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Ā· Learning the HARD way...
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I have this note ferreted away. I found it somewhere in my online reading.

A Diesel Engine produces about 20lb of thrust per horsepower

Wind resistance for a 35' boat (my boat) 225lbs of thrust at 15 knots, and 1200lb at 30 knots. A smaller boat will require less thrust per given wind speed. (there is a chart somewhere)

Detail; the very approximate calculation goes like this:
Take the engine power in Watts. If it is given in HP, then multiply by 746 to obtain the power in Watts. Call this power BSP (brake shaft power).

Multiply BSP by transmission and propeller efficiencies. You can assume around 0.5 as a maximum total efficiency of such a small prop. Call this power PE (effective power).

Convert the speed to meters per second (m/s). To do that, multiply the speed in knots by 0.514 . Then multiply again by 0.95, to approximately take into account the eventual effect of the hull wake. Call this speed Vp.

There might be no wake effects if this is a planing hull with props clear of obstructions ahead. But still I'd keep that 0.95 factor above, because it will give a safety margin to the calculation of the thrust.

To obtain thrust given by the prop, divide PE by Vp: T = PE / Vp .

This will be the available thrust in Newtons, for that speed and engine power. I'm using all SI units, to avoid errors related to inconsistent units. To convert this thrust to lbs, multiply by 0.225 .

That's all.

If you think you'll need to do more such calculations in future, and/or have them more precise, I suggest you to buy the excellent Dave Gerr's book "The Propeller Handbook".
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