the hull that meets your performance requirements and finish it out as your needs and likes dictate. Certainly an alternative to be considered.

Best,

Leo

[kengrome]

"I think the prop is sucking water into the tunnel section from forward then pumping it out of the tunnel section fast enough to create a suction between the hull bottom and the surface of the water."I think the efficiency comes from the water already accelerated by the forward part of the boat, not suction. A prop is a pump, and all pumps push better than they suck.Sorry for the confusion here Fred. When mentioned suction I was talking about stability, not efficiency. Personally I don't think the suction has anything to do with efficiency, but I think it has everything to do with the "feels like the boat weights 10,000 pounds" stability Robb White experienced while under way.

I believe the submerged box keel will not generate surface waves, as a submerged submarine does not.I don't think it matters much if the box keel's stern wave breaks the surface or not. The box keel will still generate a wave, and that wave will be trapped beneath the hull -- which means the aft hull can "get a free ride" on this wave. Surfing your own stern wave is a "good thing" in terms of efficiency I believe ... :)

I'm not sure the reversed V has much more wetted surface than a stock V boat.Looking at the hull I'm building now, I can tell you that *if* the inverted vee has any more surface area than a regular vee, it is certainly minimal.

The two write ups on Atkin hulls that were overpowered said the steering got wonky at top speed. Atkins cure was a simple lifting strake to allow the bow to rise to the height of the stern.I think I read this too although I don't remember where at the moment. This brings up an interesting point. If you take a good look at Shoals Runner in profile you will see that its chines are in the water by design:

http://www.boat-links.com/Atkinco/Utilities/images/ShoalsRunner-2.gif

Now take a look at Rescue Minor and note that RM's chines are above the waterline:

http://www.boat-links.com/Atkinco/Utilities/images/RescueMinor-2.gif

Why the difference? Maybe William Atkin discovered something about RM (design #500) that was "not so good" and maybe he thought he could fix it by keeping the chines in the water in future boats -- such as Shoals Runner which came along several years later as design #786. Probably not the case but it's a possibility. In any case here's one of my current theories on this issue:

If the aft chines are in the water by design, the tunnel won't need as much downward hook in it to counteract squatting because the chines in the water will help float the aft section of the hull all the time instead of only when enough power is added. This will help prevent squatting earlier. This also means the prop wash could be directed more horizontally by using less downward hook in the tunnel, and then the boat won't try to bury its bow as much.

I suspect that this could be one of the reasons for the "in the water" chines of SR. Of course we have no way of knowing this, but we do know that SR is a later -- and therefore theoretically or potentially better (?) design than RM, which is why I'm designing a new tunnel-stern Seabright skiff based more on the lines of SR than RM.

My design uses built-in spray rails to help lift the bow (and keep the spray down of course). The spray rails are designed after the Tolman Widebody and Jumbo skiffs which incorporate them into the hull itself rather than just tacking them onto the outer hull later. Not only does this strengthen the hull using less material, but it also creates more interior volume above the spray rail line:


http://www.bagacayboatworks.com/linkfiles/xbbskiff22rails.png


http://www.bagacayboatworks.com/linkfiles/xbbskiff21small.jpg


My cure would be to use OTS trim tabs to adjust the bow hight. At "normal" speeds the tabs would be part down to allow the stern area to perform as the stabelizing lifting surface and carry weight at the proper angle. At the highest speed the tabs might need to be thinned to reduce the aft lift, stop depressing the bow, and allow the bow to run on the normal waterlines.I think this sounds like a great solution because it also means not building as much hook into the aft tunnel section of the hull bottom. Then again, avoiding the expense and requirement for trim tabs means a lower price and simpler operation with fewer things to go wrong ... and I like 'cheap and simple' whenever possible.

[fcfc]

I don't think it matters much if the box keel's stern wave breaks the surface or not. The box keel will still generate a wave, and that wave will be trapped beneath the hull -- which means the aft hull can "get a free ride" on this wave. Surfing your own stern wave is a "good thing" in terms of efficiency I believe ... :)


I do not understand. There is a direct relation between a wave system wavelength and its speed. v = 1.34 sqrt (L). or ( u= sqrt(gL/2PI) is SI units).
So at speed above 8.5 kts, the next crest is more than 40 ft aft the first one. If the hull is 20 long, it is totally on the aft slope of the bow wave. I do not understand how it can surf something.

If the speed is below the hull speed, the wave system is shorter than the hull, and there is something to surf or recover. That why the resistance is considerably lower for speeds under hull speed. (or expressed the other way, the resistance is considerably higher for speeds above hull speed).

And if you are speaking of staggered hulls, perhaps there is a speed at which wave system cancels, but you should know that there are other speeds at which wave systems do ADD. This technic is known for nearly one century with bulbous bow design. And although this technic is known and mastered for big ships, no small boat under 70 ft has really gained something measurable in real conditions with this.

[kengrome]

I do not understand.I'm not sure I do either, but Robb White said he thinks his boat surfs on its own stern wave ... and if it does, maybe that's one reason why it gets such great fuel economy.

There is one hull on top of another in this boat. The bottom hull is long and skinny and ends in a sharp edge. Most people refer to as a "box keel". The other hull sits on top of this and ends behind the box keel, so it is the upper hull's aft end that might surf on the lower hull's stern wave.

There is a direct relation between a wave system wavelength and its speed. v = 1.34 sqrt (L). or ( u= sqrt(gL/2PI) is SI units). So at speed above 8.5 kts, the next crest is more than 40 ft aft the first one. If the hull is 20 long, it is totally on the aft slope of the bow wave. I do not understand how it can surf something.Robb said "stern wave" not bow wave so maybe you and he are talking about different waves? Or maybe the boat is surfing down the face of the first crest, not the second? Or maybe the tunnel-stern "reshapes" the wave in such a way that it produces the feeling of surfing?

We do have a propeller thrusting water backwards aft of the box keel and forward of the aft end of the inverted vee hull, too. I don't think we can ignore the influence of this item, although I have no idea how to predict what unusual effects it may create. in terms of its interaction with the boat-generated waves.

If the speed is below the hull speed, the wave system is shorter than the hull, and there is something to surf or recover.I would guess that hull speed for a double ended box keel is pretty high since it is so long and skinny. Do you know what might be a reasonable guess for a hull speed here? I think it is probably about 2 feet wide and maybe 16 feet long or perhaps a bit longer.

And if you are speaking of staggered hulls ...If you mean one hull on top of another with the lower hull ending forward of the upper, that's exactly what we have in a tunnel-stern Seabright Skiff.

... perhaps there is a speed at which wave system cancels ...Maybe this is what's going on then. All I know is that there are lots of good things said about tunnel-stern Seabright skiffs and economy is just one of them.

[Excalibur]

Looking at the lines above, I am struck by how the curve of the hull aft of the box keel describes a kind of expansion section. Is it possible that the lift comes from the sudden lessening of flow rate aft of the keel as the water flows into the larger area? There is certainly incidental lift from the hooked stern, but there may be induced lift there as well, which is a more efficient way of generating lift.

BTW, the one thing I can comment on a little is submerged hulls. Submarines most certainly DO create detectable waves, and under certain conditions can be tracked by those waves, even though they are submerged.

[kengrome]

Interesting concept, I never thought of lift being generated like that before. I wonder how the propeller -- being in the tunnel between the box keel and rudder -- might relate to the generation of lift in this manner?

[SAQuestor]

Looking at the lines above, I am struck by how the curve of the hull aft of the box keel describes a kind of expansion section. Is it possible that the lift comes from the sudden lessening of flow rate aft of the keel as the water flows into the larger area? There is certainly incidental lift from the hooked stern, but there may be induced lift there as well, which is a more efficient way of generating lift.

Hmmm… Isn’t it interesting that a comment like this can set off a chain of thoughts. Mine follow.

Instead of flow rate, let’s think about pressure.

We know that liquids are (for all intents and purposes) incompressible. But we do know that we can apply pressure through various means to a closed system and cause at least a reaction, if not real work to be done, in a different part of the closed system.

Thinking back to my basic hydraulic instruction and differential areas of pistons in a closed system. If you’ve ever studied hydraulics you probably remember the diagrams. Essentially, they have a large piston moving a small distance causing a large movement in a smaller piston – or vice versa.

The key here is the differential in area of the two pistons. A 10 sq in area piston will move a 5 sq in area piston twice the distance the other one is displaced. – And vice-versa.

That’s all fine, well and good. But what about lifting capacity? If piston 1 exerts a force of 10 psi on a fluid, piston 2 also has 10 psi working on its area. But if the area of P2 is larger than P1, P2 will exert more force on an external load.

Seabright hull form. Water, being virtually incompressible, and in this case virtually immovable, the boat being both movable in the horizontal plane via a propeller and moveable in the vertical plane via lift.

Water moving around the box keel – which at its aft end comes to a point – gains some amount of pressure as it first passes by the hull form (box keel and immersed chines) and then by and through the turning propeller. As it passes the propeller, the higher-pressure water has a “cavity” with which to dissipate pressure into. Since water can’t expand or compress, all it can do is be affected by pressure.

Hypothetical numbers only…

Passing the end of the box keel and through the propeller the water column has had imposed on it a pressure of 10 psi. That 10 psi affects an area of – for arguments sake only – 314 sq in – which happens to be the area of a 20” diameter propeller.

Once past the propeller that 10 psi has the opportunity to dissipate into a cavity - bounded on the bottom by incompressible (mostly) stationary water (that the hull is passing over) and on the top by a concave area of the hull. The sides of this column of pressurized water is bounded by water that has had some velocity imparted by the moving boat hull, but still at some lower velocity than the hull is moving at.

So we have 314 sq in of 10 psi water moving into an area of lower pressure and therefore the 10 psi can begin to dissipate into the larger area. Some of said 10 psi is going to exit the rear of the tunnel providing some forward thrust and, via the hook in the hull form, imparting some upward component to said thrust, thus forcing the bow (the other end of a lever) down as observed by White with his RM.

Let’s speculate that the area of the 'tunnel' is 10 times the size (in square inches of area) of the aforesaid water column in square inches – 3,140. Let’s further speculate that this water column doesn’t lose all its pressure when being thrust out the back of the tunnel by the force of the turning propeller, but only 50%. So if my math is correct, that 10 psi column of water would dissipate 50% of its pressure into an area of 3,140 sq in, which at the remaining 5 psi, would exert a force of 15,700 pounds.

Logically, there is not 15,000+ pounds of force pressing the hull skyward. Some amount of that pressure is going into the surrounding water and some percentage of that is exiting out from under the sides of boat as it moves along. What percentages? I couldn’t even begin to take a SWAG – and I doubt that there is anyway to actually measure something like this without extensive and expensive equipment.

But even if 10% of that 15,700 pounds were exerted against the hull – that’s 1,570 pounds of positive lift.

All of this speculation means that if the Seabright hull form is creating some sort of pressure wave in a relatively small area and that said pressure then expands into a larger area, then the result would be the behavior that has been described by Robb White, i.e. the solid feeling and the bow assuming a nose down attitude as the boat increases speed.

The key must be the box keel some how produces some hydraulic pressure.

Convoluted. Yep. But let’s see if this very speculative hypothesis will hold together after some examination by the denizens of this keep.

Best,

Leo

[FAST FRED]

I think Leo has the idea.

Some of the boats resistance comes from the bow forcing the water aside.

But the speed handicap is when the stern sinks , as the boat climbs the bow wave.

With the aft section being held up by the prop wash , the boat doesn't suffer the handycap of climbing a wave , simply pushing it aside, as it runs "on top" all the time.
Displacement being carried by the submerged box keel.

The Atkins boats were reputed to be similar to the Sea Bright Skiffs in being able to haul immense weights of fishing gear, at speed thru breaking inlets.

So light weight might be required for the best NMPG , but not to operate the boat.

I would speculate the stock Atkin power requirements were to get economy of operation , with out the higher stresses of high speeds , which would be harder and more costly for a home builder.

FF

[Excalibur]

It doesn't seem that it would be that hard to test the basic theory. A pipe and valve extending above the waterline fitted to a through hull just in front of the prop would either jet water when you opened the valve, or suck air. I'm betting it will jet water. I'm also betting that opening the valve at high speed will spoil some of the lift and help the boat remain stable at the limit.

[kengrome]

A pipe and valve extending above the waterline fitted to a through hull just in front of the prop would either jet water when you opened the valve, or suck air. I'm betting it will jet water.I will take the opposite bet because I think it will suck air, here's why ...

The propeller suctions water that's in front of it, then pressurizes water that's behind it. This suggests (to me anyways) that a pipe installed in front of the propeller will suck air since that portion of the water is under suction, not pressure.

[FAST FRED]

A different way to look at it is with jet aircraft engines.

Long ago a very small diameter engine took some air and pushed it aft at tremendous speeds . noisy and not very efficient.

Same as our std boats , where a prop pushes a stream of water , the bigger geared prop does a better job , but its still the same. Just an un-ducted water pump, tip losses, shaft angle losses ...

A modern turbofan uses a 10ft diameter prop , in a nice ducted fan cage , with the engine exhaust a very minor portion of the propulsion effort.

Both are just air pumps , but the later units move more air , at a lesser speed , with far more efficiency.

I think the reverse deadrise is fed accelerated water and the propeller accelerates the entire water volume thats contained in the aft section by the hull sides.

Result is a larger water mass moving aft more efficiently that also expends some energy in preventing the transom from going down,reducing drag.The large water mass may be the stern wave of the box keel.

So we get the pluses added from more efficient propulsion , a better performing hull aft , and the lessened wave making of the submerged box keel.

Since the boat runs flat , we get good sea keeping and a sea kindly ride.

Add everything up and it would seem Atkin was on to something !

FF

[kengrome]

I think the reverse deadrise is fed accelerated water and the propeller accelerates the entire water volume thats contained in the aft section by the hull sides.I think this is true, but I still wonder if leaving the aft sides of the tunnel section higher (Rescue Minor) is better or worse than putting the sides down in the water (Shoals Runner).

Robb White is the one who claims such great fuel economy and he built "something like" Rescue Minor ... but was it really like the RM sketches and plans? We know he created soft chines instead of using the hard chines spec'd by Bill Atkin, but were the bottom of these chines higher or lower than those in the RM drawings?

[FAST FRED]

The sides of the tunnel will need to be IN the water (to contain the thrust) when the boat is operating.

The static at rest condition might be more comfortable with slight immersion to stop roll.

FF

[kengrome]

The sides of the tunnel will need to be IN the water (to contain the thrust) when the boat is operating.Hi Fred, thanks for your thoughts. I wish I were as convinced as you are. Rescue Minor's design appears to shows the sides of the tunnel out of the water or possibly just barely touching the waterline, so I cannot help but wonder if there is more to it than this.

I did a silly little boat design in another thread that you might want to take a look at, it shows the tunnel sides partly open in the forward portion of each "half tunnel", then the sides drop down further aft when the two half-tunnels become one:

http://www.boatdesign.net/forums/showthread.php?t=17562

Granted this is a short fat little boat with a very wide box keel, so maybe this is the "right thing" for this type of boat but not for other boats ...

I think it's the right way to design this particular boat because those chines are still "in the water" even though I forgot to show the waterline in the design. Basically I left some space along the widest portion of the aft box keel (where the two half-tunnels begin) for water to flow back under the hull after it is pushed aside by the front portion of the hull, then I dropped the chines down even further toward the transom.

Maybe on a longer boat this wouldn't make sense -- especially one with a long slender box keel -- because in a long slender boat the water will be able to flow under the chines and into the tunnel area with much less difficulty than in the short fat wide box keel boat.

That's my theory anyways, at least until someone else comes up with some new info to change my mind ... :)

[kengrome]

Isn’t it interesting that a comment like this can set off a chain of thoughts ...Hi Leo, thanks for the great explanation, I think you're right on the money. Your analysis brings up more questions (for me anyways):

1- What happens if we move the prop further forward -- or aft? Shall we assume that Atkin got the prop location perfect?

2- Is there a formula for positioning the prop (such as a certain percentage distance between the box keel and the transom so we can be sure to "get it right" when we design our own modern tunnel-stern Seabright skiffs?

3- Will two smaller props, one in each hull near the aft end of the box keel, perform better than a single prop aft of the box keel?

Note that this last question might make the difference between a taller tunnel and a shorter one ... and I suspect the shorter tunnel might be more efficient since less water will need to be pumped up into the tunnel, and therefore more pressure can be used for forward thrust.

[FAST FRED]

Rescue Minor's design appears to shows the sides of the tunnel out of the water or possibly just barely touching the waterline, so I cannot help but wonder if there is more to it than this.


MY guess would be that at RUNNING TRIM , every boat of this style will only work with the sides of the tunnel capturing the wash.

Weather the sides touch un-powered is not the question, weather they allow the design to perform IS.

FF

[FAST FRED]

MY guess would be that at RUNNING TRIM , every boat of this style will only work with the sides of the tunnel capturing the wash.

I have read that the Dave Gerr version of the Sea Bright skiff was unsuccessful at obtaining good operating speeds as it ventilated the prop at speed.

So a model test might be a good idea.

FF

[FAST FRED]

cOPIED,
August 1, 2002
Yes, Atkin Was a Genius
By Robb White

We launched the Rescue Minor June 20th and she ran most marvelously. There was no ceremony to it. My wife and infant redheaded granddaughter and I just wheeled her down to Lake Lamonia about twelve miles down the road and untied the jackleg lines that held her on the trailer and she rolled off into the water.

