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

    Ok David, but...

    Once you have established the geometrical proportions of the rowing shell (which are given by the racing rules, for example), there is nothing you can do regarding the semi-circular motion of the oars in the horizontal plane.

    The only thing you can control is the motion of the oars in the vertical longitudinal plane, and the angle of attack of the blades relative to the water surface. Hence, that is what I am discussing here.
     
  2. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Interestingly, there are some similarities between oarblade AoA and those
    of a vertical axis wind turbine at tipspeed ratio = 1, i.e. when the wings
    are travelling at the same speed as the oncoming wind. They go through
    the range -180 < AoA < 180.

    I hope you realise that somebody has already produced very useful diagrams of oarblade angles! There is no need to make your own mistakes :)

    Oarblade vectors
    http://www.atkinsopht.com/row/bladvect.htm

    Oar lift and drag
    http://www.atkinsopht.com/row/liftdrag.htm

    Oar efficiency
    http://www.atkinsopht.com/row/bladefcy.htm

    I like most of Atkinson's work, except I don't think he is very good with the
    boat hydrodynamics and athelete's "bio-energetics".
     
  3. DCockey
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    DCockey Senior Member

    See the attached vector diagram for the motion of the water relative to the blade at one instant. Neglecting the transverse component of the velocity of the blade can lead to erroneous conclusions about the direction of the water relative to the blade, particularly if the speed of the boat is a significant fraction of the linear speed of the blade.

    Edit: The first link that Leo provided above has a similar diagram.
     

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  4. DCockey
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    DCockey Senior Member

    Not the only things. Also you can control the sweep angle the stroke starts at, the sweep angle it ends at, and the speed of the oar during the stroke. These factors have a major effect on the motion of the water relative to the blade.

    For much of the stroke the "angle of attack" of the water relative to the blade can be in the neighborhood where the blade can generate significant lift (lift defined as force component normal to the waterflow relative to the blade), and the lift will be in a direction where it contributes to the propulsive force.
     
  5. daiquiri
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    daiquiri Engineering and Design

    Ok, I am getting back to wings, propellers and sails. :D
     
  6. DCockey
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    DCockey Senior Member

  7. jehardiman
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    jehardiman Senior Member

    I can't stand it anymore! I have to stick an "oar" into this...

    http://www.boatdesign.net/forums/hydrodynamics-aerodynamics/there-no-lift-47727.html#post641595
     
  8. DCockey
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    DCockey Senior Member

    Deleted my post of a few minutes ago after I saw the thread jehardiman just started.
     
  9. daiquiri
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    daiquiri Engineering and Design

  10. Leo Lazauskas
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    Leo Lazauskas Senior Member

    I'm a bit confused by what you actually want.
    Can you tell us what you don't like about this graph which show the "efficiency"
    at different times during a typical stroke?
    http://www.atkinsopht.com/row/bladefcy.htm
     
  11. DCockey
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    DCockey Senior Member

    Let's look at the hydrodynamic force acting on an oar blade, the resulting force propelling the boat and rowing efficiency without talking about "lift", "drag" or "slip". The starting point is the vector diagram of the velocity of the water relative to the blade from post #48. It's the first attachment. To this is added a vector for the hydrodynamic force on the blade shown in the second attachment as Ftotal. The direction of this force is limited and cannot be against the flow of water relative to the blade. (Blade is assumed to not have any jets, heating, etc.) It won't align with the direction of travel of the boat, the direction normal to the oar blade, the direction of the water movement relative to the blade, etc for most of the stroke when the boat is moving forward.

    Next the hydrodynamic force on the blade, Ftotal, is split into two components in blue in the third attachement. One component is aligned with the direction of travel of the boat, and that component is labeled Fthrust. Fthrust is the portion of Ftotal, the force on the blade, which contributing to propelling the boat forward. The instantaneous effective power propelling the boat from the blade is the product of Fthrust and the velocity of the boat through the water. The other component in blue is normal to the direction of travel of the boat and is labeled Flateral. Flateral is the component of Ftotal which is pushing the boat laterally.

    Also shown in the third attachment is another split of Ftotal into two components which are shown in brown. One component is aligned with the axis of the oar and labeled Faxial. This component is pushing or pulling along the shaft of the oar. The other component in brown is normal to the axis of the oar and is labeled Frow. This component is what the rower is working against, and the instantaneous rate of work of the rower on the oar equals to Frow multiplied by the speed of the blade relative to the boat.

    If instantaneous rowing efficiency for the oar is defined as
    e_row = effective propulsive power from the blade / work done on oar​
    then
    e_row = (Fthrust x Vboat) / (Fblade x Ublade)
    Vboat = speed of boat through the water
    Ublade = speed of blade relative to the boat (not relative to the water.) ​

    The overall rowing efficiency for the entire cycle will be:
    (time integral of the instantaneous effective power) / (time integral of the instantaneous rate of work done on the oar)​

    The direction of Ftotal directly affects the rowing efficiency. Closer alignment of Ftotal with the direction of travel of the boat results in rowing efficiency.
     

