" Brain,
With a prop
" Brain,
With a prop you have two measurements, Pitch and Size. Size doesn't mean all that much as the manufacturer will try and optimize that, depending on pitch and you have to buy a prop that is meant for your engine etc.. Pitch is the "gearing" of the prop. Higher pitch results in lower RPM at higher speeds and lower pitch results in higher RPM at lower speeds.
Every engine has an operating range; at wide open throttle (WOT) your engine should be at the top of the operating range with a light load. ie - if the operating range is 5000-5500, you should be at about 5500, but no greater, with one person at full throttle. Operating over or under the operating range at full throttle can damage the engine, so having the right prop is very important.
Every 1 pitch change usually results in a 150-250 RPM difference. So, if an engine at WOT was at 5000 RPM's with a 15 pitch, with a 13 it might be at 5400 etc...
They also make "cupped" props, which have a sorta cup in them to grip a little better. This results in about a 200 RPM change, look at it like an additional 1 pitch.
So, figure out the operating range of that engine; and assuming that prop was good at one time, that gives you starting point for what prop you need. Also,
www.miwheel.com has a prop calculator that will give you a recomendation.
As for 30 MPH, sounds right for a heavy tri-hull, but not a bass boat.. depends on more than I know about your setup. Having a bad or wrong prop also hinders performance etc.. Good luck,
Jon "
