A boat propeller has two basic dimensions: diameter and pitch. These dimensions are used to describe the propeller, usually in inches, and always stated as diameter x pitch. For example, a propeller described as These dimensions are often stamped or cast right on the propeller. Recreational boat propellers are usually offered in two-inch pitch increments within a prop model line, but some high-performance props are offered in one-inch increments to allow for fine-tuning boat performance.
Propeller diameter is simply the diameter of a circle scribed by the blade tips of the prop. Propeller pitch is the distance the prop would move forward in one rotation if it were moving through a soft solid—think of a screw being turned into wood.
A higher pitched prop allows the boat to move faster by travelling a farther distance with each rotation. When choosing a propeller, choose a pitch that will keep the engine RPM in its recommended operating range.
Going under the range will cause the boat to lug, while going over can cause engine strain. Blades have other important features built into their design. Rake is the angle between the blade and the hub. The rake angle can be flat, staying the same along the entire blade, or progressive, meaning the angle increases as it approaches the tip.
Props have between 0 and 20 degrees rake. The rake angle determines how much the bow lifts out of the water. High rake propellers are ideal for light-weight, high speed boats. Too much rake, however, will strain the engine and reduce overall performance. Some propellers are cupped, which means they have a lip on the edge of the blades. The cup creates more space between the sides of the blades, reducing ventilation and slippage.
If the cup is on the tip of the blade, the blade has an increased rake angle and a higher bow lift. If the cup is on the trailing edge, it effectively increases the pitch size, and the propeller will create more power at the cost of engine speed. Blades are also designed to prevent ventilation and cavitation. Ventilation is when air is drawn around the blades between the blade surface and the water.
Ventilation increases RPM but reduces speed, especially during hard acceleration. The anti-ventilation plate mounted on the outboard helps to prevent this problem. Cavitation is caused by water flow disturbances around the blades. Disturbances cause water bubbles to boil and burst against the blades, which damages the surface and reduces performance. The fewer number of blades a propeller has, the more efficient it is.
However fewer blades will also increase the "vibration" you will feel. Three blades props provide the best combination of performance, speed, and efficiency. Some manufacturers also create four and five blade propellers to increase acceleration and reduce ventilation. Menu Close. Michigan Wheel. Zimar Zinc Anodes. Tides Marine. SOLAS boat propellers. ACME Marine. Volvo Penta Duo Props. Buck Algonquin Marine Hardware. Inboard Propellers. Outboard Propellers. Stern drive boat props.
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