Confused with hull design and engine HP..

Discussion in 'Powerboats' started by SoNew, Nov 18, 2012.

  1. SoNew
    Joined: Nov 2012
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    SoNew New Member

    Hello All!,

    I'm very new to the "technical" side of boats and boat building so please excuse any questions that may seem ridiculous or redundant to those that "know"

    I've chosen a boat which I like from an aesthetic and size point of view which is the Benford "Baten" and I'm now trying to learn about hull design and engine power based on that particular boat to see if it fits my "need'' of a economical coastal putterer with dreams of more.

    In the description, the Baten is referred to as a displacement hull, which is what I want. However later in the description of its engine it talks about having a 50hp engine and "her flat enough hull" and getting it up to semi displacement speeds?

    So now my question is, which sorry, is more of a two part question.

    One, can this boat be ran as a displacement hull or is it truly a semi displacement hull?

    Second, I've read that a diesel engine should have about 2-3hp per ton(2240 lbs) of boat. Does this mean that for example a 2 ton boat can be well off with a 6hp engine? It seem strange to me because there are a lot of boats under that weight yet I don't see many diesel engines in that size range. Please tell me what I'm missing from my equation of choosing a proper sized engine for a displacement hull and more specifically for the Benford "Baten"

    I'm trying to learn about about boat "weight" to engine HP while having the engine run in or at the proper RPM range for its size.

    Thank you for any responses.

    P.S. Sorry I suppose I should have separated the questions and put the engine question in the propulsion section but thought it would make more sense this way.
  2. gonzo
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    gonzo Senior Member

    About 10 HP is the smallest diesel engine available in the market. That doesn't mean that light boats need that much power, only that there are almost no buyers for a diesel that small. Outboards are much cheaper, and small boats don't usually go on long trips where an engine runs non-stop.
  3. Milehog
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    Milehog Clever Quip

    Stretch her out by about 2 feet (10%) and put the extra length into the cabin for comfort in inclement weather. That would increase it's season quite a bit. Have you looked at Sam Devlin's designs?
  4. tom28571
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    tom28571 Senior Member

    No one really uses power plants as small as the minimum required for calm water and wind for practical boats. As is typical of Jay's designs, this is a nice looking little boat. To me it is a bit cramped for more than daily excursions. If you expect to spend nights aboard though, those bunks are very short and the pilothouse is a bit tight with no place to sit comfortably. In the same type (fairly heavy, semi-displacement), Sam Devlin's "Surf Scoter 22" provides more in the way of comfortable cruising for little more investment in time or effort. Thomas Firth Jones also has a similar classic small cruiser in his book "Boats To Go" called "Slider". I think the title of Jones's book may have been changed from "low Resistance Boats".
  5. CDK
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    CDK retired engineer

    That is just a rule of thumb, saying that the average displacement hull needs 2-3 hp per ton to reach hull speed. It does not include currents, wind, waves etc. and is certainly not intended to select a specific engine.

    But you can use that rule to estimate fuel consumption: a 2 ton boat at hull speed will need 6 hp so a diesel engine will burn 6 x 250 = 1500 grams of fuel per hour. The actual engine may have 18 or 25 hp and achieve a somewhat higher speed with accordingly higher consumption or is capable of maintaining hull speed under bad weather conditions.
  6. SoNew
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    SoNew New Member

    I really do appreciate all these replies!

    Ok, so now I understand that the 2-3 hp per ton is what you can call the minimum and can be used to estimate fuel consumption under ideal conditions. Now how does a person not go overboard on HP on a certain boat design where the engine is pushing the boat at the required speed but is not under enough load which is of course not good for the engine or one's pocketbook to begin with!

    I checked out Sam Devlin's Surf Scooter and though I do like it, I'm trying to go for diesel and to go slow. My dream boat is the Sam Devlin "Kingfisher 26" but i think it's just too big for one guy?
  7. michael pierzga
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    michael pierzga Senior Member

    Also concider logic.

    If you had a one ton boat with a two hp diesel ,as soon as you clicked the boat into gear the drive train friction would overpower the idle output of your little 2hp and stall it.

    As a result 2hp per ton for a small boat doesn't work. As the boat increases in size the ratio makes sense.

    Also as Gonzo pointed out Marine diesels dont come in an infinitely variable output range.

    With a small boat you would be using what was available .

