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Try in the 10 x 12 range or i dunno...look on the side of the prop you already have and get one like it.
Should have numbers stamped into it. Sometimes it's visible from the end too.

Keep the same diameter. Change the pitch. Less pitch=more grunt or torque. Taller pitch=more speed. Can't have both.

Hey man, the way you figure out what prop is best is a process.
You need a tachometer.
You run the boat full speed, GPS it to get the speed, write down the RPM's and go to the calculator.
This is where it gets ??? for me.

There is a formula somewhere that helps you convert the prop pitch and RPM's to a theoretical speed.
You compare theoretical to actual and then you can calculate "slip".

Once you know what you got...making adjustments in size gets easy.
Trying to do this without the right tools means "buy a lot of props and figure out which one".

Another thing you should know is that trim and motor height is everything.
I got my little 15hp to go from around 16 to well over 20 just by raising my motor by two inches.
Little stuff adds up to slow you down.

(RPM's x pitch x 60)/ (63,360 x gear ratio) is your theoretical speed in mph assuming ZERO slip.
Your actual speed will be less due to "slip".

5500 rpm with 12 pitch prop , gear ratio 2.42:1 (my boat) I get a bit more than 5500 rpms but will use that as an example

(5500 x 12 x 60)/(63,360 x 2.42) = 25.826 mph

I got 24 mph in mine but will admit I was going down stream with about 3 mph current.

It takes me about 500 yards to get going that fast. 12 pitch is too much for my setup.


The gear ratio on a 1999 Johnson 25 horse is 2.15:1. You'll need that number.