See, now that tells us a lot. You have a small range of 500 rpms to work with, that is the difference in your 5250 and 5750. The middle would give you a range of 5500 to shoot for. Based on approximately 1inch/400 rpms, you would have to go down approximately 3 inches in pitch to raise you to about 5400 rpm. Since you are at 21 pitch, you would need about an 18pitch. Thats a good place to start especially since you have a heavier deep V boat. Try an 18 and if rpms are too high then go to a 19 and vice versa. Cupping or removing cupping can adjust your rpm a bit more if you like. Just get it in that range and you will be good. 5500 would put you right in the middle of the recommended range. Getting it to run about 5750 would allow you more torque to push heavier loads, you know...buddies, coolers, rods, anchors, kicker, full tank of gas, etc. Go with aluminum for now. You may be able to sell or trade your existing prop or keep it as a spare but take out the cupping if you do to raise your rpm, Cupping adds about 2 inches of pitch, which I didn't mention but by now you probably figured out. Once you get it dialed in then you can go to stainless as your primary prop, They are more fuel efficient but you may not notice the mileage but you may pick up 1-2 mph which again equates to more speed because of effiency. Stainless blades are thinner and stronger. Aluminum blades actually flex a little under heavy use. Dont believe it? Remember that skyscrapers bend in heavy winds. Your diameter on the 18 may go up to about 14.25. You wouldnt notice unless you looked. That brings us back to the "loading up the motor" factor. Now you should pretty much have your answer to what prop to use, or at least get you very close. By the way, where and what are you fishing for. Great lakes fishing may keep you from hitting bottom with the prop, etc.