    The little 14 hp Kubota is the smallest marine engine I know of , so you would probably fit it to your one ton boat. 14hp pe ton.

    Best to follow your designers recomendation.

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  8. tom28571
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    tom28571 Senior Member

    A Surf Scoter is an ocean duck and not a scooter. Many have used diesel outdrives and would meet your speed very well. The ability to run a bit over hull speed is a big plus and you give up practically nothing to get it, especially in the size of boat you are interested in.
  9. Eric Sponberg
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    Eric Sponberg Senior Member


    A good reference book is Dave Gerr's "Propeller Handbook" that will more fully explain the speed-power relationship. In there he describes his "displacement speed formula" which is:

    Speed/Length Ratio = V/Lwl^0.5 = 10.665/(Displ/SHP at prop)^0.333

    In words, this reads: Speed to length ratio equals 10.665 divided by the cube root of the quotient of displacement divided by the SHP delivered to the prop. For the engine horsepower, you have to increase SHP by about 5% to account for bearing, transmission, and internal engine losses.

    So, applying this to the Baten: Lwl = 19.0'; Displacement at full cruise load is 4,600 lbs = 2.05 Long tons (a long ton = 2,240 lbs). So the boat weighs 2.05 long tons.

    We can use the equation above because we know all of the factors except speed and SHP. So we pick speeds, and calculate horsepower. Displacement speeds will be anything below a speed/length ratio of 1.34, and a boat like this might travel at speed/length ratio = 1.10. We can also test 1.34, and let's try two more at 1.5 and 2.0. These latter two are in the semi-displacement or semi-planing range. When you get up to speed/length ratio = 2.5, you are generally considered to be in full planing regime. These numbers will vary slightly for different hullforms, but those are the basic ranges.

    So, we specify a speed, calculate speed/length ratio (which I'll label V/L for simplicity) plug it into the equation, and turn it around to solve for the unknown, SHP:

    V/L = 1.10; V = 4.8 knots; SHP = 5.05 HP; HP/Displ in L. tons = 2.46
    V/L = 1.34; V = 5.8 knots; SHP = 9.12 HP; HP/Displ = 4.45
    V/L = 1.50; V = 6.5 knots; SHP = 12.80 HP; HP/Displ = 6.24
    V/L = 2.00; V = 8.7 knots; SHP = 30.34 HP; HP/Displ = 14.80

    So here you can see the relationship between vessel weight, speed, and horsepower which is a pretty valid tool that we naval architects use.

    Sometimes, if you are dealing with a particular class of hull shape, this formula may not work precisely, and you can modify the constant 10.665 to another number based on actual experience (empirical data). For example, on my Moloka'i Strait motoryacht designs, I reduced it to to 9.8 or thereabouts for the hull class in order to come up with more accurate predictions for intermediate hullforms based on actual test running of the boats we had already built.

    I explain speed and power formulas more fully in my write-up that I did on this forum about 3 years ago called "The Design Ratios." I am posting it again here. You can also download this article from the ARTICLES section of my website.

    I hope that helps.


    Attached Files:

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  10. Frosty

    Frosty Previous Member

    All diesels have governors Micheal. But in any case there is no such thing as a 2HP . Yanmar make a TS7 industrial that is 7HP. It is not marine but depends on your conversion skills.
  11. lewisboats
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    lewisboats Obsessed Member

  12. michael pierzga
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    michael pierzga Senior Member

    Gee Frosty, you've been up in your tree too long. Stand up straight, stop dragging your knuckles and have a look..

    plenty of mini diesels. Dont know if you can add your custom Chav PVC exhaust to it. Might have to use expensive metal pipe.


    Capacity: .056 cubic inch

    Bore: 7/16"
    Stroke: 3/8:
    Weight: 2.7 oz

    Timings: Int. 134°, Xfer. 110°, Exh. 130°

    Recommended Prop: 6 x 3.5 or 6 x 4

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  13. PAR
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    PAR Yacht Designer/Builder

    I have a supplier for air cooled, electric start 5 HP diesels.
  14. SoNew
    Joined: Nov 2012
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    SoNew New Member

    Thank you all!

    This site and these replies have answered most of the questions I've needed to know to decide what boat is right for me.

  15. Frosty

    Frosty Previous Member

    Does that look like a diesel to you Micheal with a mini little carburettor on the front of the 2 stroke cylinder. Does it have an injection pump and injector !!!!!!!
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