Ventilation is when air is drawn around the blades between the blade surface and the water. 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.īlades are also designed to prevent ventilation and cavitation. If the cup is on the tip of the blade, the blade has an increased rake angle and a higher bow lift. The cup creates more space between the sides of the blades, reducing ventilation and slippage. Some propellers are cupped, which means they have a lip on the edge of the blades. Too much rake, however, will strain the engine and reduce overall performance. High rake propellers are ideal for light-weight, high speed boats. The rake angle determines how much the bow lifts out of the water. Props have between 0 and 20 degrees rake. The rake angle can be flat, staying the same along the entire blade, or progressive, meaning the angle increases as it approaches the tip. Rake is the angle between the blade and the hub. Going under the range will cause the boat to lug, while going over can cause engine strain.īlades have other important features built into their design. When choosing a propeller, choose a pitch that will keep the engine RPM in its recommended operating range. A higher pitched prop allows the boat to move faster by travelling a farther distance with each rotation. A lower pitched propeller will create more power due to more engine RPMs, but the boat will move slower. Pitch is the distance the propeller will move the boat after one revolution (not accounting for slip). Your prop selection will be focused on the pitch.
Diameter is predetermined by the manufacturer for a family of products. A large diameter propeller can push more water than a small diameter propeller, creating more power. Diameter is the overall size of the blade as measured from the tips. Those numbers are the diameter and pitch. All props will be described by two numbers, such as 13 x 21.
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