Oar angle of attack

Discussion in 'Hydrodynamics and Aerodynamics' started by rfleet1066, Jan 4, 2019.

  1. rfleet1066
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    rfleet1066 rfleet1066

    I'm designing hydraulic oar modules for my recently constructed riverboat. I know very little about boats and fluid dynamics. I'm wondering if variable angle of attack of the oar blade as it passes through the submerged part of the rotational cycle is something to be pursued. The vessel, 97' OAL , would require 10-12 oars on each side and a separate hydraulic power unit as the longtail drive engine only produces 85 HP. Ok, I know this makes me subject to some heckling...…..but I'm a builder, not just a dreamer. Photo of the Sebastian Marie, launched in October of '18. P1070987[1].jpeg
     
  2. rwatson
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    rwatson Senior Member

    There are heaps of studies you can find on Google about Oar efficiency. The few I have read seem to indicate a constant vertical line until the blade is feathered.
    eg Blade Path and the Ideal Blade https://www.concept2.com/oars/how-made-and-tested/blade-path

    One experiment that a can't find easily was a guy who tested oar blade efficiency where he tried oars with and without a curve at the end.

    His opinion after testing was that the slightly curved end caught more water at the correct angle at the start of the stroke, but even better, created some lift at the end of the stroke, just before feathering, as the water ran over the curved outer surface of the blade.

    I guess a bit of testing would show if he was right.
     
  3. alan craig
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    alan craig Senior Member

    That's a big project for someone who "knows very little about boats"!
    I'm glad you asked the question because I did my longest row last year, about 50 miles of the river Thames and I felt that my traditional symmetrical rhomboid section (squashed diamond) oars worked best with 75-80 degrees angle of attack (assuming an oar flat in the water is 90 degrees). At this angle the oar is still producing lift, in the sense of a wing section or a kite still producing lift even though it is almost completely stalled at that angle. In other words there is still some flow along the low pressure surface of the blade; it is not completely turbulent. If my comments are confusing I'll post a diagram.
     
  4. alan craig
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    alan craig Senior Member

    p.s. rwatson's link shows a sliding seat oar which is pulled through a very large angle when viewed in plan - very interesting. There must be considerable flow from the tip of the blade towards the shaft so the curved blade is a bit like a wing with a chord which is larger than the span. Totally different from my limited experience with a fixed seat.
     
  5. DCockey
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    DCockey Senior Member

    What do you mean by angle of attack? Do you mean the angle of the blade relative to a vertical line, ie twist of the oar about the axis of the handle? Another definition of angle of attack is the angle between the blade and the resultant velocity vector.
     
  6. gonzo
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    gonzo Senior Member

    When you account for the losses in a multiple hydraulic system, that would be the equivalent to maybe a 50 HP outboard. However, if oars are required, a chain drive will be cheaper, lighter and more power efficient. Whether you output power through hydraulic or a chain, the oars will have to be connected to a rather complex cam system to operate properly.
     
  7. messabout
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    messabout Senior Member

    Perhaps puzzling is that Olympic level oars as used with racing shells have flat blades that are not symmetrical. That concept is counter intuitive but seriously competitive oarsman use what they believe works best for them.
     
  8. rwatson
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    rwatson Senior Member

    His idea was along the lines of this rough illustration


    Oars.png
     
  9. DCockey
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    DCockey Senior Member

    Have you heard directly from rfleet1066 about what he means by "angle of attack"? Or is you post your assumption of what he means?
     
  10. rwatson
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    rwatson Senior Member

    I understood he was referring to the vertical direction of the oar blade, which his why I said that I couldn't find any mention of changing the vertical blade position.

    My reference to "angle of attack" was clearly the blades curved end at the start of the stroke.
     

  11. alan craig
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    alan craig Senior Member

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