Displacement Speed Question

[Tug]

Displacement Speed Question

I understand that when you have a long boat with a narrow beam your boat will go at a faster max displacement speed then a short boat with a wide beam...
I understand that adding length to a boat while keeping the same beam will increase the max displacement speed for the same boat....
What i dont understand is that there must be some length where there is a diminishing return for length increase vs speed increase....??
Thanks in advance for any ideas???
Cheers
Tug

[mydauphin]

True, look at some fast large ships like old destroyer, etc... The longer the boat the faster displacement speed. Look at container ship their displacement speed can be 30knots. The narrow beam lets you use less horsepower therefore higher speed. Narrow boat can also be easier in rough water not so steady at anchor.

[PAR]

Once you get over 5 or 6:1 length/beam ratio, things go out the window.

[mydauphin]

Once you get over 5 or 6:1 length/beam ratio, things go out the window.

Depends on boat. Look at some ocean liners they had 13 to 1. My big canoe is 18 to 2.5 or a venetian gondola.

[Ad Hoc]

"...What i dont understand is that there must be some length where there is a diminishing return for length increase vs speed increase.."

It's a practical limitation, usually cost, or stability.

Since eveything just depends upon the length/displacement ratio and also the speed length ratio.

Going further back in history one can see this notion being tried by CM Ramus's hydroplane. in 1872 he was investigating a 360ft hull form with speeds up to 130knots!

[PAR]

It's a much more complex subject the beam/length ratios would suggest, though the rules do begin to change at 6:1. You also have to have the power, a prismatic coefficient suitable for the S/L desired, etc., etc., etc. Is there something specific you need, Tug?

[Ad Hoc]

PAR

Where do you get this 6:1 ratio from?...what is the criterion for this "change"?

[Tug]

Nothing really specific....just trying to balance my wants vs needs and costs vs comforts....
I wish to eventually build a boat to live on...
I have been reading as much and learning as much as i can from this wonderful forum so i could try to make an intelligent choice of which one to build....
I have learned that they dont have a boat that will do everything i want and an operating cost that i will be comfortable with.....
So from this tread i am trying to learn that if i can live with a certain beam what would be the length where the max displacement speed vs cost of construction would be an inefficient use of money..
Thanks
Tug

[Guest625101138]

Nothing really specific....just trying to balance my wants vs needs and costs vs comforts....
I wish to eventually build a boat to live on...
I have been reading as much and learning as much as i can from this wonderful forum so i could try to make an intelligent choice of which one to build....
I have learned that they dont have a boat that will do everything i want and an operating cost that i will be comfortable with.....
So from this tread i am trying to learn that if i can live with a certain beam what would be the length where the max displacement speed vs cost of construction would be an inefficient use of money..
Thanks
Tug

Tug
I started out constrained by this thinking but ASM set me on a new path. It is what I have termed a faux-tri. It is a trimaran only at the water surface. Above it is a conventional looking monohull and below it is a long slender hull.

It provides a good layout for accommodation. It requires very low power for speed. It has better initial stability than a conventional monohull of the same beam and can be made to be self-righting.

It is the extreme variation of somethink like the Atkins box hull.

You see a similar concept being used on some modern military craft.

There is discussion of the idea on this thread:
http://www.boatdesign.net/forums/boat-design/long-skinny-power-boats-5073-7.html

Rick W

[gonzo]

Once the length gets excessive, surface friction increases more than wave resistance.

[Ad Hoc]

Tug

Best to draw up in terms of a list and also in terms of a general arrangement of what you want. A layout provides so much data intuitivity, such as a bunk that is 2m long, but the hull is only 1m long, as a simple example. In words,m it sounds great, have a bunk..but the reality once drawn up is very different. Then slowly you can tick off each one as a "yes" or a "no".
You can't get an ideal perfect solution, but you can get what you want and what you can afford and what is ultimately practical for your needs.

[mydauphin]

Once the length gets excessive, surface friction increases more than wave resistance.

This is true but it size far easier for a longer boat to travel across any wave than a shorter boat. The bigger the boat the easier in scale, and real life it cuts thru the waves. Been on many a cruise ship doing 28knots, in 40 foot sea, just plowing along, with a little sideways actions, and up and down.

[mydauphin]

Nothing really specific....just trying to balance my wants vs needs and costs vs comforts....
I wish to eventually build a boat to live on...
I have been reading as much and learning as much as i can from this wonderful forum so i could try to make an intelligent choice of which one to build....
I have learned that they dont have a boat that will do everything i want and an operating cost that i will be comfortable with.....
So from this tread i am trying to learn that if i can live with a certain beam what would be the length where the max displacement speed vs cost of construction would be an inefficient use of money..
Thanks
Tug

Went through same thing myself. Decide on 70 foot mono boat, 15 beam, 3 foot draft, 500 hp total engine power. Narrow hull less hp, easier to find dockage. Long enough for comfort and big engine room. Look at some boats like Maccgregor 68 way too narrow for comfort. Everything is compromise.

If I had to do again, I would get a little shorter boat, like 55 footer, same 15 foot beam.

Work on what you want boat for and then work backward.

[FAST FRED]

"Once the length gets excessive, surface friction increases more than wave resistance."

True but the skin friction does not go up by a square or cube power, wave making does.

The skinny boat simply makes a small enough bow wave to power through , rather than climb up on, dragging the transom .

The huge hollow of a true displacement boat doesn't show up.

6-1 seems to be the minimum for a boat to create a small enough bow wave to sail or power thru.

This is NOT plaining.

FF

[Ad Hoc]

friction goes up by the power 1.825, nearly a squared!

[tom28571]

A simplistic way to look at the long vs short boat resistance thing is to consider two extremes. Take a boat shaped like a hockey puck with a length/beam ratio of 1. As it moves forward, a lot of water has to get out of the way very fast to allow its passage and the round stern has to suck this same water back in after the puck passes. Take the same weight and volume and make it very long with sharp ends (say length/beam of 10 or more). As it moves forward, there is much less water that has to be moved sideways to allow its passage. If the ends are stretched out to a sharp bow and stern, this smaller volume of water is accelerated sideways at a much slower rate, meaning that there is a much lower bow or stern wave. Both of these actions take far less energy than is required to push a hockey puck of the same displacement through the water.

Some people have looked at exploiting this phenomena in a monohull of normal (wide) hull with a length/beam ratio of 3 or so. A half model of one of my versions of this boat is attached. At full scale, the lower canoe hull would contain about 75% of the total displacement while the much bigger upper hull only needs to support the remaining 25%. The idea is that the canoe hull has a large length/beam ratio of 12 or so and can thus run in the high teen knots with low power like a similar catamaran hull would. The upper hull is flat in the aft sections and, with very low loading factor of weight per sq ft of area, can plane at low speed.

It's only an idea and a model and but the numbers look promising. The intention is to develop a cruising boat with very nice accommodations of about 30' LOA and beam of about 9.5' with displacement of 6 or 7K lbs that will cruise in the teens at low power and low fuel use. The initial goal would be 25hp and 1 gal per hr fuel use. Type of power would dictate the aft length of the canoe body. Ideal is a diesel inboard sunk in the canoe hull with a large slow turning prop on a horizontal shaft..

