The loss of some RPM's going to a stainless 4 blade vs an aluminum 3 blade is not unusual. The stainless has an additional blade and is heavier. You could go to a stainless with less pitch to get your RPM's back. I don't know how that would affect the overall performance of your motor.
From what i've heard a general rule of thumb is:
80 HP and less stay with aluminum
90-120 can go either way
over 120 hp go stainless
Just what I've been told. Hope it helps.
Thanks for the info.
It would be nice to see HP, thrust and RPM all plotted on the same graph with different impellers on the same engine, just to see.
I got the optimax 150hp. And that is exactly what mine did, i talked to a few shops, and they said i was right were I was supposed to be. So it sounds like you are where you are supposed to be also. Give A1 prop a call in anchorage, they seem to know what they are talking about.
On a jet drive OB 5200-5300 rpm would be about right, if you start seeing max rpm it would mean its time to reshim the impeller or replace the cone or both.
In my limited experience, a properly adjusted impeller, of either kind, will limit RPM to around 5200. As the impeller and sleve wear, RPM will gradually climb.
When it passes 5,600 RPM, it is time to move a shim from the bottom to the top. This will cause the RPM to drop to around 5,000 t0 5,200, indicating a properly adjusted impeller.
When all the shims are on top, it is time for either a new liner, new impeller, or both.
BTW, with the rocks I chew, I never get to use all the shims.
PS; You are looking for the maximum efficiency of the impeller, not peak hp from the motor.
Pretty much what i had been thinking, but good to here it from other sources.
Originally Posted by snowtigger