76 Johnson 85 Prop???

[Glastron76]

HI all, I have a great little 1976 Glastron 155 trihull with original Johnson 85 hp. No tilt/trim......still running like a top. Question is the prop. It has the original 13 1/4 X 17. Lots of low end hole shot but tops out about 38 mph by gps. NO tach. Would you OMC guys recommend the 17 pitch or try 19 pitch for a little more speed.....if any??? I fish, no ski or tubers going on. Trying to stay up with my buddy's boat....sometimes he is faster sometimes I am (he has sterndrive Volvo). Any comments welcome. I love the V4 Johnson...she runs well, never fails to start first crank.

 

[etracer68]

Re: 76 Johnson 85 Prop???

I would think your about where you are out of HP. Also without WOT RPM, I cant recommend a pitch for a differant prop.

 

[steelespike]

Re: 76 Johnson 85 Prop???

Assuming I have the right gear ratio at 38 mph with a 17" prop your turning about 6000 rpm.500 over recommended.
Without actual tach readings to go with the prop size and speed.
A Tinytach is about $47 and will work on any motor. Hand held or installed.
If your actually turning 6000 then a 19 might work.
Without the tach we can't work accurately and can't check the results or suggest a prop.
Its important to have the tach info for the health of the motor as well.
Refigured with a 2. to 1 ratio rpm is closer to 5300. Rated at 5500.
So as etracer suggests your about there. Still need your rpm to be sure.
Only sure numbers we have are your pitch and speed.Pretty slim pickins.
If your prop is the old tired original a new one same pitch might help.