pontoon prop

[59Hustler]

I am repowering my 20 foot 1999 Monark pontoon boat with a 1989 Force 85 HP, It currently has a 1999 Force 40 hp on it both have power tilt and trim.
The new motor comes with a Michigan prop P539 13 1/4 x 17 stainless prop 3 blade. I think that this prop will probably be too much prop for this pontoon. I am at 1445 foot elevation in central Texas. The pontoon probably weigh about 2500 pounds.
I cannot find pontoon boat recommendations on any of the prop web sites. What size prop would you recommend for this engine the boat is rated for a 90 hp engine.

 

[harringtondav]

Trial and error may be your best bet. See how your pontoon performs with that prop. You'll need a tachometer. If the boat isn't equipped with one, photo tachs are fairly cheap, and useful to have in your shop.

The rule of thumb is 200 rpm change for each 1" change in prop pitch at WOT. Adjust your prop pitch until you hit your target. I think O/Bs are in the 5000-6000 max rpm range.

 

[59Hustler]

Thanks
I have a tach on the boat now and am not sure if it will work on the new motor. I found out that the earlier Forces use a 20 pole stator so I may have to buy a new tach. I may get a photo tach just to have around.

 

[ahicks]

I think the 17" being too big is a safe bet, but it's not so far off you can't use it's rpm data while running that prop to help get the right one. The question will be how much too much? The only way to figure that out will be with reliable tach readings. 13" or 15" could both be what you end up with.

 

[59Hustler]

I still do not have the 85 installed yet. A lot of little issues and life got in the way. I have installed a tach made for the 85HP Force. Changed the impeller, plugs and rebuilt its carbs and fuel pump. I am just waiting for some additional help in removing the old motor and installing the new. I can't lift 255 lbs by myself.

 

[JerEazy]

IMO 4 blade props do wonderful on pontoons - i am a big fan of the Spitfire props.

But i'll echo everyone else so far - get the engine on there and get it WOT and see what the RPMS are at. After that you'll know more.

 

[ahicks]

Local rental company with a "cherry picker" maybe? Or, if you might use it a few times and have a place to keep one, Harbor freight has one 1000 lb capacity that you can get for 100 bucks (including 20% discount) if they're on sale.

 

[59Hustler]

I got the 85 up on an engine stand today. I plan on taking the 40hp off tomorrow. My brother brought over his engine hoist t but isn't able to help for a few more days do to recent surgery. so tomorrow the 40hp will come off and go on my 1959 Hustler runabout. its old 1960 merc 40 will go on a stand as will the 40 hp merc on my 1959 red fish. Then I can rebuild both of those.

 

[59Hustler]

I finally got it in the water this week and max RPM is 3700. I have ordered a Piranha prop with a set of 13 p and a set of 15 p blades. It should be in tomorrow. I was having problems finding props for an 85 HP Force with less than a 17 P I found a few but Piranha will let me return the blades that don't work for the correct size or if they both work I may keep both sets if the 15 p gives me at least 4500 and a higher top speed use it when I am all by myself or just my brother and use the 13 P.