Larry Z wrote: << Most naval architects use a "rule of thumb" specifying 1 HP per 500 lbs. displacement to reach hull speed. Hull speed, of course, varies as the square root of the LWL. So if two boats have the same displacement but one is longer than the other, both will require the same horsepower but the hull speed of the longer boat will be faster. Your boat has a 39' waterline and weighs 17 tons (34,000 lbs). Hull speed should be about 8.4 KTS. The recommended power is 68 HP Larry, thanks for the interesting calculations. I can see how you calculated the recommended HP (430000/500), but there is obviously something else to the formula used to estimate hull speed, since the square root of 39 is not 8.4. Could you supply the formula for this, and also for the second set of calcs on displacement hulls, please? Secondly, is there any truth to the idea that a bigger engine that runs at lower RPM to achieve the same power as a smaller engine running at higher RPM will require less frequent overhauls and will outlast the latter? Pierre

Saturday, March 6, 2004, 6:32:26 PM, Pierre wrote: > Larry, thanks for the interesting calculations. I can see how you calculated > the recommended HP (430000/500), but there is obviously something else to the > formula used to estimate hull speed, since the square root of 39 is not 8.4. > Could you supply the formula for this, and also for the second set of calcs on > displacement hulls, please? Hull speed is 1.34 times the square root of the waterline length. -- Peter Bennett, VE7CEI Vancouver, B.C., Canada GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver-webpages.com/van-ps