great post from our fellow airboaters here. not from a point of view but from actual experiences.
the thing is to match the prop with the engine. when you are already set at the engine rated power at the rated rpm then you you're ready to go. in some cases, like aircraft engine powered airboats, repitching the prop (less pitch) above the rated rpm will result to more thrust, say 100 to 200 rpm more. this is because a/c engines have a rating called take-off power which is greater to a certain amount than the rated power.
on the other hand, we pitch our props more until a point we arrive at the rated rpm, usually 200 to 400 rpm less than the max rpm at static depending on the set-up. pitching more beyond this point will hold the engine down to a lesser rpm, resulting to less thrust. you loose thrust in two ways. one is that you loose thrust simply because you held your engine to less than rated rpm where it produces less power. the other is that you loose thust by simply adding pitch, more pitch less thrust. you can only gain additional thrust from more pitch if you feed your prop more power.
generally, fine pitched props have greater thrust than coarse (aggressive pitch) pitched props. the trade off is that fine pitch is better in push and coarse pitch is better in speed.
technically, to match the engine, get a prop wide enough just to get a fair amount of pitch. this is way better than a narrow prop with very high pitch. btw, consider factors such as redrive ratio, tip speed, etc.
examples:
- say 72" wide prop with 36 inch of pitch spinning at 2300 rpm is just about to loose bite at 55 to 60 mph.
- another 72" prop with 48 inch of pitch, on the same engine, much narrower so as to get the same 2300 rpm is just about to loose bite at 70 to 80 mph.
.... the second prop may be able to bite up to 70 mph BUT can your engine provide enough power to get that speed? you also suffer low end performance. the first prop is ideal because it is pitched just enough and perhaps, 55 mph is already too high for you unless your up for speed and you have a monster engine to provide you the power to do so.