19' Starcraft Islander Prop help

Ive got a 1984 2.5L OMC Stringer drive powered Starcraft Islander. It is the same body / hull style as the new islanders. It came with a 13 3/4" diameter 15 pitch 3 blade aluminum prop. It currently tops out at about 23 mph at about 4200 rpm and has about a 30% prop slip if I am calculating it correctly. I believe the drive ratio is 1.71:1 from what I have I read it is a 400 series drive. I have more throttle available and can over rev the engine beyond the 4600 RPM recommended WOT. I purchased a 4 blade Solas Amita 17p 4 blade 12.8" diameter thinking it would improve my slip numbers and now the boat has a hard time getting on plane and still has some serious slip issues. At 4200 RPM I'm getting similar top end speeds. I don't expect this thing to be a speed demon but I can't see why I should not be able to break 30 mph on the top end and cruise at mid 20's. Is this unrealistic? What are you other islander guys running prop wise. Anyone got a better suggestion for me on a prop to try?

Re: 19' Starcraft Islander Prop help

I think you have some faulty info my guess is the tach or gear ratio is not accurate.
Its highly unlikely the Solas 4 blade would have slip issues unless something
is disturbing the water in front of the prop.Speedometers are pretty commonly inaccurate too. Parhaps you can borrow a shop tach and a gps.
We also need actual wot wide open throttle rpm and speed.
If it struggles with the 17 it isn't likely to actually overev with the 15.
Some tachs have a selector switch on the back; note the setting and rotate it a few times to clean the contacts and try it.

Re: 19' Starcraft Islander Prop help

Good Call!

I have a multimeter with an inductive pickup that I will verify my RPM with the stock tach. All speeds stated are GPS numbers so I know you are getting correct numbers there. I'll double check my ratio again but I'm pretty certain that it is correct.

I'll double check and post back.

Weird results I agree.