Prop size & pitch

[Patience]

I have an older Irwin 10/4 with a Yanmar YSE8 that was installed about 1985. The boat originally had a Volvo mb10a with a 2 blade 13/6 prop. the current prop is a 3 blade 12/8. I had the prop and bottom cleaned and the engine completely serviced.(valves,injector etc.) before this tune up the motor would not exceed 2400 rpms without overloading, since the tune up it will easily reach 3000 rpms without any black stuff at the transom. I still can only get about 3.5 knots of speed oout of the old girl. I think I need to re-prop or re-pitch and am, looking for input...I would like to only do it once.PeacePatience

 

[T. OReilly]

1" pitch =200RPM

One inch of pitch on a prop generally equals 200 rpm. If ypur redline is 2800 rpm and you can do 3000 with the current prop, then get one with one more inch of pitch, i.e. if you have an 8" pitch get one with 9" etc. Tour redline should be your top speed and should you should not be able to exceed it, at least not by a 100 rpm or so.Good luck

 

[Steve Dion]

Get help fast.

Patience:Get ahold of Kilian Propeller in San Leandro, CA. They can probably tell you the EXACT prop that you need for your boat. You will need to weight of the boat, the length, the engine and the transmission ratio. They can tell you if you need to repitch your existing prop if that is really the problem. What are you basing your speed on (knot meter or GPS?)