The boat sits about an inch and a half down by the stem at rest but it is actually that she is up by the bow.... the toe of the stem is right at the water. It is because the boat is so light. I knew it would be like that and hoped it wouldn't be worse. When my wife and the baby got in up in the bow, she sat right down where she belonged. I hadn't brought the engine house, so, when we fired her up, she cackled pretty loud (about like a Kubota tractor) but didn't vibrate or shake the boat at all. While she was warming up, I checked all around to see how much exhaust water the little Shurflo diaphragm pump I adapted to run off the camshaft was giving (plenty) and what the oil pressure was and all. I think I was a little scared to put the power to the wheel and see what was what.

As soon as the propeller began to revolve, the stem picked up what felt like three inches and the boat began to move much faster than I would have expected from any planing boat at dead idle. Lake Lamonia is one of those lily pad lakes we have down here and there is only a narrow trail through the bonnets out to the clear water in the middle and I had fooled around looking at the engine and let us blow off so that we were heading for the lily pads but just a hint of rudder brought the idling boat right around. I have never seen an inboard boat turn like that.

There ain't no idle zone, so I ooched up on the throttle a little bit and the boat picked up speed just like a regular boat. I gave her a little more and she gave me a little more. I ran her on up and she ran on up. There was no perceptible rise to the bow at all and the wake never changed. The little engine smoothed out so that it was hard to detect any vibration at all when I put my hand on the cylinder head to see if she was warming up or not. The boat steered so stably that I could hold the tiller and walk all around the engine to check on my doings. Which, the copper tubing wrapped exhaust manifold ran cold and the inlet pipe from the keel-cooler stayed cold. There were no oil leaks and no hint of a diesel fuel stench or exhaust but I did have a damned tiny coolant leak from the plastic overflow reservoir. The outlaw graphite ceramic well pump shaft seal never gave a drop and the belt-drive transmission ran smooth as all get out.

Which, I hope I ain't ruined my credibility too bad. She ran 18.6 knots on the gps and that with the 10" pitch propeller that I put on there to make sure I didn't lug the engine while it was breaking in. That wheel let the engine run up to where the governor backed her off at 3,600 rpm. Me and Atkin think she 12-1/2" of pitch and I have that prop standing by ready to put on there. You know, I have the jackleg push-button prop nut and can change wheels by just reaching up under there with one hand. As an aside, that's a wonderful fig. I can take the propeller off a sailboat while she is luffed up in the mouth of the river. I wish I could lay claim to the invention, but it ain't nothing but something like a quick-disconnect like on a garden hose.

The boat ran most marvelously. I would have been satisfied with 12-1/2 knots (my speed) and a slightly tender feel (about like a deep "V") but the boat was so stable that my wife and I could hardly alter the running trim by both us, and the baby, sitting on the same side. She turns about level and, even then, weight distribution doesn't seem to affect the trim. I think the dynamics of the hull that control the wake hold the boat in a tight grip. It feels like it weighs about 10,000 pounds. There was never much wake at all but, like Alex's, there was a sporty looking rooster tail erupting about 8' astern. I have a little clamshell water pickup right behind the prop to give a little supplementary exhaust water at speed, and it is mighty effective (I could probably eliminate the engine-driven pump). The beautiful exhaust-water rainbow around the rooster tail made a most charming sight.

All my fears are put to rest. She ain't tippy at rest and she don't rise up by the bow and try to skitter off on that little pirogue she carries on her belly. I couldn't make her cavitate to save my life and the boat will turn, at speed, shorter than any outboard boat I ever had. I tried to make a wake to run back across so I could see what was what with that, but the boat doesn't make enough wake for a valid test. I ran across the wake of an aluminum butt head-skiff with a nine point nine that was much bigger than the wake of the Rescue Minor. I don't need no wake in no lake to tell me how she'll do in rough water. I know a sea-boat when I see one.

It is a wonderful boat and, as Alex said, "Atkin was a genius".

[SAQuestor]

Why select a semi-planer? (http://sagaboats.no/content/view/28/39/lang,en/)

Most of us are familiar with the terms "displacement vessels" and "planing craft". We know that the maximum speed of a genuine displacement hull is limited by the length of the waves it generates. And we also know that the function of the nearly prismatic shape of the aft bottom of a true planing hull is to eliminate the speed barrier by creating hydrodynamic lift as water is forced underneath the bottom, to minimze negative pressure, and to ventilate the transom.

When accelerating at intermediate speeds, however, most planing hulls experience a rather pronounced "resistance hump" combined with steep trim angles and unpleasant wake wash. The consequence is that the driver normally will try to avoid the "hump" by passing it as quickly as possible either by speeding up or slowing down.

However, in any chop, while runninng above hump speed the planing hull may experience heavy wave slamming and accelerations, and the driver may be forced to throttle back to avoid making the trip too uncomfortable for the crew. Consequently, under certain conditions it may be necessary for planing craft to proceed at close to hump speed, which means generating a lot of waves and excess fuel consumption. Nevertheless, it is precisely the difficult transition speed range that is attractive to many people, in particular when cruising long distances. Under such conditions, boats designed specifically to operate in the so-called "semi-planning" speed range would be ideal.

Since the appearance of te original Saga 7000 design more than 30 years ago, the basic concept of the Saga wide-bodied "semi-planning" craft has been continuously refined. The present family of Saga boats demonstrated the same qualitites of outstanding seaworthiness, manoeuvrability, directional stability and seakindliness as the Saga 7000.

The Saga boats are intentionally designed to operate withing the tricky transition speed range. They are by origin true "semi-planing craft" characterized by a full length keel, ample lateral area and proper balance between the forebody and afterbody. Today the word "semi-planing" is occasionally misused when characterizing boats which are more like planing craft in that they lack the power needed to get past the resistance hump. The genuine semi-planing hull, however, performes as well at dead slow as at full throttle. The Sagas are providing comfortable and safe cruising right up to the maximum speed recommended by the builder.

Eivind Amble
Naval Architect, C. Eng, Mrina, Msname

*********************************************************

Found on the Saga Boats web site (http://sagaboats.no/component/option,com_frontpage/Itemid,1/lang,en/). Thanks for the link goes to Raggi Thor in the Scandinavian boat design (http://boatdesign.net/forums/showthread.php?t=17890)thread.

Best,

Leo

[kengrome]

Most of us are familiar with the terms "displacement vessels" and "planing craft". We know that the maximum speed ...

Eivind Amble
Naval Architect, C. Eng, Mrina, MsnameI could not find a single picture of the bottom of this boat on their web site which is overloaded with hundreds of other pictures of the boat. Why not any pictures of the bottom so we can see what al the hype is about?

[fcfc]

I have neither see what is the installed power. All I have seen is that some are available as "semi planning" with deep keel and straight shaft, and as planning with stern drive (Saga 26).

[SAQuestor]

I could not find a single picture of the bottom of this boat on their web site which is overloaded with hundreds of other pictures of the boat. Why not any pictures of the bottom so we can see what al the hype is about?

Doesn't appear to be anything special. Try this on Yachtworld.
(http://www.yachtworld.com/core/listing/pl_boat_detail.jsp?currency=USD&units=Feet&id=1715147&lang=en&slim=quick&)

I have neither see what is the installed power. All I have seen is that some are available as "semi planning" with deep keel and straight shaft, and as planning with stern drive (Saga 26).

OK. Try this one on Yachtworld. (http://www.yachtworld.com/core/listing/pl_boat_full_detail.jsp?slim=quick&boat_id=1725289&ybw=&units=Feet&currency=USD&access=Public&listing_id=74556&url=) Says a 230 HP Yanmar.

Then see all 16 on this search of Yachtworld. (http://www.yachtworld.com/core/listing/cache/pl_search_results.jsp?cit=true&slim=quick&ybw=null&sm=3&is=&type=%28Power%29&man=saga&hmid=0&ftid=0&enid=0&fromLength=&toLength=&luom=126&fromYear=&toYear=&fromPrice=&toPrice=&currencyid=100&city=&pbsint=&ps=30)

Best,

Leo

[Pierre R]

Why select a semi-planer? (http://sagaboats.no/content/view/28/39/lang,en/) Today the word "semi-planing" is occasionally misused when characterizing boats which are more like planing craft in that they lack the power needed to get past the resistance hump. The genuine semi-planing hull, however, performes as well at dead slow as at full throttle. The Sagas are providing comfortable and safe cruising right up to the maximum speed recommended by the builder.
Found on the Saga Boats web site (http://sagaboats.no/component/option,com_frontpage/Itemid,1/lang,en/). Thanks for the link goes to Raggi Thor in the Scandinavian boat design (http://boatdesign.net/forums/showthread.php?t=17890)thread.

Best,

LeoI am just not sure I buy anything said about semi planing hulls anymore. I have not seen anything yet that lives up to the hype. They seem like motorsailers to me. Not good displacement boats and not good planing boats. I have not seen fantastic fuel savings in the hump range of speed over properly designed planing hulls. The only thing semi displacement seems to offer is a good ride and controllability in the hump range of speed so most people with a semi displacement hull operate in that speed range.

Convince me that I am wrong.

[SAQuestor]

Convince me that I am wrong.

Nah. I'll pass. Thanks for the offer.

As an aside, this is certainly an interesting way to enter a conversation that has been on going for over 2 1/2 months and has a 150 pretty informative posts from folks that at least have an open mind.

But, different strokes for different folks.

[kengrome]

Try this on Yachtworld.
(http://www.yachtworld.com/core/listing/pl_boat_detail.jsp?currency=USD&units=Feet&id=1715147&lang=en&slim=quick&)OK. Try this one on Yachtworld. (http://www.yachtworld.com/core/listing/pl_boat_full_detail.jsp?slim=quick&boat_id=1725289&ybw=&units=Feet&currency=USD&access=Public&listing_id=74556&url=)
Then see all 16 on this search of Yachtworld. (http://www.yachtworld.com/core/listing/cache/pl_search_results.jsp?cit=true&slim=quick&ybw=null&sm=3&is=&type=%28Power%29&man=saga&hmid=0&ftid=0&enid=0&fromLength=&toLength=&luom=126&fromYear=&toYear=&fromPrice=&toPrice=&currencyid=100&city=&pbsint=&ps=30)Hi Leo,

I checked all the pix in the links you posted, and this the best view of the bottom that I could find:

http://newimages.yachtworld.com/1/6/2/1/3/1621393_1.jpg?1168964118000

... but of course this shows absolutely nothing useful to a person who wants to learn more about designing boats that run efficently at low-end planing speeds, or semi-displacement speeds, or whatever you want to call it when a boat runs at 12-20 mph or so.

It looks like I'm not going to learn anything new by trying to see the bottom of this particular boat. That's fine, I'm happy to work on new designs based on Atkin's tunnel-stern Seabright skiffs anyways. At least I can understand how they work and how to build them ... :)

[Pierre R]

Nah. I'll pass. Thanks for the offer.

As an aside, this is certainly an interesting way to enter a conversation that has been on going for over 2 1/2 months and has a 150 pretty informative posts from folks that at least have an open mind.

But, different strokes for different folks.You are assuming that I have a closed mind and that in itself signifies a closed mind. The question was genuine.

I have two degrees in engineering and make my living as an inventor. I cannot afford closed minds. I am in the process of thinking out what I want for my next boat.

The statement that you made that raised and eyebrow was "The genuine semi-planing hull, however, performes as well at dead slow as at full throttle."

Performs well means what to you? It tracks nice or delivers the goods on efficiency at S/L ratios of 1.0 to 1.25? What does that mean. Performs well at full throttle. Does that mean that its great in downwind swells to the equal ability of a properly designed planing hull or does that mean it delivers good efficiency on fuel?

You hang the shingle out as Naval Architect and therefore have some sway with me. Most boats sold today have hull designs as an afterthought. The marketing department hates hulls and naval architects. They get in the way of great interiors. Those same folks market these as semi displacement all the time. It means nothing to me. You come along and claim that there is a true semi displacement hull just like a true displacement or planing hull and didn't appear from under a rock.

My guess is that 98% of the hulls out there are damn poor designs as far as what I would consider delivering performance.

I looked at the Saga web site. Frankly there are things I liked very much about the boat and things I think were more for marketing an interior with safety as an afterthought. I realize you have to sell boats and I realize the general public thinks they must take their house afloat with them.

If I want all the convinences of a house I will stay in my house. First and foremost I want a boat that performs well throught its designed range of speed in all conditions that is pure fun to operate. Damned all the convinences of home. Let me move anywhere on the boat and work on anything on the boat with ease and give me a fun quality built boat to operate.

I have actually got to be a naval architect's dream client. Are you sure you don't want to take another shot at it instead of dismissing me as another critic?

[Pierre R]

... but of course this shows absolutely nothing useful to a person who wants to learn more about designing boats that run efficently at low-end planing speeds, or semi-displacement speeds, or whatever you want to call it when a boat runs at 12-20 mph or so.

It looks like I'm not going to learn anything new by trying to see the bottom of this particular boat. That's fine, I'm happy to work on new designs based on Atkin's tunnel-stern Seabright skiffs anyways. At least I can understand how they work and how to build them ... :)On the contrary. I think this picture does raise some eyebrows with me.

I don't see much of a keel under the forward two thirds of the boat yet I see a substantial short keel and skeg protecting the prop and rudder. I see a rudder that is large by semi displacement standards and looks well balanced.

These attributes would indicate to me that this hull would be more nimble at planing speeds than most semi displacement boats on the market and that translates into fun. I don't want a boat that handles like a 1974 U-haul at planing speeds. This is certainly different from what I am use to seeing.

Given what I see as reasonable but high AB ratios up front I would opt for a bow thruster to take away the frustration in tight marinas with wind and current.

[SAQuestor]

The statement that you made that raised and eyebrow was "The genuine semi-planing hull, however, performes as well at dead slow as at full throttle."

Pierre - in the quote above I have highlighted 3 words that are erroneous. I DID NOT make that statement - I merely quoted from the Saga web site.

Are you sure you don't want to take another shot at it instead of dismissing me...

Actually, when a newbie comes into a forum with "guns ablazing" making demanding statements like "Convince me that I'm wrong" and then tops that by attributing something that was not written by me... and then adds high octane gas to the conflagration by spouting about engineering degrees and supposed "inventor credentials". Seems like all we have is an individual that likes to set up straw man (http://en.wikipedia.org/wiki/Strawman) arguments to prove their own "superiority".

I once worked with a fella that was a legend in his own mind. Seems like another one has revealed their true colors.

[SAQuestor]

Hi Leo,

I checked all the pix in the links you posted, and this the best view of the bottom that I could find:

http://newimages.yachtworld.com/1/6/2/1/3/1621393_1.jpg?1168964118000

... but of course this shows absolutely nothing useful to a person who wants to learn more about designing boats that run efficently at low-end planing speeds, or semi-displacement speeds, or whatever you want to call it when a boat runs at 12-20 mph or so.


I can't disagree with you Ken. As I said in my initial post, about all the information I have found is mostly marketing hype.

But it appears to me that the hull form is a deep vee in the forefoot and then the bottom shallows out to a moderate vee from midships back. The deep keel seems to be kept with a bit more protrusion to protect the wheel and rudder.

As other folks have pointed out, that forward section seems to be a key element in getting these sorts of hull forms to be sea kindly.

But still, these Saga boats appear to need those several hundred horsepower engines just like many others of their modern ilk. So please do keep on exploring your Seabright hull forms. Maybe we'll get away from mere speculation and informed commentary to where we actually have some empirical data. Gosh, what a concept. Data. How's that go? My Kingdom. My Kingdom for some data... :D Oops, maybe not quite that way. ;)

Best,

Leo

[Pierre R]

Well SAQuestor it seems that I may have missunderstood your post. Perhaps I should have wadded through the 10 pages of posts before responding.

I am sorry to have fanned your flames and will not request you opinions again.

[kengrome]

I can't disagree with you Ken. As I said in my initial post, about all the information I have found is mostly marketing hype.Maybe some of the hype is true, but it's hard to tell these days when everyone's boat seems to be the answer to everything -- at least according to the marketing talk.

It appears to me that the hull form is a deep vee in the forefoot and then the bottom shallows out to a moderate vee from midships back.Right, and this is common to what ... hundreds or thousands of planing power boat designs? Nothing new here so far ...

The deep keel seems to be kept with a bit more protrusion to protect the wheel and rudder.I wouldn't even call that thing a keel myself, although that's apparently what the marketing guys are calling it. To me a keel is a very long protrusion from the bottom of the boat, not a short thing like that "skeg" they call a keel. Then again, maybe there really is a keel under there somewhere -- but without decent pictures it's all just theory and conjecture.

As other folks have pointed out, that forward section seems to be a key element in getting these sorts of hull forms to be sea kindly.My take on this is that you need to keep the forefoot "in the water" if you want to deal with rough or choppy conditions effectively. By insuring that the bow cleaves the water nicely, the bottom will never present itself to a wave, and it will be impossible for the boat to "pound".

The problem with this approach is excess spray when compared to a planing boat that runs bow-high and skims over the surface on the aft half of its hull bottom. But I think spray can be mostly controlled by the intelligent use of spray rails located in the proper position to prevent water from climbing too far up the hull sides.

These Saga boats appear to need those several hundred horsepower engines just like many others of their modern ilk. So please do keep on exploring your Seabright hull forms.I intend to do just that. At least with the Seabright hull form I can understand how and why it can be efficient. I'm not claiming that these hull forms are efficient -- although Robb White certainly thought so -- but if they are I won't have any question why, that's all.

