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

27 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

  6. Mercury Enertia Eco im interested in this prop,im running a 26 tempest to 6100 at 74-75 mph on gps. boat is a12 250 pro xs 12 Nitro Z8 10 inch hydralic jp i like the 16 inch diameter would this prop work looking for more speed and what pitch would be recommened thank

    1. Sam,

      Consider the 28 pitch Bravo I XS. The Nitros have been performing very well with the Bravo, both top end and handling. The fourth blade and 15.25 inch diameter will help you make the most of your hydraulic jack plate. The 8 vent holes on the Bravo I XS will help with planing performance.

      Nick

  7. Hi,
    I am about to receive (in one week) new Quicksilver 875 sundeck with SINGLE Verado 300 V8 XXL.

    QS 26,3 ft (8m) hull length
    9,8 ft (3m) beam width
    8500 lbs total weight (boat+engine+gear+fuel+people on board…)

    Mercury Verado 300xxl V8 max. rpm 6000, gear ratio 1,75 or 1,85 – not sure.
    As I spoke to the local dealer, they are not sure which prop to use, as this is relatively new boat and new engine for them, and have no experience. 🙂
    So , after reading prop school from 1-6, I am thinking about :
    Enertia ECO 17P or
    Revolution 17 P. ??
    I am not in position to test props before, so I would like to be as close to “very good” as possible. Later when I will see actual data it will be easier to upgrade – if needed. Your advice will be of great help !
    Best Regards,
    Ivica

    1. Ivica,

      Congrats on the new boat. That is a pretty large boat for a single 300, if your gear ratio is 1.75, I would consider going down to a 16 pitch Rev 4 XP. That is based on an estimated 41-43 mph top speed.

      Nick

      1. Hi Nick,

        thank you for answer,
        Gear ratio in fact is 1.85:1 !
        My goal is to have best fuel economy at cruising speed (23-28 mph), and top speed 39+ mph .

        Would you still consider REV 4 XP 16 Pitch as good choice, or ?

        Than you in advance.

        BR
        Ivica

        1. What is main difference between REV 4 and REV4 XP ? I think REV 4 is only available in 15 and 17 pitch (not 16) ?

          BR

          Ivica

        2. Ivica,

          The 16 pitch will offer you great handling and top end speed will be about 40 mph with the 1.85 gears. We offer the Enertia ECO XP down to a 16.5 pitch, but I think that would be a bit too big of a propelller for you. If you are outside of the US, you may have an easier time getting your hands on the standard Mercury 15 pitch Revolution 4.

          To answer your other question, we take the Revolution 4 and fine tune it at Mercury Racing, in doing so we are able to offer different pitches than standard so that we can dial in motors to their peak RPM range.

          Nick

          1. Nick,
            thanx a lot.It is much more clear now. I will try with standard prop (?) coming with boat and make some testing.
            Then will see..
            BTW I was checking REV 4 XP 16 RH + flow torq ss adaptor for 5/4″, but did not find anybody offering it. Mostly everybody offers 18 + pitch. Any suggestion for dealer ? or could be ordered from you ?
            BR Ivica

          2. Ivica,

            You would be looking for part number 8M0142116 for the Rev 4 XP, and a FLO-TORQ SSR HD hub kit, part number 8M0101601. For your setup, the standard 15 pitch Revolution 4 should be a great starting point.

            Nick

  8. This has been a great series and I have learned a great deal!
    I have a 1175 Impact with a 2013 115 four stroke, it has the 2.33 gear ratio.
    I am having trouble identifying the best prop for overall use with this boat and motor. It currently has a 17P aluminum 3 blade at WOT it maxes out at 6100 RPM and 36 GPS measured MPH. This is a pretty empty boat (1/2 tank of gas, just me operating and a 9.9 kicker. Looking for any advice.
    Thank you!

    1. Jeff,

      You are doing pretty well with the current setup, but you can gain some performance from the Spitfire X7 in a 17 pitch. The four blades will help keep the boat on plane at slower speeds and improve handling. Happy boating!

      Nick

  9. I’ve been experimenting with my setup under different conditions and have some concerns. Currently running an Optimax 200 on a Recon 985 with a 22P Bravo 1 XS. Motor height mounted with anti-cav plate 2.25″ above boat bottom. When boat on plane and at speed, anti-cav plate is above water surface sort of skimming the surface. Hole shot is outstanding. Top speed and RPM at WOT are as follows:

    Me alone in boat, 1/8 tank fuel, modest amount of gear: 57 – 58 mph, 5600 RPM

    Me with fishing partner, full fuel, full gear: 55 – 56 mph, 5450 RPM

    Since most of the time I am running with a partner, gear, and fair amount of fuel, my RPM’s are 300 below max WOT for this engine, although still well withing the recommended range of 5000 -5750. I would like more top speed without losing hole shot. Is there any benefit to adding RPM by tweaking things such as going to lower pitch prop or raising engine further? Since the Bravo 1 XS is not offered lower than 22P, I would have to go to the FS prop. My thinking is raising the engine one hole (+ 3/4″) would put it too high at 3″, and dropping to 21P would add 200 RPM but maybe not more top speed due to lower pitch.

    Thanks for your time.

    Jason

    1. Jason,

      I wouldn’t change a thing. You do not need to be out near the rev limiter to be making peak power on that engine. You can try raising the engine up one hole, I’m weary that the prop will start to slip. 58 mph @ 5600 rpm is awesome for that setup.

      Nick

  10. Nick,

    At my current motor height putting the anti-cav plate above the water and skimming the surface, this results in the prop blades running just barely below the surface. Is this ok, or do the prop blades need to be more submerged?

    Jason

    1. Jason,

      It sounds like it is in the optimal position. I am doing my calculations based on slip. It is a tell tale sign that an engine is too high when the slip number starts to increase (higher RPM without increasing speed). You are at 14% slip which is awesome for a Bravo I XS setup.

      Nick

Leave a Reply

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