[Ad Hoc]

tom
you are basically decsribing the well known laws of Froude.

[tom28571]

tom
you are basically decsribing the well known laws of Froude.

Of course, that was the intention. Much better than being in opposition to Froude. On the other hand, where is the boat that exploits Froude in the same way as this model? At this stage, I am never going to be able to build such a boat but am looking forward to some larger scale model tow testing.

I should add to the second paragraph that the aft wide hull can plane at low speed because so little dynamic lift is required. This means that the needed dynamic lift can be developed over the large area at a very low trim angle. This low trim angle is required for the canoe body to do its thing. The aft bottom would have a built in down angle of one degree or so and the whole boat can then run at zero trim angle.

[Ad Hoc]

"...On the other hand, where is the boat that exploits Froude in the same way as this model?.."

Don't understand your point?

[tom28571]

"...On the other hand, where is the boat that exploits Froude in the same way as this model?.."

Don't understand your point?

The point is, to my knowledge, no such boat has ever been built.

Are you aware of one?

[jehardiman]

"...What i dont understand is that there must be some length where there is a diminishing return for length increase vs speed increase.."

It's a practical limitation, usually cost, or stability.

Since eveything just depends upon the length/displacement ratio and also the speed length ratio.

Going further back in history one can see this notion being tried by CM Ramus's hydroplane. in 1872 he was investigating a 360ft hull form with speeds up to 130knots!

Actually, it is strength and arrangements that limits a hull as the actual hull is not a large cost of the vessel. So much of what works in thoery by extension does not work in practice.

Also look Tug, you need to look closely at what you are comparing. You are really comparing the location of the first wavemaking hump and the power needed to get there. While a longer, narrower, vessel may use less HP/ton, it may have more tonnage for the same carring capacity and therefore go slower. Or it may be so long that forefoot emersion from pitch would be an issue in anything but calm seas. There is no one best hull form, only the best hull form for the mission requirements.

As a side point, there is no such thing as "max displacement speed". The boat will always go as fast as the power input will allow. The idea of a "hull speed" is left over from the early theoretical work by Froude and Havelock and propagated by some magazines in the inter-war years. What it best represents is the first or second discernable wavemaking "hump" for a Wiggly shaped hull. Other effects such as block, prismatic, midships coefficient and length-displacement distribution contribute much more the location of this hump which can vary between a Froude number of 0 and infinity (yes, there are some hull shapes that have no wavemaking hump....they are called infinite planks...).

However, as far as maximum speed is concerned, the "economic" speed will depend on wether the hull has enough power to climb over the first or second wavemaking humps (actually, they are the third and fourth, but the first two are negligible for most real vessels) and into the hollows between/beyond them. The first hump for most displacement hulls occurs about Fn ~0.27-0.3, the second ~ 0.47-0.5. Most displacement hulls do not see a decrease in power in the hollows, but rather a "flat spot" where very little power is required to increase speed. Some hull forms like SWATHs and powercats have such deep hollows that they become uncontrollable in speed as they clear the first hump and launch themselves across the powering curve at the next upslope.

[Ulf]

This one is very simmilar: http://www.alsphere.at/dg/index.shtml
And it seems to work.
But wouldn't the planing part be sensetive to changes in displacement for examle if the displacement increase by 12%, the planing part would have to carrie 50% more load...
And what happends in waves? I would expect the drag to increase a lot...

[tom28571]

Yes,

The DG is similar. The lifting body is quite a bit different and I don't know how that affects the performance. They do claim some magic for the shape of the hull. Loading increase would affect the upper hull of my model in the same way it would a normal flat bottom planing boat. Changes in loading would probably have less effect on the DG. However, the bottom loading on the lifting hull starts out so low that it could increase quite a bit and still be much lower than "normal" planing boats. I still think it has promise as a low power, moderate speed cruiser.

[Willallison]

Ahhh - I thought this idea showed too much promise for you to just just let it slip by Tom;)
As you know, I've been playing around with similar hullforms myself - "fat" displacement catamaran shapes, that I've coined the 'monomaran'. In fact, I think there are a few people lurking about the place that are considering similar vessels. Once I get over my current build, I hope to experiment with a large scale model myself.
All my research suggests that this does indeed hold the potential to produce a very efficient platform at low to moderate speeds. It's not an entirely new idea, of course - there are so few of those these days! - some of the box-keel shapes share similarities and of course the displacement cat form made popular by Malcom Tennant is similar too.
The displacement glider is an interesting case - though the shape of the 'upper' body would, I expect, only be optimised to a specific speed, though their website suggests otherwise...

[mydauphin]

You guys don't know my canoe. 18 x 2.5 feet, 25 knots on 5hp... Runs all day on 2 gallons. You can make a canoe plane, you can make a destroyer semi-displacement, But it helps for it to be narrow....

[tom28571]

We hear of such boats but I have never actually seen one quite that good. I expect it needs a lightweight driver to get that speed and mileage. Those are probably 5 large horses too.

My old Destroyer was 396' LOA with a beam of 35' or L/B = 11.3. It could do 36 Kts with all four boilers straining for a speed length ratio of 1 .8. The Japanese had some that did 40 Kts.

Will, maybe someone will get interested enough and actually commission such a boat.

[Tug]

I built a 17'6" cedar epoxy glass canoe a few years back....was extremely fast....
I researched this build as well.....
Of course in those days research came at a much higher cost then this wonderful forum...
I found an OLD marathon canoe racer and arranged a meeting....
He spent about 1 hour talking about designs and then dragged me out to paddle and talk...for about 4hrs i tried to talk and keep up paddlin with this old fart.....it nearly killed me...
He invited me for a 4 day paddle/camping with his family to try all his own different designs....
He forgot to mention that his family were ALL marathon canoe racers...4 days of workin my ass off chasing canoes dissappearing ahead of me....
It was a great lesson...i learned every different design particulars from actually paddling them and sitting around the fire listening too him explain why he made each choice for that design....
He made a great teacher....
I even went out and cut my own white cedar tree to enjoy the whole process...
So the new funky looking magic hulls might claim to be more efficient i think i will wait till some old guy has made 8 or 9 hulls himself and has figured out all the little problems assosiated with them...
Thanks
Tug

[mydauphin]

We hear of such boats but I have never actually seen one quite that good. I expect it needs a lightweight driver to get that speed and mileage. Those are probably 5 large horses too.

My old Destroyer was 396' LOA with a beam of 35' or L/B = 11.3. It could do 36 Kts with all four boilers straining for a speed length ratio of 1 .8. The Japanese had some that did 40 Kts.

Will, maybe someone will get interested enough and actually commission such a boat.

My canoe is doing the speed on Plane with my wife and me. About 400lbs and engine 5hp Nissan 4 stroke. Honestly any more power would be dangerous. My wife scoots in at mid canoe. I have be very careful at speed. With One person alone it really flies but becomes a handful.

I would really like know what the new destroyers do. They say only 33knots, but with all that power, they might do more.