[fcfc]

OK. Try this one on Yachtworld. (http://www.yachtworld.com/core/listing/pl_boat_full_detail.jsp?slim=quick&boat_id=1725289&ybw=&units=Feet&currency=USD&access=Public&listing_id=74556&url=) Says a 230 HP Yanmar.


Best,

Leo

Searching with google, I have found the powering option of their lastest offering (the 310, available as semi displacement only ), is a volvo D4-225 or D4-260.

I find this model surprisingly close to http://www.nimbus.se/modelPageCoupe2007.aspx?pageid=686 . Same powering, about same external size, about same layout.

Top speed of the 320 coupe is advertised as top speed 24 kts with 225 hp and 26 kts with 260 hp.

In one page the Saga 310 is advertised as cruise speed 20 kts, on the other page 14-24 kts.

The 320 coupe is nowhere classified as a "semi displacement" or "semi planning".

So I guess this term for the Saga 310 is just pure marketing blurb to differenciate from competitors.


BTW, when you try to read in D GERR propeller handbook entry for 10 000 lbs / 260 hp (38 lb/hp), you are outside the scope of displacement/semidisplacement boats (1000 lb/hp to 50 lb/hp) . But you are a bit below the middle of planning boats (60 lb/hp to 2 lb/hp) .

[FAST FRED]

"When accelerating at intermediate speeds, however, most planing hulls experience a rather pronounced "resistance hump" combined with steep trim angles and unpleasant wake wash. The consequence is that the driver normally will try to avoid the "hump" by passing it as quickly as possible either by speeding up or slowing down.:

This Hump is the product of the stern going down as the bow begins to rise .

With the ATKIN inverted V aft , the pressure created by the prop fills the aft section , and the boat remains level.

The bow stays level as the ENTIRE boat rises , as it gains speed.
The only "problem" seems to be when too much power is used , and the pressure raises the stern too high , depressing the bow , causing wonky steering.
As stated the solution seems to be forward chines to lift the bow as much as the stern , or perhaps my , suggestion of shrinking trim tabs to lessen the accelerated water lift astern , maintaining the stock trim.

The great sea worthy ability reported for these boats probably comes from the fact that the full hull is still mostly immersed at speed, so slamming is eliminated.

There might be some extra fuel burn from the extra wetted surface (compared to a true plaining boat) but the soft ride would allow higher speeds at lower G loads . An OK trade to me.

There might NOT be extra fuel burn as all the water the prop gets was accelerated before the prop increasing the prop efficiency to pump already accelerated water aft. The boat might NOT be plaining on its stern wave as some guess, but simply more efficient at recycling energy already paid for.


FF

[tom28571]

"When accelerating at intermediate speeds, however, most planing hulls experience a rather pronounced "resistance hump" combined with steep trim angles and unpleasant wake wash. The consequence is that the driver normally will try to avoid the "hump" by passing it as quickly as possible either by speeding up or slowing down.:

This Hump is the product of the stern going down as the bow begins to rise .

_________________________________________________________________
>>>>>Fred, I am pretty sure that these are symptoms rather than the cause of this behavior. As a planing hull accelerates above “hull speed” lift must begin to transfer from buoyancy to dynamic. The hull will always assume the trim angle needed to produce dynamic lift for the speed it is running. Thus as speed increases, the angle required will normally decrease as lift is proportional to both trim angle and speed. More speed = less trim and less speed = more trim. Some boats are much better at this because of lower bottom loading. More bottom area means less trim angle needed to produce the required lift at any given speed above hull speed.<<<<<
________________________________________________________________

With the ATKIN inverted V aft , the pressure created by the prop fills the aft section , and the boat remains level.

The bow stays level as the ENTIRE boat rises , as it gains speed.
The only "problem" seems to be when too much power is used , and the pressure raises the stern too high , depressing the bow , causing wonky steering.
As stated the solution seems to be forward chines to lift the bow as much as the stern , or perhaps my , suggestion of shrinking trim tabs to lessen the accelerated water lift astern , maintaining the stock trim.

The great sea worthy ability reported for these boats probably comes from the fact that the full hull is still mostly immersed at speed, so slamming is eliminated.

There might be some extra fuel burn from the extra wetted surface (compared to a true plaining boat) but the soft ride would allow higher speeds at lower G loads . An OK trade to me.

There might NOT be extra fuel burn as all the water the prop gets was accelerated before the prop increasing the prop efficiency to pump already accelerated water aft. The boat might NOT be plaining on its stern wave as some guess, but simply more efficient at recycling energy already paid for.

_______________________________________________________
I agree with some of your speculation but, like a lot of the other speculation, it is just that since none of us has any empirical evidence to support any of it. Clearly the down turn on the hull aft of the prop will produce stern lift. On the other hand, that water must first be lifted which may need energy from hull forward of the prop. Perhaps you are correct in thinking that the hull shape gives that lift without any negative effects. If you look at the start of the tunnel as the stern of a “normal” boat, then the rise in the water in the tunnel can be the normal upsurge behind the stern.

After a while this can create a headache. I will be glad when some real data is developed to put some of the speculation to rest. Lots of people claim unusual performance for the Atkin tunnel but no one seems to have any real data to offer. Not to mean that discussion should be cut off but just a caution that we are mostly just “speculating”. Tom
________________________________________________________________

F

abcdefghij

[kengrome]

There might NOT be extra fuel burn as all the water the prop gets was accelerated before the prop increasing the prop efficiency to pump already accelerated water aft.Hi Fred,

You've mentioned this before but I still do not understand what you're talking about. Can you please explain how the water gets accelerated BEFORE it reaches the prop?

Are you referring to the suction created by the propeller, which is nothing unusual since all propellers do the same thing?

Or is something else going on here -- something totally independent of the propeller's suction, and possibly unique to this particular hull design -- that accelerates the water in front of the prop?

[SAQuestor]

Tom did something strange to hide his reply from being quoted directly, but copy and paste works real good too.:p
Not to mean that discussion should be cut off but just a caution that we are mostly just “speculating”.

Indeed. As much as I'd like to think that one of these Atkin Seabright hulls will be built by someone, reality does rear it's ugly head.

In this thread, (http://www.boatdesign.net/forums/showpost.php?p=134081&postcount=49) fcfc slaps me around soundly with: "But there will be no comparison possible for resale value in future time."

This concept of resale value was reinforced today when perusing the YachtWorld listings. How's this one? (http://www.yachtworld.com/core/listing/pl_boat_full_detail.jsp?slim=quick&boat_id=1692915&ybw=&units=Feet&currency=USD&access=Public&listing_id=75650&url=) I don't know who the builder is, but I would suggest that the build quality and fit and finish is comparable to what I would be able to produce. I've gotta believe that the builder has a lot more toil, trouble and sweat into that boat than $39k (eventual actual sales price probably much lower) will ever return. The sales price may just about cover the materials used. Sad. And yes, I know it's a sailboat.

But for the same $39k - how's this semi-displacement power boat? (http://www.yachtworld.com/core/listing/pl_boat_detail.jsp?currency=USD&units=Feet&id=1325876&lang=en&slim=quick&)

And since I am comparing wooden vessels here, how about this selection (http://www.yachtworld.com/core/listing/cache/pl_search_results.jsp?cit=true&slim=quick&ybw=null&sm=3&is=&man=&hmid=104&ftid=101&enid=100&fromLength=&toLength=&luom=126&fromYear=&toYear=&fromPrice=38000&toPrice=40000&currencyid=100&city=&cint=100&cint=101&pbsint=&ps=100) at about $39k asking price?

But going back to fcfc's admonition, how about these fiberglass power boats at about $39k? (http://www.yachtworld.com/core/listing/cache/pl_search_results.jsp?cit=true&slim=quick&ybw=null&sm=3&is=false&type=%28Power%29&man=&hmid=102&ftid=101&enid=0&fromLength=&toLength=&luom=126&fromYear=&toYear=&fromPrice=38000&toPrice=40000&currencyid=100&city=&cint=100&cint=101&pbsint=&ps=100) Certainly not a lot to choose from, but representative of what is available at that price point.

My point in this post is to reinforce Tom's point and fcfc's point.

Tom's is that we don't have any data. And fcfc's is that even if we did have good data and someone built one, the resale value would be hard pressed to allow us to recoup our materials costs.

So we can speculate all we want - and sometimes that's interesting too - but if and when the $$$ fly from the bank account, we'll be able to see what there is to see regarding actual performance of boat bigger and heavier and much more expensive than Robb White's Rescue Minor. Until that time, I agree 100% with Tom.

Best,

Leo

Then I run across these for a few $$ more than the $39k mentioned above. These also fit in with fcfc's idea.

32' Vinette Custom Trawler (http://www.trawlersmidwest.com/32vinette.asp)
40' Vinette Steel Hull Trawler (http://www.trawlersmidwest.com/40vinette.asp)
29' Prairie Coastal Cruiser Sedan (http://www.trawlersmidwest.com/30prairie_coastal.asp)

[Excalibur]

I can't disagree with you Ken. As I said in my initial post, about all the information I have found is mostly marketing hype.

But it appears to me that the hull form is a deep vee in the forefoot and then the bottom shallows out to a moderate vee from midships back. The deep keel seems to be kept with a bit more protrusion to protect the wheel and rudder.

As other folks have pointed out, that forward section seems to be a key element in getting these sorts of hull forms to be sea kindly.

But still, these Saga boats appear to need those several hundred horsepower engines just like many others of their modern ilk. So please do keep on exploring your Seabright hull forms. Maybe we'll get away from mere speculation and informed commentary to where we actually have some empirical data. Gosh, what a concept. Data. How's that go? My Kingdom. My Kingdom for some data... :D Oops, maybe not quite that way. ;)

Best,

Leo

Hey All,

I live behind a VERY restrictive firewall. I can see US based YachtWorld pics, but I can't see any that are on the server that carries the Saga hull pic. I don't know why the Navy blocks it, but they do. Would someone please send me an email with the pic attached? I'd love to compare it to my 40 year old Chris Craft hull. My Sea Skiff is a true semi displacement hull. The best pics I have of it are Here (http://www.picturetrail.com/gallery/view?&p=999&uid=6976154&gid=13369318&&imgid=198651440&offset=0) and Here (http://www.picturetrail.com/gallery/view?&p=999&uid=6976154&gid=13369318&&imgid=198592282&offset=0) . You can almost make out the keel that extends 2/3 of the length of the hull from the point where the hull curves up to the props. She's 38 feet, and I've seen the displacement written as anywhere from 17,000 pounds to 22,000. I'm pretty sure it's over 20,000. 700 horsepower. It cruises (3,000 RPM) at 14.4 knots, and tops out at 24.8. What makes me characterize it as semi is the fact that bow rise is undetectable from either helm and it cruises with the forefoot still in the water. I personally like the characteristics of the hull; a very smooth ride, no "hump" and an ability to cut through chop without pounding. But she is certainly wet! Spray to the flybridge in 2 foot chop. A maxim in the aviation community is that there is nothing more worthless than altitude above you or runway behind you. I would say that the ability to run 30 knots only in smooth water runs a close second. SA, one of my winter projects will be the addition of a good fuel flow meter to my gauge set. Engineering data will be taken! In addition to an FF meter and a GPS, what other data gathering instruments are needed for a decent report? Will empirical observation and published reports coupled with backup runs be enough for weather and tides? How many dimensions should the graph have?

[tom28571]

Excalibur, those photos don't reveal much of anything about the bottom shape so they wont help much.

There is an infinite range of boat hull shapes between full displacement and full planing. Just where to place a given hull shape is often problematic. It is no surprise that the definition of a semi-displacement or semi-planing hull is vague. Nowhere is this more evident than in sailboats where great changes in performance have taken place in the last 50 years or so. 50 years ago, there were almost no large sailboats that could claim the ability to plane. With evolution in hull shape and lighter weight design, current designs can tripple the speed of older boat of similar proportions. Gradual flattening out of the aft bottom sections with greater waterline beam, lighter weight (often through the use of exotic and expensive materials) have changed forever what was the mantra of boatspeed in the 1950's.

The same thing is true (in a design sense) in powerboats. Take a displacement hull, start flattening out the aft run and lightening the hull and the performance potential starts to rise. The semi hull will fit somewhere in there and will always have some rocker. Full planing boats will usually have no rocker at all and the aft buttock lines will be parallel. Very heavy boats of good hull design will still plane fast but will require lots of power and a have wide transition zone between displacement and planing speed. WWII PT boats are like that as well as many of today's powerboats. Some other boats of good design and light weight can almost ignore the transition "hump" and run at any chosen speed the power will allow.

I see no obvious magic in the shape of the Saga boats and doubt there is any. If they are good boats, I see no problem with their hype but don't expect any miracles, because there ain't any. Because there are so many slugs on the water, a decent manufacturer must be forgiven for trying to differentiate their boats from others. Caveat Emptor :confused: ?

[Excalibur]

I don't expect miracles; I'm just curious as to whether there is any hydrodynamic progress at all. Powerboats and sailboats differ in this way as well. For powerboats, most of the progress has been in the area of power. We can now give a hull all of the power that it can handle (and we can afford). The main driving force of sailboats (wind) has not changed (except to get dirtier). Thus, sailboat hulls have evolved at a great pace, and have far less compromises in terms of cubic footage and toys vs efficiency. Why? Because efficiency matters when driving force is limited.

I strongly suspect that if my 40 year old hull were rendered in modern weight saving materials and given modern power, that you would be hard pressed to find a more efficient recreational cruiser hull. I don't think my hull is that great; I just don't think that recreational cruiser hulls have become more efficient. I too, wish I had the resources to build a large seabright hull. I would love to see data on the Atkins Naiad design. 28' and 15.5 kts on 52 hp. Now THAT would look like magic :)

But, I have what I have. I will take data for my boat. I am taking out excess weight for better performance. I will experiment with her trim. I can't help it; it's the engineer in me. She may be a 40 year old termite condo, but I want her to be the best termite condo she can be ;)

So now a question. The importance of a clean bottom was showcased for me just last week when Thetis came back from the yard. Cleaning the hull and adding fresh bottom paint netted me a cool 2 kts at cruise. But I noticed that there is still an old unused transducer from an ancient chart recorder faired into her hull, with a prop driven speedometer drive still attached. Do small interruptions like this have a real world measurable effect on performance? If I glass the bottom will the improved fairing give me enough gain that I will see it? Other than the obvious (get rid of more weight) what else can I do?

[tom28571]

So now a question. The importance of a clean bottom was showcased for me just last week when Thetis came back from the yard. Cleaning the hull and adding fresh bottom paint netted me a cool 2 kts at cruise. But I noticed that there is still an old unused transducer from an ancient chart recorder faired into her hull, with a prop driven speedometer drive still attached. Do small interruptions like this have a real world measurable effect on performance? If I glass the bottom will the improved fairing give me enough gain that I will see it? Other than the obvious (get rid of more weight) what else can I do?

Dunno if you could detect the difference a small thing like you extra fitting on the bottom or not. I expect you will remove it at the next haulout and then we will know.

As for the difference an even slightly dirty bottom can make on a planing hull, my experience is the same as yours. Doesn't take very much to make an easily detectable loss in top speed.

[Willallison]

As for the difference an even slightly dirty bottom can make on a planing hull, my experience is the same as yours. Doesn't take very much to make an easily detectable loss in top speed.

I worked as an instructor for a time in a waterski school in Greece. A couple of guys arrived in a fair sized boat with a 23ft Beneteau powerboat in tow. I was so horrified at the growth on the boats bottom, that I offered to clean it for them (Big job.... ever tried breathing underwater through a piece of garden hose...?:eek: ) Anyway, the end result was that their 12 knot boat became a 30 knot boat!

[charmc]

My Sea Skiff is a true semi displacement hull. You can almost make out the keel that extends 2/3 of the length of the hull from the point where the hull curves up to the props. She's 38 feet, and I've seen the displacement written as anywhere from 17,000 pounds to 22,000. I'm pretty sure it's over 20,000. 700 horsepower. It cruises (3,000 RPM) at 14.4 knots, and tops out at 24.8. What makes me characterize it as semi is the fact that bow rise is undetectable from either helm and it cruises with the forefoot still in the water. I personally like the characteristics of the hull; a very smooth ride, no "hump" and an ability to cut through chop without pounding. But she is certainly wet! Spray to the flybridge in 2 foot chop.

Excalibur,

My 30' Pacemaker Sport Fisherman was an earlier carvel sea skiff. Smaller and not as much flair in the bow, but otherwise very similar hull to your Corinthian. Pacemaker and Egg Harbor built some excellent carvel hulls with great seakeeping characteristics. From your photos and descriptions, your hull is similar: deep and narrow forefoot, rounded chines, sharp vee carried well aft, transitioning to flat stern, rocker bottom, deep keelson/skeg. Classic semi-planing hull. My home port was near the head of Barnegat Bay, long and narrow like the Chesapeake. Steep 2-3' chop whenever the wind was northerly or southerly. Coming back from Opsail 76, we were exiting NY Harbor with a long run in open waters, onshore SE wind vs ebbing tide created steep 6-8' seas. Next day several skippers of hard chine glass 40- 46 footers complained of pounding, broken glass, and sick passengers and crew. We came through that same stuff at around 17-18 knots, easy rocking horse motion, all my landlubber passengers fell asleep. Single 200 HP gas Palmer gave 6.5 - 9 gph @ 18-23 knots. She would hit 30, but wide open you could almost see the fuel level dropping in the tanks.

Your description of the easy transition to planing, forefoot in the water slicing through waves rather than lifting and pounding, and soft ride in rough waters sounds familiar. Offshore, I discovered that she tracked very well in quartering and following seas. When the odd wave from a different bearing hit, she was easy to hold on course. Heavy construction helped that good ride. Like yours, she was very wet.

