11/29/2023 0 Comments Little rock inboard propeller![]() If your prop has too little pitch, the engine can rev past its red line, which is like driving your car on the interstate in second gear. ![]() ![]() Does Your Engine Over-Rev While The Boat Seems Slow? A general rule of thumb is that a 1-inch pitch change will result in a 175 to 225 rpm change at WOT. Most props can have the pitch adjusted at a prop shop, which can save you from having to buy a new one. If your engine can’t get to within 100 to 200 rpm of rated wide-open throttle (WOT), you probably have a prop issue, assuming your engine runs fine and minimal growth on the hull. Hence, stainless steel props are more efficient. Stainless propellers are five times stronger than aluminum, which is why they can be thinner and still retain adequate strength. Thickness: Blades should be as thin as possible because it takes more power to turn a thick blade. The typical compromise is three blades, although four blades are becoming increasingly more popular. Number of blades: More blades are smoother, but slightly less efficient. Many sterndrives and even some newer outboards have two counter-rotating props. Most outboard and I/O propellers rotate to the right. Rotation: Propellers can rotate to the right (clockwise when viewed from astern) or the left (counterclockwise). The second number stamped on the prop is pitch. But too much or too little slippage reduces efficiency. Without it, the prop couldn't move the boat. So instead of moving 18 inches, a propeller in water moves maybe 15 inches. The reason a propeller moves less than 18 inches is because it operates in a liquid medium, which creates slippage. A prop with an 18-inch pitch would move 18 inches through a solid medium with each complete rotation. Pitch and slippage: Pitch is the distance a prop moves through the water in one revolution. Diameter is the first number listed in a prop size, such as 14 x 17. As a general rule, when performance suffers, you change pitch and not diameter. Diameter: An important factor in thrust - the larger the diameter of the prop (everything else being equal), the more water that gets moved.
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