how much difference 9 vs 11 pitch?

[rkuz]

I recently spun a prop, a local shop gave me a replacement that is a 9 pitch 10" diameter, and told me it was the replacement for what I had. Now I am only reaching 12 mph on gps and RPM's sound much lower, I used to hit 22 mph. I see Michigan Match makes 2 props for my engine. The other is an 11 pitch 9.875" diameter. Could I have the wrong prop? Could the difference from the 9 to 11 pitch make that much of a difference to get me up to 22 mph?
Boat = 16' starcraft aluminum rowboat 60" beam (would guess 600 lbs)
Engine 1976 Mercury 20 hp

While I was waiting on the prop, I cleaned the carb, and installed a new fuel pump kit. So I am not certain if it is prop or fuel issue. But engine does start, idle and accelerate fine, just top speed of 12mph on GPS and and RPM's sound much lower.

 

[steelespike]

Re: how much difference 9 vs 11 pitch?

The 9" pitch should be revving more than the 11.
We could really do better if you had a tach.Check out the Tinytach.they have a web site.
The 9" should jump on plane and run at least around 20.
I suspect a plug wire off or messed up throttle linkage.
If your 16' Starcraft is truly a row boat,no floors or casting decks its probably less than 300 lbs.
Is the 22 buy a gps?