By the way, I like the looks of your Corinthian. Beautiful shear, and that design did a good job of matching clean, modern looking topsides with a seaworthy hull.

I understand your desire to build. There have been several good suggestions here. The heavy construction of the traditional skiff hulls helps the ride, but reducing weight via modern construction methods will definitely help fuel consumption. I'll send you the Sea Saga photo.

[charmc]

I was so horrified at the growth on the boats bottom, that I offered to clean it for them (Big job.... ever tried breathing underwater through a piece of garden hose...?:eek: ) Anyway, the end result was that their 12 knot boat became a 30 knot boat!

When I was young and stupid, I cleaned a hull by free diving; coming up for air 40-50 times made it pretty slow going. I got it done, but every time I think back on it, it was pretty stupid. :o The boat did pick up noticeable speed, though. :)

[FAST FRED]

"Can you please explain how the water gets accelerated BEFORE it reaches the prop? "

As the water gets pushed aside , skin friction drags some of the water along.

On a 35 ft boat with smooth bottom it can be 3 or 4 inches thick.This water contains ENERGY whose capture by the ATKIN reverse V shape may add to the energy the prop produces.AS accelerated water gets further accelerated by the prop, the total energy consists of what the prop produced as well as the "free" energy from the already accelerated water.

Its ALL that pushes out the rear that makes the boat go , so reusing energy that simply heats the ocean (on other hulls) makes sense.

Depressed sterns,

"I am pretty sure that these are symptoms rather than the cause of this behavior. As a planing hull accelerates above “hull speed” lift must begin to transfer from buoyancy to dynamic. The hull will always assume the trim angle needed to produce dynamic lift for the speed it is running. Thus as speed increases, the angle required will normally decrease as lift is proportional to both trim angle and speed. More speed = less trim and less speed = more trim. Some boats are much better at this because of lower bottom loading. More bottom area means less trim angle needed to produce the required lift at any given speed above hull speed.<<<<<

Agreed , but in FL we have loads of "canal patios", 25 ft Pontoon boats with 20-24 inch aluminum pipes that power well.

They hardly rise a bit , yet speeds of 30mph are common with a modest engine.In terms of SL ratio there ar 5 or 6! with no plaining loss of hull LOA.

They have the "right" plaining angle as none seem to ever have a trim tab, although the engine tilt IS adjustable. The point is fast speeds do not require plaining on a small spot in the stern, simply power and light weight.

Efficiency is a different trade off , as the long immersed hulls give a better ride in chop compared to bouncing off the wave tops.

The ATKIN concept might work to allow a long slender , light boat to cruise at better efficiency, (lower fuel burn) at moderate hi teens speeds. The concept of 16 -18K at 5 mpg would work for me , really well.The RM 35mpg won't work in an 8000lb boat.

Leo's thought,

"But there will be no comparison possible for resale value in future time."

Might be correct if the idea doesn't work, but IF it does it will surely be copied instantly.
Not many cruisers may want a 38ft boat with 7,6ft beam to fit into a std shipping container. BUT , it only takes one individual that wants to cruise world wide , sans the bother and expense of crossing oceans to sell my dream boat after cruising.

The concept tho of a high teens cruising boat with excellent fuel economy would be grabbed world wide .(If folks can be convinced not to want floating apartment blocks.

The Austrians as well as the US "creator" of similar designs claimed great speeds with minimum wake and low fuel use.

Perhaps if they had copied ATKIN better , not just the box keel, but the reverse V also , the world would not be ignoring their boats?

FF

[tom28571]

"Can you please explain how the water gets accelerated BEFORE it reaches the prop? "

As the water gets pushed aside , skin friction drags some of the water along.

On a 35 ft boat with smooth bottom it can be 3 or 4 inches thick.This water contains ENERGY whose capture by the ATKIN reverse V shape may add to the energy the prop produces.AS accelerated water gets further accelerated by the prop, the total energy consists of what the prop produced as well as the "free" energy from the already accelerated water.

Its ALL that pushes out the rear that makes the boat go , so reusing energy that simply heats the ocean (on other hulls) makes sense.

Depressed sterns,

"I am pretty sure that these are symptoms rather than the cause of this behavior. As a planing hull accelerates above “hull speed” lift must begin to transfer from buoyancy to dynamic. The hull will always assume the trim angle needed to produce dynamic lift for the speed it is running. Thus as speed increases, the angle required will normally decrease as lift is proportional to both trim angle and speed. More speed = less trim and less speed = more trim. Some boats are much better at this because of lower bottom loading. More bottom area means less trim angle needed to produce the required lift at any given speed above hull speed.<<<<<

Agreed , but in FL we have loads of "canal patios", 25 ft Pontoon boats with 20-24 inch aluminum pipes that power well.

They hardly rise a bit , yet speeds of 30mph are common with a modest engine.In terms of SL ratio there ar 5 or 6! with no plaining loss of hull LOA.

They have the "right" plaining angle as none seem to ever have a trim tab, although the engine tilt IS adjustable. The point is fast speeds do not require plaining on a small spot in the stern, simply power and light weight.

Efficiency is a different trade off , as the long immersed hulls give a better ride in chop compared to bouncing off the wave tops.

The ATKIN concept might work to allow a long slender , light boat to cruise at better efficiency, (lower fuel burn) at moderate hi teens speeds. The concept of 16 -18K at 5 mpg would work for me , really well.The RM 35mpg won't work in an 8000lb boat.


FF

Fred, please,

That free energy is just not there. The accelerated boundary layer water you mention is actually moving along with the boat contributing to drag and is going in the wrong direction to be "free energy" for the prop. Its effect is negative rather than positive. If there is magic in the Atkin, that is not it.

Those pontoon boats are not planing and have L/B ratios, from your numbers, of 10 to 12. 30 MPH! I'm skeptical although a well designed non-planing catamaran of similar proportions could probably do that. To be planing, a boat hull must have some angle of incidence to the water or it cannot develop any dynamic lift. Dynamic lift is the definition of planing.

[Willallison]

Sorry Fred - Tom's quite right. The boundary layer is a contributor to drag: it takes energy to move the water along with the boat. Imagine a very smooth bottom (of the nautical variety). It stands to reason that it will 'drag' only a thin layer of water along with it. Now imagine a very rough bottom....the opposite would logically be true.

As for pontoon boats, they would probably be operating as a relatively inefficient displacement catamaran. With a length:beam ratio of around 12, a well shaped displacement cat is very economical up to a SL ratio of around 2.8. Beyond that the planing cxat comes into its own, though the lines are very definitely blurred. Conversely, with that kind of length:beam ratio the hulls aren't really wide enough to provide sufficient lift for the boat to plane as the bottom loading is too high.

[charmc]

As the water gets pushed aside , skin friction drags some of the water along.

On a 35 ft boat with smooth bottom it can be 3 or 4 inches thick.This water contains ENERGY whose capture by the ATKIN reverse V shape may add to the energy the prop produces.AS accelerated water gets further accelerated by the prop, the total energy consists of what the prop produced as well as the "free" energy from the already accelerated water.

Its ALL that pushes out the rear that makes the boat go ,
FF

Sorry, Fred, most times I agree with your insights, and I'm not meaning to pile on here, but ... on this one you're wrong. You're describing a boundary layer, and that's a net energy loss, not gain. Energy is used to drag that water along as you describe. Total energy is what the prop produces less what is consumed in accelerating (dragging) that water.

[FAST FRED]

"You're describing a boundary layer, and that's a net energy loss, not gain. Energy is used to drag that water along as you describe. Total energy is what the prop produces less what is consumed in accelerating (dragging) that water.'

My thinking is the boundary layer of water is not stagnant , and stuck to the hull, with the same water in the same place place hour after hour.

Rather I would imagine its replaced constantly as it gets dragged aft .

AS it departs aft it still will have some velocity , therefore some energy .

Sure the prop would see all the water as pure drag to be overcome , but its what happens to the energy /water velocity being dragged in the boundary layer the that is trapped by the inverted V that may be being captured.

One hulls shapes induced drag might ? be a different hulls source of energy recovery.

Since wave drag would be low , due to the majority of the displacement in the box keel , with the surface hull forward simply working as an end plate over the box keel and the skin friction (boundry layer =induced drag ) perhaps being reused , it could answer the many efficiency claims.The pressure of the prop wash holds the stern from sinking , weather its plaining becomes moot.
*
The Good old Boys with the canal patios install 45 to 150 hp engines on 20ft boats!

30K is easily done , although the fish killers with 2- 250's are way faster, but they do plane, while the patios only take up an angle of attack with bow raised and then skoot.

FF

DO pile on , I am an recovering New Yorker, and can take the hit.

Also it will be 2 years of expensive and hard work to get results.

First I'm planning on building a powered scale model, that can record the power (electric motor) used at all speeds.

I can weight the model and get realistic impressions of what weight does to the hull form/ power requirements.. Additionally it will be easy enough to borrow an underwater camera and video scale speeds to see what gives.

Full sized will be in 3 stages,build a bare hull and power it with a throw away car engine to get real power requirements.
Install a mock up interior to maximize the space (its a small 38ft).

With the knowns , known repower with "correct" size diesel, and have interior done by craftsman , rather than painted ply of mock up.

FF

[Jimboat]

Interesting discussions. The latter descriptions of boundary layer theory are getting closer to the reality. What seems the most important observation re: your discussions/questions, however, is that the lift/drag of any hull can be analyzed...so while it may be complicated through different hydrodynamic stages for each hull, it is still quite possible to calculate the lift/drag for the hull. It just takes the work to do it (but it's fun...if you like that sort of thing!)

[tom28571]

Fred,

Relative to the speed of the boat, the water at the surface of a boat underway has a velocity of zero or exactly that of the boat. This velocity reduces throughout the "boundary layer" until it is at the same speed as the boat beyond the boundary layer.

All the energy required to move this water in the boundary layer must be supplied by the propulsion system and is therefore a drag on the boat speed. We might wish it were otherwise but that is just the way it is.

Not having any experience with those pontoon boats, I can't make any further comments on them.

[FAST FRED]

"All the energy required to move this water in the boundary layer must be supplied by the propulsion system and is therefore a drag on the boat speed."

Understood , the question is weather there is any recapture caused by funneling the accelerated water under the aft lifting section.

The water IS accelerated , therefore contains energy already paid for, which the uinique shape might be recaptyring?

A skin of water 3 or 4 inches thick covering the front of the vessel should have high energy from the closest to the skin and lesser out to 4 inches where its minor to non existant.

The hull "should " be doing something with this energy , if its ducted aft its presence must have an effect.\\

The SL speed of 2.8 would be dandy for my purposes , its 16 or 17K with a plumb bow , fast as I'm willing to buy fuel to run.

FF

Interesting that we can carry this conversation on as we transit the NY State Barge canal, via Wi Fi , complements of nearby sources.

FF

[tom28571]

"All the energy required to move this water in the boundary layer must be supplied by the propulsion system and is therefore a drag on the boat speed."



The water IS accelerated , therefore contains energy already paid for, which the uinique shape might be recaptyring?

Interesting that we can carry this conversation on as we transit the NY State Barge canal, via Wi Fi , complements of nearby sources.

FF

Fred,

I see where you are going with this. I have seen this idea expressed before and think there is merit to it. Whether the Atkin design gains some particular advantage from this accelerated water shed from the boundary layer that other boats do not is not so clear to me. There must be some data on this phenomena somewhere in the books.

Lots of Wi Fi along your route huh? Amazing that you can get the use of it while traveling.

Just watched a recording of the latest race in the AC. While I was pulling for the Kiwi's, I am not sure they deserve winning the Cup. Had the race handed to them three times and insisted on giving it back. I should talk. Sailed in a race today and am not sure whether we finished at the top of the bottom half or the bottom of the top half. At least we did not screw up as much as they did but were just not fast enough. Felt like a bucket was tied on the keel.

[kengrome]

Here's something else to think about ...

What if it takes more energy to "bring the water back together" behind the box keel than to simply allow it to flow straight back from each of the side tunnels? In this case the boat in this picture might be even more efficient than the Atkin tunnel-stern designs:

http://www.bagacayboatworks.com/linkfiles/xbb-tunnel-twin-67.jpg

This one uses twin propellers, one in each tunnel, to provide the thrust.

When Fred asked earlier about how to possibly accelerate the water in the individual side tunnels even more than it is already accelerated, I said a couple of smaller props in those tunnels should do the job. Now I'm thinking that this design might not even need the main propeller if two side-tunnel propellers are used instead ... and it might be even more efficient than the Atkin designs.

With two small props instead of one large one, the tunnel section can be shallower and the boat will still have the same draft -- but with less lifting of water up into the tunnel section. Of course means more power available to push the boat forward and thus more efficiency, right?

But will the widening side-tunnels aft create less efficiency?

Instead of funneling the water to a smaller area this design APPEARS to let the water that is trapped in each tunnel spread out a bit toward the aft end of the boat. I would think that the best efficiency might be found in just the opposite condition ... where a wider tunnel narrows down so the prop can be located in the narrowest part of the tunnel.

Or maybe it just "looks like" the tunnel gets wider aft when really it doesn't?

Another issue to look at here is that the box keel extends all the way to the transom -- which makes it longer and more efficient, right? So how much more efficient does this make the boat compared with one that has its box keel 2-3 feet shorter as in the Atkin designs?

[FAST FRED]

This indeed might recapture the energy in the wash , but the surface avilable for the prop pressurized water is far smaller , so there would be less lift aft , and perhaps the stern would not be held up.

Long and slender is wonderful when there is lots of SURFACE wave making. With the box keel supplying much of the displacement , I'm not sure the added skin friction would be an improvement.

Looking at the submerged surfaces of other boats that get buoyancy from under water surfaces , most don't seem to be very long .

Perhaps Navy research on the many under hull protrusions on modern warships could answer the question , although most go over the 20K envisioned .

*

Now in Oswego NY , enjoying a day off , abet with 4 hours of "field day" (general task & clean up time). Again free Wi Fi fills the air waves , to my delight!!
Monday its off to Canada to pick up Trent Severne waterway.


FF

[tom28571]

Hi Fred,

Here is an account of the cruise my wife and I made on the Trent Severn in 2003. http://www.bluejacketboats.com/ontario_waterways.htm

Great trip, have a good time.:)

[Excalibur]

I'm not at all sure Fred is wrong. Boundary layer water is created regardless of whether the energy is recaptured or not. I think the word "free" is what is throwing folks off. The energy contained in the moving boundary layer is not "free", but if it is recaptured as it flows through the prop, it's a net gain.

As far as pontoon boats are concerned, there are different kinds. Some have pretty much "pure" displacement shapes (ie, round bottomed logs), but others are designed with 3 logs and lifting strakes. Those are the fast ones.

Check the pic in this article

http://boatingworld.com/Articles/2007/04/Boats/Lowe_Suncruiser_Trinidad_220XL.html_491904626.html

You can clearly see the optional lifting strakes tacked to the outside.

[tom28571]

I'm not at all sure Fred is wrong. Boundary layer water is created regardless of whether the energy is recaptured or not. I think the word "free" is what is throwing folks off. The energy contained in the moving boundary layer is not "free", but if it is recaptured as it flows through the prop, it's a net gain.


I put this question to a couple people who should know, if anyone does, and did not get a definitive answer. The question being whether the forward moving water from the boundary layer feeding into the prop was a net positive or negative effect?" I don't know yet but feel that it may be positive.

[Excalibur]

There are conflicting forces in play here. Generating boundary layer laminar flow creates drag. The Atkins design has more surface area than a round bilge boat with the same beam and displacement would have, so drag should be greater. This is born out by the tank tests that reported that the Atkins hull had more drag than a conventional hull when towed. The only way that the design could be more efficient under power is if some if the the energy used to generate the boundary layer is recovered by the prop. Someone hurry up and build one so we'll know!

The twin tunnel design, BTW, has at least one significant difference that will not enhance efficiency. The hub of the prop generates no force, and is in fact a source of drag, as are the struts and shaft. The Atkins design has no strut, and puts the prop hub right at the end of the box keel where water can most easily flow around it.

[kengrome]

I put this question to a couple people who should know, if anyone does, and did not get a definitive answer. The question being whether the forward moving water from the boundary layer feeding into the prop was a net positive or negative effect?" I don't know yet but feel that it may be positive.Tom, it has to be positive. It's the same as motoring downstream. When you're motoring downstream you're moving in the same direction as the water and you're going faster than if you were motoring in still water ...

With these boats (and with any other boats for that matter) the boundary water is already moving in the same direction as the boat, so the effect is positive. But in the Atkin designs much of the moving boundary layer water is being directed into the propeller where it can be "further accelerated". I think this is the main thing that makes these boats more efficient.

I can finally understand what Fred was trying to describe when he kept saying that the water is accelerated before it gets to the prop. It is already moving in the direction of the boat because of the skin friction effect which drags some of it along. The main difference between the Atkin designs and most other motorboats is that Atkin takes advantage of this moving boundary layer of water instead of ignoring it.

In this respect my twin-tunnel will do the same thing -- take advantage of the moving boundary layer of water by trapping it beneath the hull and directing it toward the propeller(s). Obviously there are other factors going on in my design that will result in more or less efficiency, but there are so many potential things going on here that it seems only a test will tell us whether or not the twin-tunnel is "more efficient" than the Atkin design. I suspect that it is not.

However ...

