[MHml] Engine power

[roy mills]

Hi Bill,

        Sorry to be late at the fair but I have been away for a couple of days. So 20 hp, with I don't know what prop efficiency, pushes AB at around 7 knots. Resistance is partly air drag and partly hull drag.  The air drag we know increases as the square of the speed. The square of 20/7 is say 8.2. so that is the increase in air resistance. Hull resistance is a little more complex, with peaks at certain speeds, however AB with a probable length beam ratio in the order of 16 would not be much troubled by those. According to what I can get from graphs in Bruce's "Design for fast sailing" the drag increase from root L ( about 6.6 knots for AB) to 3 root L - say 19.8 knots is in the order of 10 times.  Since hull drag is larger than air drag you should probably use the multiplier of 10 which gives 200 h.p.  Of course it might be prudent not to cruise it at full throttle, maybe a 250 hp engine running about 200 hp would do well. However you may not need 200h.p., its thrust you are needing not necessarily hp. I dare say that if you had a sled mounted diesel with a good sized reduction gear between it and the prop, a prop guru could almost certainly design a more efficient prop for running at 20 knots than your present Honda turns at about 7, and surely more efficient than the props fitted to outboards of over 200hp when running at 20 knots.

    I am in favour of sleds, Peter Spronk used them on his 60 footers, though not for motoring at the speed you are looking for, and I have one on G & S which profiles over the waves nicely and does not  cavitate/ventilate provided that I have lowered it enough in the first place. The bottom of such a sled could provide the location for cooling tubes for the engine as they will be under water.

    Are you quite sure you want to go at 20 knots?  14 would be twice as fast and would only need provide about 4.5 times as much thrust instead of 10. Besides 90 hp is not as big a challenge to find.  You were concerned that a tall standing outboard would bang up against the underside of AB. If you did not use an outboard that height would not be a problem. Sure Diesel is heavier, but fuel is heavy too and you would burn a good deal less. Prop efficiency is the key factor in converting hp to thrust.

    Cheers anyway   Roy, who looks forward to a similar blizzard of technical advice to what you got with your initial query.
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