Hello Craig, welcome and thanks for the good words. I ran your suggestion though the program today, and if your speed reference data is correct, then increasing the hull factor to 177 isn't going far enough. Actually, to replicate the number for a hull of 20.500 and 30 mph with (only) 575 hp, the hull factor number needs to go to 185. By using that factor, it puts the "standard (baseline for my numbers) 38 Commander Express at 17,000 pounds) running an equivalent of 33.88 mph. Therefore I am wondering about the accuracy of your numbers, as I feel sure CC would have been bragging about that 33.88 mph speed, instead of quoting 31.
AS a point of information to anyone studying the numbers here, the hull factor of 170 is assigned to the average planing runabout. 215 is achieved by a very light high speed runabout. 240 is for the moderate performance race boat, 250 for a hydroplane, and 260 for an all-out top end race boat.
C = a constant determined by boat configuration
LB = boat weight in pounds.
SHP = horsepower at the shaft (.95 x engine power)
The basic formula consists of this, an numbers are all affected by deviations such as semi displacement, strakes, keel, lapstrakes, etc.
Boat speed (MPH) = C/(LB/SHP) 0.05
Craig, what boat achieved the numbers you quoted, just curious. If it was a smaller or larger hull than the 38, then this may come into play too. Hull resistance will vary, and the formula is only good to a point. For instance, it takes more power to get a slalom skiier out of the water, than it does for the same weight person to be on a 4x8 aquaplane sheet of plywood.
Regards, Paul
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