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For more prolonged trips against wind and tide you may wish for a bit more power. Time will tell. In the meantime you'll appreciate the lightweight 4 hp.

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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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Looks like you may be on a lake? Depends on how much headwind & chop you may encounter? The 4HP should work in all but heavy wind & chop conditions?Is there a formula for how much HP you need per foot of boat? Just got a 24 foot and have a 4hp outboard Mercury. I assume that will be fine.

Just put it on & see what happens.

Paul T

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Here is the factory site for the boat http://www.rlyachts.net/index24.asp

An excerpt from the page Designed to sleep 4, the interior is open and clear of unnecessary bulkheads to provide comfortable living. The midship toilet, galley and icebox areas are convenient and the cabin can be subdivided if required. Auxiliary power is provided by a 4 to 10 hp standard shaft outboard sited in a special well designed to prevent cavitation.

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