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I can't remember the exact RPM, I'm thinking right around 5,000-5,100 with the engine trimmed out. The original engine was also installed all the way down which did not help performance. The 19 gave much better all around performance with more than just a person or two in the boat. Both of these were aluminum propellers, the 21 was an older Quicksilver that was on the boat when I bought it, the 19 was a Solas.

I'm currently running a 90 Etec with a Stiletto Advantage II stainless steel 15 pitch and 2 holes up. (It could go up another hole, I just haven't taken the time to do it.) I remember when doing the research it being stated on here that the Stiletto props acted like they were a couple of inches more than the stated pitch. If you can find a 19 pitch Stiletto. it might be a good choice (it will act similar to a 21 pitch). They'll run well with the engine up as high as you can mount it. Beyond that, I'll defer to those that have run the Optimax. Other than the gear ratio, I'd think the Optimax would be a completely different and much improved engine over what I had. The 21 inch pitch stainless steel recommendations above are likely a good match to it. Good luck!