Re: 1982, 90hp merc - 36mph at 4900rpm - can i get more?
It's doing pretty good pulling a 21 pitch at 4900 rpm. I bet you could pick up a few rpm and speed with a 19". Worth a try, anyway. If you have a cooperative propeller shop nearby they may let you try a prop and return undamaged for a different pitch, if it doesn't work out.
BTW if you like kneeboarding you'd love wakeboarding, it's way more fun and easier on the knees! I'm 54 and still on the board, I need to get back in shape if I want to get up on a slalom ski again but can still ride the wakeboard.
We used to kneeboard as a diversion, if you have the type with an adjustable strap across the board, you can strap both legs in, then deep-water start that way. This works best on the hollow-type boards that tend to float a bit. It's fun jumping the wake but my knees can't take that anymore! Nice thing about the wakeboard is you're going much slower than a slalom ski, so it don't hurt so much when you crash! And lots more planing surface, you'd pop right up without too much strain.
For more serious watersports, you might want to drop down to an 18 pitch, which might work out better than a 17, since a 17" pitch could be a touch too small for the motor and limit your top speed. It'll pull like a tractor out of the hole, however, with the lower pitch.
It all depends on what this particular motor "wants" and the only real way to know is to experiment with different props. Every one is going to behave slightly different due to changes in pitch, rake, cup, diameter, etc. You can even have two props of the same size from different mfr's run differently. When you find one your motor really likes, you'll know!
Anyway, have fun in the water, I'm certainly looking forward to getting wet this weekend!....ed
