The optimum boat propeller for your particular application is one that will allow your boat's engine - with your normal load aboard - to develop full-throttle RPM within the engine manufacturer's recommended maximum RPM range.
Every engine manufacturer states the recommended RPM range as part of the engine specifications. Damage to the engine can occur if a propeller that allows the engine to over-rev (attain a full-throttle RPM higher than the specification) is installed. Damage can also occur if the installed boat prop causes the engine to lug - not reach the minimum specification. Overall best acceleration and top-end speed will be achieved when your full-throttle RPM is near the midpoint of the manufacturer's recommended maximum RPM range.
Outboard motors 30 HP and higher, and Sterndrives require the most attention to correct propeller selection. Smaller outboards come with an all-purpose size that meets most requirements. An onboard tachometer to monitor engine RPM is also commonly installed with the larger engine installations. Most boats are delivered from the dealership or factory with propellers that have been properly matched for the most common usage. Make note of the part number that is usually cast into the metal itself and you'll be able to order an exact replacement using iboats' Propeller Part Number Finder.
The two important propeller dimensions that you'll need to consider are pitch and diameter. In most cases the boat propeller and engine manufacturers have pre-selected the diameter range needed for your engine. Your selection can be focused almost exclusively on pitch. Your vessel will respond to different pitches the way your car responds to different gear positions in your car's transmission. Moving to a higher pitch will result in a decrease in your full-throttle RPM, just like the effect when your car's transmission shifts to a higher gear. The reverse is also true: move to a lower pitch and your full-throttle RPM will increase. Most propellers are sized by two-inch intervals, example 17", 19", 21", etc, and a 2" pitch change will result in an average 300 to 400 full-throttle RPM change.
It's always a good idea to keep a spare prop aboard, but here's a way to get double-duty from that investment. If your family is into tow watersports, it's likely that your vessel's performance can benefit from a change of 2" less pitch whenever you head out for a day of skiing. The improved acceleration will really be appreciated by the water skier you're pulling. Just don't forget to change back to your usual size standard prop on those days that you're out cruising.
Most boat props constructed of either aluminum or stainless steel and utilize a softer hub material to protect your engine's drive train. Aluminum propellers are less costly, but more prone to damage. Stainless steel propellers are usually about 3 times more expensive than their aluminum counterparts, but far more durable. Some boaters also experience better overall performance with stainless steel because the material's tensile strength allows the manufacturer to cast the blades thinner.