# Help With Economical Semi-Planing Designs

Discussion in 'Powerboats' started by SAQuestor, Apr 5, 2007.

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### SAQuestorSenior Member

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

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### PericlesSenior Member

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### RonWJunior Member

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.

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### PARYacht Designer/Builder

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.

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### longliner45Senior Member

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

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### SAQuestorSenior Member

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

Best,

Leo

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### SAQuestorSenior Member

Nice boat... but:
"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

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### longliner45Senior Member

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

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### SAQuestorSenior Member

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 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

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### SAQuestorSenior Member

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

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### longliner45Senior Member

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

12. ### FrostyPrevious Member

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.

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### fcfcSenior Member

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."

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### fcfcSenior Member

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### FAST FREDSenior Member

"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

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