What is the propulsive efficiency of oars?

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

  1. daiquiri
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    daiquiri Engineering and Design

  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    It depends on when during the stroke you measure that "efficiency".
    Here's another reference for your library
    http://espace.library.uq.edu.au/eserv/UQ:134879/UQ_AV_134879.pdf

    There are several others you should consult before committing to the findings of Ruina et al. :)
     
  3. thudpucker
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    thudpucker Senior Member

    I like to row!
    I put a Rolling Rowing seat in my 12' Boat. I get a pretty long stroke by pushing the seat back the length of my legs.
    I won a Mile Race with a guy in a similar boat with a 1.5 Hp motor.
    So whatever the numbers are, the longer the stroke the easier it is to Row for long distances.
     
  4. Petros
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    Petros Senior Member

    I skimmed the report and I think they made many basic errors in their assumptions. All of their equations, while impressive, are meaningless if they do not match reality.

    A much more realistic and reliable way to measure this would be to determine the drag of the boat at rowing speed by pulling it, loaded with a rower and oars, behind another boat and measure the drag with a simple scale. And than measure the rowers output on rowing machine dynomometer (typically a fit human can sustain about .2 or .25 hp output). When doing this it is easy to measure the rower's oxygen uptake, and than you put him in the row boat and verify the rowers O2 uptake is the same. Same 02 uptake, same power out put. Than you would have directly measured energy in vs. energy out. I suspect it will be much lower than 84 percent.

    There were so many assumptions in all those fancy calculations I would say as an experienced engineer, there is little chance that it actually matches reality. It looks more like a intellectual exercise to try and use a lot of numerology to try and mathematically explain the operation of a human rowing. There were a number of assumptions I have found in the essay that are just flat out wrong. For example they assume that "lift" on the oar is zero and that all of the forward motion comes from drag. This is just flat out wrong, the higher the L/D of the blade, the more forward thrust it produces. If oars worked by drag than a tennis racket would make a good paddle, just not so. As one of my engineering professors used to say, one simple test is worth more than a thousand expert opinions.

    And lastly, I suspect the efficiency of rowing can vary greatly with different seating configurations and oar designs. they used a competitive rowing scull and oars, which I suspect are pretty good. That is not the typical oar one finds in most row boats.

    I wonder what they would find doing a similar test with a kayak and double ended paddle. And compare the Greenland, Aleut and typical "white man" Euro-paddle designs for efficiency.
     
  5. Leo Lazauskas
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    Leo Lazauskas Senior Member

    But they do match "reality", just not as closely as you would like. :)

    (Remember George Box's Dictum: "All models are wrong: some are useful".)

    I also agree with you that there are a lot of assumptions, but it is a
    very complicated flow problem.

    Your proposal does not match reality even closely here.
    Rowing shell velocity varies very significantly during the stroke.
    Experiments at constant velocity, like the one you have proposed, are
    useless.

    Some attempts with unsteady experiments in towing tanks have been
    performed, but they too are subject to a lot of experimental
    uncertainties and scaling difficulties.

    I agree. Mathematical modelling of such tough flow problems does look
    like numerolgy if you just skim a report.

    Trying to get all the important inputs to the model is not easy and
    requires considerable expertise and expense. I found that you need to
    have good anthropometric data (e.g limb lengths and weights, centres of
    mass etc) good measurements of oar angles and forces, videos of
    the rowers' movements in the shell, and lots more.
    Once you have all that, a mathematical model can do reasonably well at
    predicting the shell acceleration and velocity. See, for example,

    http://www.cyberiad.net/library/pdf/rsn07sep10.pdf

    If there are significant effects from waves and wind, then all bets are
    off :)

    I agree with you here, but I would also say that lift is not of great
    significance except at the start and finish of the stroke. By far the
    greatest component is the drag at square-off.

    In fact, I use a method closer to Macrossan's. Like him I think that the
    division into lift and drag components is pretty useless and tends to
    confuse the issue. Eventually, you need to use components that are in the
    direction of motion, and at right angles to that, so why frig around with
    lift and drag?

