[FONT=Arial, Helvetica, sans-serif]The diameter of a prop decides how much water you will disturb while the pitch determines how quickly you push the water displaced out of the way. A prop's pitch is similar to an airplane wing as well, with high and low pressure sides. So a prop with a large diameter prop with no pitch would not displace any water until pitch is introduced. The boat moves because of a combination of the differing pressures and Newton's Law that every motion has an equal and opposite reaction.[/FONT]
[FONT=Arial, Helvetica, sans-serif]So if you have a barge or a sailboat (neither are going to get on plane), then you want the biggest diameter you can swing with the flattest pitch. This allows the prop to take advantage of the torque the engine develops. For these types of boats the important thing is to have as much power (thrust) available for use, rather than trying to develop speed[/FONT].
[FONT=Arial, Helvetica, sans-serif]If you have too big of a prop you won't reach maximum RPMS. This relationship is the same at whatever RPM you are operating at, but is easiest to see at full RPM where the torque, horsepower and fuel consumption efficiency curves cross and your engine is working harder than it should to reach any RPM. It is inefficient and in the long run (very long run) can damage your engine. You'll have a terrific hole shot, but won't be able to run at full RPM-- like driving a car uphill in fourth gear when you should shift down to third.[/FONT]
[FONT=Arial, Helvetica, sans-serif]If you have too small of a prop it means that you can go over your maximum RPMS and that your engine over-revs to get to a specific speed. It also means that you may have trouble getting out of the hole or it may take forever to get up to plane and you won't be able to go very quickly as you will have the prop turning so quickly that cavatation develops. Cavatation destroys the wing lifting component. (You can actually get the water boiling on the low pressure side.) It is like driving down a hill in third gear when you should be shifting up to fourth gear.[/FONT]