prop sizing question on new installation

[pauljennings]

I have a 1967 34' with 300 hp cummins 6bta. (new installation) Can anyone help with a correct prop for this set up.
I am having a difficult time getting the 2900 rpms with 20 X 19.5 X 1.5 nibrals on 1:5-1 ZF220a gears.
I started out with 20 X 21 and could only get 2600 rpms. After reducing the pitch to 19.5, I saw only an increase of 75 rpms. currently 2675rpm.
al comments are welcome

[jim rosenthal]

I moved this thread to Tech Talk. It will get more attention there, and hopefully advice that will help you.

[Cricket]

Best bet would be to contact a decent prop shop for help.
The bigger/better ones often have test props to try, especially in the smaller sizes like yours.
To use your current props you're probably going to have to cut the diameter down.
Btw, I'm not a fan of under square wheels. especially on a sportfish.
There's some forum members with 34's, I think similar power that will chime in.

[bobk]

Do your engines make proper no load RPM? You didn't gain much by taking 1.5" out of the pitch...

Bob