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    Last edited: Jul 25, 2013
  12. Petros
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    Petros Senior Member

    I had a chance to sketch up the action of oars in the water. I attached a sketch of the perceived oar blade movement through the water (I could not figure out how to post it as an image). It shows the profile of the blade as seen from the side as it moves first down into the water at the catch or beginning of the stroke, at mid stroke and at the end of the stroke. the "leading edge" of the blade is the lower edge, the trailing edge is the upper edge. the lifting surface is the forward facing face of the blade, the aft facing surface (often incorrectly called the "power face") is the high pressure side of the blade.

    I drew on the blade the relative angle of attack (alpha), the "lift" (force perpendicular to the relative motion of the water), and drag (the force in the same direction as the relative water movement) the resultant total thrust is in the direction of movement of the boat. this is the force that the rower is pulling against. Also note I call the path of the blade the "perceived path" because the motion of the water over the blade is something quite different. When you pull against the total thrust, you feel like you are moving the blade backwards through the water, but in reality it is you that is moving forward and the blade moves very little backwards, it mostly is moving downwards during the first half of the stroke. This is what causes the circulation around the blade that generates the lift and drag, and the total result thrust that allows the rower to pull the boat forward.

    as the oar is swept from front to back the angle of the length of the blade changes relative to the center-line of the boat but I do not think this has much effect on the relative angle of attack of the blade profile against the water, all this would do is reduce the efficiency of the stroke since the thrust vector is angled slightly from the direction of travel (left and right oars would cancel the component of the sideways thrust). All of the thrust "action" comes from the blade slicing downward through the water to generate the necessary thrust on the blade to allow you to pull the boat forward.

    The difficult part to conceptualize is to look at the movement of the blade through the water, not the perceived movement relative to the rower. When you are rowing you feel like you are pushing the blade backwards against the water, that is because the hull is moving forward as you pull on the oars. but the relative motion of the water over the blade is not quite the same, and not obvious from the point of view of the rower. This is what I think trips people up in trying to analyze the travel of the oar blade through the water.

    And as can be seen, the oar blade does NOT operate by drag, but the drag component is useful in generating part of the forward thrust. Though the "lift" component (the component perpendicular to the flow direction) is the main source of thrust. If much higher L/D blades were developed, it means you would get more useful thrust for the effort, and the blades would have to be held at a slightly different angle of attack. Also notice that if you pull an oar straight back through the water the blade will stall and flutter, that is why it is important that there be some cross flow over the surface, the downward movement of the blade is critical in generating efficient thrust. Anyone that has spent any time with oars or paddles is familiar with the flutter that occurs if the blade stalls, the solution is to put more crosswise flow over the blade (sweep it downward) and/or change the angle of attack of the blade. both of these corrections can be felt and an experienced paddler automatically learns to adjust both the downward sweep and angle of attack by "muscle memory" or by the feel of the blade in the water. It is quite natural, like learning to ride a bike, while the motion and physics is quite complicated, once you learn the feel of it it all feels natural and normal.
     

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  13. DCockey
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    DCockey Senior Member

    Follow-on to my post above.

    Another way to look at the hydrodynamic force on the blade of an oar, Ftotal, is to look at the component of Ftotal aligned with the water flow relative to the blade, Fl in the attachment, and the component normal to the direction of the water flow relative to the blade, Fd in the attachment. This set of force components is the system which hydronamacists and aerodynamacists frequently use to describe and study the hydrodynamic/aerodynamic force on a body in a flow. The direction of the flow relative to the body is usually given as the "angle of attack". A common convention is to use the term "drag" for Fd, the component of the total force aligned with the direction of the flow relative to the body, and the term "lift" for Fl, the component of the total force normal to the direction of the flow relative to the body. This convention originated with studies of airfoils and wings of airplanes, and has since become common for other types of bodies in other configurations . Fd and Fl could equally well be called the "force component in direction of flow relative to the body" and "force component normal to direction of flow relative to the body".

    What causes these force components, particularly Fl, has been the subject of an immense about or research, contemplation, analysis, discussion and argument. A variety of explanations/theories/models have resulted with some more valid and useful than others. It has been discussed on this forum in the past and undoubtedly will be in the future. My experience has been that no single explanation/theory/model is better than all others.
     

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  14. daiquiri
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    daiquiri Engineering and Design

    Very good drawing, thanks. Shows very well the difference in reference frames we were using while discussing this this thing. You folks were fixing your reference frame to the blade, and hence had lift and drag forces involved in the analysis, I was considering the reference frame fixed to the boat. And hence lift (relative to Vinf, which generates the boat resistance) has disappeared from the considerations.

    And besides that, I truly envy the amount of time you guys seem to be able to dedicate to this stuff. I am struggling to find some time (stolen from my overwhelming work tasks) just to write these few lines. Still didn't manage to read all the informative texts in Leo's links at the previous page, and will have to come back here 3 times today just to manage to read the entire reply by DCockey... :mad:

    Cheers
     

  15. daiquiri
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    daiquiri Engineering and Design

    Leo, as soon as I find some time to do it, I will read your links and will reply. Thanks!
     
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