comprop size

			PROPELLER SIZE SELECTION WHY & HOW
Why worry about propeller size? Not only does your propeller push your boat, it also functions as a single speed transmission. Your boat is not like a car with many "speeds" or "gears" that can be used while underway. The propeller size (diameter and pitch) determines the "gear" you're in all the time - whether pulling a skier, pleasure cruising, or going for all out speed. Most boat manufacturers use a simple performance test that determines the correct propeller size for almost all applications. It is explained below. We have also included a few definition and hints. If you have further questions call us.

You need to know three things:
	The size propeller (diameter and pitch) currently on your boat.
		You can sometimes determine this if you have the propeller's model number of some other identifying marks on the propeller.

	Your engine's recommenced RPM limit (red line).
		This information is in your engine owners manual. Common RPM limits are: sterndrives 4200 to 4800 RPM; outboards 5000 to 5800 RPM.

	Test your current propeller. How much above/below are your engine's RPM's vs red line?
		This test should be performed at full throttle, full trim and with a light boat. The propeller should be in good shape. Don't count on what you remember from last season, perform the test before you buy any propeller.

Interpreting the results If your propeller is the proper size for your boat, your actual RPM during the above test should be within 100-200 RPM of your engine's RPM upper limit. You want to be able to develop full power without going over the limit. A correct sized propeller on your boat will let you run at full power but will not allow you to exceed the RPM limit. A general rule of thumb is each inch of pitch change will result in a change of 150-200 RPM. If your boat test shows you need higher RPM's, then select a propeller with lesser pitch (shifting down). If your test shows you are over the RPM limit, choose a propeller with a higher pitch (shifting up). Examples: A 21" pitch propeller turns 4200 RPM on your boat, but 4600 is the upper RPM limit. Try a 19" pitch propeller to "shift down" and increase your RPMs by 300-400. If your outboard is turning 6100 RPM and your red line is 5600, "shift up" and try a propeller with 2" more pitch. Propellers are labelled diameter x pitch. A 13x19 propeller is 13" in diameter and is 19" pitch. Diameter - the distance (in inches) across the circle made by the tips as the propeller rotates. Pitch - the distance (in inches) a propeller moves forward in one 360deg revolution (assuming zero slip).