What is the formula for Max. HP on a flatbottomed boat?

Discussion in 'Boat Design' started by willfishforbeer, Jul 7, 2006.

  1. willfishforbeer
    Joined: Jun 2006
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    Location: Florida

    willfishforbeer Junior Member

    I've heard there is a different formula than the 2xLxW-90 for boats with flat bottoms, is this true? Or is it safe to use this one?
  2. Tim B
    Joined: Jan 2003
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    Location: Southern England

    Tim B Senior Member

    Go and use a Savitsky power prediction. Enter all your boat details and calculate the power you will need for your desired speed.

    There is a spreadsheet available on this forum.

    Personally, I don't usually like rough equations. The only one I take any heed of is the one for "Hull-speed" but only then as a rough guide.

    Good Luck,

    Tim B.
  3. Ike
    Joined: Apr 2006
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    Location: Washington

    Ike Senior Member

    If the boat is under 20 feet in length then the Coast Guard formula is, 1/2 the length times the maximum transom width - 15. See http://www.uscgboating.org/safety/boatbuilder/part1/reg33_183_4a.htm

    If it's 20 feet or over you can put what you want on it. I would do a manuevering test or use Saitsky. Personally I prefer the manuevering test.

  4. lewisboats
    Joined: Oct 2002
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    Location: Iowa

    lewisboats Obsessed Member

    The CG says it a bit different but it works out the same. The CG says it is the factor that is multiplied by .5 and the factor is Length times the max transom width (including rails etc) eg (16 * 4 = 64) so 64 * .5 = 32 -15 =17 (same as what Ike says, just so the different wording doesn't confuse anyone) ...rounded to the next highest 5. So this boat has a max hp of 20....according to the USCG...Provided it isn't remotely steered or has a transom of over 20 inches...then its ????????. They have a bunch of problems with these formulii. Use common sense and slow down in turns with a flat bottomed boat so you don't trip on the chine and you should be fine.

    BTW, for the same boat and your formula it works out to 38 hp...a bit excessive for a 16 x 4 ft flat bottom.

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