Pitch is the number of inches your boat and motor should move forward in the water when that prop makes one complete revolution. Diameter is the width of a circular column of water flowing off the face of the prop.
Props are initially classified by numbers showing diameter and pitch. But PM wanted to go beyond theory and show how different props can change boat performance, so we tested six props (on a single boat) that incorporate a variety of materials and designs, from the most basic to the technologically advanced. Whether you're interested in props with names like "chopper" or "cleaver," or if you just want to know the differences between sizes, materials and trade names, your dealer's an expert. Besides, having more than one prop on board is cheap insurance and good common sense. Choosing a prop with the correct pitch, diameter, rake, surface area and contour will keep your engine within factory-recommended wide-open throttle (WOT) rpm ratings. Fishing, cruising and waterskiing all place special demands and different loads on your engine. Boat Repair: Choosing a Marine MechanicĮditor’s Note: This article was updated in November 2022.Even if you're happy with your boat's performance, you and your family might participate in more than one watersport, and you may benefit from changing props.However, a lower pitch will be the better choice if you load your boat with a significant amount of gear. When selecting a boat propeller, you should go for a higher pitch if you care about boat speed. The best bet is a compromise between these extremes, a prop size that puts the engine in the sweet spot of its RPM range for everyday use. A prop with too much pitch may deliver more top speed because the prop moves further forward with each rotation, but acceleration may be poor, and the boat will struggle to get on plane. Moving up or down two inches in pitch will usually change WOT engine speed by approximately 400 RPM.Ī lower-pitch prop is like low gear in a car or a bicycle-you'll accelerate quickly, but top speed will suffer. If the prop pitch is too high (higher in inches), the motor may run below the WOT range-called "lugging the engine"-which can also put undue stress on the engine and gearcase components. If the propeller pitch is too low (lower in inches), the engine can run over the desired WOT RPM, leading to engine damage or the engine protecting itself with a rev limiter. That RPM may go up with a light load of fuel or passengers and lower with a heavier load-by propping for the middle of the range, there is leeway in either direction.
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Ideally, a motor with a WOT range of 5000-5800 RPM (as noted on the boat tachometer) will reach 5400 rpm with the boat running wide open and trimmed out for optimal performance, with a full load of fuel and water and an average passenger load. Read Next: Boat Propellers: Repairs & Replacements A boat should be "propped" to operate within its wide-open throttle (WOT), which can be found in the motor specifications or the owner's manual. Propeller pitch determines the final gear ratio between the engine and the water. Progressive pitch improves performance in high-speed applications. The pitch number assigned to a progressive-pitch prop is the average across the entire blade. Progressive pitch starts lower at the leading edge and increases to the trailing edge. Some propellers have a constant pitch, meaning the pitch is the same at all points from the leading edge to the trailing edge of the propeller blades. The blades on a propeller are analogous to the threads on a screw. Propeller pitch is the distance the propeller would move forward in one rotation if it were moving through a soft solid-think of a screw being turned into wood. Propeller diameter is simply the diameter of a circle scribed by the blade tips of the propeller. Now that you understand how propeller pitch works, the next step is calculating your boat's propeller pitch. Still, some high-performance props are offered in one-inch increments to allow for fine-tuning boat performance. Recreational boat propellers are usually offered within a prop model line in two-inch pitch increments. These dimensions are often stamped or cast right on the propeller. These dimensions describe the propeller, usually in inches, and are always stated as diameter x pitch.įor example, a propeller described as 14.5 x 19 has a diameter of 14.5 inches and a pitch of 19 inches.
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This post will explain how to calculate propeller pitch and why all boaters should understand thisĪ boat propeller has two basic dimensions: diameter and pitch. In addition, when it comes time to replace your propeller, you'll need to consider the propeller pitch as part of your decision. Even if you own a boat, you might not know why propeller pitch matters or understand how it affects boat performance.
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