prop adjustment

[keithm1961]

I finally got the boat out on some smooth water and wound it out to see what the tach readings were. Its a 1976 85hp Johnson, an 18ft Baja. Top rpm was steady at 4900 rpm speed about 35. I have the standard prop that came on the engine. I dont recall what that was but can find out if necessary. My question is shouldnt the engine be turning about 5500, if so should I go smaller diameter or less pitch or both?

Keith

 

[ezeke]

Re: prop adjustment

The motor came from the factory with a 3 blade aluminum propeller 13-1/4 X 17.

The best top WOT RPM for the engine at your average load would be 5800.

Lowering the pitch will help get you there.

 

[keithm1961]

Re: prop adjustment

so i understand that if i decrease pitch by 1" i pick up about 200 rpm's so would I be better off dropping from a 17" to a 13" or should I drop diameter
I need to pick up roughly 800 rpm's

 

[ezeke]

Re: prop adjustment

Numbers such as those are "rules of thumb", at best rough estimates. Use a calculator; your gear ratio is 2.0: http://go-fast.com/Prop_Slip_Calculator.htm

 

[steelespike]

Re: prop adjustment

Diameter is a secondary consideration except in the extreme.
Whether a prop change is needed will depend on the size now.Typically
your motor would have a 17" around 5000-5500.