[mydauphin]

Is a destroyer doing 33 knots planning, semi-planning?

[apex1]

Overforced displacement rather than semiplaning. Say 112meter LWL

square root LWL = 10.58 by 2,43 = 25,71 the so called hull speed.

Regards
Richard

[mydauphin]

Overforced displacement rather than semiplaning. Say 112meter LWL

square root LWL = 10.58 by 2,43 = 25,71 the so called hull speed.

Regards
Richard

You lost me on this one. Explain Overforced displacement please.

My guess, you mean that given enough HP any speed is possible...

[apex1]

You lost me on this one. Explain Overforced displacement please.

My guess, you mean that given enough HP any speed is possible...

They hardly can be called semiplaning, but with a L/B ratio above 10/1 they are fast as if they where a semiplaning vessel. But need a hell of a lot of power to achieve their target speed. At least in the old German Literature I have they are described "überforcierte Verdränger" which I (correctly) translated as "overforced". My shoolbooks call them "hybrid" not really semiplaning, not just only displacement vessels.
That was my ability to explain that, I hope it was sufficient.
And to your question:
No, there is a limit, I was told some hundred years ago, above that, additional power would sink the vessel. Let me guess thats just a theoretical one. But I cannot elaborate on that, I know nothing about it!

Regards
Richard

[jehardiman]

Is a destroyer doing 33 knots planning, semi-planning?

Depends on the particular ship class and hull form. Apex1 is sort of correct about overdriving a hull, and if you read my previous post in this thread you will see that if you have enough power you can start to climb the 3rd major powering hump. However, that slope is almost vertical (i.e. large hp increase for almost no speed gain) for many "normal" displacement hull forms. This leads to specalized hull form for the smaller (150-300 LwL) combatants. Large combatants don't need to plane to achieve significant speed and can do it while devoting much less volume and tonnage to machinery.

Check out the US Navy Cyclone PC's: 157' LwL x 25', 315 t, 13K shp, 35 knts. Not really planning, nor really displacement. Compare this to a non planning Spruance class destroyer: 550 LWL x 55, 9100t, 80K shp, 33 knts. The Spruance uses 1/5 the power per ton. On the other hand compare a Osa class missle boat: 120' LwL x 24', 210 t, 12K shp, 38 knts, which is a planning boat and uses about 50% more hp per ton.

[Ad Hoc]

Just checkout any FastFerry.....they are running at very high Froude numbers, they are not planning, nowhere near it.

HUll form dictates whether it will plane or not.

[mydauphin]

I am confused. I understand planning, I understand displacement. I understand fast ferry and cats that split water and ride over bow wake. I don't understand Spruance that uses 1/5 the power per ton, could it be that Spruance gets more power to water because of bigger prop turning.

[mydauphin]

Amazing little boats... Big fancy props, lots of power. Hull long and skinny, still displacement?


USS HURRICANE (PC 3)

PATROL COASTAL

UIC: 21932
Class: PC 1 Fleet: Atlantic
Status: Active, in commission Homeport: NORFOLK, VA (LITTLE CREEK)
Date status changed: 10/15/1993 Berth:
Maintenance Category:
Force: Auxiliary MARAD Type:
Builder: BOLLINGER MACHINE
Delivery Date: 07/21/1993
Award Date: 08/03/1990 Age (since delivery): 15.9 years
Keel Date: 11/20/1991 Commission Date: 10/15/1993
Launch Date: 06/06/1992 Inactivation Date:
Age (since launch) 17.0 years Decommission Date:
Years from Commission to Decommission:
Stricken Date:
Overall Length: 170 ft Waterline Length: 0 ft
Extreme Beam: 25 ft Waterline Beam: 0 ft
Maximum Navigational Draft: 8 ft Draft Limit: 0 ft
Light Displacement: 288 tons Full Displacement: 334 tons
Dead Weight: 46 tons
Hull Material: Steel hull, steel superstructure.
Number of Propellers: 4
Propulsion Type: Diesel Engines
Accommodations: Officers: 5 Enlisted: 23

Custodian: US NAVY Ships Program Manager: 325
Planning Yard: Puget Sound (DET) Boston, Boston, MA

[mydauphin]

These destroyer have some strange ways to get that speed


Stern Flaps

A stern flap (Figure 3) is a relatively small plate that extends behind a ship's transom, lengthening the bottom surface of the hull. A stern flap alters the water flow at the stern in ways that reduce the ship's resistance and increase fuel efficiency by a few or several percent. A stern flap for a Navy surface combatant in 2000 cost about $170,000 to fabricate and install.11 Preliminary tests of stern flaps on DDG-51s showed an annual fuel reduction of 3,800 to 4,700 barrels, or about 6.0% to 7.5%,
--5--
per ship.12 As of November 2004, the Navy had installed stern flaps on 98 ships (primarily surface combatants) and planned to install them on an additional 85. The 98 ships equipped as of November 2004 had accumulated 403 ship-years of service and saved $44 million in fuel costs.13 The Department of Energy stated in 2003 that by 2005, stern flap installations on Navy ships would save 446,000 barrels of fuel, or $18 million, per year.14

[Ad Hoc]

mydauphin

Im not sure what you're getting at...can you be more specific.

[mydauphin]

I am trying to understand these high-speed non-planning boats. I think these boat are super skinny length to beam, have flat bottoms at rear quarter to half of ship. So the front acts like a knife, the back as a planning boat. The destroyer ship is using bow appendages to get lift and rear flap for lift. They may also be getting some lift from props.

[FAST FRED]

"The point is, to my knowledge, no such boat has ever been built.

Are you aware of one?"
__________________
Tom Lathrop


Would seem that most of the Atkin Box keel reverse deadrise are quite similar, however Atkin goes two steps further in an attempt to capture the velocity of the water dragged by the canoe body and hull above and feed it to the prop.

And in using the reverse deadrise to lift the stern from the water , perhaps reducing drag even further.
No stern flaps anti squat required.

We all dream of a series of tank tests to see if this old design is as efficient as claimed 50 years ago , and weather a modern version would indeed make a super efficient SL 2.5 or 3 cruiser .

With modern materials weight and maint could be minor , compared to planks.

FF

[Ad Hoc]

mydauphin

It is more than just looking at hull shape and saying...hmm..flat bottom, with a Vee, must be a planning boat...or wow, boat is fast so why is it not called a planning boat.

Any "body" moving through the water, ie at the air-sea interface, will create pressure variations around the 'body'...these pressures variations manifest themselves as waves. These waves are a measure of energy and hence drag.

The shape of a hull can affect this pressure distribution considerably. In a nut shell to cut a long story short, the stern experiences suction pressure fields. Speed then also begins to play a part....ie trim/squat, the faster one goes.

For a "normal" boat, the faster one tries to go, the more trim and the greater the power required for little gain. The back is sucked down and dragging creating a lot of wash/waves. Too much energy is being used in making waves. The reason is the hull shape and its length displacement ratio. This is seen in the resistance curve by humps, and the main prismatic hump. A hull must over come this main hump to go faster, ie make "lesser waves" or better still no waves. Long thin hulls, hydrodynamically, behave differently to short fatter ones.

