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.

 

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57 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

  11. Nick or Scott,
    See below for my test results. Thinking next try should be Bravo 1 LT in 20 pitch to achieve most efficient hole shot and mid-range fuel economy for the day long take offs associated with musky fishing. Your thoughts greatly appreciated.

    Boat 1 – 2017 Ranger 619 FS, 225 Verado, kicker
    Tempest 21 – full of fuel, driver only, musky gear, empty livewells
    Cav to hull 2.75 in.
    Slight chop on the water
    5940 max rpm @ 53 mph – couldn’t over trim as Verado limit hits 8.1 and stops
    Opposite direction – 5900 rpm @ 50mph
    Time to plane approx. 6 sec count

    Bravo I FS 20 – time to plane approx. 1-1.5 sec better than Tempest 21
    Full fuel load – couldn’t touch 6000 (5980-5990) rpm but 1.5 mph gain

    Bravo I FS 21 – time to plane matched the FS 20 (down 7 gallons of fuel from full) and top speed .5 mph gain over the 20 with no difference in top

    Boat 2 – 2018 Ranger 619 FS, 225 Verado, kicker
    Tempest 21 – ¾ fuel, driver and passenger, musky gear, livewells
    Slight chop on the water
    5860 max rpm at 49.9 mph – max trim limits at 7.9
    7-8 count to plane

    Bravo 1 FS 20 – no change in time to plane, 5980 rpm @ 52 mph
    Bravo 1 FS 21 – 1.5-2 sec less time to plane, 6000 rpm @ 53 mph

    1. Chris,

      Awesome data. I think you are spot on with the 20 pitch Bravo I LT. The longer, flared barrel will help with hole shot and carrying heavier loads.

      Nick

  12. Nick, Scott, I want to dial-in a new bay boat, Blackjack 256 Coastal / 300 V8 Verado, 1:85LU. 6 in Bobs. TM on bow. Still in break-in with 4hrs. Now running a 19 EcoEnertia. WOT = 54.1 mph on my Simrad gear @ 6,050 and really about what I expected for this size rig with a T-Top.

    Now that Eco like to be run deeper on the JP by 1 to 2 inches it seems than my B1FS props on prior boats. As we know, it turns like a 2 inch stiffer prop – and thus the slip #s range from 8 to 16% depending if you use the 19 or 21 in the slip calc tool.

    I have a 22 B1FS laying around to try and wonder what y’all think about that or the L, or other props for this rig?

    Many thanks.

    1. Todd,

      That ECO is hooked up so well I wouldn’t change a thing. The 19.5 pitch ECO XP from Racing could give you 1-2 mph increase. That hull definitely likes all of the lift it is getting from the ECO.

      Nick

  13. Just picked up a little Bayliner Vr5. 4.5L 200 to haul the kids around with. Topped out 48mph@4800 full of gas one passenger. Nice cruising boat with a 17P Vengeance prop. She very lazy though trying to get a skier up, me 240lbs @6’ I draw a bit of water lol. I’m hearing keep the pitch same and try the high five. This might not be the place to ask but seems this is where the knowledge hangs out.

    1. Brian,

      Sorry for the late response. I think you are right on the money with the 17 pitch High Five. It will be great for pulling skiers, those props have great forward thrust. Please note that they do not hook up as well in reverse as your three blade, it will take a little longer to stop the boat and get it going in reverse.

      Nick

  14. Question on my center console: I am currently using standard Rev4 17P on the new 200HP Mercury 4 strokes (1.85 Gear Ratio).

    At WOT, I am seeing 5800 RPMs (Max for engine) and 50 MPH (verified on vessel view) which is 1% slip.

    Others with identical setup are running as much as 22P Bravo 1 and seeing 5500 RPMs and 57 MPH which is 8% slip.

    So, what should I do??? Take on more slip for higher top end, faster cruise but lower RPMs, or, leave it alone and stick with what I got?

    I guess I don’t understand the issue some have with losing RPM if you’re gaining 10 to 15% top end.

  15. Brian,
    I am running a 2015 Ranger Z119C wih G1 E-TEC 225HO and 26P Raker HO. I’m running 65.4mph at 6000RPM, which is about 18% slip. I see a lot of other guys getting low 70’s out of the same set up. Do you think I can achieve better results with a Bravo 1 XS or other prop? I have had holeshot issues; had to open up the Raker HO holes a little more than factory to get results.
    Thank you,
    Ken

    1. Ken,

      Consider the 27 pitch Bravo I XS, you can expect to gain 3 mph and hole shot will improve. You can open up all eight vent holes.

      Nick

      1. Hey Nick, it’s been about a year since you recommended a 27 Bravo and since then I have tried a 26 Fury 4 and 27 Bravo 1 XS. The Fury 4 is faster, about 68 mph was my best, but still over my RPM range at about 6300. The Bravo was slower at about 63 and also about 6300 RPM. Puts my slip % pretty high at about 27. My rig is a 2015 Ranger Z119C with 225HO ETEC G1. I have a manual 10″ plate and twin Talons. It has holeshot issues with a three blade, so have been trying the 4 blades. Should I jump up to a Bravo 29? Will I have holsehot issues going that big? I have all the plugs open on the 26 Fury. I had to close the bottom holes and use the reduced opening plugs on top of the 27 Bravo. Any idea why the slip numbers are so different between the two? Thank you for any insight!

        1. Ken,

          Sounds like you are hitting a rev limiter at 6300? The Bravo will appear to have more slip if you are hitting the limiter because it is essentially a lower pitch than the Fury. The Fury’s run two inches of pitch taller than the Bravos. Hole shot will definitely be slower going with the 29 pitch, but if you are very happy with the current hole shot, it could be manageable.

