In a message dated 3/7/04 12:03:44 AM, Pierre asks: << 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, The formula for hull speed is: Hull speed in knots = 1.34 times the sq. rt. of the waterline length in feet. In this case the sq. rt. of 39 is 6.24. Hence 6.24 * 1.34 = 8.4. An approximate formula for speed of displacement hulls as a function of length, power, and displacement is given in Skene's "Elements of Yacht Design." Knots = sq. rt. of LWL * 1.18 * ((Horsepower * 1000)/Displacement in Lbs.) Finally, all else being equal, engine life is usually a function of piston speed and rpm. A larger engine running at a slower speed will last longer than a smaller engine running at a higher speed. On the other hand it will be heavier and take more space. The additional weight may require a greater power output to equal the performance of a smaller faster turning engine in a given boat. Larry Z

Considering the differences between sail and motor reminded me of Kenneth Grahame's famous, if clichid, quote from his Wind in the Willows: "There is nothingabsolutely nothinghalf so much worth doing as simply messing about in boats. . . ." However it's the remainder of his thought that's interesting: "In or out of 'em, it doesn't matter. Nothing seems really to matter, that's the charm of it. Whether you get away, or whether you don't; whether you arrive at your destination or whether you reach somewhere else, or whether you never get anywhere at all, you're always busy, and you never do anything in particular; and when you've done it there's always something else to do." Sounds a lot like Trawlerists! Personally, boats is boats, I love 'em all, sail or motor, big or small. FrankB Kimmell, IN