hdb wrote:
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.

The "right" prop depends on what you want the boat to do. For example,
many people that like to waterski place more emphasis on being able to
accelerate under load, and the right prop for this costs a bit of top
end speed & fuel economy.

As others have said, a good guide is your engines RPM at wide open
throttle. Give it at least 5 minutes to wind up, water is incompressible
and it will take a while for it to reach true top speed. If the engine
is not reaching it's rated RPM then you need less pitch; if it winds up
quickly and doesn't make good top speed, you need more.

It may be that your boat has gained weight over the years and the right
prop back then has too much pitch now.

Hope this helps
Doug King