1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does pumping for speed really work?

  1. May 14, 2009 #1
    Surfers would say it does but they only believe it does because it feels like it does. No one has ever taken a radar gun out to test it.

    Pumping for speed is where you move up and down the wave face to try and increase your speed parallel to the wave. The belief is that you will gain a higher maximum speed quicker by doing this than simply riding the wave normally. But I am unsure of whether this is actually mathematically possible.

    The example scenario i am choosing is a wave with a perfectly circular form (looking at it from the side you will see 1/4 of a circle). The wave also moves at a constant speed and is perfectly uniform with no break. After x amount of time both scenarios will end with the surfer having a final position on the same part of the wave with the same heading.
     
  2. jcsd
  3. May 14, 2009 #2
    I've never surfed in my life, but I did think of one thing: if you average out your speed, shouldn't it be the same as the speed of the wave crest?

    I think if you pump (I assume you mean bending your knees) you will gain a little speed. I notice the same effect on a skateboard, snowboard, whatever. However, you will then outrun the wave slightly, and you will slow down again. You get up slowly, and the wave catches up with you again. Then you pump again and the cycle starts again.

    So I think 'locally', you can sometimes go a little faster. But if you average it out, you won't (otherwise you could surf on a wave that was actually like 30 meters behind you! Doesn't make sense...)
     
  4. May 14, 2009 #3
    Speed of the wave crest? What do you mean by that?

    I mean pumping as going up and down the wave face. The thing people do where they bob up and down on their board doesn't do anything if they aren't in a turn.
     
  5. May 14, 2009 #4
    Well, the wave moves forward of course, probably with a roughly constant speed (I know the speed will get higher once it reaches the shore, but I assume you are surfing a way out into the sea, and not on 3 inch waves near the shore..)

    Anyway, I believe my interpretation of 'pumping' was incorrect. I was thinking of the 'pumping' you see skateboarders do in a half-pipe. If you simply stand up straight you will lose much energy and come to a stop fairly quickly. But if you bend your knees and really 'drop' your weight into the pipe, you can easily increase your speed with every 'pump'.
     
  6. Oct 17, 2009 #5
    For simplicity, imagine the reference frame of the propagating wave in a conservative system. As a surfer slides down the face of the wave, he/she gains speed because potential energy is turned into kinetic energy. Of course, the unique nature of the surfboard in a liquid also allows for "gripping" to go down the line. Pumping is, in effect, the surfer doing work, by "stepping" back up the wave to gain potential energy and immediatly sliding back down turning this into kinetic energy, thereby gaining speed. In real life there are also losses. The skill is to generate more energy than is being lost. I hope that helps.
     
    Last edited: Oct 17, 2009
  7. Oct 18, 2009 #6

    Cleonis

    User Avatar
    Gold Member

    There is an example that is a clear case of pumping to gain speed.
    Check out this youtube video of http://www.youtube.com/watch?v=KP7IBUWdeZg", eventually building up to a 360. (Of course a 360 is possible only with a swing that doesn't have cables for suspension, but rods.)

    Notice how the acrobat squats down on the descend, and stretches out when ascending again. As the acrobat stretches out he moves his center of gravity closer to the pivot point, and that pumps energy into the existing motion. For maximum efficiency the pumping must be precisely in tune with the existing swing.

    Now, I never surfed, but I can imagine some mechanism that is similar to pumping a swing. In order to pump the surfer must move to a more upright posture when the board is climbing the wave, and during the descend the surfer must squat down.

    I'm guessing that if the surfer gets in a good pump every three or four turns he will still gain. I'm guessing that most of the shifts of posture will be towards maintaining balance, and I suppose that all of that jostling for balance will tend to "hide" the squatting and stretching that do the pumping.

    So it's conceivable to me that surfers feel they gain by pumping, while unable to pinpoint exactly how the gain is achieved.

    Cleonis
     
    Last edited by a moderator: Apr 24, 2017
  8. Oct 19, 2009 #7
    I am with Nick89; you can't possibly go faster then the wave on average, but I suppose you could pump to move further down / forward on the wave should that be your desire…. I am guessing that is the reason, just for initial positioning, or to catch up with the wave once/if it starts to get away from you, but its all relative... at the end of the day you are either on the wave or not on the wave, and the wave does what it wants :-)
     
  9. Oct 20, 2009 #8

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    This need not be true - in the same way that sailing boats can easily move faster than the speed of the wind by moving at an angle across it.
    The surfers I have seen are moving across the wave and can go a lot faster than the wave's forward speed. The forward speed of the wave is set by the Physics of the wave but their speed is limited by rate of Potential Energy that they can transfer and the friction / drag of the board in the water.
    They are constantly falling down a slope which is being raised by the advancing wave (providing more Potential Energy for them). I imagine that the pumping is effectively 'walking' them higher up the wave slope a bit, giving them even more extra Potential Energy, which they can transfer to Kinetic Energy.
     
