Dave Martin
Northern News Services

NNSL (Jun 22/98) - There really is no way to tell which propeller works best on which boat. There are just too many variables in boat and motor combination, and nearly as many variables in propellers.

Let's start with propeller basics. The diameter of a propeller is the outside limit of the propeller's blades describing a circle. Pitch is the theoretical distance the propeller would move forward in one revolution, not allowing for slippage. Propeller sizes are normally expressed by numbers such as 15x17, meaning diameter and pitch expressed in inches.

Aluminum propellers are the most common and have good durability in most cases. Stainless steel propellers are the strongest. A propeller with a clock-wise rotation is known as a right-handed propeller. Counter-clockwise is left. From here on in, things get a little vague.

Which propellers you choose for your boat will depend on the service your boat is likely to see. Generally speaking, the larger the diameter and pitch of the propeller, the more thrust. Shallower pitch propellers and smaller diameters offer higher top speed and generally run at higher engine revolutions per minute (rpm). The Outboard Marine Corp. Web site offers these tips to remember when choosing propellers.

At wide open throttle your engine r.p.m.must be within the recommended operating range with the rated horsepower r.p.m. as the target number.

Increasing or decreasing propeller pitch directly affects engine load throughout its r.p.m. range. High propeller load (high pitch and/or diameter) will result in lower engine r.p.m. Low propeller load (shallow pitch and/or diameter) on the engine will result in higher engine r.p.m. Your boat's hull design may limit its overall performance. Transom height, angle, and engine trim angle all effect propeller performance as well.

The best advice anyone can give is to consult with your local dealer and determine the requirements of your boat. With a little luck, you can come up with a solution that will keep your motor operating efficiently and your boat moving.