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Selecting HP for given hull (outboard)
"Gene Kearns" wrote in message ... On 4 Oct 2003 07:21:34 -0700, (roy) wrote: Am looking at a new boat approx 20-21 ft in length with 15 degree deadrise, sort of a flats type hull shape for shallow water. The manufacturer rates the transom for up to 200 HP, the dealer claims that 150 is plenty and that anything larger is a "waste of money" in both initital cost and operating costs. He also claims a speed increase at WOT of only 2-3 MPH. I have looked over the Yamaha website for performance on specific power packages but can't seem to find the same hull with two different HP performance tests. My previous experience indicated that by having maximum rated HP on the transom that one could turn a larger prop, say a 21 inch pitch while the less powerful engine would force the prop pitch down to a 19 or less. I realize that at WOT a 200 HP will burn more fuel but it is my intent to operate for the most part in the 30-35 MPH range on a hull that will offer in excess of 50 MPH. Will the reduction in RPM on the larger motor to achieve a given cruising speed provide adequate additional fuel economy and performance increase to justify the increased initial expense? 50 HP isn't going to make a difference of a mere 2-3 mph. Bear in mind that not only will the HP change, but so will the pitch of the screw. You will most likely be turning fewer RPMs at your desired cruise with the larger engine due to this.... and, again, burning less fuel. If you really want to burn max fuel and go flat out... you have the option.... You are in the "driver's" seat.... if the dealer wants to sell the boat, he must let you sea trial both models. My guess, extrapolating from your description, is that he has a smaller engine he doesn't want to get stuck with through the winter. Be from Missouri...... say, "Show me....." The rule of thumb is that max speed is proportional to the square root of hp. If you increase the HP by a factor of 1.333, you increase the top speed by about 14 %. If the top speed is about 50, that is 7 mph. But if you are only going to go 35 because of waves or other factors, then that should be taken into consideration. The bigger motor won't burn less gas at the same speed fer sure. Maybe not much more, but not less. del cecchi |
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