Which prop? Please help me.

I am trying to find out what prop to use without spending a fortune buying them. I have a 1968 Starcraft tri hull fiberglass fishing boat 15.5' long. I just bought a 1989 Mercury 4 cylinder 40 horse classic that came from a 20 foot pontoon boat. The owner says the prop on it will probably not work the best as you have to keep the right rpms. I am new to anything but an electric motor so any help would be great. The prop is about 10.5" in diameter and it states it's an 8 pitch. He said I might need to be around a 11-12 pitch prop. Does anyone have a similar boat who could help me get close and how do I know if it's a good prop. I have tried the prop calculators but they do not give me the right options. I have two guys in the boat and I'm not looking to ski, just an all around prop to go fast to the fishing hole. I have had bad winds here to try the boat out yet.

Thanks in advance.
David

Re: Which prop? Please help me.

I'm not to sure about those smaller outboards, however, as a rule of thumb for us I/O guys, the lower the pitch the more RPM's the motor can turn out, and the reverse. However, if you want a faster prop, you need something with a steeper pitch, and this will in turn drop the RPM's

I'm sure someone with more knowledge will chime in here soon.

Good luck, and happy boating!

-Mark

Re: Which prop? Please help me.

Yes you will need to change your prop.There are a number of prop calculaters on the net.My guess is a 12" pitch will be close.You shouldn't select a prop without a Tach. Increasing pitch (higher number)lowers rpm.
If it is a little under propped(too low a number) you can use it but be sure you stay within the recommended rpm range.If it is over propped (too high a number)you can't use it.The experts may be along with more specific info.I believe there is a prop sticky at the top of the forum.

Re: Which prop? Please help me.

I would try a 10 first,
but you need to test out the 8 first.
see how easy it is to over rev.
If you reach max rpms at half throttle, you may want a 12, if you reach max rmp at 3/4 throttle, go for a 10.
it is really all guess work until u put the prop on.

Re: Which prop? Please help me.

What should my max rpm be with this motor? I'm getting a tachometer today before I go out.

Thanks

Re: Which prop? Please help me.

An 8P prop will over rev the engine by a ton. 12 or 13P is in order. Maximum RPM will be in the 5000 - 5500 rpm range.