How much is the minimum clearance between the tip of a prop blade and the AV plate? If you have something like 1.5 inches or so you have room for a 13+ inch diameter prop. The Mercury 75 hp of that year runs such a prop. Not familiar with that year's similarities between the two badges. I think some Force engines of the later production possibly such as yours use Merc lower units to achieve the thru prop exhausting. Is yours thru prop exhausting? But have no idea as to the gear ratio used.
Next question is your gear ratio. The Merc 75 runs a 2.33:1 ratio meaning the engine turns 2.33 times faster than the prop. That means you will normally run a higher pitched prop for a given rig than you would with a 2:1 which is used in the 115 hp Merc engines for one.
Going to the BAM prop slip calculator and playing with a couple of numbers let's look at both gear ratio's and your indicated speed.
Assuming 5000 rpm which should be the center or so of the recommended rpm range:
Pitch, 14, gear ratio 2:1, estimated rpm 5000, indicated actual speed 33 mph. Assume a prop slip for that rig of 15% with what you said about light loading which is reasonable: Calculator nails it at 33 which is right on what you have.
Assuming a 2.33 gear box with everything else the same you get: 28 mph.
So, just hip shooting you have the first guess of a 2:1 gear ratio.
Now as to the question can you take on more prop and possibly improve your top speed, you will need to get a tach and tell your WOT rpms when you are trimmed out at your 33 mph. If you are short on thrust like your hole shot and have the room then a larger diameter prop can help that....if the engine has the ponies to turn it.
Mark