          Nick

  16. I have a 2005 Checkmate Pulsare 2100 w a 2005 promax 300x with a sportmaster 1.62 lower.currently running 27p Trophy Plus that will turn 5900 at 80mph…it seems like this combo should run faster to me.. what prop would you recommend?? I have a 27 Tempest, but the boat has a nasty chine walk with this prop…

    1. Tim,

      It’s going to be hard to beat the Trophy for your setup. A 26 pitch Pro Max may come close but it won’t be any faster.

      Nick

      1. Thanks for the response Nick…I have seen several boats get into the upper 80’s low 90’s with similar setups and was just curious what your recommendations would be.

  17. I am trying to prop a 2019 Ranger 521L with the Mercury 250 pro xs4., loaded light. The boat has reacted quite differently than any boat previously owned. I had a 23p Fury 3 blade on it running 5580 RPMs and had to bottom out the jack plate to hit 64.6. We tried a 24p and it was much the same. I am currently trying a Fury 4 25p that is allowing me to raise the jack plate and give some transom lift but I am only turning 5500RPMs at 68.5. Prop slip appears to be low but it won’t hardly come out of the hole and is running 300-400 less RPM’s than I would have expected. Your help would be greatly appreciated!!

    1. Devin,

      Consider the 25 pitch Bravo I FS. Hole shot will improve and you will be able to run the engine higher. Expect RPM to raise to 5800-5900. Top end gains will be small, maybe 1 mph over your 25 Fury. If you are mostly concerned with top end, the Fury is hard to beat.

      Nick

  18. Does anyone know where you can buy a copy of the “Everything You Need to Know About Propellers” guide online?

    Thanks.

  19. Hello,

    Ideally should prop slip be the same throughout all RPM’s. It appears that I have more slip at mid range speeds than I do top end. Is that normal? Also, how do the number of blades effect slip, 3,4,5?

    1. Max,

      It really depends on the application. For most bass, bay boats, and runabouts the spread between cruising slip percentage and top end slip percentage should not be big. In some cases a guy will have 35% slip at cruise and 18-20% at wide open, there are ways to address this such as changing props, moving weight around in the boat, etc. The number of blades on a prop has become very important over the last few years because everything has gotten bigger- engines, boats, and gear cases. Four and five blade props have taken the place of many three blade props because they offer more grip at higher engine heights, better boat control, and more lift.

      Nick

  20. This is an interesting read. I have a heavy gauge west coast aluminum boat with a 150 seapro with a 19p tempest plus prop. I am barely getting into the max rpm range at 4800rpm @ 40mph. I care most about fuel economy do I stay with a tempest and go to 17p or would a enertia eco be better.

    1. Ryan,

      With how well the Tempest is hooking up, I would stay with that prop family and switch to a 17 pitch. It should free up the motor for much better hole shot and acceleration as well.

      Nick

  21. I have something very interesting going on that from past experience is not adding up. The boat is a Bass Cat Pantera Classic. Motor is a 200 Pro XS V8 with the Mercury Monitor Gauge. Prop is a 24 pitch 3-blade Fury LT. The most RPM’s I am able to turn is 5350 according to the gauge. My max speed (GPS) is 68mph. Hole shot is very good.

    If you take the rpm’s, gear ratio (1.75), and the max speed according to the prop slip calculator I am only at 2%. The GPS speed was measured using my Hummingbird unit and a phone app for GPS speed, and they both matched. I believe the monitor gauge is correct as at idle I am at 650rpm’s.

    So if all of the variables are correct (gear ratio, rpm’s, and speed) is 2% prop slip even achievable?

    Thanks

    1. Greg,

      Yes it is possible, especially since the Fury props run tall in pitch. Realistically, you are running a prop that is closer to 25 or 25.5 pitch, plug that number in and you are closer to 7%.

      Nick

  22. Nick,
    I have a 21′ Tritoon with a 150 Pro XS. I am running a 15.6 X 13 Enertia prop. I get 35.5 mph at 6030 rpm at 4200 ft of elevation. When I calculate prop slip I’m just over .5%. Per the article written I don’t have enough slip. My question is am I too efficient? Should I be running a smaller diameter prop even though I’m at optimum WOT?

    Thanks, John

    1. John,

      It sounds like you have your rig dialed in perfectly, low slip on a tritoon is a great thing, I wouldn’t change a thing! The Enertias run a little tall in pitch, try plugging in 14 for pitch and slip goes up to 7%.

      Nick

  23. Hi
    I have a Very light light RIB, 7 meters and about 370 kg withiut engine. The hull is air wented with two steps. Engine is Mercury 115 CT PRO xs 2017, ratio 2,38. For the moment i have enertia 22” and Looking for a economical cruising speed. At 3700 rpm the speed is about 26 mph (23,5 knots). Is the slip to high 17 %, is there any better choice for a propeller. I also have hydraulic jack-plate with 4” set back.

    1. Tomad,

      For mid range cruising, 17% slip is actually not bad. One prop to try would be a Revolution 4 of equal pitch. The fourth blade should help you stay hooked up.

      Nick

  24. hi
    I have a light 5 m RIB and recently i bought a Spitfire X7 17 pitch prop. My gear ratio is 2.15 and at around 5000 rpm I have 33,8 Kts speed, measured with my lowrance gps. With previous aluminium props I had measured the slip and it was around 3.5% at higher than 5000 rpm’s. With this prop the slip is negative, so is there by any chance an issue with the prop’s pitch ? According to my calculations the actual pitch is larger than 18.

    Petros

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