20 foot pontoon 1988 85hp force motor prop question

[busted-bayliner]

Ok guys i got this 03 20 ft pontoon with a 1988 85hp force motor on it. The toon didnt come with this engine but i got it this way. Ive took the prop off and can not see any markings what so ever. Its a 3 blade, one blade is missing about a inch of the blade. So my question is what prop should i be running?? Going to get a new one. I use the boat for fishing and pulling tubes any help is greatly appreciated like to get the most out of the engine..

 

[steelespike]

A total guess would be an 11" pitch.There is a 4 blade available here.
A 4 blade will grip better and control better around the dock.
If your messing with props you need to have a working tach.
Its important the motor run within its rated rpm at wot.
You can run at what ever rpm you want once propped correctly.
If the 11" over revs you can just throttle back a little.
Your better off a little under propped until yopu can get a prop.

 

[busted-bayliner]

A total guess would be an 11" pitch.There is a 4 blade available here.
A 4 blade will grip better and control better around the dock.
If your messing with props you need to have a working tach.
Its important the motor run within its rated rpm at wot.
You can run at what ever rpm you want once propped correctly.
If the 11" over revs you can just throttle back a little.
Your better off a little under propped until yopu can get a prop.

the motor is rated at 4500 rpm to 5500 max the bot does have a working tach if this information helps any

 

[buckinrut25]

I would take the prop to your local boat shop and see if they can match it up. If the engine was on the boat when you got it hopefully the previous owner did some of the work to set it up. In any case it will beat a pure guess. I just went through about the same thing and my local boat guy was willing to loan me a prop to test and get the nessasary info to order the correct prop. With any luck you may be able to do the same.

 

[jestor68]

My 90/24 ft pontoon turns a 15 pitch at 5300 rpm. Your 85 on a smaller boat should be able to do a 13 pitch.

 

[Jeffromxer]

I have a very similar setup as you do. 97 Fisher 20 ft toon with force 75. Mine has a working tach and I just got done propping it. When I got it it had a 12 1/4x9 prop on it. It was underpropped as it would rev to 6k rpm wot. I now have a 11.6x12 on it and it tops out at 4900 rpm and works nicely. My max rpm is 4750-5250. If I were you I would get either an 11 or 12 pitch. As far as diameter goes, you are at the mercy of the engineers who make the prop that will fit your motor.