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Posted by member Peter Cox on Monday 26 April 2021
The performance figures on this website can be misleading; some seem to refer to cruising speed and some to maximum speed. For semi-displacement and planing boats, the fuel consumption per mile increases once past the hull speed that Julian mentions, but when planing speed or semi-planing speed is reached, the fuel consumption per mile decreases, then starts to increase again. On my semi-displacement Hardy 32, the fuel per mile is better at 3000 rpm.17.7 knots than it is at 2500 rpm/10.5 knots because around 15 knots it begins to plane, or at least semi-plane.
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Message 2 of 3
Posted by member Julian Gregory on Monday 26 April 2021
MPG can apply to any speed but obviously the faster you go the less miles per gallon you get and in my experience people tend to give an mpg for a particular speed.
Maximum displacement speed will depend on the waterline length of the boat. A rule of thumb calculation to establish maximum hull (displacement) speed is that hull speed in knots equals 1.34 times the square root of the waterline length in feet. So a comfortable displacement speed will vary between different models / length of boat. In practice you can sense when the engine is working to try to push the hull beyond hull speed and backing off a little usually finds the sweet spot.
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Message 1 of 3
Posted by member Gary on Monday 26 April 2021
Looking at the speed and mpg performance figures listed by fellow owners, am I right to assume the mpg refers to displacement rather than planing speed ? What would a comfortable displacement speed be ?
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