How much HP do I really need?

Discussion in 'Outboards' started by Martell_Florida, Jul 29, 2007.

  1. Martell_Florida
    Joined: Oct 2005
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    Location: Florida

    Martell_Florida Junior Member

    I have a skiff that (when finished) will weigh around 300 Pounds. Add me, gear, 6 gallons of fuel etc... and I estimate 650 pounds loaded. The skiff has a 15 degree dead rise and is 13' long and 60" wide at the transom. Would a 15 hp (another 74lbs) get this boat on plane and move along in the 20 mph's? Is there any weight:HP formula to reference? Thanks in advance.
     
  2. timgoz
    Joined: Jul 2006
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    Location: SW PA USA

    timgoz Senior Member

    Hi Martell,

    My 14'5" aluminum skiff did approx. 17mph with a 9.9hp 2-stroke Nisson. I am not sure of the weight but I also had 6gal. fuel, oars, small anchor & line, my approx. 200lbs., ect...

    I would think a 15hp would be all you would need. For a good 13' general purpose skiff, that would be my maximum. Unless you really need the speed, a 9.9hp would most likely meet your needs and be more efficient.

    Take care.

    Tim
     
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  3. Guest625101138

    Guest625101138 Previous Member

    It should do it with a bit to spare.

    The hull drag for a boat on the plane is usually around 1/8th of the total weight. So lets say you end up with 800lb by the time the motor is added. The hull drag is therefore around 100lbf. The speed is 20mph or close to 30ft per second. The required hull power is therefore 3000ft lbf/s.

    A well set up outboard with matched prop should get better than 60% efficiency so lets say 60% to be safe. Required motor power is thus 5000ft lbf/s. 1HP equals 550ft lbf/s so 10HP will do the job at full throttle. However the boat will plane at less than 20mph so 10HP would get on the plane but only just make the speed.

    It is good to have some margin as the trim can make it slow to plane. If you add two more people you could be adding 400lb so this will take another 3HP. By these estimates I would say 15HP is a good choice. Should be adequate without being overpowered.

    Rick W.
     
  4. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    I think 15 hp will be overkill on such a small boat. Also many areas require registration (tax) on boats with over 10 hp, so some of the 9.9's seem rather underrated for obvious reasons. boatdiesel.com has calculators for power needed, not sure if it would be applicable to a craft this small, though. Don
     
  5. blared
    Joined: Aug 2005
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    Location: Orlando

    blared ALFA

    Get 15hp 2 stroke if you think it can handle it , the more powerfull the more fun. If the bow rises too much make some trims.
     
  6. charmc
    Joined: Jan 2007
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    Location: FL, USA

    charmc Senior Member

    15 hp might be more than needed for just the load you described, but somehow every boat, whether it be 8' or 70' ends up sooner or later weighing more than planned. The extra power will come in handy more than once, and it's nice to get good performance without having to open the throttle all the way.
     

  7. charmc
    Joined: Jan 2007
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    Location: FL, USA

    charmc Senior Member

    Martell,

    As you might know already, Florida registration fees are based on hull length, regardless of HP. Yours is a Class A2 (12-16'). Registration would be the same if it was a 16' Formula 1 racer with 400 HP. (Don't get any ideas, now) :)
     
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