What is the propulsive efficiency of oars?

Discussion in 'Hydrodynamics and Aerodynamics' started by daiquiri, Jun 27, 2013.

  1. Leo Lazauskas
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    Leo Lazauskas Senior Member

    No, I think you are on the right track.
    I think it is more correct to say that you are actually moving the Earth
    backwards (or retarding its rotation) but that isn't always helpful either. :)

    Petros is correct too, there must be some movement of fluid (i.e. circulation)
    just as there has to be a starting vortex and viscosity for a wing to take off
    and fly.
    I'm not sure what Petros's beef is. If he drew us a diagram, or annotated
    one of those in the papers I attached, showing what he sees as
    fundamentally incorrect it might help.
     
  2. Mikko Brummer
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    Mikko Brummer Senior Member

    Interesting... I didn't realize rowing and paddling was so different. Rowing would seem like a perfect challenge for a dynamic CFD simulation... if I could lay my hands on typical oar angular movement data vs. time, I might try to do something... thanks for the great papers, Leo.
     
  3. thudpucker
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    thudpucker Senior Member

    I got far enough in Engineering to realize nothing involving Energy is Free.
    Especially when somebody's sitting in the boat, while I row, telling me I'm not really moving the Earth beneath the boat.

    "Oh Yeah"! "Then tell me how come that Dock's getting closer!"
     
  4. Leo Lazauskas
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    Leo Lazauskas Senior Member

    There are several papers around that could get people started on CFD
    investigations of these very difficult flow problems, for example,

    Alban Leroyer, Sophie Barre, Jean-Michel Kobus, and Michel Visonneau,
    "Influence of free surface, unsteadiness and viscous effects on oar blade
    hydrodynamic loads",
    Journal of Sports Sciences, 2010, pp. 1–-12.

    For those without access to journals, the two attached theses should be
    helpful.
     
    Last edited: Aug 12, 2015
  5. thudpucker
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    thudpucker Senior Member

    I didn't mean to be critical Leo. I was making a joke.

    Mentioning 'free surface' reminds me of my Grandson who couldn't keep both Oars in the water at the same time. Splash splash...
     
  6. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I didn't think you were being critical or mean in any way, thudpucker.

    And I'm still trying to think of whether the "earth moving backwards" idea is
    correct. I know that is the correct viewpoint for some analyses, but I'm not
    sure if it is valid for propulsion on the water.

    If only there was an engineer around who could could offer an opinion ;)
     
  7. thudpucker
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    thudpucker Senior Member

    It seems easy to me. The pressure on the oar, is against the Boat's weight, Drag etc.
    IE: the pressure is a small footprint in either case.
    The shallow water over the Oar gives way, to make a slip feature.
    The Shallow Water all around the boat gives way to make a slip feature.
    That large square area of the boat bottom has nothing but friction to fight the pressure from the oars.
    Otherwise the Oarsman would fall over backwards at the end of his stroke :)
     
  8. Lister

    Lister Previous Member

    Leo, you can prove one thing and its contrary. It happens all the time. It is what does the scientists.
    Of course it depend of the age of the rower, his physical condition, if is divorced, married or have a mistress. In that later case, he will be exhausted after rowing like a mad man all night long on top of his mistress.;) ( Theory of Lister on lovemaking efficiency before rowing )
    It is tough to know the efficiency of a human being. The word efficiency should even not apply to human.
     
  9. Petros
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    Petros Senior Member

    efficiency is applied to humans all the time. there labs all over the world where they measure O2 up take, blood pressure, brain waves, etc. while they are on a dynamometer that measures hp output. It could be a stationary bike, tread mill or rowing machine.

    If you can get a measure of the total calories burned (O2 up take is a good way to estimate that), and total power out put, you can measure efficiency just like any other heat engine. It is done all the time. the better condition the human, the more efficient they are (less calories burned for the same power output as compared to a less fit person).

    If you use the same person that maintains his same fitness level, than you can compare speed or calories burned over a give distance for various paddles, hull shapes or even different techniques. And get a reasonably accruate comparisons of which of the variables are beneficial or not over a base line configuration.

    I worked for the US Olympic Bicycling team in 1981-82 doing such measurements trying to improve the efficiency of the bicycles they used in the '84 Olympics. Our group designed the first of the "funny bikes" used in international competition. For the first time in 1984 the US team won 16 medals, including 4 golds, using the bicycles and equipment I helped design (it was team effort). All previous years combined they had won only 4 medals and no golds. the following year every top bicycle and component manufacturer had copies everything on our bicycles (including some of the mistakes).

