Aim for the midpoint or higher of the recommended operating range. Each inch of pitch size will change the RPM by 150-200 RPM. If the RPM is higher, switch to a propeller with higher pitched blades. If the RPM is below the recommended operating range, switch to a propeller with lower pitched blades. To test the prop, run the boat at WOT and observe the maximum engine RPM on a tachometer. Set the trim angle so the boat has optimum speed on top of the water. Test the props under the same conditions that you would typically use the boat-same load, gear, and water. Choose a range of propellers to perform a water test. Once you know the operating range, consult the propeller guide for your engine to pick a combination of pitch, blade number, and material. Refer to your engine operating manual for the operating range. This operating range will translate to the highest horsepower available from your outboard. Choose a propeller that puts the engine RPM at the midpoint or higher of the wide open throttle (WOT) range with a normal load. The propeller choice directly affects the engine RPM and therefore its performance. If you will be using the boat for multiple applications, you may need to switch props. Consider your average speed, load, and boating location. How to Choose the Correct Pitch and Blade Countīefore you dive into choosing a propeller, understand your boat’s intended use. Stainless steel props will outlast aluminum multiple times over. Stainless steel propellers offer better acceleration and are less likely to be damaged from striking objects in the water. Stainless steel is five times stronger than aluminum, dramatically improving performance and durability. Aluminum props are a great balance of cost, performance, and durability. Five blade propellers are good for rough water conditions and tow sports.įinally, material is an important consideration for your propeller. A four blade propeller will improve handling and control for high-performance applications. Some manufacturers also create four and five blade propellers to increase acceleration and reduce ventilation. Three blades props provide the best combination of performance, speed, and efficiency. That’s why three blade propellers are the most common for general purpose boating. However fewer blades will also increase the "vibration" you will feel. The fewer number of blades a propeller has, the more efficient it is. Disturbances cause water bubbles to boil and burst against the blades, which damages the surface and reduces performance. Cavitation is caused by water flow disturbances around the blades. The anti-ventilation plate mounted on the outboard helps to prevent this problem. Ventilation increases RPM but reduces speed, especially during hard acceleration. 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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