    I have attached a bundle of Macrossan papers that I think are much better
    at explaining the forces and avoid the atrificial separation into lift and
    drag components.

    Certainly, but you have to take into account for for how long lift is
    important. There is no point in trying to maximise L/D if it is important
    for a few milli-seconds and is then counter-productive for a longer
    duration.

    Yes, but it has to be the correct test. :)
    Your attempt to define such a test was way off the mark, so I could also be
    as glib as your professor and say, "But first make sure you know what you
    are talking about".

    Again, we agree.

    Seating arrangements can affect squat, pitching, heave and roll. They can
    also affect whether one blade enters the water where another blade has left
    "puddles".

    Remember too, it is not just oar efficiency that matters, but its effect
    on the rower. Some competitive rowers have had problems with the newer
    large blade oars: there are many cases of rowers breaking their own shoulder
    blades and ribs when the blades "lock on" to the water at blade entry.

    Yes, but to be fair, they were focussed on competitive rowing, not
    recreational.

    There are several studies of paddling that I can provide if you are really
    interested. It is just as complicated as rowing, both numerically and
    experimentally.
     
    Last edited: Aug 12, 2015
  6. rxcomposite
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    rxcomposite Senior Member

  7. Petros
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    Petros Senior Member

    This is an incorrect statement, and understandable. Because it is a matter of requiring better observations of the actual movement of the water over the blade surface. the problem is that the action of the blade through the water is not intuitively obvious. I had spent some time ago considering this movement both by swinging a kayak paddle around in my living room and many hours in a sea kayak.

    It suddenly occurred to me that why most models results in very wrong assumptions, as you have displayed above. The ones that most people make when considering the blade movement through the water.

    most will consider the movement to be "most efficient" if you insert the blade into the water and pull it straight back. that is our intellectually model and where everyone is wrong. If you pull a blade straight back through the water it will stall, flutter and rock back and forth as you form a vortex street of alternate shedding vortexes first off one edge and than the the other as you pull it back. So the experienced paddler or rower will move the blade through the water sideways, making one edge the leading edge, and the other the trailing edge to prevent the flutter and the vortex street. they can feel the higher trust being generated and not really need to understand what is happening, just that they get more forward motion when they alter the blade sweep.

    both an oar blade and a kayak paddle works the same way, you have to SLICE the blade through the water at the correct angle of attack to get the most forward thrust. You are moving a foil downward to generate the thrust, and backwards because of the movement of the hull over the water, to get thrust. With experienced and skilled paddlers they can feel when the blade is at the most optimum angle to get the most thrust, and they unknowingly make the micro adjustments to stroke motion, angle of attach and other fine tuning based on the feed back through the shaft. Too little angle of attack, and you get less thrust, too much and the blade starts to stall and you feel it so your hands back off the angle slightly. On a kayak, the wing paddle is sliced outward away from the hull, the conventional paddle works best if sliced downward. on oars the motion of the blade makes your hands not move strait back, but upward and away from each other as you pull it back, causing the blade to slice downward.

    This is why L/D is important, the lift component is actually perpendicular to the primary direction of the blade through the water, pointing forward (the forward facing surface is of course the low pressure side of the foil) and the drag component would be in this downward movement of the blade, and is small compared to the thrust. You can not generate a force from a fluid unless you accelerate it, the most efficient way to do that is with a foil.

    That is why smooth foil shaped blades make the most efficient thrust. If it was the drag component than a tennis racket would make a better paddle. consider taking a kayak paddle and wrap one blade with carpet, that one will have more drag. but if you go out and paddle it, the smooth one will generate more thrust.

    This is where most of these models fail, they miss the important parts of the action of the foil, the blade aspect ratio, and the movement of the blade through the water as a foil.
     
  8. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sorry, but you are completely wrong in assuming that an oarblade and a
    kayak paddle work in the same way. An oarblade doesn't move very far through
    the water. If your idea was correct you would see the trajectory of the
    oarblade through the water. You don't. You see fairly small puddles where
    the blades have slipped a little through the water and created vortices.

    Ideally, you want the oarblade to remain almost anchored in place and to
    lever the boat past the blades. Some blade slip is inevitable.
    (You certainly don't want to be moving large volumes of water backwards as some people maintain.)

