Prop School – Part 6: Slip

Response to my Prop School series has 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.

A wing moving through air produces a pressure differential: low pressure above the wing, with high pressure below it, creates lift.

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.

Marine propeller blades need to move through water with an angle of attack to create thrust.

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.

Prop Slip 

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:

And can be shown by a vector arrow.

Rotational speed.

Blade tip speed can be calculated using the following equation:

Forward speed is shown by an arrow in the direction of travel. The length of the arrows reflect speed in MPH for both the measured speed and the theoretical (no slip) forward speed.

Forward speed.

 

The original Quicksilver prop slip slide rule calculator.

Prop Slip Calculator 

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 Quicksilver Propeller Slip Calculator.

We Have an App for That

Today, you can get all of your prop information with our Prop Slip Calculator App. Download it for free from Google Play Store or Apple iTunes.

The Mercury Racing Prop Slip Calculator App is available for free download from Google Play Store or Apple iTunes.

Click here to see some real world examples on how I use the app.

I hope you have found my Prop School blog series both educational and useful. I’ve enjoyed sharing with you.

 

Share Share

11 thoughts on “Prop School – Part 6: Slip”

  1. Back in the day that is how we did it. It worked well and thanks for putting the tools out there to help customers to get a better understanding of how props work. Way to go Scott!

  2. I guess I don’t understand how you can come up with a theoretical boat speed that does not depend on the pitch of the prop?

    1. Simply, it does depend on prop pitch: Theoretical speed = pitch x prop shaft rpm (with unit-of-measure conversion constant). Here’s our prop slip calculator: http://mercuryracing.com/propellers/propslipcalculator.php
      Without pitch, but with a wealth of installation history, one can estimate likely boat speed – based on power, weight, hull type, drive and prop type, and drive installation parameters.

  3. Hi
    Is there (or what is) the maximum speed at which a propeller can be rotated in the water before it starts to lose efficiency – presumably this varies with the diameter of the prop – so I am probably asking what is the maximum usable velocity of the tip of the prop??

    thanks – very interesting article

    1. James,

      With the thousands of different hull deisgns, it is impossible to say. Every hull has a different balance and water flow, we adjust our propeller designs accordinly.

      Nick

  4. Scott, long time no see.
    You helped me prop my 2015 Ranger with the Bravo XS and I have loved it from the beginning and have turned several other guys on to the Bravo and have cured set up nightmares.
    Now I’m moving to a 2019 Ranger z520L..I’m told I need to run a 23 or 24 pitch fury….nobody is mentioning a Bravo XS or why…just like in 2015 I’m guessing nobody is educated on the benefits gained in the Bravos design. Question…Will the Bravo set up work with the new 4stroke?
    and like we did in 2015 run a slightly higher pitch like a 25-25.5?

    1. Dene,

      Great to hear! Scott says hi as well. With the new four stroke, you can migrate over to the Bravo I FS line, same design minus the extra vent holes. The Fury could still be faster on top end with a light load but the Bravo will carry loads, as you have found out. You are spot with pitch, there is generally a two inch pitch difference between the Racing Bravo family and the Fury family. If you still have a 27 pitch Bravo I XS, you can plug the 4 forward vent holes and test it. Having baseline data always helps with propping.

      Nick

  5. Nick and Scott, Thanks a bunch.
    Crunching the numbers I was pretty confident that there would be a Bravo fit. And nice tip on the baseline, I will pass on the data FYI……….See ya

Leave a Reply

Your email address will not be published. Required fields are marked *