However, just making the aft section flat with/out a vee, doesn't mean that is all that is required just to go fast, or get over the "hump" in the resistance curve. That is just hull shape.

Making the hull longer and thinner, reduces the 'near vertical' curve in resistance of a 'normal' hull form...ie its length displacement ratio. Not only does the resistance curve become less step, but also the main hump is much less pronounced. The longer and thinner one makes it (that is light for its total length), the curve slowly approaches a smooth curve and almost no discernable pristamtic 'hump'. This is why fast ferries, for example, go fast, have a high froude number, but are not planning. They are long and thin.

The length displacement ratio (ie long and thin) allows the hull to be driven faster than would normally be the situation. The wave making reisatnce gets less and less, the longer and thinner the hull becomes.

The down side is, one of these long thin hulls is generally far too unstable to be used effectively. Solution, put two side by side, ...a catamaran. Utilising the benefits of the long thin hydrancimc effects, but providing a stable platform for use in almost any application safely.

[mydauphin]

Ran across this reverse transom, it work its way to the front were it became a traditional V hull. So the boat had a large reverse planning area in between chimes. Anyone seen this type of hull, what is it called and do you anything more about it? thanks

[Ad Hoc]

there are 3 lines, showing er...what?...is this plan, profile, body plan...not very clear...which is centreline, or baseline etc....????

[mydauphin]

there are 3 lines, showing er...what?...is this plan, profile, body plan...not very clear...which is centreline, or baseline etc....????

Sorry, The wavy line is the bottom of hull at the transom. The vertical and horizontal line mean nothing...

[Ad Hoc]

still confused, in what plane is the view?

[Village_Idiot]

Would seem that most of the Atkin Box keel reverse deadrise are quite similar, however Atkin goes two steps further in an attempt to capture the velocity of the water dragged by the canoe body and hull above and feed it to the prop.

And in using the reverse deadrise to lift the stern from the water , perhaps reducing drag even further.
No stern flaps anti squat required.

We all dream of a series of tank tests to see if this old design is as efficient as claimed 50 years ago , and weather a modern version would indeed make a super efficient SL 2.5 or 3 cruiser .

With modern materials weight and maint could be minor , compared to planks.

FF

There is an outfit in Texas that is building a shallow-water marsh/flats boat that incorporates some semblance of the inverted V / Hickman design.

http://duckmasterboats.com/lagunatiger.html

http://duckmasterboats.com/lagunatiger2.html

[mydauphin]

still confused, in what plane is the view?

Your in the back of the boat looking at bottom looking forward.

[Ilan Voyager]

A bit more infos on the USS Hurricane


Length, Overall 51.62 M (169 ft 4 1/4 in.)
Beam, Maximum 7.62 m (25 feet)
Length at design waterline (DWL) 48.00 m (157 ft 5 3/4 in.)
Draft above Bottom of Keel Corresponding to Full Load Displacement 2.14 m. (7.02 ft)
Displacement, Full Load 315.32 tonnes (SW) [310-34 L Tons (SW)]
Height of highest projection above baseline to lowest projection below baseline 17.77 m (58 ft 3 3/4 in.)
Superstructure Material 5086 Aluminum
Fuel Capacity 47,772 Liter (12,620 gallons)
Fresh Water Capacity 4,701 liter (1,242 gallons)
Lubricating Oil 568 liter (150 gallons)
Boats (1) 20' RIB, (2) Combat Rubber Raiding Craft (Large)
Crew 30 persons
Capacity (including crew) 39 persons
Maximum Speed
(sea state 1, 50% fuel capacity) 35 knots
Cruising Speed
(sea state 3, 50 % fuel capacity) 25 knots
Minimum maneuvering speed 3 knots
Seaworthiness Survive through sea state 5
Minimum Range 2000 nautical miles at most economical speed over 12 knots
Endurance 10 days
Propulsion Engines Manufacturer: Paxman, Type: Valenta 16CM Diesel, Model 16RP200M, Rating 3350 bhp @ 1500 engine rpm, QTY: 4
Reduction Gearbox Manufacturer: Reintjes, Model WVS 2232, Reduction Ratio: 2.025:1, QTY: 4
Propellers 5 Bladed Fixed Pitch Nickel Aluminum Bronze
Generators Manufacturer: Caterpillar, Model 3306 DITA, Rating 150 Kilowatts @1800 rpm, Quantity: 2
Air Conditioning Air Conditioning Plants 83.33 kW (23.7 Tons) Total
Desalinators Reverse Osmosis Desalinators 1514 liter (400 gallons) per day
Builder Bollinger Shipyards, Inc.

[Ad Hoc]

Looks like small "tunnel recess" for prop's...since it doesn't really show much else with one line and no reference to a baseline and CL nor how the shape changes as the lines go fwd in body, profile and plan view.

As for a name..pick one...does it matter so much?

[Village_Idiot]

Ran across this reverse transom, it work its way to the front were it became a traditional V hull. So the boat had a large reverse planning area in between chimes. Anyone seen this type of hull, what is it called and do you anything more about it? thanks

Here is a similar, though extreme, version of that hull bottom that is in production (pay attention to their logo design).

http://www.shallowsportboats.com/body.htm

I'd consider it a modification of a tri-hulled monohull.

[mydauphin]

Yes, village it looks very similar, the one I saw is a little flatter and wider.

So my question is... Is a reverse planning area in between chimes typically better to worse at planning(lift) that a regular planning hull?... It would seem to entrap air better.

[Village_Idiot]

The best planing hull would be a true flat bottom. I believe an inverted-V-hull (e.g. - reverse chine) would plane easier than a standard V-hull of the same deadrise, and likely hold plane at a slower speed. The added benefit of the inverted-V would be raising the motor/prop for shallower operation. There is also apparently less spray from the hull since most of it is directed inward, although it may suffer from sneezing. Not sure how their ride would compare to a V-hull of the same deadrise configuration. I'd also think handling would get squirrelly in high-HP situations. However, I don't have any personal experience with these inverted-V hulls, so I'm just judging based off of simple physics (ya know, physics says a bumblebee cannot fly :D ).

A number of flats boats designed for extreme shallow water operation incorporate some sort of air entrapment under the hull. As another example, look at the flatscat www.flatscat.com although its purpose is to entrap water rather than air, and bring it up into the center of the hull where it can feed the prop above the waterline. Other flats boats that use air- or water-entrapment technology are Ultracat, Transcat, Laguna Tiger, and I'm sure many more that I cannot think of at the moment.

[Leo Lazauskas]

...so I'm just judging based off of simple physics (ya know, physics says a bumblebee cannot fly :D ).


I heard that was just another urban myth perpetuated by village idiots :p

[mydauphin]

Thank you village...
Question - why on earth did you pick a name like Village_Idiot ?

[Ad Hoc]

Having chines lower than the keelson would mean the boat is very likely to be too directionally stable, so much harder to turn. May also induce a broach too. Also, the point of a chine and a keel is to separate the flow to provide a lifting surface, which is why round bilge hulls don't plane without spray rails. If the keel line is higher, the flow of water will be mixed, not separated as would be the case on a normal Vee hull.

