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Thanks mtown and Finnegan.

mtown, With the 17 I was thinking he would max out his RPM, but maybe his 90 runs a lower gear ratio... If it's like 2.0, a 17 would probably be just right. Mine is a 2.33 ratio.

Finnegan, thanks for all the details, exactly what I was looking for. I'm thinking that if I can actually push one of those 21's at 5750 and 10% slip, I could hit 44. That would be awesome if I can do it, but even if I can turn a 20 that fast I should get 42ish. The boat currently has a 40 and will push it to 27 or 28 on a good day with a 13 pitch on it. Which is like 16% slip, but I think the motor is just way too small, so it's going to slip more.

I will probably go aluminum only because I do tend to do some rivers and explore unknown places, and beach the boat pretty often, so I'm probably more likely to hit bottom than most folks. If I'm more interested in efficient cruise and top end than hole shot, do you think the Opti would turn the 3 blade or 4 blade better, and if I went all the way to 21, would the 3 or 4 blade put me near the top of the RPM range better? I could see how a 4 might get you out of the hole faster but might be harder to turn, but really don't know enough to know better either way.

Also, when you say mount in the 3rd or middle set of holes, I think mine may be old enough that it might not be the same hole number on the top and bottom set of holes. Do I want to be 2 holes up from having the mount flush to the transom, or 1 hole off the transom? I think the way my holes are drilled, I can't really mount it flush, but the current motor is pretty close to where 1 hole up (maybe 3/4" off the transom) should have been.

Sounds like I should just go for the 21" black max as my best first shot, but if you think the 4 blade aluminum might be a better first choice, maybe I should try that. Any advantages of one over the other?

Thanks again!