Response to my Prop School series has been been gratifying. It has generated a lot of good discussion (online and off) regarding propeller design, function and application. One of the most common questions is about prop slip. It is the most misunderstood of all propeller terms.
Propeller blades work like wings on an airplane. Wings carry the weight of the plane by providing lift; marine propeller blades provide thrust as they rotate through water. If an airplane wing were symmetrical (air moves across the top and bottom of the wing equally), the pressure from above and below the wing would be equal, resulting in zero lift. The curvature of a wing reduces static pressure above the wing — the Bernoulli effect — so that the pressure below the wing is greater. The net of these two forces pushes the wing upward. With a positive angle of attack, even higher pressure below the wing creates still more lift.
Similarly, marine propeller blades operating at a zero angle of attack produce nearly equal positive and negative pressures, resulting in zero thrust. Blades operating with an angle of attack create a negative (lower or pulling) pressure on one side and a positive (higher or pushing) pressure on the opposite side. The pressure difference, like the airplane wing, causes lift at right angles to the blade surface. Lift can be divided into a thrust component in the direction of travel and a torque component in the opposite direction of prop rotation.
Slip is the difference between actual and theoretical travel through the water. For example, if a 10-inch pitch prop actually advances 8-1/2 inches per revolution through water, it is said to have 15-percent slip (8-1/2 inches is 85% of 10-inches). Similar to the airplane wing, some angle of attack is needed for a propeller blade to create thrust. Our objective to achieve the most efficient angle of attack. We do this by matching the propeller diameter and blade area to the engine horsepower and propeller shaft RPM. Too much diameter and or blade area will reduce slip, but at a consequence of lower overall efficiency and performance.
Calculating Rotational Speed, Blade Tip Speed and Slip
Our propeller engineers study props at the 7/10 radius (70% of the distance from the center of the prop hub to the blade tip). The 7/10 radius rotational speed in MPH can be calculated as follows:
Blade tip speed can be calculated using the following equation:
Now let me walk you through a real-world example using formulas to determine theotetical boat speed and slip. I’m trying to determine which props (stock 15.25″ diameter x 34″ pitch Bravo I vs. 15.625″ x 34″ Lab Finished Maximus) will run best on a 2005 Donzi 38 ZR. It is powered by twin 525 EFIs coupled to Bravo One XR drives with 1.50:1 gear ratios. Max engine RPM with the Bravo props is 5250. Max engine RPM with Lab Finished Maximus props is 5200.
Turns out the ST Maximus 5 blade performed the best on paper and in water, offering lower slip with increased top end.
Back in the day when the Everything You Need to Know About Propellers book was published, the Internet didn’t exist and you had to actually use these cumbersome formulas or rely on the handy dandy Quicksilver Propeller Slip Calculator. Today, you can get all of your prop information with our online prop slip calculator. Try it out!
I hope you have found my Prop School blog series both educational and useful. I’ve enjoyed sharing with you.