[Willallison]

The Hickman Sea Sled featured an inverted 'v' and was reputedly an excellent sea boat

[Ilan Voyager]

Inverted V are planing boats. This shape is useless in displacement boats.

We are getting far from the initial question...I'm a bit more familiar with fast displacement boats as I've worked on them. A lot of research has been made about this kind of boats for fast ferries.

Myself I had a very small part in a research made by the French Navy about 20 years ago; the conclusions were simple: the lightest , the longest, the slimiest with the sweetest shapes (elliptic, plumb bows with no flats, no bumps, no hollows) were the winners. No miracle, no magic shapes... and I can say that a good number of shapes were tried. The better were rather close to the amas shapes of the actual 60 feet trimarans but with a different center of displacement (it's normal as sailboats have the engine -sails- several meters above the deck). Ratios LWL/BWL from 12 to 18. Chine hulls were not explored.

The difficulty with these shapes were 2;
- not enough stability so the multihulls were mandatory.
- very difficult to place the propellers and engines. Surface props, waterjets or pods were also mandatory.

None of this solutions were of the taste of the french navy. In fact no navy was ready to go that far 20 some years ago. Thus the classical monohull shapes were kept.

We can see the problem of the propellers with the USS Hurricane; a true forest of shafts and supports making a big drag plus 4 propellers that are robbing power each other. Not very clean in a hydrodynamics point of view. 13400 HP for getting 35 knots on a ship of this size and weight is not outstanding (but warships are designed with other requirements than only efficiency).
The ferry catamaran Patricia Olivia II, shorter (45 meters), and heavier achieves 50 knots with 15600 HP...

Both, USS Hurricane and Patricia Olivia II are displacement boats. At 35 knots the catamaran ferry is using far less than 13400 HP. There no competition.

It's funny that a situation similar to that happened in aviation in the States around 1935, when the civilian DC3 and other transport planes were faster than the fighters and bombardiers, is happening now; multihulls ferries are faster than warships of similar size, and the monohull ferries very close.

The catamaran ferry Luciano Federico makes 110 NM travels in Mar de Plata in 2 hours...http://www.ship-technology.com/projects/luciano/

Another link (in Spanish, but pics and ciphers do not need translation):
http://www.rkviajes.com/buquebus/flota.html

The navies have to change their minds...

Adrian Thomson draw a patrol boat and a prototype made with a very slim hull and 2 flats (what's the word :chines?) I do not know if it was succesfull or not. I do not find anymore a link.

The displacement boats with the highest Froude numbers are sailboats: catamarans and trimarans. A Tornado catamaran (20 feet) can attain 25 knots and the best 18 feet are very close, a Class C catamaran can reach a bit more than 30 knots knots (Victoria 100 has been measured at that speed), a 40 feet trimaran can reach 27 knots on 1 NM (Data General a 40 feet catamaran with oceanic ability could easily go to 20 knots on only mainsail and 26-27 with full sail and spi in 1987), a 60 foot trimaran can reach 35 knots over 3 NM and more than 24 knots during 24 hours.

And the big boys like Groupama III (trimaran 103 feet 794 NM - 1470 km or 914 US miles in 24 hours so 33 knots of mean speed, the Atlantic at almost 30 knots...) or IDEC (trimaran 30 meters, 11 metric tons, 26 400 NM -49000 km or 30385 US miles- at 19,09 nœuds SOLITARY, yes a guy alone around the world in every state of sea...)

As you see the recipe is simple: light, slim, multihull.

When applied to motor multi yachts that gives: Ilan Voyager 21 meters 5 to 7 metric tons trimaran 28 knots with 240 HP only. Main hull, elliptic shapes, 17 to 1 ratio. Oceanic ability.

[Willallison]

The Adrian Thompson boat was called the VSV and it has spawned several variants since. Most recently the pleasure boat Mary Slim. All were quite succesfull I believe, though I think all operated in the planing mode, even though they were round bilged

[Guest625101138]

.......

As you see the recipe is simple: light, slim, multihull.

.....

There is an exception to this.

If you go below the surface and consider submerged buoyancy or full submarine there is a significant reduction in drag over what is achieved with high speed surface craft.

As noted on the current SWATH thread the optimised submarine Albacore AGSS 569 achieved 33kts with 7500HP. It was a 1540t vessel.

A single hull submerged buoyancy vessel has significant advantage from a drag perspective but making it practical for common use poses many challenges. I have first hand experience of this.

Rick W

[Ilan Voyager]

The Adrian Thompson boat was called the VSV and it has spawned several variants since. Most recently the pleasure boat Mary Slim. All were quite succesfull I believe, though I think all operated in the planing mode, even though they were round bilged

Thanks for the news. Very interesting boat. Beginning at displacement mode, ending planning when the "flats" are on the surface of the water. You're right it's more a planing boat.

[Ilan Voyager]

There is an exception to this.

If you go below the surface and consider submerged buoyancy or full submarine there is a significant reduction in drag over what is achieved with high speed surface craft.

As noted on the current SWATH thread the optimised submarine Albacore AGSS 569 achieved 33kts with 7500HP. It was a 1540t vessel.

A single hull submerged buoyancy vessel has significant advantage from a drag perspective but making it practical for common use poses many challenges. I have first hand experience of this.

Rick W

You're right, but submarine yachts and ferries are not common so I got stuck on ordinary displacement boats...the big problem of boats is the interface air/water. More I get old more I think that the best is to leave the surface and you have 2 options; up and down.

[Guest625101138]

.... More I get old more I think that the best is to leave the surface and you have 2 options; up and down.

Maybe a combination of both. The accommodation in the air and the displacement deep in the water.

Would possibly be a very good option for bulk carriers. Need some means of reducing draft in harbours but once at sea they go deep and into high speed economy mode.

Rick W

[Ad Hoc]

"...There is an exception to this.."

Not when you're referring to a submarine.

The resistance of deeply submerged bodies/vessels is given by:

Rt = Rpv + Rf

Which becomes

Ct = Cpv + Cf

When Cpv is very much greater than Cf it is considered “bluff”, since boundary separation produces a large wake. For example, a sphere or a flat disc across the flow. Whereas a streamed line shape is shaped because it does not experience large pressure gradients, or separation. The boundary layer actually becomes thicker and separation occurs much further aft, in some cases even avoided. When this occurs, the wave making component is sufficiently small that skin friction CF predominates the total resistance.

In submarine design this is termed ‘slenderness ratio’ and coupled with its diameter defines how stream lined the vessel or sub is. The USS Albacore was the first to adopt a tear shape, because it is a nuclear powered and operates for very long periods under water, and hence ignores the air-surface interactions. All to reduce the Cpv.

However as the slenderness ratio increases, for a constant immersed volume, the surface area also increases, which increases the Cf. But, the adverse pressure gradients over the aft section decreases as does the wake, hence Cpv is less. Contrary to this, when the ratio is reduced both the wake and Cpv are increased. The optimum ratio has been defined as 6, by trials, by Arentzen & Mandel in 1960.