If the boat must accommodate twin engines, my twin-propeller design may be the most practical even if it isn't quite as efficient as the typical Atkin tunnel-stern designs. It should still be more efficient than the vast majority of other semi-planing hulls out there these days which of course do nothing to use the accelerated boundary layer water to their advantage.

As I have discovered recently, there are not many cheap or easy ways to turn a single screw with twin engines either. This makes me want to design a boat that is powered by the twin 10 HP diesel engines that I can get over here in Asia for $200 each. I think they are a great solution for small boats that needs 15-20 HP to reach low-end planing speeds and want to have two engines for redundancy. These engines are already EPA-certified too, so they can be imported to the USA -- a big factor for me since this is what I would like to do sooner or later (probably later).

Personally I like redundancy on the water, especially in a boat that's going places far enough from land that a potential disaster might be avoided with more than one engine. If one engine dies in this system you can still get home on the other one with no problem -- and it will NOT cost you an arm and a leg to get one of these engines fixed either. If worse comes to worst you can simply pull out the bad engine, throw it away, and bolt in a new one for a few hundred dollars.

[tom28571]

Tom, it has to be positive. It's the same as motoring downstream. When you're motoring downstream you're moving in the same direction as the water and you're going faster than if you were motoring in still water ...


I also think the accelerated water in the boundary layer is a positive effect on a propeller but am afraid your example is irrelevant. A boat in a steady current is completely unable to tell whether it is in a current or not without some outside reference.

In any case, the effect will be most prominent in a displacement boat and all but insignificant in a planing boat. The easiest way to prove or measure this effect would be to calculate the slip of the propeller on an Atkin boat and compare to a comparable case on another well established "normal" hull where the boundary layer effect on the propeller is minimized..

[kengrome]

I also think the accelerated water in the boundary layer is a positive effect on a propeller but am afraid your example is irrelevant. A boat in a steady current is completely unable to tell whether it is in a current or not without some outside reference.Understood, but I was actually trying to get at the relationship of the boat with the ground, not the boat with the water. Obviously I didn't explain this well enough which is no surprise to me. I know what I'm thinking but getting it down in writing so other people can understand it is not always the easiest thing. Maybe an example will help:

Let's take a look at the theoretical performance of Boat A and Boat B:

Boat A - an Atkin tunnel-stern or similar design
that funnels the accelerated boundary layer
water to the propeller

Boat B - a typical boat that does not funnel
the boundary layer water to the prop

My guess is that when both boats are run in completely calm water Boat A will reach a speed of 14 knots under the same power that Boat B will only reach 12 knots.

Why?

Because the prop in Boat A is running in the accelerated boundary layer water, and that water is already going 2 kt faster than the water being pushed by the propeller on Boat B. So the Atkin design simply ADDS the speed produced by the engine/prop in still water to the speed of the moving boundary layer water. The result is that the overall speed is higher in the Atkin boat.

It is common knowledge that for the best efficiency and performance propellers should be run in water that has not been disturbed by the boat. But guess what? I believe this "common knowledge" is wrong, especially for boats designed to run at low-end planing speeds and slower ...

I think Billy Atkin somehow realized that *IF* the prop could be made to run in the disturbed (and accelerated) boundary layer water -- without cavitating and without any other problems -- he would have a more efficient boat. His tunnel-stern keeps the air out of the tunnel so it seems he eliminated the biggest potential problem right there. But that's not all ...

He also managed to prevent the stern from squatting at the same time, which of course suggests that the boat should be more efficient in the speeds between displacement and planing since it doesn't have a big "hole in the water" to climb out of like most other planing powerboats do when they actually start to plane. Smaller holes (or no holes at all) will mean less energy to push the boat up and therefore more energy to push it forward.


In any case, the effect will be most prominent in a displacement boat and all but insignificant in a planing boat.Granted, and since those Jersey Seabright skiffs were originally nothing more than displacement boats in the first place, Atkin seems to have found a way to modify them in such a way that he was able to make them plane ... while also using the accelerated boundary layer water to great advantage.


The easiest way to prove or measure this effect would be to calculate the slip of the propeller on an Atkin boat and compare to a comparable case on another well established "normal" hull where the boundary layer effect on the propeller is minimized..I don't know about prop slip as a measurement, how is this done? My other question is how do you determine which normal boat to use in the comparison? Any suggestions for one that is similar?

[kengrome]

There are conflicting forces in play here. Generating boundary layer laminar flow creates drag. The Atkins design has more surface area than a round bilge boat with the same beam and displacement would have, so drag should be greater.At what speeds?

I don't think you can talk about drag unless you also talk about speeds at the same time. They are related to each other, aren't they?

There may be a cross-over point where the less efficient hull at lower speeds becomes the more efficient hull at higher speeds. But even if not, I think you're missing a point about these long slender box keels ...

Personally I think the long and narrow box keel supporting most of the boat's weight will reduce the total drag on the boat over that of a round bilge hull that (in my opinion) has nowhere near as efficient an underwater shape.

My gut tells me that when most of the weight is held up by a long skinny underwater section the boat is simply going to move more efficiently -- at the low-end planing speeds (or semi-displacement speeds) we are talking about -- than any other type of hull except for a catamaran. And catamarans generally have two "even more efficient" hulls than these box keels, with no other hulls to drag on the waves at the surface.


This is born out by the tank tests that reported that the Atkins hull had more drag than a conventional hull when towed.I never saw any of those tank test that writer reported. Some of his conclusions did not make sense to me either, but that's really beside the point:

We are not talking about towing these hulls, we are talking about powering them. Thus those tank test results are (in my opinion) rather useless with the possible exception of this conclusion:

The only way that the design could be more efficient under power is if some if the the energy used to generate the boundary layer is recovered by the prop.I agree with this statement but not because of those tank tests. I agree because I think I understand what's going on with the water under and around the hull in these boats. If I'm wrong I will know sooner or later I guess ... :)


Someone hurry up and build one so we'll know!Robb White built one but no one seems to believe what he said about his. Yes I KNOW he did not built it precisely according to Atkin's plans -- but who cares if he didn't follow the plans exactly? His son said he built the tunnel section precisely according to plan, and it was a VERY efficient boat according to both of them. I guess this only confirms the design if you believe them.

Nevertheless, I would like to build TWO BOATS -- one with the Atkin style tunnel-stern and one without. This doesn't mean I am actually going to do this. After all, it costs money -- not as much in the Philippines as in other places -- but it is still an expense that I do not feel comfortable shouldering on my own at this time.


The twin tunnel design, BTW, has at least one significant difference that will not enhance efficiency. The hub of the prop generates no force, and is in fact a source of drag, as are the struts and shaft. The Atkins design has no strut, and puts the prop hub right at the end of the box keel where water can most easily flow around it.That's for sure!

I think the factors that may work to counteract this drag and improve efficiency in this design are the longer box keel --- arguably one of the primary reasons why these boats are reported to be so efficient in the first place -- and the straight flow of water aft from the boat's mid-section rather than making it bend around the box keel as much as in the Atkin designs.

The twin-tunnel looks like it will have more drag from hull friction as well, although to tell the truth I haven't calculated the surface areas and I'm not sure I will bother at this point since it is just a theoretical design.

My basic goal is to build hulls and test them on the water once I come up with a design that appears to have a favorable combination of features. The labor and materials are both inexpensive here (relatively speaking of course) and I really think there are few places on Earth that would be better for prototyping and testing boats.

I only wish I had a wealthy benefactor to cover my costs for such testing. Oh well, maybe in the next life ... :)

[Willallison]

I think you guys are over-analysing this. No doubt, Aitkin was a clever fella, but I think he developed a boat who's primary goal was shoal draft operation with relative economy. The boats' clearly evolved over time... elements like the "side-skirts" and kick down at the transom were different from boat to boat, suggesting this was more of a trial-and-error approach than some kind of high-tech boundary layer experiment.
Nothing wrong with that of course - most boats are improved this way. And it may well be that the end result did indeed re-use the energy expended in creating the boundary layer in the 1st place, but I think it'd take some pretty serious testing to prove one way or the other.....

The boat has a number of other benefits - particulary for a containerable craft like FF's.... hurry up and build the bloody thing will you.... we are all eager to examine the results!

ps ... Tom... did you get my PM, or email...?

[charmc]

There are conflicting forces in play here. Generating boundary layer laminar flow creates drag. The Atkins design has more surface area than a round bilge boat with the same beam and displacement would have, so drag should be greater. This is born out by the tank tests that reported that the Atkins hull had more drag than a conventional hull when towed. The only way that the design could be more efficient under power is if some if the the energy used to generate the boundary layer is recovered by the prop.

Fred and Ken,

Earlier, Fred, I said you were wrong about the net energy effect of boundary layer drag. As this discussion has progressed, I've started thinking that the boundary layer effect was not the only answer, and that there may be more even to boundary layer effect than I realized. My original analysis had the layer of water dragged forward adding to the energy requirement in two ways: first because it had to be accelerated to the hull's speed, drawing energy, and second because it had to be stopped, then reversed, which would use more energy. I was wrong about the second way (not so hard to say, after all :) ). That analysis would be correct only for a stationary prop. For a vessel underway, however, the prop is moving forward at the same velocity as the hull. The difference in velocity (delta V) between water and the hull/prop is equal to the vessel speed minus the water speed, normally zero. So for a vessel moving forward at 10 knots, delta V = 10-0 = 10. For that portion of water in the boundary layer, however, there is some forward velocity imparted by drag. Assuming boundary layer average velocity is 50% of the vessel velocity, delta V = 10 - 5 = 5. Boundary layer water will offer less resistance to the prop, so will require less energy. In addition, focusing a channel of water to the prop without any aeration reduces cavitation and adds to efficiency.

At least one patent has been granted, I found, for the design of a tunnel to channel and focus boundary layer water into the prop.
"This dynamic boundary layer arises at the interface between the hull of the vessel and the water. Part of this dynamic boundary layer has the speed of the vessel and energy is required in order to obtain this acceleration. According to the present invention provision is made that this dynamically accelerated water is as far as possible concentrated at the propeller, as a result of which an improvement in efficiency can be obtained." (Bouwe Prakken, June, 2006) http://www.patentstorm.us/patents/7066776-description.html

Another aspect, mentioned by Ken, and by Atkins, is the fact that the stern does not squat. I believe this may be because the tunnel geometry actualy imparts lift to the aft section of the hull. In a conventional V hull, the deadrise angle represents a deduction from lift (why deep V hulls need strakes and other aids to run on plane; the ideal planing surface is perfectly flat). A tunnel, however, regardless of its precise shape, functions as an inverted V, with lift added by the reverse deadrise angle and by the increased surface area supporting the same weight.

An example of an efficient cruiser incorporating a variation on the Atkins design is found in Walter Schulz's Shannon 38 SRD (Schulz Reverse Deadrise), a semi-planing design which reaches 20.5 knots max and 18 knots cruise with less than half the power of typical cruisers of the same hull length. What Walter calls reverse deadrise is actually an inverted V forming a tunnel leading to the prop (actually there are 2 tunnels, as the boat has twin diesel power). Design benefits include the progressive and level transition from full displacement to planing mode, level ride, excellent seakeeping, and 2 mpg fuel economy.

http://www.shannonyachts.com/38SRD.html

BTW, Fred, I thought you'd be interested in Walt's straight bow and hull length, although his boat has way too much beam to fit in a container!

[tom28571]

I think you guys are over-analysing this. No doubt, Aitkin was a clever fella, but I think he developed a boat who's primary goal was shoal draft operation with relative economy. The boats' clearly evolved over time... elements like the "side-skirts" and kick down at the transom were different from boat to boat, suggesting this was more of a trial-and-error approach than some kind of high-tech boundary layer experiment.
Nothing wrong with that of course - most boats are improved this way. And it may well be that the end result did indeed re-use the energy expended in creating the boundary layer in the 1st place, but I think it'd take some pretty serious testing to prove one way or the other.....

The boat has a number of other benefits - particulary for a containerable craft like FF's.... hurry up and build the bloody thing will you.... we are all eager to examine the results!

ps ... Tom... did you get my PM, or email...?

Yes, I do think this issue is being over analyzed but hopefully some light will shine in here somewhere. It already has with the general agreement that the boundary layer water can provide some advantage to a trailing propeller.

I am shocked that a patent has been granted on this phenomena that has been known for some time. Lindsay Lord has quite a bit of information on it in chapter 12 and I think I read about it in Gerr's books.. I was a bit skeptical but this discussion has helped to reinforce the idea of positive contribution of the boundary layer. This has been hard to come by since I know that the energy in the boundary layer has to be supplied by the propulsion system, engine or sail, and detracts from boat speed.

I also wonder just what Schulz's patent contains. Negative deadrise has been around for a very long time. His hull bottom does resemble Atkin's in principle. I have been using a similar shape on the aft bottom of my boats for about 10 years. My design arose from material I read from Weston Farmer. I guess it could be looked as a double tunnel as mentioned above. There is very little new in boat design.

Most of what we see as "new" rests on work or suggestions done by others earlier.

Will, I don't check my PM often enough. The at-rest CB of the 24 (dry hull) is calculated as 57.7% aft of LWL. This is close to what you are shooting for. Adding engine and fuel aft of dry hull CB, two crew, water, stores and other gear forward of dry hull CB should keep it close to that.

Fred, Sorry I missed your PM also. Like most of the rest of you, I wish I had the loot and time build all these designs we talk about here. Discussing them here does no harm even if some of the thoughts are probably completely off the wall.

Added: OK, I looked up Schultz's patent. It looks a lot like the famous Hinkley patent fracas in that it is a combination of 6 claims that taken alone are already used or known by the boating fraternity. So it appears to me that it could be infringed only if someone built a boat including all of these claims since there are existing boats that already have the features of some of the individual claims. http://www.patentstorm.us/patents/6994049-claims.html Any patent experts here?

[Willallison]

Yes - sorry - I re-read my post and it comes across as a criticism of the discussion in general. I didn't intend it that way by any means - the pursuit of improved efficiency is of course a valid one....

I haven't done any research on this, and I'm sure it varies from boat to boat, but I wonder just how far from the hull surface the boundary layed extends. If it's but a few inches then it's doubtful the much if any of the prop is running in this pre-accelerated water.

Oh - and thanks Tom

[Tad]

I guess the ultimate development of a reverse deadrise hull would be the Hickman Sea-Sled.

That a patent has been granted to Schulz doesn't really mean much until he has to defend it in court. So far no other builder has seen the need to challange this patent. When the SRD story was first published in 2004 I ran some numbers looking at the transport efficency and could not find anything special about the performance of the boat.

I don't believe the claimed 13,500 pound displacement for the SRD 38' so I upped it to 15,650 at half load. At 21.5 knots with 320 HP I get Et = 3.22 at a Volume Froude Number of FNv =1.02

An American Tug 34 (1980's semi-displacement hard-chine Lynn Senour design) at slightly lower FNv =0.79 achives a Et = 3.34

A 40' traditional wooden Lobsterboat designed by Pete Kass and myself achives Et = 3.88 at FNv = 0.93.

Looking at the Et of Rescue Minor is dificult because of the hazy numbers quoted by Rob White. He mentions 550 pounds as weight with 2.5 gallons of fuel, presumably that is without crew. So I'll guees at a weight of 750 pounds with one person aboard. He mentions 22 knots but claims a useful speed of 18.6 for a FNv = 3.67. He does not mention how this speed was measured. Unfortunatly the power required is unknown, though he mentions it takes "about half the available HP". I don't really believe that, but if it takes 12 HP to achive 18.6 knots Et = 3.56, or if it really did only take half the installed 18 HP, Et =4.75

At a FNv = 2.82, Whio's Et = 2.27

Looking at some really efficient boats.....

Harry Bryan's Rambler (16') reportedly runs 5.2 knots on 3.2 HP for an Et =7.998. Small and slow equals efficient.

LF Herreshoff's Piquant topped out at 20 knots with twin 16 Hp gas engines for an Et = 5.88. Long (47') and light (6700 lbs) equals efficient.

W. Garden's Clam uses 10 HP for 6 Knots, Et = 17.13 at FNv = .34. Very long (40') and slow equals efficient.

Phil Bolger's Slicer design achieves 18 knots with a 15 HP outboard for an Et = 6.26 at FNv = 1.8. A very light and narrow hull can be fast and efficient.

All the best, Tad

[Willallison]

Hmm - some real numbers... thanks Tad
Incidentally, how is the Transport Efficiency calculated?

It brings us back to a comment made by one of our very wise contributors quite early in the piece...;)

FF-
I actually think the SBS has great merit for your application. As you say, there are many other attributes that make it a worthy contender. I would expect that the box keel configuration ought to suffer less from the sort of overloading that becomes so common on board cruising boats too. I know there are a number of people - myself included - that have at least toyed with the idea.

But, there's been a great deal made of its apparent fuel economy And as it was designed (and I certainly believe it was designed to be light, as fcfc has demonstrated) I'm just not convinced that it's any more efficient than a simple lightweight boat of 'normal' form would be.

I can't wait till you have yours in the water - then we'll all know!!:p

[kengrome]

Looking at the Et of Rescue Minor is dificult because of the hazy numbers quoted by Rob White. He mentions 550 pounds as weight with 2.5 gallons of fuel, presumably that is without crew. So I'll guees at a weight of 750 pounds with one person aboard. He mentions 22 knots but claims a useful speed of 18.6 for a FNv = 3.67. He does not mention how this speed was measured. Unfortunatly the power required is unknown, though he mentions it takes "about half the available HP". I don't really believe that, but if it takes 12 HP to achive 18.6 knots Et = 3.56, or if it really did only take half the installed 18 HP, Et =4.75Hi Tad,

I wish I knew more about these Et's and FNv's and how they are calculated so I could join in the technical part of this discussion in an intelligent manner -- but I don't (at least not yet) so I will stay away from trying to interpret those numbers in any more than a simplistic manner.

