Re: which pitch is what?
In a perfect world, a prop with 10 inches of pitch means the prop will push the boat forward 10 inches with each full revolution. A prop with 20 inches of pitch means the prop would push forward 20 inches. A prop with high pitch (say 20) would provide a faster top speed (roughly double) of a prop with 10 inches of pitch. Here is where it gets a little complicated. Since props work in water, not a solid, there is slippage so one can figure about 10 to 12% slippage. Ok you say -- why not stick a 40P prop on my six horse motor and blow the windshield off my buddies 115 HP motor that only runs a 21P prop. Unfortunately physics comes into play here. The weight of your boat, the load it carries, the type of hull (V-hull, flat bottom, pontoon, cruiser, etc.) and the HP of the engine all determine the prop required. An engine develops its maximum power somewhere in the 5000 - 6000 rpm range. A prop must therefore be selected that allows the engine to run in that range at wide open throttle. Your six HP motor would therefore not get you away from the dock with a 40P prop because it simply does not have enough power to spin it. Unlike a car, a boat does not have a multi-gear transmission so it starts out in high gear all the time. Therefore prop selection is a data gathering exercise. If you have a given prop to start with, you make several wide open throttle runs with a normal load and you not engine rpm and speed. If the engine tends to over rev (revs beyond the manufacturers wide open throttle rpm) you would install a prop with slightly more pitch. Each inch of pitch increase slows the engine by about 200 rpm. If the engine does not reach the recommended rpm, you would decrease pitch slightly. When the engine is running at its wide open throttle rpm, you have what you have as far as speed is concerned. It's all about hull, load, power and RPM. If you tow a lot of water toys, wake board, ski, you would benefit by lowering pitch by one number. Consider it a power prop. But that might make the engine overrev but you can throttle back. Many people have two props. One for water sports and one for cruising.
