advice on the correct prop for my 1976 85hp mercury outboard

[celacross]

hello everyone, I'm new to the forums and have a couple questions that hopefully someone can help me out with. I bought a used 1976 85hp mercury outboard and it doesn't have a prop on it. My old caravelle boat currently has a 65hp 3 cylinder mercury outboard on it and I'm at a loss as to what size and pitch prop I will need to get for the 85hp. My boat is a 17' older caravelle that is fiberglass and I'm guessing weighs pretty close to 1000lbs. I won't be using it as a ski boat like in years past but rather fishing and mainly trolling at 2-3 mph. Any suggestions on a prop that will get me some one slow speeds and not over rev the motor? The 65hp worked perfectly speed wise and all but it needs a new distributor ( I dropped it and broke the rotor off). Also I'm curious as to if a midsection off an inline 6 will fit on the 85hp? I don't have power tilt and trim, but have found a really good deal on a lower unit and midsection with the power already on it and a pump with it also. Thanks in advance!

 

[jimmbo]

9 times out 10, a 15 in pitch will be what you need for your setup. A 13 would be slower but would probably result in over revving A tachometer will be needed to determine if it keeps the engine in the proper rpm range 4800 - 5500. As for your midsection question. It would be less work to transfer the power trim cylinders to your 85 than moving the power head, exhaust tuners, and gearcase to the inlines midsection, of which I don't know if the bolt patterns match.

BTW, the props from your 65 will not fit the 85

 

[Texasmark]

9 times out 10, a 15 in pitch will be what you need for your setup. A 13 would be slower but would probably result in over revving A tachometer will be needed to determine if it keeps the engine in the proper rpm range 4800 - 5500. As for your midsection question. It would be less work to transfer the power trim cylinders to your 85 than moving the power head, exhaust tuners, and gearcase to the inlines midsection, of which I don't know if the bolt patterns match.

BTW, the props from your 65 will not fit the 85

Didn't both those engines have the 4" gearcase?

 

[Texasmark]

I think you need to start with a tach and GPS speed source and a "guesser" for a prop. Best thing to do is go aluminum and go to a prop shop that will let you test before you buy. However since your usage is what you said, it really doesn't matter all that much and I agree with jimmbo for a starter.

I had a mid 70's 85hp 850 on a 16' Glass bass boat running a 13+ x 19. Ran out about 39 as I recall on a SS Merc prop. Not sure which part number or if it had a name like Laser for example. I remember the speed because I dearly wanted 40 mph and it just wouldn't get there without taking a lot of my gear out. At the time I had no tach and an after market speedometer but I did have PTT, the one with the pistons outboard on each side of the clamp brackets..

 

[Chris1956]

1970s Inline 4 and 6 cylinder motors share props and can share gearcases and midsections. They also share PTT units.

 

[jimmbo]

Didn't both those engines have the 4" gearcase?

A 4 cylinder 65 did, the 3 cylinder 65 uses the same gearcase and props as the 50hp

 

[Texasmark]

A 4 cylinder 65 did, the 3 cylinder 65 uses the same gearcase and props as the 50hp

I asked the question because he is talking about a 4 cyl cross flow 85 which I had and thought I recalled running a 13+ " dia prop. But Chris answered my question anyway. Thanks.