Here's what I think in layman's terms:

Most of us who want efficiency are focused on running at certain speeds that will satisfy us. As long as we can run at these speeds, our next goal is to find ways to use as little fuel as possible at our favored speeds -- given all the OTHER requirements we have for our individual boats.

Leo tried to focus this discussion a bit in his first post when he said:

Can anyone provide any examples (other than Whio) of low power semi-displacement/semi-planing boats that can cruise at the same 12-15 knots and achieve at least 6 NMPG while doing that speed?I think you may have done this with your Slicer example:

Phil Bolger's Slicer design achieves 18 knots with a 15 HP outboard for an Et = 6.26 at FNv = 1.8. A very light and narrow hull can be fast and efficient.... but then again I don't know how Et's and FNv's convert to knots and NMPG's so I could be way off on this one. One thing I do know is that I ran my father-in-law's full displacement 22 foot Philippine banca boat on an 8 HP B&S engine at speeds which I believe were probably at least 15+ mph ... and I wasn't running WOT either, so I can easily believe that Slicer can run at 18+ mph on almost twice the HP.

But Slicer is a very simple and lightweight open boat, nothing like Whio. So don't we have to define the boats in this discussion in a more focused way, perhaps by specifying some parameters that make our comparisons more "comparable"?

My first thought in this regard is to specify not only a speed range like Leo did, but also a displacement range, and maybe a couple of other parameters as well. For example, here's a question we might ask which would focus our attention on a more specific set of parameters:

What hull design and engine combination will provide the greatest propulsion efficiency under these conditions:

1- Continuous cruising speed of 15-17 mph
2- Cruising displacement of 2000-2500 pounds
3- Propeller/rudder draft no more than 12 inches
4- Easy trailerable -- no more than 30 feet long

I'm not trying to focus the discussion on these arbitrary parameters, I'm only asking if we might end up with a clearer understanding of the issues involved here if we did?

I noted this in Leo's first post too:

Other than Whio, I have yet to see a lightweight semi-displacement/semi-planing boat that specs out an engine of less than 100 horsepower to achieve the same cruising speed as a Bluejacket, 14 knots.It seems like Slicer does this even though it is a dramatically different boat than Whio, so I don't think Leo was thinking of a boat like Slicer when he posed his original question. Slicer could very well be "full planing" boat too, which should actually leave it completely out of this "semi-planing" discussion -- unless of course we get rid of the focus on"semi-planing" and replace it with a specified speed range -- not a bad thing to do in my opinion.

As people are beginning to learn, I have a particular interest in the tunnel-stern Seabright skiffs modeled after those of the Atkin's. But this interest is not solely based on my desire for fuel efficiency or that these boats might run best in the semi-planing speed range. I actually believe they have several more important features to offer ... extreme shallow draft, upright beachability, protected props and rudders, etc. to name a few.

Do they provide the fuel efficiency Leo is seeking? I'm not sure and I have my doubts, but I certainly hope so.

[FAST FRED]

" extreme shallow draft, upright beachability, protected props and rudders, etc. to name a few."


For my concept of a cruiser the ability to be installed in the box with simple pipe rollers , or to slide on a simple flat trailer for land movement is also a plus.

A bigger plus is the ability to RUN aground with no hassles, damage.

Cruising in new places , a grounding is simply part of the game.

But BEST!! of all is the ability in many places to avoide the overcrowded moorings and packs of boats.

If the boat draws 2 or 3 ft , and is happy as a clam taking the ground , many near shore places become suitable anchorages.

So far the claimed "efficency" of the Atkin boats comes from 3 items.

1.The box keel supports the boat creating LESS wave making drag.
2.The reverse deadrise keeps the stern from droping at higher speeds.
3. There may be some added efficency from capturing the skin drag.

One other thing that Atkin chose was is 1-1 tranny no reduction gears.
This gives a smaller diameter , higher pitch prop , perhaps suited to the already accelerated water aft of the box keel.

The other is weather the bottom of the Box Keel serves as a plaining surface , providing some lift.

Comments gentlemen?

FF

Tom , read your T?S adventure , will be somewhat different in out 50 ft Navy Utility launch!!

FF

[tom28571]

I have used something similar to the Et that Tad mentions but have no idea of how that one is calculated. Would be interested in how each of those numbers is arrived at. None of my books mention an Et although Gerr uses something similar.

Fred, since your boat is a Launch, what kind of cabin structure do you have? Any photos?

[Tad]

Quick paste from an old piece of writing.

The comparison of MPG without regard to either load (displacement/weight), or time (speed), seems an incomplete discussion. There is a very useful comparison of speed/power/weight called transport efficiency (Et ). With this coefficient, plotted against a dimensionless speed, we can compare the efficiency of displacement, semi-displacement, and planning hulls.

Transport Efficiency takes into account not just the power required (in effect MPG), but also the weight (displacement in the case of a boat) being moved, and the time required to move that weight (speed).

Et = W * v / Pd * 550

W is operating displacement, in pounds.
v is speed in feet per second, (knots * 1.6889).
Pd is the horsepower required for the above v.

For Rambler I guessed at a cruising weight of 1600 pounds.
6 miles per hour is 5.2 knots is 8.798 feet per second.
I took the power as stated 3.2 HP.

Et = 1600 * 8.798 / 3.2 * 550
Et = 7.998

Plot this against Volume Froude Number

FNv = v / (g * disp.^.666)^.5

v is speed in feet per second.
g is acceleration due to gravity, (32.2)
disp. Is displacement, this time in cubic feet.


Tad

[kengrome]

For my concept of a cruiser the ability to be installed in the box with simple pipe rollers, or to slide on a simple flat trailer for land movement is also a plus. A bigger plus is the ability to RUN aground with no hassles, damage.Well, if we are going to discuss live-aboard cruisers like the one Fred describes, these seem to be desirable features:

upright beachability
extreme shallow draft
protected props and rudders
run aground frequently without damage
bottom designed for easy flatbed trailering

These features are all offered in the Atkin style tunnel-stern Seabright skiffs assuming the bottoms are built thick and/or strong enough. These are all features the average everyday boater will benefit from on smaller boats as well, which is why I like these boats so much regardless of whether or not they are any more efficient than similarly sized non-tunnel stern boats.

One other thing that Atkin chose was is 1-1 tranny no reduction gears. This gives a smaller diameter, higher pitch prop, perhaps suited to the already accelerated water aft of the box keel ...... and most definitely suited to a tunnel that is shorter in height!

As I mentioned previously the prop does not have to work so hard lifting water up into a shallower tunnel, and therefore more energy is available to push the boat forward. Small diameter props are also cheaper, and so are 1:1 transmissions, and the boat will have more useful volume inside with a shallow tunnel. These are all "good things" in terms of economy and features.


The other is whether the bottom of the Box Keel serves as a planing surface, providing some lift.I kind of doubt this when talking about the Atkin-style bottoms which are all perfectly flat. The Atkin boats are designed to keep the forefoot in the water, which suggests to me that they are really designed to perform more like displacement boats than planing boats, so I don't think much planing comes from the flat bottoms.

In some of my designs (including the twin tunnel on the previous page although you can barely see it in those pictures) I use a very shallow vee on the FORWARD bottom of the box keel only. I think this will soften any pounding that might occur when running in a chop. It is not there to induce planing, but when running at speed it might lift the front of the boat a little since this very shallow vee actually warps into a fine entry.

I do not think the planing effect here will be noticeable though, and I certainly do not want the bottom to ride on top of the waves -- but the recurved aft tunnel should lift the stern and push the bow down to prevent this anyways, just like it does on the Atkin boats.

This shallow vee on the forward bottom will provide a benefit not available on the Atkin boats. It will concentrate wear and tear on bottom of the vee since it is lower than the rest of the bottom in this section. If damage occurs here more than in other places on the forward bottom it will be easier to find and fix. The bottom can be strengthened along this vee as well to deal with the higher stresses that focus here.

[Willallison]

The box keel may well be operating primarily in the displacement mode, but given that the boat speed is well byond the displacement speed of the boat as a whole - and provided the bottom is offering a positive angle of attack - then it would definitely produce a degree of lift.
The down-angle of the stern , however, may counteract any natural tendency for the bottom to assume a natural trim angle.

Thanks for the info too Tad.

[kengrome]

Hi Tad,

Thanks for the info!

Using Et's is clearly a more 'universal' or 'theoretical' way to discuss efficiency since it dispenses with the need to define both displacement and speed. But I always think in terms of numbers I can relate to -- like 2000 pounds and 17 miles per hour for example. These Et's and FNv's are so new to me that it's hard for me to get a grip on them ...

The comparison of MPG without regard to either load (displacement/weight), or time (speed), seems an incomplete discussion.I think this is why I suggested that we define the displacement and running speed of a particular "discussion boat". Then we can talk about real numbers like MPG's ... and pick out some real boats that match our "discussion boat" and compare MPG's very easily.

Sorry, I'm just a simple guy I guess ... :)

[kengrome]

The box keel may well be operating primarily in the displacement mode, but given that the boat speed is well byond the displacement speed of the boat as a whole - and provided the bottom is offering a positive angle of attack - then it would definitely produce a degree of lift.I agree PROVIDED the bottom is actually offering a positive angle of attack -- but I question whether or not this positive angle of attack actually exists?

Robb White said the faster he runs his boat the closer the forefoot gets to the water. So if there's a positive angle of attack at slow speeds, it appears to diminish when the boat speeds up -- effectively reducing or eliminating the positive angle of attack needed to make the boat plane.

So how can we resolve or explain the apparent inconsistency here?

A lowering angle of attack suggests less and less planing as speed increased -- yet the boat is moving INTO (not out of) the planing realm while this is happening.

Seems like a paradox to me.

[Willallison]

Actually, the faster a boat goes (once on the plane) , the lower the required angle of attack - or trim - in order to supply the required amount of lift. So, in fact you could expect this to happen regardless of anything else that is going on.
Having said that, I agree that as far as the SB skiff goes, significant lift is going to be created by the aft sections. Indeed, if you go fast enough, the bottom of the keel would be presenting a negative angle of attack and may produce a corresponding negative lift! A sort of amplified porpoising action, if you will. Let me observe from a nearby boat when this starts to happen...

[kengrome]

Indeed, if you go fast enough, the bottom of the keel would be presenting a negative angle of attack and may produce a corresponding negative lift! A sort of amplified porpoising action, if you will. Let me observe from a nearby boat when this starts to happen...I have a Toyota engine and transmission that I may end up installing in my Tolman Seabright if I don't sell it to someone else first. That engine must be at least 40-50 HP or so, way more than the 20-25 this boat should be able to handle, so I think it will provide plenty of power to run the boat too fast and experiment with this effect.

Why don't you join me on the boat? Then we can both experience this "super-porpoising" together! It will be fun to talk about for a very long time, assuming we live through it ... :)

[Willallison]

I'll bring my wetsuit!:D

[Tad]

Graph of Et against Volume Froude Number from Donald L Blount and Associates.

14613

[tom28571]

Tad, Will, Fred, Ken,

In a fit of (probably false) bravery, I will throw in a couple numbers of my own.

2500#(2 adult crew), 20.5kts, 50hp - (Et = 3.14), (FNv = 1.78)

Same boat 3175#(5 adult crew), 19.1kts, 50hp - (Et = 3.72), (FNv = 1.53)

I wish I could give some numbers at lower speed but haven't the faintest idea of what the power factor would be.

I attribute the improvement at higher load to having the 2 largest crew at the stern. This is a trade of higher wave making for lower wetted surface at higher trim angle. This was an experiment. Trim of the motor has very little effect on hull trim angle. My usual cruising speed is between 10kts and 15kts.

In defense of Schulz, I think his boat is probably designed to operate at higher efficiency in the "transition" speed range as is mine. Like mine (and Atkin), his stern shape will depress the bow resulting in a lower top speed than otherwise might be expected.

Have at it.

Without taking into account the actual use that the boat is put to, these numbers don't mean very much. If we are talking about a boat that can transport people in some degree of comfort for a period of time, space and other desirable characteristics will negatively impact these numbers. Efficiency is a good comparison tool only for boats that are intended for the same purpose. Many of these design objective have been listed above. My goals are little different in that they are not so dominated by the shallow draft requirement but are also pretty shallow.

[FAST FRED]

Our current boat is not bad on fuel use, about 3gph at 8K.

If the box boat could double the speed at the same fuel consumption I would be delighted.

16K to 18K on 3 gph.

LUCY is 50ft loa 15ft bwl, about 30,000lbs with simple structure and all cruising gear.

FF

[kengrome]

Tad, Will, Fred, Ken,

In a fit of (probably false) bravery, I will throw in a couple numbers of my own.

2500#(2 adult crew), 20.5kts, 50hp - (Et = 3.14), (FNv = 1.78)

Same boat 3175#(5 adult crew), 19.1kts, 50hp - (Et = 3.72), (FNv = 1.53)I presume that this was a test to determine the highest speeds you could obtain in these two different configuration. Do you happen to know what your fuel consumption (mpg) was in each of these situations?

I attribute the improvement at higher load to having the 2 largest crew at the stern. This is a trade of higher wave making for lower wetted surface at higher trim angle. This was an experiment. Trim of the motor has very little effect on hull trim angle. My usual cruising speed is between 10kts and 15kts.It's an interesting test that clearly demonstrates the fact that you can get higher efficiency with more weight in the boat if you ALSO adjust the boat's attitude or pitch angle to improve its planing performance by reducing wetted surface area. It's like testing two different boats when you think about it.

In a heavier cruising vessel, fully loaded and ready to go on a trip that might last months, adding more people aft like you did might not change the trim angle enough to get the bow out of the water -- in which case the efficiency will undoubtedly be lower instead of higher.

It would be interesting (to me anyways) to see how your mileage differs at a constant cruising speed of say 15 mph with the two different displacements you mentioned. A test like this might be more interesting to a person like Fred who will likely be cruising for hours at a time at these lower speeds that everyone likes to refer to as "semi-displacement" or "semi-planing".

Without taking into account the actual use that the boat is put to, these numbers don't mean very much.They don't mean much to me anyways because they are not real-world numbers like miles per gallon ... but this is my problem not anyone else's ... :)

I agree that taking the boat's use into consideration is important to figuring out how to develop the most efficient hull for that particular use. After we define all our other parameters such as length, beam, height, displacement, bottom strength, draft, beachability, seaworthiness, acquisition cost, etc. the owner's primary concern is probably going to be:

"What kind of hull design can I choose that will give me the highest mpg most of the time?"

So after defining a "concept hull" that meets all the other requirements, fuel efficiency is the one thing I am most interested in learning how to optimize.

[tom28571]

[QUOTE]I presume that this was a test to determine the highest speeds you could obtain in these two different configuration. Do you happen to know what your fuel consumption (mpg) was in each of these situations?

Actually, no. This hull has a chine configuration aft that is intended to lift the stern to promote early planing at lower speed than "normal". The test was to see how much this configuration detracted from top speed by depressing the bow. I don't have means to measure fuel use at particular speed or loading but am now working on a gage that should be able to tell me that. I plan to run the boat at different speeds over a measured 2/10 mile course and measure fuel used.

In a heavier cruising vessel, fully loaded and ready to go on a trip that might last months, adding more people aft like you did might not change the trim angle enough to get the bow out of the water -- in which case the efficiency will undoubtedly be lower instead of higher.

In my experience, this would not work on any of the "normal" boats since they are mostly bow high anyway. Lowering the stern would not likely help matters. I don't think it would have any positive effect on Fred's boat.

They don't mean much to me anyways because they are not real-world numbers like miles per gallon ... but this is my problem not anyone else's ... :)

Oh contraire. I think many of us have the same interest.


I agree that taking the boat's use into consideration is important to figuring out how to develop the most efficient hull for that particular use. After we define all our other parameters such as length, beam, height, displacement, bottom strength, draft, beachability, seaworthiness, acquisition cost, etc. the owner's primary concern is probably going to be

"What kind of hull design can I choose that will give me the highest mpg most of the time?"

The other critical element to add to your list is speed. At what speed do you want to optimize mpg?

So after defining a "concept hull" that meets all the other requirements, fuel efficiency is the one thing I am most interested in learning how to optimize.

Right on.

Although Fred's fuel use if a lot more than I want to pay for, he owns the boat. You have to balance the cost of a new boat against that. Fred, what kind of engine do you have?

[SAQuestor]

Although we've been mostly talking about performance over the past few weeks, a commenter in the Dutch Barge thread (http://www.boatdesign.net/forums/showthread.php?t=11316) said in message 120: (http://www.boatdesign.net/forums/showpost.php?p=150255&postcount=120)

{with snippage} An important consumable when boat building is enthusiasm. It may get expended quickly along with frustration and slow progress.

I think that this is an extremely important point. I've built a house, a barn and several outbuildings and in each case the enthusiasm waned about 2/3 of the way finished and it was a chore to regain the focus necessary to complete the project. Why should it be any different with a boat build?

And I doubt that I am alone in this. My co-worker just had a house built, but he did the painting (inside and out), landscaping, driveway, built the separate garage/shop and a garden tool storage building. His enthusiasm (and his spouse's enthusiasm) waned when the painting was weighing heavily on the build schedule. He'd drag into work after being up 'till 2 am painting the interior with the prospect of days (and nights) more to go.

