Question for the Prop Masters - Too High of an RPM!

[Mercury60]

Hello gents! I began a thread in the engine repair forum but its more appropriate here. I have a 1986 17ft Smokercraft walkthrough bowrider with an aluminum hull. It has a 60hp 3-cyl Mercury 2-stroke outboard on it. My gear ratio (according to my Seloc "Manual") is 1.64:1. My RPM's are too high. Mercury says I should be running between 5300 and 5800 WOT.

The current prop is a QuickSilver 2074R-12, which turns out to be 10.625" in diameter with a 12" pitch. Its a 3-blade aluminum.

With the boat completely empty and just a couple gallons of fuel I top out at 6200rpm. This is flat calm, no wind and fully trimmed. This equates to a GPS speed of 35mph. With a passenger, fishing gear, and 15 gallons of fuel (maybe 400lbs total including passenger, not including me) I top out at 5900rpm with a speed of 32mph.

It looks like I should increase my prop pitch. How much should I increase it? It looks like I need a 400-500rpm WOT reduction. Also, how will this effect my top speed?

 

[jopes]

Re: Question for the Prop Masters - Too High of an RPM!

best advice I can give you is to load your boat the way you are going to run it most times. Then see where the RPM's land and go from there.

 

[Mercury60]

Re: Question for the Prop Masters - Too High of an RPM!

Typically a passenger, fishing gear, and 15 gallons of fuel (maybe 400lbs total including passenger, not including me) I top out at 5900rpm with a speed of 32mph....

 

[steelespike]

Re: Question for the Prop Masters - Too High of an RPM!

A 12" prop doesn't seem excessively low;I think I would check that the tach is accurate.
Perhaps borrow a shoptach.
Tachs usually have a selector switch on the back.Note the position and rotate the switch to clean the contacts.