Howdy!

I know my engines are a bit worn, but I can't do anything about that until winter.

My port engine runs maybe 4200 RPM wide open. Starboard hits about 4K at half throttle, and falls on it's face if I give it any more fuel than that.

I am running 50-51 MPH trimmed up just to the point of slippage and backed off, wide open.

Rapture z257 props.

I see other folks posting significantly higher speeds/rpms with similar boats/engines.

Are my props way off?

I assume that rebuilding my engines this winter will help, but they start and run very well so I don't think that is all of it.

Thanks in advance.

The engines will need to be up to their peak performance in order to determine the correct prop sizes and geometry.
Once that is complete, baseline info with the existing props is important. From there we can recomend sizes or other models with your goal in mind.
We also offer test propellers.

https://bblades.com/propeller-test-program/

Brett