correct propeller

[vanstee]

I have a '88 Donzi Ragazza 17' with a 3.0L OMC on it. The boat seems extremely slow to get on plane and I'm wondering if I have the correct prop on it. It currently has a 14-19. I see in Overtons Michigan Propeller makes a 13-19. Would that help? It's also only WOT'ing at about 4200. Thanks!

[MOP]

I would try to borrow a 17 inch pitch prop, I think thats a better route. Try for 44-4600 RPM.

[BUIZILLA]

Keep the same prop, and find another 3 liters to turn it.. :D

J

there is NO substitute for cubic inches :)

[vanstee]

I'll try that. Any idea where I can get some performance spec's from ? I'm only getting about 36 MPH out of her. My parents have a bayliner about the same age and getting about 43. True it's a lighter boat but I'm also 2' shorter.

[Rootsy]

to begin with, what is the WOT operating range of the 3 litre according to OMC? This is important for you want to operate in that RPM range and not lug the motor (less rpm) which is actually worse than over revving it a small amount. Once you know this information you can begin propeller selection given your current baseline. This is assuming that your motor and drive are up to par on power and performance... without that none of this amounts to a hill of beans.

Secondly, what are you trying to accomplish? skiing? Holeshot? Top End? Cruising efficiency? you have to determine this so you can select an appropriate propeller.

by reducing diameter you will increase rpm but you seriously run the risk of excessive slippage due to that small a diameter not having enough blade area to carry the weight it is trying to push. 14 inches is probably just about as small (for a 3 blade) in diameter as you want to go in this situation, with a sterndrive, especially at those rpm's.

Apples to apples (same propeller of the same diameter etc) for every inch of pitch you decrease you will gain approx. 200 rpm. The same holds true for i believe every 1/4 to 1/2 inch diameter decrease. the opposite holds true if you increase pitch and diameter.

The smaller the pitch number the better it will be getting on plane... but you will turn more rpm and you should be at roughly the same top end speed as the larger pitch prop. Midrange speed will decrease for a given rpm though.

ultimately many factors other than hp and prop determine top end speed and cruising efficiency. Bottom condition, cleanliness, paint or lack of, Lift of the propeller on the hull and trim. the more hull that is out of the water and the cleaner it is the less drag there is.

answer a few of these and you will be further on your way to getting her propped correctly plus we can offer more suggestions and assistance :)

[BUIZILLA]

You should have the Cobra drive with 1.84 gears...

I have the SAME drive. You need a 15x17 prop in aluminum or a 15x16 in SS.

Wheel the engine to 4500-4600 with 2 persons and full fuel.

Hope this helps.

J

[vanstee]

I guess skiing primarly or just pleasure cruising with 4 people in the boat. The WOT range for the boat is 4200 to 4600 per the engine's manual. With that range I should be able to drop down to a 17" pitch and stay in that range. The way it's set up now, if there is more than 2 people in the boat while I'm trying to ski, I really struggle unless the person not driving is sitting clear up in the bow. What's your thoughts on a hydrofoil?

THANKS!!!

[BUIZILLA]

I used the plastic DolFin when I had the V6 in the boat and it was worth doing...I bolted to the top of the cavitation plate with 2 screws on each side. Used it for 8 years like that. It was a MUST when trying to pull skiiers.

J

[vanstee]

Tried the hydrofoil and works GREAT! Rechecked the RPM's and only getting 4000. Switching to a 14-17 or a 14-15 4 blade comprop. Any further suggestions?

[MOP]

A lot of people who put the fins or plates on experience RPM lose from the drag that it causes when the cav plate is low or near the bottom. On a few I have seen them cause the stern to go down when you trim up, like a dive plane. I have raised them up many times a 1 to 2 inches using ½ blocks of plastic. That usually gives you back the RPM lost and helps with the diving problem. You are much better off with trim tabs, they will give even a little more lift and a lot more overall control of the boat. Even if you just get the fixed type they will work better than anything mounted on a lower unit any day, I have always felt fins add a little stress on the drive.