Propeller questions on 85 Force 85hp

[ToonTy]

Hello All!

I swapped out a 40 hp Mercury/Force motor on my 24' pontoon for an 85 hp US Marine/Force this spring and I finally have it running well. That said the motor came with a 21 pitch and a 19 pitch propeller. Originally it had the 21 pitch on it (admittedly I thought it was a 17 pitch at first) and at WOT I could only get about 3000 rpms. If my math is correct I need to get down to a 13 or 11 pitch prop to get my rpms up in the operating range to fully maximize my motor. Does my math seem correct? I put the 19 pitch on it yesterday and plan to see how many rpms I pick up today.

Where can you find an 11 or 13 pitch prop for this motor? I think I found a 13 pitch on eBay for $150.
Also, do you think a 4 blade would be good given my large pontoon that weights a considerable amount? Do they even make a 4 blade for this motor?

 

[jerryjerry05]

They do make a 4 blade.
But before you spend $$$$$.
I'd send the specs to Michigan Propeller
They can tell you what pitch you need and can sell it to you too.

You'll probably need a 12-13 p ?????

 

[ToonTy]

I took the boat out on Sunday and with the 19p prop it turned about 3600-3800 rpm depending on if I was going with the current or against it.

I will give them a call, but is the name Michigan Wheel because I didn't find a Michigan Propeller?

My guess is that a 13p prop will be about perfect for getting me in the operating range for rpms at WOT.

 

[ToonTy]

I installed a 14 x 13 prop on the motor yesterday and took the pontoon out for a run. At WOT it was between 4800-5000 rpm which is right in the sweet spot of the operating range. It was also able to push my heavy 24' pontoon 22mph which I am pretty happy with. I can set at 3600-4000 rpms and the motor just hums along nicely at about 16-17mph. This is a huge improvement from my old 40hp mercury force. Getting the motor running right and the right prop for your situation is important!

 

[ToonTy]

I do now have a used 21p prop in good shape and a basically new (only used twice) 19p prop for this 85 Force 85hp motor. Any suggestions on where to sell them? I could put them on craigslist and/or eBay I suppose.

 

[jerryjerry05]

E-bay or Craigslist.

You still have another 500 rpm's not utilized?
Wide open 5200-5500

I had a 21 with twin 88/85s
Came with 17p.
I wanted more, speed and performance.
Changed to 19p.
Got more, 2-3 mph at wide open(I never run wide open) and
my fuel economy went down.
Used about 15-20% more fuel.
I changed back.

 

[ToonTy]

I found a refurbished 13p prop on eBay for $110 shipped to me. I was really struggling finding anything lower pitched. It gets right up against 5000 rpm which is the meat of the operating range and where the engine is rated for 85hp. I am happy at this point.

I too do not run it at WOT hardly at all. I like to cruise between 3600-3800 rpm.

Before I put the 13p prop on I would have black oily residue drip out of my exhaust port after running the motor for the day on the water. Now with the 13p prop I pull the pontoon out of the water, park it next to my garage, and the exhaust port doesn't drip at all. I hope that is a sign that the motor is actually burning the fuel/oil mix as it should now that I can actually obtain rpms within the operating range at WOT.

 

[jerryjerry05]

The older 2 strokes leak.
It's usually oil coming from the carbs and working it's way down.

If you leave the motor tilted up while parked? It gets worse.