LRZeitlin@aol.com At 01:34 PM 3/7/01 -0500, you wrote: >Given that the engine you describe is a slow turning, normally aspirated >power plant, output power is fairly linear with RPM. At 80% RPM you will only >be getting about 75 hp. Speed at this power should be 8.4 kts. This is not a >bad cruising speed but it may fall short of your requirements. Larry, Note that the hull speeds you are talking about are only 1 knot apart. From about 8.5 to 9.5. But, that it may require almost twice the power to get that extra knot. This means twice the fuel consumption for 1 extra knot. Or about twice the fuel for about 12% more speed. All this being just crude approximations. At what is approaching max theoretical hull speed, factors like hull friction, wind friction begin to have a significant impact, since any increase in them will require a lot more horsepower at the upper end. All of which argues for an engine that runs at about 85-90% max, providing about 1.1 to 1.2 times the square root of the waterline length, in speed. If I recall, 1.5 times the waterline square root, would require about 250 hp. In the boat you are talking about. That's about 10 knots or just a little bit more, and that is the theoretical max for displacement speeds(the 1.5). It's nice to have the extra power available, but then the engine most likely does not run at an optimum 85-90%. I suspect the 94 hp is about right, but might be a little on the low side. But it ought to be really frugal to operate. Most times the boats I get for delivery, have enough power to achieve 2.0 times or more the waterline square root. Some of them plane, and some don't under these conditions. The ones that don't are might impressive, with their bows in the clouds. Usually if I want to get really good mileage and save fuel, I am forced to run the engines way slower than what is "good practice". Turboed engines are not happy at 1200 rpm. I brought a 70 Formosa Sailboat in from Tahiti. It had a 275hp Turboed Lehman. We ran it at 1200 rpm for 3 weeks straight. When it would get to whining, a real subtle whistle, we would run it up for 15 minutes to about 1900. But, I didn't like it one bit. 1200 rpm was about 1.5 gals per hour and about 50 horse, I think. That gave us 5 knots water speed, except when the wind blew above 18 knots, when we would lose about 1/2 knot. And we had 2-5 knots of current from behind, coming down the equator. On the subject of turbos, if I recall my list of things that can go wrong, a turbo is somewhere on the list, if not near the top. On single screw boats, I would much rather not have the turbo. And by the way, I buy about 50 thousand gallons of diesel a year. Do you suppose that delivering boats for owners who can afford all this is just a little bit crazy. ----- Don't answer that question. Regards, Capt. Mike Maurice Near Portland Oregon.

Burkhardt, <<I will do some seatrials. If this vessel (94 hp, 51', 78,000#) reaches her figured hull-speed with about 80% of maxRPM I'll calm down - otherwise she's not my vessel of choice.>> I seriously doubt if 94 hp will move your trawler at its 9.6 kt. hull speed in any but the most benign conditions. Given the weight and size of your boat, its estimated speed will be about 9 kts. The usual rule of thumb for most naval architects is that it takes an engine delivering approximately 1 hp per 500 lbs displacement to push a displacement hull at its hull speed. In your case this works out to 156 hp. Actual power at the propeller is reduced by losses in the reduction gear and drive line. Using Keath's calculation method, as found in Skene's "Elements of Yacht Design", the required power at the propeller is 111 hp. Given that the engine you describe is a slow turning, normally aspirated power plant, output power is fairly linear with RPM. At 80% RPM you will only be getting about 75 hp. Speed at this power should be 8.4 kts. This is not a bad cruising speed but it may fall short of your requirements. Then again, these are hypothetical figures. Nothing beats a good sea trial for accuracy. Larry Zeitlin