I have a new to me 2001 Sweetwater 20' with a 91' Mercury 90hp. I have had it for a couple of months and the motor runs great, knock on wood, but would only hit 4800rpm at WOT @ 20mph with a 13-1/8x16P prop. I assumed that the low RPM was the over pitched prop and the high slip, +35%, was the small diameter. Last weekend I installed a turning point 14x13P prop and was able to hit close to 5500rpm which is great but only got to 17-18mph which still equates to +35% prop slip. I have extremely high slip at any engine speed and didn't see much difference between the two props outside of sounding different. I now believe my motor may be too low in the water. At the moment the cavitation plate is about 1.5-2" lower than the bottom of the transom plate, I could possibly lower it one more hole if that was the issue. I have liked a YouTube video that has some motor pics as well as video showing the boat running at wot, it's really turbulent back there! If anyone has a suggestion on making motor adjustments or anything else I would appreciate it. Thanks.