Pitch, slip and cup vital terms for props

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Last month’s column just barely scratched the surface as it pertains to propeller technology. In order to better understand propeller designs and their applications it will be necessary to understand the language of propellers. As I explained last month, a propeller can be best described as a screw. The pitch is measured along the front face of your propeller blade. If you have a 19-inch-pitch prop, your prop would theoretically travel 19 inches forward with each revolution of that screw.

Depending on application, you may purchase a prop with a constant pitch or possibly one with a progressive pitch. The constant pitch prop blade is the same degree of angle across the entire blade from the leading edge to the trailing edge. The progressive pitch blade starts at a lower pitch angle on the leading edge and increases to a larger pitch at the trailing edge. The pitch number that is assigned to the prop is the average of this progressive increase.

Constant pitch propellers are generally used on slower boats and smaller engines. As boat performance and speed increases, the progressive pitch propeller will generally be a better choice. When testing propellers, keep in mind that the smaller the pitch, the higher the engine RPM and the greater the acceleration.

If you test a 21-inch prop and find that your boat is sluggish accelerating and the RPMs are less than the factory recommendations, a decrease in pitch will help. A general rule is that for every inch of pitch change you will see a 200 RPM change.

Diameter works hand in hand with pitch. You can determine the diameter of a prop by drawing a circle around the prop from tip to tip. The distance across this circle is the diameter. A larger diameter means more blade area. It is somewhat like adding wider tires to your truck. You get more traction.

In this case, the propeller can create more thrust. Larger diameter propellers with their increased thrust take more power from your engine to push the boat. As engine size and boat speed increase, you will generally use a smaller diameter prop.

Rake is one of the most misunderstood terms in propeller language. Simply put, rake is the angle that the blades slant in relation to the hub of the prop. Rake can be forward (negative) or aft depending on whether the blades tilt toward the leading edge of the hub (forward rake) or the trailing edge of the prop (aft rake).

Most outboards will have aft rake ranging from 5 to 15 degrees, and some high-performance props can have 30 or more degrees of rake. Aft rake gives the engine the ability to create bow lift. As you trim up, the bow of your boat lifts out of the water to lesson drag and increase performance. Negative or forward rake is used to hold the bow of the boat down, and is found primarily in work boat applications.

Cupping can be found on most outboard propellers. A cup is formed when the trailing edge of the blade is curled or bent over. The cup helps the propeller blades to grip the water so that there is less slippage as the engine is trimmed higher and the prop runs closer to the surface of the water. In other words, it will decrease ventilation. The cup will also reduce full-throttle RPM by 150 to 300 revolutions.

Angle of attack and propeller slip work together to maximize your boat’s performance characteristics. Propeller slip sounds like an evil thing, but just the opposite is true. Slip is not a measurement of how good or bad your prop performs. It is simply the difference between the actual and theoretical distance of travel that your propeller makes with each revolution.

Propeller blades are similar to the wings of an airplane. If an airplane wing were to move through the air perfectly flat (zero angle of attack) there would be no difference in the air pressure above or below the wing. There would be no lift and the plane would not fly. By creating the proper angle of attack, the airplane wing creates a low-pressure area above the wing and a high-pressure zone below. The wing now has lift, and the plane can fly.

An outboard propeller does the same thing in the water. The angle of attack decreases the pressure on the leading-edge side of the blade and increase pressure on the aft side of the blade. You now have thrust, and you are on your way to the fishing hole.

Angle of attack also creates a slight difference between the actual distance of travel and the theoretical distance of travel that your prop will make for each revolution. Most propellers will have 10- to 20-percent slip depending on the angle of attack.

You can use your computer to determine the amount of slippage your propeller has. Google search “propeller calculator.” On this web site, you will be able to plug in your engine RPM, the gear ratio of your lower unit, the propeller pitch and the boat speed (use a GPS for accuracy). When you click the compute button, you will be given the percentage of slip for your prop. This can be helpful in choosing between two props that perform similarly.

Two other propeller terms frequently used and confused are cavitation and ventilation. If you check your September issue or go to the Louisiana Sportsman website, you can check out the column I wrote about these often-misunderstood terms in the propeller dictionary.

There is still one more variable that has to be addressed in choosing a propeller for your boat. There are many different styles of boats. Each has specific characteristics and matching a propeller to the boat’s characteristics can be just as important as matching the prop to your motor.

Check in with me next month for a look at the various boat hulls and the handling features of each.

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