JA,
I went thru the same years back, I had an old 5411 with a 2 blade folding prop
that I changed to a 2 blade (same pitch) fixed prop. I had no experience of the forces & how the worked at the time.
A boats horsepower at the flywheel is very different from the HP at the prop.
Depending on prop length, the HP from flywheel to the prop diminishes greatly.
Example: my 5411 HP was only 6HP at the prop at full throttle.
I couldn't even get out of my way. I put in a 3M-20A & did this test. I found
this can be done at the dock or while underway.
Simply put, if your wide-open throttle does not reach MAX RPM's as described in the engine manual, you are over-pitched. If exceeding max RPM's, you are under pitched.
My first offshore trip to Key West was arduous to say the least. Between current wind-n-wave, I was lucky to get 3-4 KTS. I did the prop change in Key West &
unlike my trip down, I flew back!.
Do the test & contact the manufacturer for the recommended diameter & pitch for a 3-blade.
Then contact a local prop guy to see what he says. Then decide on the prop.
I now have a kick-ass 3 blade & sometimes, I feel like I'm a powerboat up on a plane!
Hope this helps.........