  10. Oct 20, 2009 #9
    sorry, I did mean perpendicular to the wave but I didn't actually say that so fair comment!

    another viewpoint is that the opposite to what you say is true in that by trying to traverse across the wave, the wave maybe wants to move underneath you more (more than if your board was longitudinally aligned I mean, because the rudder is then sideways on to wave and hence less likely to be 'skidded/slipped') and hence by pumping you are perhaps trying to move 'down' the wave to stop this happening (ie the pumping could be 'lowering' the centre of gravity)
     
  11. Oct 20, 2009 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    In a (breaking) water wave, a lot of the water at the front of the wave is actually moving out to sea and upwards (as in the precursor to a Sunami and you also can see this as the foam just under the breaking head actually moves upwards). There is no bulk movement of water towards the beach. If anything, I should imagine, the skeg, when parallel with the wave crest, would actually help to lift the surfer. This is just what he wants (or she, of course) as it gives more Potential energy. I wonder, in the light of this, whether the pumping actually produces more lift as it pushes the board under the rising water.

    We need an expert surfer to take this much further, I think.
    Personally, I always got far too knackered when just trying to stand up to examine what was actually happening during my brief few seconds. I think I would die if I tried it now!
     
  12. Oct 24, 2009 #11
    In addition to my previous answer, the sinesoidal oscillation of a small potential energy;
    [tex]{V}_{0}Cos\left[k\left(z,E_0\right)r\right][/tex], where [tex]{V}_{0}=mgz[/tex] is the amplitude of oscillation and should be much smaller than the unperturbed kinetic energy, [tex]E_0[/tex]; r is along the wave line and [tex]k\left(z,E_0\right)[/tex] is the wavevector and is dependent on the height change and unperturbed kinetic energy, will lead to an average kinetic energy reduction, to first order in kr';
    [tex]\frac{1}{8}\frac{V_0^2}{E_0}[/tex].
    But this is [tex]~\frac{V_0}{E_0}[/tex] orders smaller than the kinetic energy gained due to work done
     
    Last edited: Oct 24, 2009
  13. Oct 24, 2009 #12

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    A breaking wave isn't a sinusoid, though. How does that effect the outcome of your analysis? The water is decreasing in depth and dispersion plus energy loss will changt things a lot. After all, people only 'surf' on surf and not on 'waves'.
     
  14. Feb 4, 2010 #13
    huh? I meant the pumping action as a sinusoidal oscillation.
     
  15. Feb 4, 2010 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    What evidence is there that successful pumping is sinusoidal? That sounds extremely unlikely.
     
  16. Feb 4, 2010 #15
    I was simple making as estimation to compare how much the average kinetic energy drops if the motion goes up and down (without doing work). This was to see if it is comparable to the energy that is put in by doing the work.
     
  17. Feb 4, 2010 #16

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    The problem is that the only reason that the whole thing works is because nothing is behaving linearly or sinusoidally. The water flow on the surface of a breaking wave is very complex. The drag and general performance of a surf board is not what you'd expect from treating it like a simple displacement hull going through smooth water.
    You can surf along across a breaking wave without any pumping and go faster than the wave is advancing. This is all because the water at the front of the wave is moving uphill - towards the crest - you just have to arrange that you are going at the optimum angle so that you are constantly 'falling' down the wave whilst being lifted up at the same time. This is the same sort of process as gliding in updrafts produced by sloping ground.
    The pumping must be a way of increasing the sideways force from the rising water by forcing the hull deeper into the water and getting more lift. You can only do this in bursts, of course. Hence the pumping action.
     
  18. Feb 4, 2010 #17

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Has no one a comment (positive or negative) about my observation that the water at the front of a breaking wave is moving upwards?
    I saw it quite clearly the other day in some very impressive Atlantic breakers.
     
  19. Feb 4, 2010 #18
    it will divide your speed into going up the wave and down when going down you will get an increase in const. speed and going up it will decreas but this will seem like you are going faster because of the acceleration and relative vision of you eye
     
  20. Feb 5, 2010 #19
    I feel like no one read the whole thread.

    sophiecentaur,
    I was only making a pertubation to a simplified problem. I realize it is indeed extremely complex. But getting nowhere on an difficult problem is much worse than making a simplification.

    Yes the front of a breaking wave moves upward and over the falls. It's an interesting questions, why do waves break at all? But irrelavent to if pumping increases speed.

    This comment really frustrates me.
     
  21. Feb 5, 2010 #20

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    ExtravagantDreams
    It confuses me too. I don't know what it can mean.

    The crux of your point "But irrelavent to if pumping increases speed." is the "IF".

    IF pumping alone would increase speed, people could pump their way up and over ordinary waves. Have you ever seen them doing this? Does anyone claim to have done it?
    My explanation agrees with observed fact; always a good reason to accept things.
    The reason that waves break in the way that they do can be explained by the reduction in wave velocity as water gets shallower. The wavelength gets shorter as a consequence and the water particles can no longer move in circular paths. The wave still has a lot of energy and this energy ends up as vertical movement. It starts with the motion getting elliptical (Major axis vertical) - increasing GPE changes c/w/ KE, I think. Eventually, the wave tumbles forward (breaks). Surfers use the upwards flow to gain PE, which they convert to KE. You could do precisely the same thing on a very wide travellator - maintaining your vertical height and going faster and faster, laterally. You could, with suitable control, ride a bike facing downwards on a travellator with your brakes on just enough to stay in the same place. The brakes would get hot with the energy that was being transferred to you - or you could use a dynamo to extract the energy.
    As I said previously, pumping will give a better 'grip' (sideways drag) on the rising water and make better use of the available energy. So there is a reason for pumping to work which a sinusoidal model of a wave doesn't give.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Does pumping for speed really work?
Loading...