The blade area and pitch have as much (or more) to do with how the horse power is translated to water speed. You are worrying about gear ratio, but that works only if you use the same propeller. Generally speaking, the smaller gear case will need a higher pitch propeller when used on the same boat. (If it was running at the proper RPM with the 10/24 gearcase.) You can only find out by trying it. If the motor runs too fast, you can gain a half inch of pitch by cupping the blades (If brass or stainless steel) … Good Luck.
