Please Help Searay 220 DA

Please can someone help, i own a 1991 searay 220 DA with a 4.3 mercruiser and an alpha 1 leg i believe my prop is the wrong one for the boat it is a 5 blade stainless steel and gives me approx 8-12 knts at 3000 rpm and a max of 18 to 20 knts at 4000 rpm the boat is supposed to cruise at 20 knts with a max of 35 knts, i cannot get anymore out of the engine as it 4000 - 4200 it starts holding back and is maxed out, it revs freely past 5000 rpm out of gear and seems to run fine and tick over fine. can anyone tell me what prop i should have on this boat so i can go and buy one.
I have e-mailed Searay but no responce.

 

To start with, a 4.3 in a 22ft searay will be a bit of a pig regardless. You should have a bit more speed though. What size is the current prop? (It's stamped on the hub, like '15x19' or something like that). Don't rev it like that out of gear, you could damage it. My first guess would be to go down 2" in prop pitch. A set of hydraulic trim tabs would also help if you don't have any. Try toying around with the trim a bit too; have it full down when you first ram the throttle forward but then gradually trim up once you're on plane. Drag on the Alpha will fall off with some positive trim and you'll rev higher.

 

Thanks Matt i do have trim tabs and have tried every option, its useing these that give me the speed ive already got, Searay have now come back and stated i should have a 14-19 prop so i will have to check whats on it.
it also had another prop on board which i thought might have been the original but when fitted it gave me 14knts max?