Are you interested in the science behind outboard propellers? If so, this article is for you.

Genuine Yamaha, Mercury, Solas, Acme, and Michigan Wheel propellers are engineered to enhance the performance of your outboard. Every inch and angle—even its material—makes a difference in the way outboard propellers influence performance. Getting to know the specs of an outboard propeller will help you to select one that will help to harness the true power of your outboard.

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Propeller Engineering Terms

Diameter

This is the total width of the “circle” at the blade tips as the outboard propeller spins. The proper diameter for each propeller model is determined by the propeller’s design and intended application.

A larger diameter pushes more water and reaches deeper into the water; so they’re typically used on large, heavy boats or ones with high engine-mounting heights. A smaller diameter is usually used on lighter-weight boats, where the propeller operates lower in the water or when a gain in engine RPM is desired. Within a particular propeller series, as the diameter increases, the pitch generally decreases.

Propeller pitchOutboard Propellers Pitch

Pitch is the distance a propeller would travel in one full revolution as if traveling through a solid. Each inch of pitch is equal to approximately 150 RPM, plus or minus 50 RPM. The right propeller will allow your engine, under a normal to heavy load, to reach the upper portion of the WOT range specified by its manufacturer without exceeding it. A lower pitch will have greater acceleration and “pushing power” but a lower top speed. A higher pitch will provide less acceleration, but a greater potential for higher top speeds. Within a particular propeller series, as the diameter increases, the pitch generally decreases.

Propeller rake

Rake is the angle of the blades in relation to the propeller’s barrel, or center, and is expressed in degrees. Boat propeller rake 2 A high-rake propeller is best suited for high engine-mount applications. It reduces ventilation and increases bow lift. However, too much rake can negatively impact handling and performance by straining the engine, which will decrease hole shot. A low-rake propeller causes less strain on the engine, resulting in a potentially better hole shot and higher wide open throttle (WOT) operating RPM.

Propeller size

Outboard propeller size is characteristically expressed as its diameter and pitch, in inches. The diameter is the first number. Pitch is the second. So a 14″ x 17″ prop would measure 14″ in diameter with 17″ pitch. This same propeller may be expressed as 14″ x 17″x 3, which would indicate a three-blade design.

Propeller cut

Cup is the small curved lip on the blade tip and/or trailing edge. In proper amounts, cup reduces ventilation and slippage, allowing for higher mounting heights and greater bow lift. However, too much cup can cause excessive steering torque and bow lift and limit the engine’s ability to develop and maintain proper RPM.

Propeller blade geometry

Blade geometry refers to the actual shape of the blade or “ear.” By manipulating the blade’s shape, diameter, and pitch progression, different performance characteristics are created for each different type and style of propeller.

Propeller blade surface area

The more blade surface area a propeller has, the more water it pushes. This results in a better hole shot and increased planing efficiency. However, too much blade surface can create significantly more drag, potentially restricting engine RPM and cause negative boat handling issues.

Number of propeller blades

Three Blades Are the most common propellers. They offer good overall performance, top speed, and efficiency for most applications. Four Blades Characteristically provide increased acceleration, enhanced bow- and stern-lift, and reduced ventilation. However, their increased surface area can create drag on the engine, resulting in lower top speeds and potentially different handling. three and four blade boat propellers Ventilation is when air is drawn in around the propeller blades. Normally, this increases RPM, but lowers speed, because the propeller blades aren’t biting “clean” water. This usually occurs during hard cornering or in certain water conditions, such as following seas. Controlled ventilation can be beneficial, helping the engine gain RPM during hard acceleration. Engineered into certain propellers, usually two strokes, ventilation can appear as small holes in the side of a propeller barrel. These allow exhaust to intentionally be drawn in around the blades at hole shot, helping two-stroke engines generate the higher RPMs they need for proper hole shot performance. Four-stroke engines typically don’t use built-in ventilation.

Propeller cavitation

Cavitation occurs when pressure on the water across the blade’s surface is reduced to the point of becoming water vapor, forming bubbles. If these bubbles burst, they can cause a “cavitation burn” which can deteriorate the propeller’s surface and cause negative performance issues. Because this condition can cause an increase in engine RPM, it’s often confused with ventilation.

Propeller slip

Slip is the amount of “wasted” energy a propeller generates. In other words, the actual distance traveled in one full propeller revolution is less than its pitch measurement. Slip is usually expressed as a “percentage of inefficiency”. Though this may not sound like an advantage, a certain amount of slip is engineered into each line of propellers. It helps to create different performance characteristics.

Hole Shot

Hole shot is the rapid acceleration of the boat, from a standing rest or very slow speed until just on-plane. This is when the engine and the propeller work their hardest.

Gear ratio

A marine engine’s gear ratio refers to the gears used in its lower unit. It’s important to choose a propeller that allows the engine to operate within the manufacturer’s recommended wide-open throttle (WOT) RPM under normal loads and conditions. The higher the ratio, the more pushing power the engine will produce. The lower the ratio, the more top speed the engine can generate.