Propeller size help

Good Afternoon. I have a 1977 16′ Lund Pike with a 35 HP Evinrude motor. Last trip to the repair shop the owner suggested it was time for a new prop.

It is a light boat, and with just me in it and not much for gear, I’m only able to get 19 MPH on the GPS. The current prop is a 10-1/2×11. How do I determine appropriate prop size? It seems that it is a bit slow and should really be in the low to mid 20’s for speed. Maybe I’m wrong on that. No tachometer, so I don’t know if the RPM’s are in the right range or not.

Any suggestions?

You might want to start with a temporary tach like this

it might not be perfect but will get you close to your wide open throttle rpm.

not very expensive, easy to come by.

I ordered up a tach. Thank you.

Pardon my ignorance, but what influence does RPMs have when deciding proper prop size?

blank wrote:

Pardon my ignorance, but what influence does RPMs have when deciding proper prop size?

Too small a prop you can run too high of rpm.

Too big a prop and your at low rpm at wot. … no harm here, but that’s your sign to go smaller for better performance.

You want to prop, so you’re wot rpms are within the proper operating range.

FishBlood&RiverMud wrote:

Too big a prop and your at low rpm at wot. … no harm here, but that’s your sign to go smaller for better performance.

Too large of a prop can put extra strain on one of the pistons causing it to lose compression. Propper size prop will give you a good hole shot and top end. A smaller prop can have a higher top end than a larger prop because it is able to reach the right RPM range.

Use one of the online prop calculators. I am not a Mercury fan but they really know props and are light years ahead of most others.