As Tony said, you need to judge it based on your RPMs at wide open throttle
(WOT). You need to find the specs for your engine and see what the RPM range
is at WOT. If your boat currently revs higher than the spec, you need to
increase the pitch of the prop. A general rule of thumb is that each number
increase of pitch will reduce WOT by about 200rpm.

I bought a boat last year that ran at 5500rpm WOT. The manual said 4400-4800
was optimal. I suspect that the previous owner bought a lower pitch throttle
to pull a good sized skier out of the water. I put a 23 pitch (from a 19) on
the boat, and it purrs along at about 4600rpm WOT...perfect.

Now, if we could just have variable pitched props like airplanes, all would
be good with the boating world. g

--Mike

"hdb" wrote in message
oups.com...
I have recenlty purchased a olde double hull fiberglass speedboat with
a 1972 johnson 50, I beleive the prop that it came with is incorrect.
It was a 14" by 17 pitch all the literature suggests that is a wrong
prop but I can not find any guess on what size prop should be used. Any
help wouold be nice. Thanks
Wes