Prop on 32' Classic Irwin

Mar 8, 2014
9
Irwin 32' Fleming Island
What determines the size of the prop? The engine or the displacement of the boat? The original engine was the Atomic 4 but PO replaced it with a 3 cylinder universal. Is there a site where I can find what the proper prop should be. I measured the prop and it is 14 inch three bladed, but not sure of the pitch. Know that is the most important factor. Thanks, Pacerow
 
Mar 20, 2012
3,983
Cal 34-III, MacGregor 25 Salem, Oregon
What determines the size of the prop? The engine or the displacement of the boat? The original engine was the Atomic 4 but PO replaced it with a 3 cylinder universal. Is there a site where I can find what the proper prop should be. I measured the prop and it is 14 inch three bladed, but not sure of the pitch. Know that is the most important factor. Thanks, Pacerow
im sure there is more info here than you wanted.....

with the three blade 14" prop on that engine, i think a 10 or 11 pitch would be close... but then I have no idea of the horsepower rating either.




on a sailboat the engine horsepower@proper RPM and speed of the prop shaft are the only thing that determines the size of the prop....

the displacement of the boat determines the engine it should have installed.... but the prop criteria follows the engine specs only.

the diameter, pitch, number of blades and width of blades are all equally important in the equation.... in relation to how fast it turns.

why?.... let me try to explain.

as the diameter increases, the prop will be grabbing more water that is farther from the hub so it will cause the engine to pull harder to maintain its rpm....

as the pitch increases, it will be trying to screw itself thru the water at a higher rate, which equates to more water over the blades faster.... you need more power to make this happen efficiently...

the number of blades only means the prop will be grabbing that much more or less water, respective to the number of blades. more grab means you need more power to drive...

the width of the blades has everything to do with "slip".... slip is hard to explain but at a low rpm the prop will spin in the water without the boat moving very fast and without much driving power.... it doesnt take much to stop the boat while the prop still spins. this is slip...
a wider blade holds more water over its surface for a longer period, so there will be more push and less slip, while a narrow blade can turn and beat the water to a foam without much push at all. it will have lots of slip.
more or less power has little to do with slip, but what adjusting the slip can do is to allow an efficient use of the power that is available.

the speed of the prop shaft itself is fairly self explanatory. but just a few shaft RPM will make a huge difference in the equation...

but when it all gets put together, a proper balance between slip and grab is all that matters, and this is designed around the horsepower of the engine at its target RPM....

its not a simple process, but close is sometimes good enough, because we arent racing and we arent planing.
and when the matchup between prop and engine is close, the hull speed in a sailboat can almost always be reached well below maximum engine speed/power except in extreme cases of wind and waves...