Bernie Baake wrote:
Now all you have to do is find out if the prop pitch is what the factory recommended,because right now your under pitched. Prop pitch and wheel diameter will determine what your max rpm is. if you want greater acceleration add diameter, if you want higher top speed without over reving the engine add pitch. If the prop is factory spec you have in fact increased HP and can increase your speed. Under pitch on a prop is not a bad thing if you don't constantly over rev the engine. Over pitch however leads to problems, so make sure you have the boat loaded as you would when you normally use it before making any pitch changes.
Bernie if you don't mind I'd like to pick your brain about this a little bit. Admittedly I'm having trouble wrapping my brain around this, and the bit of reading I have done leaves me a bit confused.
First of all, you say I'm under-pitched. Is this based on the fact that I'm over-revving the engine? I actually think that I may not be that far off. 4800 is a rated WOT rpm for these motors, and I have been reading that in auto applications these motors have a 5300 rpm redline and 5700 rpm cutoff.
So I guess I first need to decide what I want the max engine rpms to be at WOT, right? Then you basically pick a prop which will let the engine run at that speed at full throttle?
Let's say I want to run at 4800 RPM at WOT. Then I obviously need to do something to add load to them motor. If I could change a gear ratio I'd do that

I was thinking that increasing diameter would add load to the motor, and probably increase top speed.
However, I read last night that increases in diameter doesn't affect speed? That it's all about pitch ?!? Doesn't make sense to me... push more water at the same RPM, same speed.
So I'm confused. It's a 14.5" by 19 pitch prop and this is the factory prop. I'm making probably 15-20 HP more than stock and am over-revving by about 200 rpm.
Thoughts?
_________________
Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1