Propeller question

"I am using a -89 40 HP Evinru

"I am using a -89 40 HP Evinrude on a 17 feet tracker. Now i know that's a bit undepowered but it came bundled that way.

My problem is that engine dont reach the right RPM at WOT. Currently i reach 3600-3800. At that RPM the boat runs at 23 knots on gps. I recon that is max speed u can get with that motor on that boat, the problem as i see it is the low RPM. So my conclusion is that im over propped.

I bougt at new propulse (Model 6901 adjustable propeller)and replaced the standard 11-3/4X17 (part nr 0386841).

Here comes the strange part.
With the propulse adjusted to 12" or 13"(wich i fugured would give me 800-1000 more RPM) the prop is cavitating every 3'd second. Adjustment from 14" to 16" gives faster speed with more pitch but RPM's are the same. Still none of them get to the speed that the orginal prop give.

I cant trim down the engine (no trim) wich i guess had helped against the cavitation.

What is going on here. It seems like changing pitch anly affects speed and that RPM stays the same. Can this be ???"

"The 200rpm/"pitch is only

"The 200rpm/"pitch is only valid when using same prop-type.
Going to a different brand with different caracteristics changes it all.
Put Your 'new' prop at 17" and then reduce to 14" and You might get the normal difference.
Unfortunately the gearhousing design of the 40/50 that You have is not good for lower pitch props. The ventilation (kavitation) plate is too 'short' in front, and the prop might suck air.Only solution might be to lower the engine or encrease the transom angle by putting a wedge between bracket and transom."

"put again your old prop and p

"put again your old prop and play with the shaft that tilts your engine closer or far from the transom. The problem may be that you are pointing the bow down and for consequence, top speed is not archived. I have a 86 40hp and when pointing bow down , top speed is impossible to do."