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Old 10-02-2018
denverd0n
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Re: Boat thrust calculator?

There is a formula that will get you into the neighborhood. I can't remember where I found this, but I believe it was a boat design book that I have. Here it is, formatted for Excel:

=(A1/POWER(10.665/1.34,3))/0.95

Where A1=displacement in lbs, and "0.95" is a fudge factor assuming 5% loss between the crankshaft and the propeller. On some boats it could be much higher than that. Also, you'll note the "1.34" in there. That is a generally accepted factor for hull speed, but it is not precise for every hull shape. Some boats, the actual number might be closer to 1.33. Some boats the actual number might be closer to 1.35.

It is important to realize that this is just a very rough ballpark number, and it assumes a properly sized and pitched propeller. If you want to be absolutely certain of attaining hull speed, round up by several horsepower.
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