By Chuck Fort
Got Juice? Lessons Learned From Your Battery
Selecting the proper prop for the job is a matter of listening to what your boat is telling you
Chuck Fort is Associate Editor of Seaworthy, the BoatUS Marine Insurance damageavoidance publication, which is filled with useful boating safety advice and lessons learned, and free to all BoatUS members. For an insurance quote, please call 1-800-283-2883 or apply online at www.BoatUS.com. (Now you can also follow Seaworthy on Facebook.)
Diameter: An important factor in thrust — the larger the diameter of the prop, everything else being equal, the more water that gets moved. As a general rule, when performance suffers, you change pitch and not diameter. Diameter is the first number listed in a prop size, such as 14x17.
Pitch and Slipage: Pitch is the distance a prop moves through the water in one revolution. The second number stamped on the prop is pitch. A prop with a pitch of 18 inches would move 18 inches through a solid medium with each complete rotation. The reason a propeller moves something less than 18 inches is because it operates in a liquid medium, which creates slippage. So instead of moving 18 inches, a propeller in water moves maybe 15 inches. Some slippage is essential; without it, the prop can’t move the boat, but too much or too little slippage reduces efficiency.
Rotation: Propellers can rotate to the right (clockwise when viewed from astern) or the left (counterclockwise). Most outboard and I/O propellers rotate to the right. Many sterndrives have two counter-rotating props.
Number of Blades: More blades are smoother but slightly less efficient. The typical compromise is three blades, although four blades are becoming increasingly more popular.
Thickness : Blades should be as thin as possible, since it takes more power to turn a thick blade. Stainless propellers are five times stronger than aluminum, which is why they can be thinner and still retain adequate strength. Hence, stainless-steel props are more efficient.