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Sea Dancer - Capt. Jerry Allen, Kailua-Kona, HI

Performance specifications

Submitted by Paul Gozewski (Capy)

diesel H.P. d knots Gas H.P. g knots prop dia D slip diesel max slip D prop dia G slip Gas max slip G
280 18 540 20 18 26% 21% 16 25% 24%
432 21 660 21 21 19% 12% 17 30% 34%
500 24.6 660 22 21 15% 10% 17 32% 33%
600 28.5 680 22 19 13% 14% 18 27% 25%

About the data,

Separate curves were generated for Diesel power and Gas power, although the curves are on the same graph. Slip was calculated by dividing the actual distance traveled by the combination and dividing by the theoretical travel (negating friction forces). Slip is the first measure of vessel performance. Lower % slip is more efficient. Horsepower is the rate at which work is done, and work is the acceleration of mass. Horsepower without a corresponding torque is useless.

The first chart shows slip vs diameter which really makes the case for large wheels. the gas boats cannot spin these wheels due to half the torque output of a diesel. although the cruise speeds are similar (chart 2), because their "small" wheels spin twice as fast. The gas combinations pay a huge penalty in efficiency, due to high slip.

Note in chart 1 The slip for Richard Miller's vessel is approximately same at cruise and at max speed, this suggests correct propping, while large differences in slip between cruise and max, suggest over propping. Consulting a marine engineering handbook, slip should decrease by 2-4% at maximum speed, for maximum cruise speeds to be obtained. Once a certain speed is obtained >40 knts, diameter should be reduced to overcome drag.

Paul Gozewski

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