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Thread: Engine sizing question for the NAs

  1. #1
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    Default Engine sizing question for the NAs

    Every now and then we have random boat discussions in the office. And every now and then, search engines are now help. I'm probably just using the wrong search terms, but don't know where else to start.

    Suppose you were designing a new displacement hull powerboat. I know, in general you can rule of thumb things like so much HP per ton (and I've heard everything from 2HP per ton to 6 HP per ton), but that's what I've been told for sailboats.

    And of course, we really know that torque, not HP is the important thing, and propeller design as well.

    But what if, instead of a single engine sailboat hull, you decided to go with a twin engine design? Do you just take the total HP required and divide by two? Do you add a bit to add for increased drag and weight?

    Our "Office Designed Boat" is about 42 to 46 feet, probably a trawler type hull (although one person in the corner insists we need a clipper bowed, fantail stern launch type). Suppose we convinced ourselves that 40 to 50 HP could do it. Could we go with two 25s?

    I know this is vague. We haven't settled enough on what we want yet for beam, displacement etc. but that's what led to the one engine or two conversation, with the accompanying how big do those two engines need to be conversation. Our "starting point" for discussion was Devlin's CZARINA, a "new" Lake Union Dream Boat, which calls for two Yanmar 18s. Later, we will argue if that hull design could be a coastal cruiser (as in a Looper Contender), but first we need to get through this argument. Enquiring minds want to know.
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  2. #2
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    Default Re: Engine sizing question for the NAs

    OK, You should be able to find the numbers for a 40 foot motorboat on line by looking in the for sale ads, so I'll talk trade-offs.

    The horsepower is set by the hull resistance at the desired speed, plus a margin. Power is work done, in this case making waves and warming the water.
    If you have enough draft you can swing a big single prop. The bigger the prop diameter, the slower it can turn to absorb the power and the more efficient it will be. But it will need a more torquey engine gearbox combination.
    If you are constrained by draft, twin screws allows for smaller prop diameters, but you lose a bit of efficiency, and hence need more power at the shafts.
    It really is quite difficult to build an ugly wooden boat.

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  3. #3
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    Default Re: Engine sizing question for the NAs

    There are a number of books around that will give you graphs for power required for displacement and speed, Dave Gerrs "The Nature of Design" is one such.

    John Welsford
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  4. #4
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    Default Re: Engine sizing question for the NAs

    There are a number of factors that need to be considered....Horse Power,......torque,....gearing.....propeller size and pitch. Obviously the nature of the hull, drag, intended speed. Normally twin engines will consume a lot more fuel for any given speed, but there is added reliability, and get home reserve should one engine go down.

  5. #5
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    Default Re: Engine sizing question for the NAs

    Quote Originally Posted by john welsford View Post
    There are a number of books around that will give you graphs for power required for displacement and speed, Dave Gerrs "The Nature of Design" is one such.

    John Welsford
    Thank you. This I understand, but it's not the question:

    Suppose the formula/graph/calculation comes up with "50HP to move the hull at hull speed". Fair enough.

    Does that mean "One 50HP engine or two 25HP engines", or "One 50HP engine or two 30HP engines" or something else?
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  6. #6
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    Default Re: Engine sizing question for the NAs

    Since you are using a crude rule of thumb to arrive at the required HP, twin 25's will equal a single 50 HP.

    One rule of thumb that seems to hold true is "One HP for every 500 lbs of displacement will give so called hull speed (1.34 * sqr root LWL)." This applied to older style slim full displacement hulls with fine ends. Modern hulls tend to be wider, higher, and lighter. These are easy to push slowly in flat water with a tail wind, but with minimum power they stop dead in current, sea, or headwind. So you need to allow for that. Sam's Czarina could be moved nicely with part of one of those 18's, but the additional HP was required to deal with actual cruising conditions.

    A Naval Architect will first arrive at EHP (effective HP) at a range of speeds. That's the power required to push the bare hull through flat calm water. This comes from resistance calculations or model tests of hull families of form similar to the subject. To EHP is added the power lost in propeller efficiency (somewhere between .45 and .7), in loss in bearings, gearbox, engine add-on's like the alternator and water pumps, etc. Then there are losses in bottom roughness, bilge keels, keel coolers, stabilizers, etc. And then there is wind resistance, sea state, current, etc.
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  7. #7
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    Default Re: Engine sizing question for the NAs

    Thanks Tad. If I understand, determine the power needed, then get to that number, but don't take into account "published specs", get there via actual power produced by the prop(s).

    That makes a little more sense. Today the argument devolved into "how much if you go electric". Fun conversations we have there at the scuttlebutt.
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