Hence long slim still dominates, even at extreme depths.

[Village_Idiot]

Thank you village...
Question - why on earth did you pick a name like Village_Idiot ?

plausible deniability

[Village_Idiot]

Need some means of reducing draft in harbours but once at sea they go deep and into high speed economy mode.

Rick W

Pumped air/water chambers for light/heavy ballast.

Would be quite interesting to have a large submarine, the size of a modern freighter, that would submerge for long voyages, with just a small helm protruding above the surface of the water, connected to the mainship via a small shaft with stairwell...

[FAST FRED]

"Would be quite interesting to have a large submarine, the size of a modern freighter"


I think they do something similar by towing a submerged bladder to the rich gulf states with arctic ice water.

FF

[Guest625101138]

.......


I think they do something similar by towing a submerged bladder to the rich gulf states with arctic ice water.

FF

Fred
Is this actually happening? I cannot find any references to it.

Rick W

[Guest625101138]

......
Would be quite interesting to have a large submarine, the size of a modern freighter, that would submerge for long voyages, with just a small helm protruding above the surface of the water, connected to the mainship via a small shaft with stairwell...

Actually I was thinking the turret would be large enough to take a high capacity bucket elevator for discharging. It would hopefully have a lift because the hull would be something like 20m deep and you need to get 3 or 4 diameters below the surface to achieve the low wave drag. So the turret would be maybe 70 to 100m high.

Certainly presents interesting challenges.

Thinking about it I have no idea how much wave drag a large bulk carrier actually experiences. I do not think it is a large proportion of the overall drag. So there may not be a benefit for this size vessel. May be better suited to smaller vessels intended for high speed operation.

Rick W

[mydauphin]

Maybe a combination of both. The accommodation in the air and the displacement deep in the water.

Would possibly be a very good option for bulk carriers. Need some means of reducing draft in harbours but once at sea they go deep and into high speed economy mode.

Rick W

My boat has that. I can change draft and trim using water tanks. It can also alter timing of rocking at anchor by raising and lower CG.

[loups1]

halo, congratulations, it`s a very useful forum-site.sorry if my english is pure. i am from greece. i have a traditional displacement fishing boat "trehandiri" made from fiberglass.it`s 6,20m long and 2m wide with an inboard nissan diesel engine of about 60hp at 4000rpm [used from an old taxi car 4cyl,2000cc-about 800,000 km] with a hurth [now zf] reverse [ratio 1:2,15] and it weights with fuel and fishing gear about 1,8ton. the max speed is 6,2mph. the question is: what can i do to gain more speed f.e.9mph or more as some other manufactures [exacly same hull design] say that their boats do. i dont know if its the right place for my question but thnks anyway.

[Willallison]

Loups1 - can you post a picture of the boat? Preferably out of the water so we can see what the bottom shape is?
The speed you're getting now is about 'hull speed' for a displacement hull of that size. If the boat is a full displacement hull, then achieving 9 knots will either come at the expense of enormous extra power input, or not at all - at least not without major hull shape modification.

[loups1]

thanks very much.i will try to put a photo .

[loups1]

here it is33284

[Ad Hoc]

loups

You'll be spending a lot of money trying to make her go faster. She is a "traditional" displacement boat. The harder you push her the more draggy she will be. This hull form does not lend itself to high speed owing to its shape. So as a rough guide/guess, it will be like doubling the power for each knot increase (not to mention the increase in weight of the ever increasing engine package)...so is it worth it?

[Ilan Voyager]

This kind of boat, called "pointu" in France, are common in all Mediterranean sea; it descends from the small boats using latin sails of the past centuries. Very well adapted to the "short" stepped seas that you can find when winds like the Tramontane or Mistral are blowing. Aegean Sea can be pretty hard in November.
Very good boats but definitely not made for speed; a 60 HP engine is already a "big" engine on a such boat and you won't get any improvement of speed with a bigger engine.

Certainly you have remarked that the speed you have now is not proportional to the RPM and the throttle; certainly at half throttle the boat is almost as fast than full throttle. Full power is only needed when going against the waves and wind. 6 knots is already a good speed on a 6.2m displacement boat.

To go faster you need a different boat; I would like to see a "Pointu" of 6.2m at 9 knots, it must be a curious spectacle to watch a semi planing heavy double ended boat. Those announcing 9 knots with a 6.2m "pointu" are abusing of the ouzo liquor, retsina and Samos Glyko wines, delicious drinks but of none help in naval architecture...Or worst they are drinking too much Turkish raki.

[mydauphin]

Would creating a curtain of air on bottom and sides of hull reduce friction or just make ship sink in air bubbles.

[daiquiri]

Would creating a curtain of air on bottom and sides of hull reduce friction or just make ship sink in air bubbles.
Diverging results from various authors:
http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA237032&Location=U2&doc=GetTRDoc.pdf
http://www.nmri.go.jp/turbulence/group/040615Energy_saving_by_microbubbles.pdf
http://wasub2.tudelft.nl/download/microbubble%20drag%20reduction%20on%20a%20human%20powered%20submarine.pdf
http://www.newscientist.com/article/mg18925391.600
http://www.nmri.go.jp/spd/drag/taka99/taka99e.htm
I also recall (but I'm unable to retrieve that paper) that one japanese author (Kawashima? some 6-7 yrs ago) has reported big problems with maintaining the layer of bubbles attached to the hull, because they tended to escape upwards at some distance from air injectors.

[mydauphin]

Wow thanks Daiquiri... Everything except a working boat...

[liki]

Here some displacement boats in previous life have been converted into speed monsters. See what has been done for the bottom shape as an example here:

http://www.friddemartin.net/fanny/8161.html
http://www.friddemartin.net/fanny/IMG_3708.html

[Ilan Voyager]

Many Thanks Daiquiri, very interesting links. Lot of research since years but none full size commercial boat has been made...between research claims and real life there is a gap.
For example the model used by the Japanese in the last link is rather unreal; 12m long, 1m wide, 0.045m draft (??!!). That gives at best a displacement of 0.340m3; the ultra-ultra-ultra light boat.

[apex1]

Many Thanks Daiquiri, very interesting links. Lot of research since years but none full size commercial boat has been made...between research claims and real life there is a gap.
For example the model used by the Japanese in the last link is rather unreal; 12m long, 1m wide, 0.045m draft (??!!). That gives at best a displacement of 0.340m3; the ultra-ultra-ultra light boat.

There have been many vessels built using that air bubble technique! Although (to my knowledge) only icebreakers. The bubble "curtain" reduces the friction between ice and vessel. As far as I know all of them have been replaced by water injectors to gain the same advantage with less power and structural issues (hullform).

[loups1]

33379

[loups1]

33377
Τεχνικά χαρακτηριστικά 6.20
Το Saronic 620 είναι ένα σκάφος παραδοσιακό, τύπου τρεχαντήρι.
Fishing μέγιστου μήκους γάστρας 6,12 μέτρων και μέγιστου πλάτους 2,20 μέτρων.
Δέχεται ένα κινητήρα έσω ντίζελ από 8 έως 35 ΗΡ και έχει maximum ταχύτητα με τη βοήθεια δύο παρατροπιδίων 9,5 knots. Το βάρος του με κινητήρα και καύσιμα αγγίζει τα 1100 κιλά.

