Oars - curved blade vs. flat blade for performance rowing skiff

Discussion in 'Hydrodynamics and Aerodynamics' started by SailorDon, Jun 15, 2015.

  1. upchurchmr
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    upchurchmr Senior Member

    NoEyeDeer,

    If you want reduced pitching, why go half way?
    Put in a sliding rigger.
    You can easily restrict yourself to what ever leg bend you want or change it on the fly and there is so little pitching you will never know it.

    I don't know about hacking up your boat, since I have not seen it (as far as I remember)
     
  2. NoEyeDeer
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    NoEyeDeer Senior Member

    If you look at the picture of him rowing, you'll notice the mechanism is kept clear of the water. Also, John isn't stupid. I haven't measured those blades, but you can bet your boots that the area was about equal to conventional blades.
     
  3. NoEyeDeer
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    NoEyeDeer Senior Member

    I'm not that concerned about the pitching, and setting up sliding riggers would be a lot more hassle.

    The way I was thinking goes like this:

    When I drew the boat, I did a rough estimate of where I thought the CB should go (based on weights of components). When I weighed the boat after completion, it turns out that I'd got it a bit wrong. Consequently, the boat needs a little trimming ballast aft to sit level.

    Also, the relationship between the thwart and rowlock blocks is a bit cramped when rowing hard. No worries when just toddling around (anything will work then) but has a tendency to stuff oar handles into guts when going for it. That means I want to sit further forward over the thwart, which obviously puts the trim further out if I don't compensate with ballast aft. Ballast slows you down and is just one more thing to deal with.

    The other thing of course is that getting a long catch angle requires an inefficient and uncomfortable torso angle, because I'm sitting forward to keep oar handles out of guts.

    One day I'm looking at the boat and it occurs to me that the obvious solution is to chuck in a short slide. This can get me far enough forward at the release to keep oar handles out of guts, and far enough back at the catch to sit the boat down slightly on its tail, with the halfway position about right for even trim.

    It's also very easy to engineer, unlike sliding rowlocks, and makes for a compact and easy-to-handle seat unit, again unlike sliding rowlocks. It also doesn't require any modification of the existing rowlock blocks (which really are very nice ones) and will work with oars of a convenient length for general use.

    Edit: Picture of my boat added beneath.

    [​IMG]
     
    Last edited: Jun 15, 2015
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Beautiful boat. Absolutely gorgeous.
     
  5. Rurudyne
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    Rurudyne Senior Member

    Seconded.
     
  6. markdrela
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    markdrela Senior Member

    The motion sketched correct. The "comma" shape of the path can be seen by watching the oar from a bridge directly above the boat.

    Most people picture oars as pure drag devices. Not so. The oar acts as a lifting wing immediately after the oar is submerged, then briefly as a drag-producing "parachute" when the oar is perpendicular to the boat, and again as a lifting wing at with the direction reversed at the end of the stroke.

    Normally one would think that a bigger oar will be better because it will reduce slip. But that's only true for the brief "parachute" part of the motion at midstroke. During the "wing" parts of the motion, a very large oar will have a significant profile-drag loss, so there's an optimum size.

    One thing that can definitely improve the oar is a proper leading edge on the outer "tip" edge perpendicular to the shaft, since this really is a leading edge during the initial wing phase. The inner edge on spade oars like the "Big Blade" is also a leading edge during the final wing phase, so that also should have a good leading edge shape. The "Macon" in effect is like a delta wing during the final wing phase, so its inner "swept" edges should also have good leading edge shapes.

    I'm amazed to see square tip and inner edges on even "high tech" composite oars. Surely these could be considerably improved with proper leading edge shapes. The attached PDF shows a good shape for an oar blade. This is a top view as would be seen overhead. It has a leading edge on each end to handle both the initial and final wing phases, although the shaft attachment will cover part of the right leading edge. The overall camber is there because its an airfoil after all.
     

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  7. markdrela
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    markdrela Senior Member

    One other thing which theoretically will improve any oar is the "span" of the blade, measured perpendicular to the shaft. Increasing this span in effect will decrease the induced drag, or equivalently the slip, during the lifting phases. This span should be increased until it starts to interfere with blade insertion and removal from the water, or until the torques on the shaft get uncomfortable. Note that the modern "spade" oars have much larger spans than the old-style "skinny" oars. I suspect that this greater span is the main feature which makes them superior.
     
  8. DCockey
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    DCockey Senior Member

  9. markdrela
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    markdrela Senior Member

    Attached is my idea of the "perfect" oar, with a decent aspect ratio and a proper airfoil. Sticking it into and out of the water cleanly might be a challenge, but hey, that's not my problem :D

    The kink in the shaft makes the lift centroid roughly colinear with the oarlock and the handle, to minimize torque on the wrists.
     

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  10. upchurchmr
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    upchurchmr Senior Member

    Looks easy enough to test out.
    Can we see a picture of your prototype?

    Theory is great, but facts seem to get in the way of ideas.
     
  11. SailorDon
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    SailorDon Senior Member

    NoEyeDeer,

    Your boat looks fantastic. Looks fast! Nice design. Excellent build.
    [​IMG]
    The only problem with that boat is I don't have one in my boathouse! :D
     
  12. SailorDon
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    SailorDon Senior Member

    With my new found knowledge of oar blade motion with respect to surrounding water on competitive rowing shell designs, I want to find out what is the oar blade motion on my cruising Thames Rowing Skiff.
    I'm going to try and beat tropical storm Bill's arrival here in Texas, and get out on the lake for a few video passes and hope to be able to capture the motion of the oar blade as I stroke down the lake.

    If you don't see any further posts from me, you can assume tropical storm Bill beat me to the punch! :D

    It is my intention to report back (soon) with video and screen capture analysis of my oar blade motion.
     
  13. Jamie Kennedy
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    Jamie Kennedy Senior Member

  14. Rurudyne
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    Rurudyne Senior Member

    Crazy Contraption Idea: The Oardrive

    A sponson that attaches to an outboard motor mount. A literal rowing machine is mounted thereon. Double or triple oar rows provide lower paddle loading for the higher power that a small engine can provide. Engineering eye candy.
     

  15. markdrela
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    markdrela Senior Member

    About 12 years ago I had a student group compare in a wind tunnel conventional oar blades with a new blade with larger span and better airfoils. The new blade was definitely better, perhaps not surprisingly -- it's not difficult to improve on a very low-AR wing with square leading edges. But they never got around to making and testing an actual oar. Except for the airfoil, the oar in the sketch is mainly brainstorming.
     
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