- How propellers work
- design considerations
- How to choose the right pitch and number of sheets
Choosing the right propeller from Crowley Marine
The right propeller gets the best performance out of your boat. Matching the propeller to your application, be it cruising, waterskiing or performance, will maximize the performance of your engine. It also helps with handling and acceleration. Use this guide to learn how different propeller designs affect efficiency, power and speed, then learn how to choose the right propeller for your boat and application.
How propellers work
The propeller has a hub surrounded by blades of equal length and size. The blades are twisted so that when they spin, they push water back. This thrust propels the boat forward through the water. Most blades rotate clockwise (clockwise) when viewed from the rear. Left-hand propellers are combined with a right-hand propeller on twin-engine systems to keep the boat stable and reduce steering effort.
All props are described by two numbers, e.g. B. 13 x 21.These numbers are the diameter and the pitch.The diameter is the total length of the blade measured from the tips. A large diameter propeller can push more water than a small diameter propeller, producing more power. The diameter is specified by the manufacturer for a product family.Your accessory choices will be pitch-focused.
Pitch is the distance the propeller moves the boat after one revolution (ignoring slip). A lower pitch propeller produces more power due to higher engine RPM, but the boat moves slower. A higher pitch propeller allows the boat to move faster by covering a greater distance with each revolution. When selecting a propeller, choose a pitch that will keep the engine RPM within the recommended operating range. Going under the band will drag the boat, while going over the band can strain the engine.
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 and remain the same throughout the length of the blade, or progressive, meaning the angle increases as you get closer to the point. The supports have between 0 and 20 degrees inclination. The pitch angle determines how far the bow comes out of the water. High-pitch propellers are ideal for light, high-speed boats. However, too much rake will overload the engine and reduce overall performance.
Some propellers are concave, meaning they have a lip on the edge of the blades. The shell creates more space between the sides of the blades, reducing ventilation and gliding. When the cup is at the tip of the blade, the blade has a larger rake angle and nose lift. When the cup is on the trailing edge it effectively increases pitch size and the propeller produces more power at the expense of engine RPM.
The vanes are also designed to prevent aeration and cavitation. Deflating draws air around the leaves between the surface of the leaf and the water. Ventilation increases revs but decreases speed, especially under hard acceleration. The anti-ventilation plate mounted on the outboard helps to avoid this problem. Cavitation is caused by disturbances in the flow of water around the blades. Disturbance causes water bubbles to boil and burst against the blades, damaging the finish and reducing performance.
The fewer blades a propeller has, the more efficient it is. However, fewer blades will also increase the "vibration" you will feel. For this reason, three-blade propellers are most commonly used on general-purpose boats. The three-blade propellers offer the best combination of power, speed and efficiency. Some manufacturers also make four-blade and five-blade propellers to increase acceleration and reduce aeration. A four-blade propeller improves handling and control for high-performance applications. Five-blade propellers work well in rough water conditions and towing sports.
Finally, the material is an important consideration for your propeller. Aluminum supports offer an excellent balance of cost, performance and durability. Stainless steel is five times stronger than aluminum, greatly improving performance and durability. Stainless steel propellers offer better acceleration and are less likely to be damaged by objects hitting the water. Stainless steel supports outlast aluminum many times over.
Left: a stainless steel propeller. Right: an aluminum propeller.
How to choose the right pitch and number of sheets
Before you consider choosing a propeller, understand what your boat will be used for. Consider your average speed, load, and browser location. If you use the boat for multiple applications, it may be necessary to swap out the propellers. The choice of propeller directly affects engine speed and therefore performance. Choose a propeller that will bring engine RPM into the mid or high end of the wide open throttle (WOT) range at normal load. This operating range corresponds to the maximum possible performance of your outboard. See engine owner's manual for operating range.
Once you know the operating range, refer to your engine's propeller manual to choose a pitch, blade count, and material combination. Choose a variety of propellers to do a water test. Test the propellers in the same conditions you would normally use the boat in - same load, gear and water. Adjust the trim angle to give the boat the optimum speed on the water surface.
To test the propeller, run the boat at WOT and observe the maximum engine RPM on a tachometer. If the RPM is below the recommended operating range, change to a propeller with lower-pitch blades. If the RPM is higher, switch to a propeller with higher pitch blades. Each inch increment changes the RPM by 150-200 RPM. Aim for the middle or maximum of the recommended operating range. Remember that high altitude reduces engine power, so choose a lower incline to get the same RPM at sea level.
Engine height also affects propeller performance. Once you find the right mount, adjust the motor mounting height to achieve the best balance of speed, handling, and acceleration. Start with the anti-ventilation panel that sits flush with the bottom of the boat. Raise the motor one mounting hole at a time until performance is no longer acceptable, then lower the motor one hole lower.
Keep backup support on board for emergencies. One recommendation is to purchase a replacement propeller that is two inches lower than the stock propeller. This pitch reduction is perfect for tow sports like water skiing due to the slower acceleration and increased power.
to go back-the low-pressure side of the blade closest to the boat
cavitationevaporated air caused by disturbances in water flow
cup (blade)-the lip on the edge of the blade
Diameter-Width of the circle formed by the tips of the blades
face (blade)-the high pressure side of the blade furthest from the boat
Leading Edge (Blade) -the edge of the blade closest to the boat
Tom-Distance covered by the propeller in one revolution
arithmeticAngle between blade and hub
root (blade)-the edge of the blade that attaches to the hub
Number of revolutions-revolutions per minute
slip-lost distance due to inefficiency
Surface-the total area of the blades
Top-the point separating the farthest leading edge and trailing edge of the cube
trailing edge (blade) -the edge of the blade farthest from the boat
Ventilation-air bubbles around the blades
WOT-wide open throttle