    It is done, it has been done and the results are always beneficial.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I agree, Petros.
    There is an enormous amount of measurement and research going into
    VO2max and other measures of human performance/efficiency.

    I have been trying to include some "bio-energetics" into my rowing
    models for the last 2 years. There are some very nice papers by R.
    Hugh Morton that describe methods based on breaking up the problem
    into anaerobic and aerobic capacity. It's not ideal, but it is
    reasonably easy to implement. Basically, (apart from the effects of
    creatine which lasts only for the first couple of seconds of a race)
    the first 45 seconds is dominated by the anaerobic system, after
    which performance is largely determined by the athlete's "aerobic"
    capacity.

    Of course there are many complications, but it is at least easy to
    understand and to line up with measurements in the lab and on the
    water. Many other variations on the theme are possible.

    Your suggestion of applying these sorts of measurements to the design
    of paddles and oars is fundamentally sound, but it requires a lot of
    experiments to statistically tease out the relevant indicators from
    the noise in field experiments.
    The Dreissigacker brothers develop their oars using repeated on-water
    tests.
    http://www.concept2.com/oars/how-made-and-tested/speed-testing
    I don't think that they go as far as to include any "bio-energetics"
    in their work.

    Another enormous and very annoying complication is that is almost
    impossible to get sets of oars with consistent bending characteristics.
    You would think that you could ask for and get say 8 carbon fibre shafts
    with almost identical flexibility so stress measurements could be made
    accurately. The Australian Institute of Sport biomechanists kept
    complaining about this inadequacy when I worked for them a couple of
    years ago.
     
  11. Mikko Brummer
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    Mikko Brummer Senior Member

    That's surprising - we get carbon fibre masts much more complicated in shape, and always to the millimeter of bending ordered. Maybe rowers should turn to mastmakers for help.

    Thank you Leo for all the valuable links - I will try to find the time to read them.
     
  12. Mikko Brummer
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    Mikko Brummer Senior Member

    I found this at www.biorow.com, by Dr. Valery Kleshnev, about oar angles. It appears that in the horisontal plane, the the curve of the oar angles is practically sinusoidal, in the vertical plane it is more uneven. Now I would need a scale for the time axis T(s): For a single scull, is a stroke rate of 30/min (2 sec for stroke) realistic?

    Then I still need the hull (I saw a lines plan in one of Leo's papers), and the oars - plenty of pictures of them available.
     

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  13. Mikko Brummer
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    Mikko Brummer Senior Member

    With CFD, I was not planning to design a better hull or oar, I'll stick to sails ;). Rather I would try to make the boat move with the oars like it should, and see what speeds the sim could attain... something like this "free sailing" sim on a Star. Here, the boat is free to translate in all axes, free to pitch, only heading & heel are locked, and it is travelling with the actual sail forces calculated by the CFD, in 30 cm waves.
     

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  14. daiquiri
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    daiquiri Engineering and Design

    That's a nice exercise and video Mikko, but how close it is to (or far from) the reality?
    I notice that there is apparently no leeway in the initial moments, when the boat starts to speed up.
    And then there is that odd hull motion between 0:04 and 0:05, like if the boat was skiing downhill, rather than moving through waves... ;)
     

  15. Mikko Brummer
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    Mikko Brummer Senior Member

    I find it surprising that the boat is sailing at all (took some time to get there!). It's very crude, the element resolution on the boat is 10 cm, so you cannot expect too much. I will run at a higher resolution when I get all things settled, but my system will not permit a good resolution anyhow (memory & processor cores). I give it a push at the start (initial velocity of 4,2 kn), else it would stall due to inertia and slide sideways. The launch is also higher into the wind (the course that the boat should be able to maintain), so it appears as there's no leeway. When that push decays, you will see some extra leeway, until the boat gets up to speed somewhere halfway. There is too much leeway, and the end velocity is higher than it should - this could be the low resolution, under estimating both lift & drag. With such a coarse element grid, the software relies entirely on its wall functions, handling the flow close to the hull.

    There are still stability issues in the free surface solver, and also mass integral issues, but I think that there are interesting times ahead with CFD and free surface modeling.
     
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