Re: will a 1985 115 hp mercury lower unit fit a 74 850 mercury? mer
Yes, the propshaft splines are identical and your 85 hp prop will slide right on the propshaft of a 115 hp Inline Six lower unit.
The issue we're attempting to point out is that the difference in gear ratios between the Four and Six gears will affect the way your motor runs.
To use an analogy to a motor vehicle, imagine if you're running around in 4th gear all the time. She runs up just fine and everything is the way you like. Now, all of a sudden you shift up to Overdrive 5th and the motor won't pull this hill or that.
The same thing would happen with substitution of the 6-cyl L/U for the 4-cyl's.
Now, if you think about it, you could get back the same level of performance in 5th gear of your car as you had in 4th, if you put smaller tires on it.
Propeller pitch is analogous to tire diameter. Prop pitch is the theoretical distance a propeller will travel in the water, with one revolution. We'll just concentrate on that, and not talk about "slippage" which of course happens or you'd never get the boat up on plane!
So, a 19" prop will travel a [theoretical] distance of 19" thru the water for every revolution.
You can see that if you switch to an "overdrive" lower unit you've just changed the "final drive" ratio same as if you shifted to 5th gear.
However, a boat motor doesn't have multiple speeds forward, you're always in the same 'gear'. So by going to an 'overdrive' lower unit you've just increased the load on your motor, and likely it won't have the power to pull the 'taller gear'.
So, you decrease the 'diameter of the tire' by getting a prop with lower pitch. Typically the gearing change we're discussing would require a prop with 2" less pitch, just to get back to the same 'final drive ratio' as you had before the swap.
This assumes that the prop you're using was correct for the boat in the first place. So what we really need to know is what's the model of your boat, how long is it, does it have a relatively 'flat' bottom or is it a 'deep vee'.
All these things (including boat weight and weight of driver, pssgr's, fuel, etc) affect performance and prop recommendations.
The only 'real world' way to figure this all out is to take the boat out, just the driver, trim for maximum speed ('trim' being the tilt of the motor, either by power trim or what hole you have the tilt pin in), and see what rpm you achieve at Wide Open Throttle.
Your motor should run up approx 5500 rpm at W.O.T. to be in the correct power band. If the prop is too big, the motor can't wind up to the rpm where it makes its HP and you've just overloaded it. Just like driving around town in 5th with too-tall tires, lugging the motor all the time.
Props sizes are expressed by diameter, then pitch. A example would be "13x19" which would be a 13" diameter prop, 19" pitch.
Unless your boat is short and light, a 19" prop is pretty big for an 85hp. Thus the other poster's concern.
At any rate, hope this helps with what you're trying to do.
Maybe it'd be easier to find another lower unit from a 4-cyl "800", "850", 80hp or 75hp, with the same gear ratio as your motor's L/U???
Just a thought!...........ed