I know, to save a few thousand he volunteered and should have understood what he was getting into. Regardless of that, he wasn't as "pumped up" for the next phase of the whole project as he could have been and that waning of enthusiasm delayed some of the other aspects that he would have liked to have done sooner.

Perhaps not all folks are like David and I, and can better maintain a steady enthusiasm level for the hundreds (or even thousands) of man hours it will take to see a major home boat build project through to completion.

IMO this should be something that we all take into serious consideration as we attempt to decide whether and how to proceed.

As always, YMMV.

Best,

Leo

[kengrome]

Perhaps not all folks are like David and I, and can better maintain a steady enthusiasm level for the hundreds (or even thousands) of man hours it will take to see a major home boat build project through to completion.Actually I think most backyard boat builders have the same problem staying focused and making regular progress. Other things in life will 'take over' and become more important if you let them ...

One guy once told me he just considers it a responsibility like a job. He goes to the garage (shop) on time, works on a schedule, and leaves on time to go home enjoy the rest of his life -- without any feelings of guilt or frustration for neglecting the project. He said he is happier and gets more done this way.

Even if you did this by scheduling your boat work for 3 or 4 hours every Monday, Wednesday and Friday you would still make regular progress, provided you do not let other things take priority over your boat building time. That's the key to success here -- not letting other things cut into your scheduled boat building time.

IMO this should be something that we all take into serious consideration as we attempt to decide whether and how to proceed.One way to have the exact boat you desire without all that work -- and still keep it affordable -- is to hire a guy who owns an overseas boat building shop and have him build it for you. Materials and labor are so much cheaper in Asia that you probably won't pay any more in the long run, even after shipping costs are included.

But even if it costs a little more, you could be using your new boat months or years earlier this way -- rather than still building it. For folks who prefer to be on the water rather than than in the shop this seems like an attractive option. There's nothing wrong with having "just the hull" built and finished for you either. Then you can install all your personal goodes and do the final outfitting yourself after the boat is delivered.

It would be even more interesting to find an overseas shop that would let you go there on a "tropical boatbuilding vacation" in the middle of winter and take part in the construction personally. Or instead of getting your hands dirty, why not just sit in the shade of a palm tree sipping one of those drinks with the little umbrellas in it as a cute local girls fans you to keep you cool ... and simply "inspect and observe" while the local craftsmen do the work of building your new boat for you!

:)

[Willallison]

There's really only two reasons why people choose to build a boat. The most obvious is that they can't afford to have someone else build it for them. The other is because they enjoy the process. This latter group are far less likely to fall into the enthusiasm abyss.
Sadly, for every story one hears about a fabulous boat built overseas, there's another about the horror of discovering that all that cheap teak is full of borers or something. Also, custom builders usually like to be involved in the process, so an overseas build is less attractive. Though visions of the cocktail under a palm tree style oversight sounds good!

[kengrome]

There's really only two reasons why people choose to build a boat. The most obvious is that they can't afford to have someone else build it for them.Actually they can in most cases, they simply need to hire an overseas builder. But most people seldom think beyond their local areas so the thought of hiring an overseas builder never even enters their minds. Or they don't trust the builder when he is so far away.

The other is because they enjoy the process. This latter group are far less likely to fall into the enthusiasm abyss.That's for sure. When you enjoy the work you usually remain interested and motivated.

Sadly, for every story one hears about a fabulous boat built overseas, there's another about the horror of discovering that all that cheap teak is full of borers or something.I treat my wood -- even the plywood -- with Ethylene Glycol and Copper Naphthenate. American builders could easily do the same and their boats would never rot or be eaten by bugs, but I don't think very many of them bother, and I'm not sure why to tell you the truth ...

There are always a few greenies around who whine about the dangers of using these and other chemicals, but as far as poison treatments go these two are some of the least likely to endanger you. Once they have been applied to the wood EG and CN are basically benign to humans, yet they are exceptionally effective at doing exactly what they are supposed to do, which is to prevent the "bad things" from destroying your wood.

Also, custom builders usually like to be involved in the process, so an overseas build is less attractive. Though visions of the cocktail under a palm tree style oversight sounds good!Chuck Leinweber at Duckworksmagazine.com asked me to write up the story/dream of my Tropical Boatbuilding Vacation concept so he could publish it in his online magazine. I haven't done it yet but I probably will some day since it does sound rather appealing, especially in the middle of winder in the snowy and frigid north country.

There may actually be a big enough market for such a unique niche offering even though it is probably one of the smallest niches in the world. I don't think anyone else is doing it, and it wouldn't take more than one or two clients a year to keep it going if the boats they choose to build are big enough. I'm still considering doing it some day ... :)

Until then I hope to keep myself busy designing and building new tunnel-stern boats inspired by the Atkin team, mostly because I think there is now -- and will continue to be in the not so distant future -- a growing demand for "go slower, go anywhere" boats like these.

[FAST FRED]

"I treat my wood -- even the plywood -- with Ethylene Glycol and Copper Naphthenate. American builders could easily do the same and their boats would never rot or be eaten by bugs, but I don't think very many of them bother, and I'm not sure why to tell you the truth ..".

One reason is many US wooden boats get a coat of GRP to keep the worms out which works fine.
BUT if a boat is slathered inside with diesel , mineral spirits,or antifreez (the vehicle in most preservatives) the stuff will run alongside even "watertight" clench nails and lift the glass bond under the nail head, in time.


Just wood is almost impossible to sell in the southern US, as the boat literally dissolves before your eyes.
The wooden canoe folks store their boats in garages , a wood boat in FL water is a VERY temporary thing.

"I'm still considering doing it some day ..."

Problem is most folks want to see many completed boats , that are standing up and performing well before plunking down cash.
A start up is a VERY high risk for a buyer , esp in a country with uncertain courts strange legal system etc.ect.

"go slower, go anywhere" boats

Speed is very relative, while some "need" 40+k cruise , most of these boats run out of fuel in 300 miles at best , unless really large.

I figure high teens will actually get someplace , and 5mpg is loads more reasonable than .5mpg.

1000 miles is only 200G of fuel , a weight to be sure , but not so large as to be difficult.

The current boat is powered by a 6-71 giving about 15 or 16 hp for each gallon of fuel.
Hopefully a new boat would be 1/4 the weight ,6-1 L/B ratio instead of 3-1 , and the engine should be capable of 22hp per gallon of fuel.

A 2007 engine will probably never have the robustness and reliability of the DD 6-71 ,1936 design , but it should weigh 1/3 the weight.

Progress?

FF

[FAST FRED]

In the #67 issue of Pro boat builder, I stumbled on an article about water jets. The author is Richard Akers.

I'm reviewing all the back issues I have looking for construction and other ideas I can steal.

On page 67, is a discussion of how the water jet works,

"In simple terms, water flows under the hull, enters the jet inlet, receives a momentum push by the pump and exits through the nozzle. A whole lot of the magic is high performance water jets is in the inlet design. Early jet inlets were just holes in the side or the bottom of the boat. The water would flow under the hull and make a right angle turn into the inlet. Much of the water in the inlet was coming from the still water outside of the boundary layer under the hull. The jet had to accelerate the still water to the forward speed of the boat, then pump it out the nozzle fast enough to go in the other direction, relative to the still water. With this design, the thrust is equal to the water -mass flow rate times the difference between the jet velocity and the ship velocity. We would have been better off with a propeller.

But the water directly under the boat is not still. The boundary layer of water, which starts at the bow of a high speed vessel and then thickens going aft, varies in speed from the still water outside the boundary layer. Although it is quite thin - at the transom of a 40ft boat going 25 knots it's about 4.2" thick- there is a lot of energy tied up in the boundary layers accelerating water, and we can derive significant benefits from sucking the boundary layer out from under the boat into the inlet (the hull bottom over the inlet itself doesn't have a boundary layer). If we angle the inlet to grab water from the boundary layer, we recapture the energy that would have been lost in a poorly designed inlet because some of the water going into the inlet has already been accelerated to boat speed.

For mathematically inclined readers naval architects describe the efficiency of a complete hull - propulsion system by defining a thrust - deduction fraction (t) and a thrust-wake fraction (w).

Hull efficiency - nH = (1-t)/(1-w).

The mid range values for a typical boat reveal that the propeller driven boat will have a hull efficiency of 92%, while the water jet driven boat will have a hull efficiency of 110%.

The fast propeller driven boat looses energy because of prop and hull interactions while the water jet driven boat recovers energy precisely because of the inlet and hull interactions."

As the water jet is only obtaining energy from a square ft of inlet area , and the box keel with reverse deadrise is getting almost ALL the energy from the hill in front of the prop, could this be the explanation for the reported higher efficiency?

FF

[SAQuestor]

As most know, there are just shy of a zillion different places where one can exchange information about boats – or any other subject – on the Internet.

One such place is an email list called “Trawlers & Trawlering”. You can Google it up if you’re interested.

Reading that list is generally informative because many of the posts are written by folks that are doing instead of wishing. Not all, but many. If I’m not mistaken Fast Fred even posts there. :D

A recent thread there was about Downeast Style hulls. And earlier in this thread we had a bit of a conversation about those style hull forms before we got off on Seabright skiffs and tunnel hulls.

The original questioner asked, “Relatively few pleasure boats have been built on this platform and I am wondering why.”

As we have determined earlier in this thread and in other threads on this site, I wrote back, ”As for why more pleasure boats aren't built on this hull form? Simple answer is they don't do well tied to a dock at the marina. They are great sea boats, but when compared to other 'name brand' boats that offer 2x or even 3x the actual living space you'll find in a proper downeast hull form - there's no contest for most folks that want a boat to entertain friends aboard and not actually go to sea in a northeast'r.”

Other various conversations took place and then this came across the list-serve.

“We have an '88 Nauset 35, built on a Stillman hull. (other stuff omitted) Our Cummins QSB 380 repower cruises her at 14-15 knots under 11gph. We do a lot at 8.5-9 kts well under 5gph.”

Gulp! :eek: I then wrote back and asked if they had ever weighed the boat – the reply was a survey estimate of about 18,500 pounds. For arguments sake here, let’s round up to 10 tons.

So… A 35’ boat weighing 10 tons consumes (at today’s prices about) $35 US to go 15 NM (28 km). My spreadsheet says that’s about 3 /4 gallon per nautical mile – or $2.33 US/NM.

So… 9 knots at 5 GPH – That equals about $16 US to go 9 NM (17 km). My spreadsheet says that’s about 1.8 NM/gallon – or $1.77 US/NM.

Given that these folks are out on the Great Loop and at that 9 knots/hr speed for 10 hours a day they’re moving about 90 NM (103 statute & 166 km) a day. Their cost in fuel alone is about $160 per day. In areas where they can go their 15 knots “cruise” that per day fuel cost JUMPS to $350. And this doesn't factor in any generator running time.

Holy Cow Batman! The first thing that comes to mind is the difference between having gobs of money versus just being in a “comfortable” economic situation. Personally, I can’t wrap my (raised poor) feeble mind around the concept of spending that kind of cash to pay for fuel for a single day. And yes, I know lots of people drive Hummers and big boats that don’t give this sort of expenditure a second thought.

But I do, and that’s the primary reason that I started this whole thread. I want a reasonably sized boat that Momma and I can spend several months aboard seeing the same sights as the folks forking out $$$ hundreds per day for fuel while I do the same trip for comparative pennies.

As an addendum to this, there are apparently some issues with the Trent-Severn waterway wherein there are a number of boats hitting debris and getting out of the channel or having some other calamity befall them. Lots of bent shafts, struts and wheels. So not only does having a large & heavy boat cost in terms of fuel usage, but it is also very expensive when a problem occurs that requires a haul out in the relatively remote areas of the T-SW.

The point here? Bigger doesn’t appear to share a bed with economical. And yes, I recognize that my definition of economical may not be the same as someone else’s. A matter of perspective I ‘spose.

Best,

Leo

P.S. I'm not criticizing the folks that have the big boats and their ability to pay for the boat/fuel/maintenance. It's just not 'me' is all.

[moTthediesel]

Leo -- I hear you!

Holy Cow Batman! The first thing that comes to mind is the difference between having gobs of money versus just being in a “comfortable” economic situation. Personally, I can’t wrap my (raised poor) feeble mind around the concept of spending that kind of cash to pay for fuel for a single day. And yes, I know lots of people drive Hummers and big boats that don’t give this sort of expenditure a second thought.

But I do, and that’s the primary reason that I started this whole thread. I want a reasonably sized boat that Momma and I can spend several months aboard seeing the same sights as the folks forking out $$$ hundreds per day for fuel while I do the same trip for comparative pennies.

I feel just the same way and I have been thinking for some time about what boat would serve for that kind of economical cruising.

What I've found so far is that there is, to my knowledge, nothing available on the market that meets, or even comes close to my criteria. So, for the second time in my life I'm thinking about undertaking a large boat building project :eek:

It's no secret what kind of boat is needed for that kind of fuel economy, it must be long, narrow, and light. That, of course, means compromises, but all boats are that way, it's just that my compromises are different then those made by the designers of commercially available boats.

I had been thinking of a beam less than 8', and a LOA of something less than 40'. I had never thought about container shipping until I read FF's idea's about it, but as luck would have it, that's just the size for a "boat in a box" too.

My first thought was a simple canoe shaped hull, and while that would give the minimum wetted surface for a given displacement, there are some serious drawbacks to that shape. In order to have standing headroom below decks in a ultralight boat of this size, you would need either very high freeboard, or very high crowned decks. One of the things I find attractive about the Atkin tunnel hull SS is that it can get your feet lower in a hull that displaces a very small amount of water. Also, I think that the relatively deep center hull area would give a smoother ride in a chop and better directional stability to boot.

I'm still just noodeling around, and a long way from lofting a shape, but I sure enjoy reading everybody elses ideas on the subject here, please keep this thread going!

moT

[u4ea32]

I spent 10 days cruising Alaska in a chartered Nordic Tug 32, single diesel, "semi-planing hull" boat.

Had a GREAT TIME!

First tank of fuel, mostly at 2200 RPMs @ 11 knots:
268 nm, 200 gallons, 1.35 nmpg

Second tank of fuel, mostly 1800 RPMs @ 8.7 knots:
396 nm, 200 gallons, 2 nmpg

Total:
664 nm, 400 gallons, 1.66 nmpg

We had glass smooth water almost all the time.

My 32 Fountain sportfish cruiser in glass smooth fresh water
gets the same mileage at any planing speed from 15 to 60 knots,
with twin gasoline big clock engines.

In rough water I need to put down my K-planes to have a smooth ride, and I get the fuel economy [!] down to the 1.1 nmpg that the Nordic Tug sees at 11 knots. Of course, I can still go any planing speed at that 1.1 nmpg, up to about 40 knots in rough seas.

The difference: the Fountain weighs about 10000 lbs, the Nordic 32 weighs about 18000 lbs.

With the fuel efficiency difference between gas engines (10 hp per gallon per hour) and diesel (20 hp per gallon per hour), its pretty clear that the Fountain is substantially easier to push through the water than the semi-planing Nordic Tug.

Yet more evidence of my earlier statement: efficiency requires light displacement. All the other stuff is minor compared to displacement.

[kengrome]

I want a reasonably sized boat that Momma and I can spend several months aboard seeing the same sights as the folks forking out $$$ hundreds per day for fuel while I do the same trip for comparative pennies.Leo, can you tell us what size this boat might be? Do you have an idea in terms of length or displacement or something?

There are apparently some issues with the Trent-Severn waterway wherein there are a number of boats hitting debris and getting out of the channel or having some other calamity befall them. Lots of bent shafts, struts and wheels.If a tunnel-stern Seabright design protects the underwater runnning gear from this type of damage, it seems like a hull that would be good for the Great Loop, provided it meets the rest of your needs too.

One of my ideas is to develop a very good hull for folks who want to do the Great Loop ECONOMICALLY, then build the boats and lease or sell them. I don't have the funds for such an endeavor, but who knows when an angel might come along to support me? Anyone with half an interest in this concept would certainly recognize the value of having me build the boats overseas, that's for sure.

I think the tunnel-stern Seabright skiff is the answer here. It protects the prop, shaft and rudder very well, and its (theoretical) efficiency would be great for economical cruising. I want the typical cruising couple to be happy with the boat while living on it for months at a time, but I do not want the fuel costs to be so high that no one can afford to use the boats!

I have questions about empty displacement, total displacement, maximum speed, cruising speed, dock and lock handling, and much much more . I know I cannot have all the answers at once, but to settle on the basic hull design concept with a rough idea of the length and beam of the boat means I can get started and make some progress while the other answers come to me gradually.

Sometimes I wonder if it makes any sense at all to go for a semi-displacement hull when pure displacement hulls are much more economical. I even started designing such a boat based on the traditional non-tunnel Seabright skiff bottom. But then I go back to wondering if anyone these days would actually be satisfied to plod along at displacement speeds? If the goal is to "enjoy the ride" rather than to "enjoy the ports of call" displacement speeds might be enough. But I still think people are spoiled enough these days to demand higher top speeds if/when they happen to feel like it.

Am I wrong about this?

[SAQuestor]

Leo, can you tell us what size this boat might be? Do you have an idea in terms of length or displacement or something?


In general terms:

Trailerable behind a HD American pickup.

LOA - 9m -ish
overall beam <2800mm
Total height from keel to PH roof <3500mm (to meet federal trailering height restrictions.)
Displacement - <3650kg
Permanent accommodations for a couple - arrangements for a guest couple for 2-3 weeks.

Good-e-nuff Ken?