Κατασκευάστηκε για να καλύψει τις ανάγκες της ελαφριάς παράκτιας αλιείας αλλά και του μικρού οικογενειακού σκάφους που θα μπορεί να χρησιμοποιηθεί ασφαλέστερα για ψάρεμα και αναψυχή από μια απλή βάρκα χάρη στη βαριά κατασκευή του.
Συνδυάζει τα σύγχρονα πολυεστερικά υλικά με το παραδοσιακό στιλ. Διαθέτει κλειστό χώρο με καντράν έτοιμο να υποδεχθεί κάθε ηλεκτρονικό όργανο.

Η διακυβέρνηση του σκάφους γίνεται από το κατάστρωμα με λαγουδέρα το οποίο μάλιστα είναι αυτοστραγγιζόμενο, μηδενίζοντας έτσι το άγχος του κυβερνήτη στις δύσκολες καταστάσεις. Το σκάφος επίσης χαρακτηρίζεται για την ιδιαίτερα καλή για τα μέτρα του πλεύση αλλά και τους αποθηκευτικούς του χώρους. Εύκολος στην πρόσβασή του για συντήρηση είναι και ο κινητήρας του.
Το υλικό κατασκευής του είναι ενισχυμένος πολυεστέρας με υαλoίνες (GRP). H γάστρα του διαιρείται σε τρία στεγανά διαμερίσματα από ισάριθμους μπουλμέδες (φράχτες) ενδυναμωμένο με ξυλεία ναυτικού τύπου ακόμη και για βαριά επαγγελματική χρήση.

Στο κατάστρωμα και στην καμπίνα πλοήγησης ανάμεσα στο GRP υπάρχει κόντρα πλακέ θαλάσσης υπό μορφή σάντουιτς για να μπορεί να δεχθεί βίντσι υδραυλικού τύπου.
Eξοπλισμός
Specifications:the saronic 6,20 is a traditional boat, trehandiri type fishing length 6,12m and 2,20m wide.it takes one inboard diesel engine from 8 to 35hp and has maximum speed with the help of two “chines”? [I don’t know the exact translation maby its something else] of 9,5 knots.it weights with engine and fuel about 1,1tons.it is build to cover the needs of the near shore commercial fishing also the small family boat which will be used safely for pleasure fishing than a small boat by its heavy construction. it combines grp with traditional design and it has a closed space where c take any electronic organ .the control of the boat is done from the deck which is self draining with a paddle? The boat also has very good -for it length-flotation and good storage. Easy to acces is the engine.

I tried to translate the specifications of a boat similar to mine from a greek boat building company. I once tried to ask them but no response.
Thanks for your time .
Also this morning I took a photo of another boat similar to mine.as you can see it has something like flaps at the rear and left and right at the middle has something like a keel? I don’t know the speed of that boat but I would like to know the use of that structure. Thanks again for your time
33378

[Guest625101138]

lou
It should do better than 6.2mph with 60HP.

Do you have any detail on the prop - diameter and pitch?

Do you know what the engine speed is at full throttle - maximum rpm?

The various photos shown are different. Some of the boats have the aft trim tabs but the first photo you posted does not. These will reduce the squat when the hull is pressed beyond hull speed. They should reduce the required power to get above 5.5kts. Does your boat have the aft trim tabs?

Rick W

[loups1]

hi rick thank you.you are right but as i mentioned are boats similar to mine in dimentios6,20x2.00 and the hull is exactly the same exept those differences which i noticed.if aft trim tabs you mean those which i call flaps no my boat had not but last year a friend of mine automotive engineer told me to put two piece of metal 30x45 under and a litle forward of the prop. after that the bow does not rise as did and the stability is better but no more speed.the maximum rpm is 4200 as the engine is from a nissan car used as a taxi. the diameter of the prop is 36cm in diameter but how i can meaure the pitch? the boat is at the marina.is it any pratical way to do it without taking off the prop?

[Guest625101138]

Is the engine reaching 4200rpm or is it loaded down too much to get to this rpm?

Is there a gearbox? If so do you know the ratio from the engine to the prop?

You need to be able to get at the prop to find markings on it or measure the blade angle at a known radius. Is the boat in the water? If so is it easy to get it out?

To measure the blade angle you need to place a straight edge across the back of the blade at say 14cm from the shaft centre. This should be 4cm from the outer tip. Then determine the angle between the straight edge and the shaft.

Rick W

[ancient kayaker]

Would creating a curtain of air on bottom and sides of hull reduce friction or just make ship sink in air bubbles.

That sounds awfully close to a hovercraft!

Enough pressure has to be exerted somewhere on the bottom of the hull to support the boat's weight. I suspect if enough of the bottom area is covered in bubbles the boat would just sink enough to push the bubbles out of the way. Under way, the bubbles would be swept back into the wake before they could do much good. To have the bubbles stick together sufficiently to stay put and exert pressure on the enclosed air the liquid would have to contain a lot of detergent or be something other than water. But I could be wrong.

[Village_Idiot]

That sounds awfully close to a hovercraft!

Enough pressure has to be exerted somewhere on the bottom of the hull to support the boat's weight.

OK, this is totally off-topic, but one of the leading theories to disappearances of vessels in the Bermuda Triangle is bubbles. Some scientists believe huge amounts of gases (nitrogen?) are occasionally released from fissures in the ocean floor, and this creates enough "bubbles" in the water to immediately sink ships, and even adds enough instability in the atmosphere above it to bring down airplanes.

Now, back to the OP's topic...

[apex1]

Enough pressure has to be exerted somewhere on the bottom of the hull to support the boat's weight. I suspect if enough of the bottom area is covered in bubbles the boat would just sink enough to push the bubbles out of the way. Under way, the bubbles would be swept back into the wake before they could do much good. To have the bubbles stick together sufficiently to stay put and exert pressure on the enclosed air the liquid would have to contain a lot of detergent or be something other than water. But I could be wrong.

Nahh, Terry it will not sink. The air "curtain" is too small (by volume) to have a big effect on displacement value. But in a turbulent flow, those tiny bubbles at the boundary layer are under some very strong shear forces, and become deformed and rather flat, like lentils: and this change in shape seems to reduce turbulence, and holds them in the layer quite a while, therefore frictional drag drops.
But there will be much more to research before we see this in every day use. If ever..

Regards
Richard

[ancient kayaker]

True. I didn't expect the boat to sink to the bottom of course, just enough to compress the bubbles, just as you describe in fact. Bearing in mind the power needed to pressurize the air, much more for a gas than for a liquid, and the complexity and weight of the delivery system required to define the bubbles and get them to the right place, it is probably the sort of thing that would only benefit larger vessels such as tankers.

There was a TV program a few months back that looked at highspeed projectiles entering water; by designing so the projectile traps and carries with it a bubble of air, it was found to travel a much greater distance and be less likely to break up on entry.