    Attached is Jackson's paper on kayak blades.

    It is easy to see the way in which rowing works by watching a sped up
    overhead view of an Olympic crew. It is very clear that the boat is being
    levered past the (almost) fixed oarblades.
     
    Last edited: Aug 12, 2015
  9. baeckmo
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    baeckmo Hydrodynamics

    Interesting discussion, which makes me remember the rowing technique the local oldtimers here along the coast used. My grandfather would never touch an oar that was fixed against rotation. The angle of attack was varied during the stroke, very much like the "wing paddle" in the report above, and you could spot a true islander on the short strokes he/she used. They could row for hours, if not days with this special "kick".

    It seems much of the thrust was generated during the vertical movements; you might say that their rowing technique used more vertical movement and less horizontal than others. Thanks Leo for the report, it gave me an answer to why this particular rowing style was favoured by those who lived their lives in rowboats here in the old days.
     
  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Glad to help, baeckmo.
    We should refer to it as "paddling" to differentiate it from "rowing" with oars or
    it could get very confusing.
     
  11. baeckmo
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    baeckmo Hydrodynamics

    You misunderstood? They were using oars, resting on the gunwale between two "pegs"; free to twist, but with the lever action. The difference lies in the variation of blade angle during the "powerstroke", just as in paddling. "Normal" rowing seems to keep the blade more or less fixed in a vertical position during the powerstroke and turning it to horizontal during the return.

    To me a paddle, or paddling means there is no leverage directly to the boat. The driving force is transferred through the paddlers body. No confusion there, I'd say?
     
  12. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Sorry for the confusion.
    In "standard" Olympic-style rowing the variation of the oarblade angle is
    quite simple, almost sinusoidal in the horizontal. There are certainly no fancy
    flicks or other tricks to rapidly change (vertical) blade angles.
     
  13. Petros
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    Petros Senior Member


    that was one of the points I was making, that you seemed to have missed here. You CAN NOT get a force off of a fluid unless you have motion of the fluid. This means that "inevitable slip" is an essential part of being ABLE to push against the ore. the word "slip" is an obsolete misnomer, the blade is not "slipping", is it creating circulation around the foil of the blade (that can be seen in the vortices you noted above), that generates the thrust that you actually push against. You get a starting vortex, and an tip vortex when the blade "slips" (I really hate that word, it implies something that is not happening when a foil moves through a fluid).

    Again, I think that these obsolete ideas and terminology has prevented those that should know better to be blinded to what is actually happening.

    A kayak paddle and an ore blade has to operate the same way to create thrust, or you violate the basic laws of physics, the navier-stokes equations, and everything else we know about the movement of fluids.
     
  14. thudpucker
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    thudpucker Senior Member

    Wow this is getting tough for an old man.
    Leo and the Engineers say the same thing about thrust. An engineer told me the (Jet) prop is moving the boat through the water it is turning in. IE: not forcing the water out the back end of the boat to create thrust.
    So my Oars are staying in one place in the lake. But the boat is moving past that 'one place' in the lake where the oars are.
    I hope I have that correct now?
    Did I miss something?
     

  15. Leo Lazauskas
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    Leo Lazauskas Senior Member

    The main problem is that we are probably using terminology neither of
    us likes or can visualise. A simple diagram showing the various vectors
    would make things much easier for all concerned. As I said, I prefer
    Macrossan's approach and his terminology to alternatives that use lift
    and drag. (For a blade operating close to the free surface, lift and
    drag are not as easy to visualise as for a wing operating in free air).
    I can't see what objections you have, apart from semantics, in the paper I
    attached earlier:
    "The direction of the water force on a rowing blade and its effect on
    efficiency", and in particular Figure 9.

    (Incidentally, Petros, I agree with you that that Cabrera et al spent an
    inordinate amount of time in some of their papers on the numerics of the
    problem. Although they only reference two of my papers, they were
    trying to reproduce some work I released in a program named
    "Michlet Rowing" about 12 years ago.)
     
    Last edited: Jul 2, 2013
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