Leo

[Pericles]

Leo,

IIRC there was mention of Down East hull forms. Your last post listed dimensions that reminded me of two vessels with DE antecedents. They are the DE25 http://www.bateau.com/studyplans/DE25Cabin_study.htm?prod=DE25Cabin and the LB26 http://www.bateau.com/studyplans/LB26_study.htm?prod=LB26

The DE 25 will accommodate two guests, the LB is a couple only boat, but either will provide an easily driven craft with the ability to speed up if necessary.

There are many other sets of plans for sale http://www.bateau.com/categories.php?cat=29 and even some with Carolina flare and flair. :D :D

I've chosen the LB26 for building in BS 1088 Okume and West epoxy, both of which are easily available here in the UK. It's the RCD certification that's going to cause the most grief. http://www.ceproof.com/recreational_craft_directive.htm

Pericles

[SAQuestor]

It's the RCD certification that's going to cause the most grief. http://www.ceproof.com/recreational_craft_directive.htm


I'm curious as to your reasoning.

From the RCD:

Exclusions
"craft built for own use, provided they are not sold for a period of five years from completion;"

Perhaps there is more to the RCD that I haven't seen, but the above is a pretty strong statement IMO.

Leo

[Pericles]

Leo,

That's the point. Keep it for 5 years after completion. The boat is worth much more with the RCD, especially as it's being fitted with VSD. It is to be sold. http://www.yellowfin.com/media_coverage.asp

The LB26 is just the first step to a big liveaboard that will incorporate many of my ideas for comfortable living. Of course, if money were no object this is what I'd go for. http://www.vicemyacht.com/v52/gallery.php

Pericles

[u4ea32]

Size limits for trailering in USA is actually rather inconsistent.

However, the absolute limits are beam and height: 8.5 feet wide and 12 feet high. Sure, there are ways to exceed these, generally requiring an inexpensive permit, but the limitations are onorous, including simply not being able to get into most launch ramps if the beam is exceeded (you can't get through the gates where you pay to launch). While the typical legal height limit is 14 feet, if you go that high, you'll be breaking overhanging tree branches with the boat -- a very, very expensive way for you to do tree trimming on public roads.

The length limit is far more fuzzy. Obviously, to cruise you want the maximum length you can get away with. 35 feet is better than 32, and 40 is better than 35, etc. You can easily get away with a hull length of 40 feet anywhere in the USA. Outboards and pulpits don't count: these are considered "ecoutrements" and are not measured as long as they obviously don't cause a problem. Yes, that's a judgement call.

In most practical cases, the limit is actually 65 feet overall including the tow vehicle. A big pickup is 20 feet long, so that leaves 45 for boat and trailer.

If the tow vehicle has a commercial plate, then the US universal highway access law says the maximum length of the trailer that must be acceptable in all juristictions is 53 feet x 14 x 8.5 feet, to allow the use of 53 foot long containers.

So you can build a really big trailerable boat for the US.

The weight then becomes in issue, but the weight must be addressed for the goal of this entire thread: efficiency requires light weight, extreme efficiency like people are looking for here requires extremely light displacement.

If the weight of the fully laden boat on trailer, including fuel, water, stores, gear, etc., is less than 10000 lbs, very many common trucks can tow. 8500 lbs is even better: that gets to nearly every full size pickup truck. 6600 lbs starts to include full sized SUVs.

[SAQuestor]

That's the point. Keep it for 5 years after completion. The boat is worth much more with the RCD, especially as it's being fitted with VSD. It is to be sold. http://www.yellowfin.com/media_coverage.asp
Cool! :cool: How much does that little ol' drive cost nowadays?

The LB26 is just the first step to a big liveaboard that will incorporate many of my ideas for comfortable living. Of course, if money were no object this is what I'd go for. http://www.vicemyacht.com/v52/gallery.php


Nice boat. Lotto's up to 44 million on Saturday. Maybe I'll spend a dollar and take the chance. ;)

Leo

[SAQuestor]

Size limits for trailering in USA is actually rather inconsistent.

However, the absolute limits are beam and height: 8.5 feet wide and 12 feet high.

David,

I snipped all that I completely agree with.

The two points above do not fit with my research.

Individual states trailering regulations do vary. But federal highways all have the same requirements and as long as you're on an federal interstate highway the legal height limit is 13'6" and legal max width is 8'6" .

That's why I quoted a max height of <3500mm (~11'5") of the boat from keel to PH roof. That allows ~24" of road clearance for bumps and hollows in the road surface.

As for width - the boat I want is not one that you'd take to the local lake for a Sunday picnic. So with permits, it could actually be up to 10' wide (in most states) before a pilot car would be needed. Some states go to 10'6" before a pilot car is required.

Regardless, a max beam of less than 9'6" will allow one to travel most anywhere in the USA with minimal hassles.

And many modern 3/4 and 1 ton pickups have towing capacities to 16,000 pounds. I chatted with a fellow towing a 5th wheel travel trailer behind a Dodge diesel dually who told me that all up his trailer weight was just shy of 17,000 pounds. Not something that I'm personally comfortable with - but he was and he is not alone.

Best,

Leo

[SAQuestor]

Seems that a new member to boatdesign.net is actually building a semi-planing design.

Take a look at his photo gallery (http://www.boatdesign.net/gallery/showgallery.php/ppuser/20928/cat/500).

Best,

Leo

[Pericles]

Leo,

The Yellowfin option is a complete package, engines, VSDs and controls for straightforward installation in a boat designed for the task. It is not retrofit.

However, the benefits are smaller engines, because of the efficiency of the VSDs, no bowthruster, no rudders. Thus, if a major manufacturer were to invest in side by side comparisons with two of their hulls, one completed conventionally and the other with Yellowfin VSDs, they will see many savings over the conventionally powered boat.

You do not have to take my word for this. Page 28 of the August edition of Motor Boats Monthly has a report by Simon Collis. The subject is the latest generation of Volvo IPS technology, but the rationale is valid. The new engines are the IPS750 and IPS850, but as with other IPS units the numbers refer to the HP of an equivalent shaft drive. Thus the IPS600 uses the 435 HP D6.

The Lazzara LSX-75 is fitted with four IPS600s. This installation weighs 2.5 tonnes less than the equivalent shaftdrive setup As the engines are mounted right aft there is more internal space. Lazzara claim the fuel economy is twice as good as the shaftdrive boat. I welcome the arrival.

Pericles

The Southampton Boat Show will see the latest Volvo IPS UK prices revealed. When Yellowfin come to market is not yet announced, but their system will offer another option for enjoying boating without embarrassment (when docking).:(

[kengrome]

I keep trying to reply but get pulled away every time before I click the Submit button ... :(

Trailerable behind a HD American pickup.

LOA - 9m -ish
overall beam <2800mm
Total height from keel to PH roof <3500mm (to meet federal trailering height restrictions.)
Displacement - <3650kg
Permanent accommodations for a couple - arrangements for a guest couple for 2-3 weeks.Leo, thanks for the dimensions, they help me to understand some of the features required in the boat you want. I think your numbers would work well for a boat designed to cruise the Great Loop with no more than 2-3 people aboard. I'm hoping to design such a boat with similar parameters to yours.

Most of my recent design work has been tunnel-stern Seabright skiffs because I really love the way they combine so many features I like into a single boat ... but this new boat is a departure from the tunnel-stern concept:

http://www.bagacayboatworks.com/linkfiles/landingcraft31small.jpg

Here's a link to the full sized image if you want more detail:

http://www.bagacayboatworks.com/linkfiles/landingcraft31large.jpg

This boat is 31 feet long and 9 feet 6 inches wide. It still has a Seabright skiff bottom, but it is missing the tunnel-stern portion. That's because my goal here is to create lots of capacity in an easily driven displacement (not semi-displacement) hull while also offering safety when beaching through the surf, sitting stable and upright when landed, and easy loading and unloading via a bow transom that opens downward to become a loading ramp.

These are special requirements for a special boat -- a commercial mini-ferry with capacity to transport 20+ people and their stuff from one beach to another between the islands. The ability to avoid commercial piers and docking facilities by using local beaches offers a much broader range of transport options than currently available ... and since it wil be built by local shops I want it to be very easy to build thus the reason for the front-rear symmetry.

As I was creating this design (it is not yet finished) I was constantly thinking of how a similar boat might be optimized for cruising the Great Loop, and whether or not Americans can accept a displacement cruiser for an excursion like the Great Loop. Would you be satisfied to run at displacement speeds while cruising the Great Loop with your mother?

Jim Michalak says a boat needs one HP per 500 pounds of displacement to move at its calculated hull speed. If this boat weighed 8000 pounds that means only 16 HP would do it. A 16-20 HP diesel inboard is relatively economical to run, but I don't know what the true hull speed might be in a boat like this. I suspect that it will be better than the calculated hull speed due to the boat's fine entry and slim lines ... but does anyone here think this boat would move at 10 mph? Probably not, but it would be nice if it did.

The two waterlines on the drawings are 12 and 18 inches. The 12 inch waterline is 4600 pounds displacement and the water is not touching the chines yet, so it should still be very efficient at this light loading. At 8000 pounds and 18 inches draft the water is just above the chines in the boat's mid-section, so a bit of efficiency would be lost at this displacement unless the hull were modified -- easy to do at this stage of the game.

Regardless, a max beam of less than 9'6" will allow one to travel most anywhere in the USA with minimal hassles.At first I was concerned that this boat's width might be a problem since it is in fact 9.5 feet wide. But if you're correct that it's really no big of a deal to trailer a wide boat like this once in a while, then I don't think this boat's width would bother too many people. It's not like this is a weekend boat so it won't be moved all that often over the highways anyways, assuming it is ever built in the USA ... and if they are ever built here in the Philippines they will never be trailered, that's for sure.

:)

[SAQuestor]

while cruising the Great Loop with your mother?


Mother's ashes are fertilizing the grass at the family cemetery.

Momma, on the other hand - Nana to the grand kids, and to me SWMBO - Momma - Admiral - Significant Other - is off on her annual summer visit to her mother in NJ.

Just to clarify that point Ken. :D

Leo

[FAST FRED]

Much of the loop is 10 Klic speed limit , so most boats putter at 6K , which is economical for almost anything with a bunk.
Were now at Manatoulin Island , and still only clocking 8statute mph , at about 2 - 2.5 gph.
Those old detroits siup fuel at 1100rpm, so fuel cost EVEN at $1.25 a liter in Canada isnt a horror.

FF

[kengrome]

Much of the loop is 10 Klic speed limit , so most boats putter at 6K , which is economical for almost anything with a bunk.I don't know what "10 Klic" means, but if you're suggesting that most of the loop is run at displacement speeds, this means my displacement hull is a good starting point for a specialized Great Loop boat. That's good. I'm going to continue developing the above design then. I really like the way the bow transom converts to a loading ramp. For people who like to pull up to a beach frequently, this is a great feature. I will probably convert the stern to a traditional wide transom with an Atkin inspired tunnel too ... so it can be driven a bit faster on occasion.

Leo, thanks for the info about Momma, you had me confused until I read your explanation of how you use the word.

Fred, thanks for this too, I think you're "right on" with this conclusion which is why I intend to continue to focus on the tunnel-stern Seabright concept for designing efficient low-speed hulls:

As the water jet is only obtaining energy from a square ft of inlet area , and the box keel with reverse deadrise is getting almost ALL the energy from the hull in front of the prop, could this be the explanation for the reported higher efficiency?

[SAQuestor]

Nice boat. Lotto's up to 44 million on Saturday. Maybe I'll spend a dollar and take the chance. ;)


Or I could be like this chap - win the lottery twice (http://www.reuters.com/article/oddlyEnoughNews/idUSL0181922120070801) in one drawing.

:D

Leo

[FAST FRED]

"Maybe I'll spend a dollar and take the chance. "

At a dollar a DREAM its pretty cheap ,

even if the odds are 1,000,000,000 to 1!



FF

[SAQuestor]

At a dollar a DREAM its pretty cheap , even if the odds are 1,000,000,000 to 1!


Actually here in Texas the odds are only about 47 million to one in Lotto Texas.

There was a guy in the NEWS that has been struck twice by lightening, 47 years apart. Lived both times. But I can't find the article right now. Now he beat the odds.

Best,

Leo

[SAQuestor]

I >>>hate<<< to post a link to that 'other place' - but in this case I think it is warranted.

Pictures of an Atkins tunnel stern design (http://www.woodenboatvb.com/vbulletin/upload/showthread.php?t=68331) for your perusal.

Best,

Leo

[FAST FRED]

AS far as size goes I think the minor loss of interior to get just UNDER 8ft , to fit in Da Box is worth the 8 inches.

Yes' I know its 8 inches out of a narrow item to start with , but the length of 38ft, if needed, can be pretty comfortable .

Sitting dead flat, any trailer , not some specialized "Boat Transport" rig could be used.

A commercial trailer is 40 inches from the ground , so staying inder 13ft 6 inches while loaded wold be not much harder than the 9 ft to fit inside Da Box.

Biggest hassle I face now is getting used to a propulsion system with lousey reliability. With a std old diesel, once its started you're home free. With the electric crap, you're on the end of a long string , battery ,alternator and some unreliable un-fixable electric box.

The concept of marinizing a truck engine sounds better and better as spare electric boxes are at the recycle yard ,with not nearly as much markup as at the Marine Pirates.

FF

[FAST FRED]

I have been reading my back issues of PBB while cruising and came across an interesting concept.

Seems the Navy uses a rudder input as a stability enhancer. Sure on any airplane kicking a bit of rudder will lift a wing , before slewing the hull around , but the thought of doing this on a boat is interesting.

I had read of simply using a dagger board to lower roll, and have asked (with no luck) about a centerboard with a flap on the back as a stabilizer .

But an anti roll input to the rudder would be even less work ,if it works on a single rudder (the Navy reference was to "rudders").

This would be a simple addition to a cruising boat , could be light AND not too expensive.

Comments or references?

FF

[longliner45]

the staelth bomber has a sophisticated on board computer ,that makes it able to fly ,,unlike boeings wing in the 60s,,what if this technology is applied to vessals?

[FAST FRED]

Having a computer operate roll stab is easy enough , the hassle is the COST!

Also boats have been holed and sunk from active stabilizers on running aground.They were "supposed" to sheer , but the hull was built poorer than advertised.

A board or center board in a case would preclude this disaster.

FF

[Excalibur]

Having a computer operate roll stab is easy enough , the hassle is the COST!

FF

As a separate system, yes making a rudder into a computer operated roll stab would be costly. But if you built it into a gyro'd autopilot, the cost would be limited to the additional software. The aircraft version of what you are talking about is called Dutch Roll. There is both a roll and yaw component, so you will get a heading change along with the roll adjustment. Differential power inputs would be needed to counteract the yaw input and not the roll. I can see this working as a stability enhancement on a vessel with computer controlled rudders and variable pitch props but not otherwise. Now your thought of placing a trim tab on the rear of a centerboard is interesting. Especially if the centerboard is balanced and allowed to rotate along the vertical axis, similar to the way wind vane autopilots are designed. You would certainly want to design in self centering tho, so that it would not be forced up into the casing cocked at an angle if you hit bottom...

[tom28571]

Last week my wife and I were returning to Poole, UK from St Malo, FR on one of COndor Ferries 300 foot fast wave piercer catamarans, otherwise known as the "Vomit Comet". This was in the remains of the storm that banged up the Fastnet Race. I noticed that the wake had some rather sharp oscillations back and forth.

I thought that this must have been a steering stabilizer system. Anyone know about these cats, probably made in Aus? The ride was not too bad but some did get sick. Propulsion is through waterjets from 28,000 Hp (I'm guessing diesels).

[Willallison]

Tom - Unless they've changed, the Condor Ferries are built by Incat, here in Tassie (I went to school with the founder's son). http://www.incat.com.au/
I think most of their cat's are fitted with foils these days, but I'm not aware of any roll stabilisation system as such.
Earlier (and smaller) Incats had steering mechanisms that could account for the wake disruptions you mention. The were fixed 'rudders', set at an outward angle that were proegressively lowered to produce increasing rates of turn. A little like interceptors, except for steering rather than trim.. The bigger boats, however I think are all fitted with steerable water jets.
Trust you had a pleasant trip.....

[tom28571]

Will, The trip was a 50th anniversary present from our son and his wife. The whole family (with another son) had 10 days on a canal narrowboat in the west midlands of England and then Liz and I went to France for 4 days. The trip was great but travel is really hard sometimes. Had to spend a whole night in Gatwick and had two flights canceled in succession. Over 40 hours in airports, boats and trains. Getting too stiff for that kind of stuff. I think the French never put in a straight road if they can possibly fit in one with curves. Beautiful countryside.

I would think that the wake excursions that I saw were too great to be acceptable unless for a purpose. There was no way to learn anything on a ferry packed with over 700 passengers and cars though. Much better than flying anyway. Wish we had a similar train service in the USA.

[Willallison]

Yes - it's somewhat ironic that we come home from a holiday in need of a rest!:p

I'll probably be out at Incat before too long... we do a bit of work for them now and again.... I'll ask the question....

[FAST FRED]

Found another good reason for a beachable boat.

On the "loop" from Chicago south the Illinois river is running 5 ft beyond flood stage . Trees , picnic tables, chairs you name it is in the water.

Of course the rapid current is helping the fuel consumption , 12 mph at 850 rpm is really nice for the wallet
But the constant eyeballs needed to avoid junk is a pain.

FF

[tom28571]

Fred,

I suppose you will be going north up the ICW next Spring. When you get to Oriental on the Neuse River, you will be only a mile from my dock. You are welcome to stop and stay awhile.