[apex1]

There was a TV program a few months back that looked at highspeed projectiles entering water; by designing so the projectile traps and carries with it a bubble of air, it was found to travel a much greater distance and be less likely to break up on entry.

Years ago I have seen a report (maybe the same), coming to the conclusion, that this "projectile effect" could in theory make a submarine be propelled at speeds in the hundreds of kn.

Bearing in mind the power needed to pressurize the air, much more for a gas than for a liquid,

As I mentioned above, the Icebreakers did refit with pressurized water systems to gain a better effect at lower power consumption.

[ancient kayaker]

Actually I missed it at first read: thanks for bringing it to my attention.

[loups1]

hi rick.the engine reache easily the max rpm, the gearbox is a hurth with 2,15/1 ratio and yes the boat is in the water.[i reache the max.speed with about 3000 rpm-a little more than half thotle open]. i took the prop off last year and went to an prop expert who gave more pitch than the original. note that the same gearbox and the same prop was used with my former engine a lister-petter ac2w [11hp/3200rpm] and my speed was about 5mph.

[Guest625101138]

lou
Is the attached photo of your boat? You said yours did not have the trimming boards.

If not your boat, have you got a similar photo of yours out of the water?

If your engine is easily getting to 3000rpm and then continues to rev faster with no increase in speed it sounds like your prop is getting air.

The deadwood in front of the prop in the photo is not very good for going fast. There is potential for a bubble to form behind the deadwood. If this bubble is open to the surface it will feed air down into the prop continuously. You may be able to see a trail of bubbles in the flow off the prop if this is happening.

If it is the case then you need to fair the deadwood so the flow leaves as streamlines or stop the air finding its way down. The trim boards may actually be designed to do this.

It would be useful if you took some photos over the stern looking down at the propeller when you are at say 4, 5, 6 knots and also at full throttle. Video clips would be even better. They do not need to be very long just 5 to 10 seconds each so the degree of turbulence can be observed - preferably with sound as well.

You have heaps of power for such a small boat. It is a little overweight but not ridiculously heavy. It should not get to full speed at 3000rpm and then not go any faster eventhough the prop spins faster. There has to be a problem that you can observe.

Is the hull clean or badly fouled? That will make a big difference. Likewise with the prop. Is it clean?

My calculations show that the hull should get to 9 knots with 30 to 35HP so I expect you have a problem that can be identified.

Take some photos and video clips and post them here so we can take a closer look.

Listen for and slurping sounds near the prop. Look for swirls down the side of the hull just forward of the prop. Look for air bubbles in the stream behind the prop. Listen for a change in engine noise as you get near top speed and the torque reduces. All these things will indicate the prop is getting air. This is the most likely problem.

Have a look at the similar boats that go faster. Do they sit differently in the water. Have you got a lot of extra weight in the bow that is trimming the boat bow down.

Rick W

[loups1]

hi rick,thanks again.1] no the photo is not from my boat.2] mine has not trimming boards like this one but i put as i was advised lastyear infront and underneath of the prop 3]my boat is in the water but i'll try to take potos of all those you asked 4] no the bow is not overweighted because without the trimming boards which i put last year the bow in 4,5-5knt points the sky and after that speed all things from the bow come to the stern because it's straight up.[the stern almost is in the water.5] yes the prop is clean but the hull is almost a year in the water so now its not clean 6]i can not find any boats similar to mine in the water near me to take a look. 7] in your calculations how i can get 9knts, with what prop?

[Guest625101138]

The immersed boards will add more power than I have calculated but it is important to keep the boat in trim if you can. So the boards are doing the right thing for trim. The extra power they absorb is better than having the bow pointing skyward and the stern submerged.

What you need to look for is if the boards are pushing the prop near the surface so it is getting air.

The speed should increase as you increase shaft rpm. If it is not doing this then the prop is either loaded so heavily that it is stalling or getting air. Without knowing the prop size I cannot be sure the props are not stalling but it is unlikely. It is more likely that they are getting air or very disturbed flow.

The power calculation assumed a prop efficiency of 50%. This is quite low but realistic without having prop measurements.

Rick W

[apex1]

The "tabs" shown are intended to avoid air sucking by the props. They do nothing in terms of performance. Almost all of those Terhandri´s are way overpowered to be able fishing far enough outside the homeport. Yours is overpowered already, but if there are no "fins", "tabs" installed, sucks air into the prop. You could install 1000 hp with almost no gain in speed. Get a set of those fins and install them as every shipwright in Greece nows how and where. Then your power installed, will get you to a more than sufficient level of speed compared to power. Every step above is not cost effective, and you will not find it in any commercially used boat! 9kn are way above a sensible level for your boat and why should you believe to do better than those making a living with these boat day by day?

Regards
Richard

[Guest625101138]

hi rick,thanks again.1] no the photo is not from my boat.2] mine has not trimming boards like this one but i put as i was advised lastyear infront and underneath of the prop 3]my boat is in the water but i'll try to take potos of all those you asked 4] no the bow is not overweighted because without the trimming boards which i put last year the bow in 4,5-5knt points the sky and after that speed all things from the bow come to the stern because it's straight up.[the stern almost is in the water.5] yes the prop is clean but the hull is almost a year in the water so now its not clean 6]i can not find any boats similar to mine in the water near me to take a look. 7] in your calculations how i can get 9knts, with what prop?

lou
Can you give me your best English translation of this:
"Δέχεται ένα κινητήρα έσω ντίζελ από 8 έως 35 ΗΡ και έχει maximum ταχύτητα με τη βοήθεια δύο παρατροπιδίων 9,5 knots."

I have attached the hull linesplan that I used for estimating the drag.

Using this and taking the initial data of 3000rpm, 2.15 gear ratio and 6 knots I can determine the existing prop is something like 12 X 12. It could be larger diameter and smaller pitch. Unlikely to be smaller diameter and larger pitch. It also assumes that at this speed the prop has not started to ventilate.

I have also shown the prop curve for engine speed of 4200rpm if it was working properly. It shows that the power rises considerably and expected top speed is 4.4m/s or just under 9kts.

The efficiency is lower than my earlier estimate so power is around 29kW or 39HP. Still within the capability of your big motor.

So you need to have a look at how the trimming boards work an other boats. Also confirm that your prop is sucking air in.

If you can confirm your prop size I can check the calculations.

The more information you collect the easier it is to validate the estimates and work out the best solution to get the speed you are trying to get.

Rick W

[Ilan Voyager]

I agree with you Apex, a part the "tabs" for getting a better efficiency of the propeller these hulls were never made for speed but for safety and ability to withstand choppy seas and the sudden gusts of wind so common in Mediterranean Sea.

I'm afraid that "improvised" modifications will adversely affect the safety of the boat, making it simply dangerous.

A 6.2m motor boat of 9 knots must have a very different hull that of these traditional boats.

[TollyWally]

Posting a bit late but squat boards were fairly common on old east coast fishboats converted with junkyard car motors looking for more speed out of displacement style hulls.