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

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

  1. SailorDon
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    SailorDon Senior Member

    I have searched unsuccessfully for an analysis (force vectors) of oar blade performance with respect to curved blade vs. flat blade when rowing performance rowing skiffs (Thames rowing skiff, Wherry, typically with gunwale mounted oarlocks).

    The flat blade analysis is more straightforward since the reaction force on the oar blade is the resultant force which is perpendicular to the motion of the blade with respect to the water. There will be a component force vector in the direction of motion of the boat and a component force vector perpendicular to the motion of the boat. If the catch of the stroke starts at 45 degrees to the centerline of the boat, 0.707 X resultant force is the driving force (and the perpendicular force). At mid-stroke, the resultant force is 100% of the driving force.

    If it was possible to continuously keep the oar blade perpendicular to the direction of motion, 100% of the resultant force would be driving the boat forward.

    I think the curved blade is an attempt to keep the resultant force on the oar blade as close to parallel to the direction of motion of the boat as possible during the entire stroke of the oar.

    Longer oars help blade driving efficiency by minimizing the angular displacement of the stroke for the same linear displacement of the oar blade. Anything other than 0 degrees at mid-stroke is going to have some resultant force on the oar blade that is perpendicular to the direction of motion of the boat.

    Has anyone done the vector analysis of oar blade forces for curved blades vs. flat blades?

    From an "experience" aspect, I have both flat oar blades and curved oar blades for my Thames Rowing Skiff. I have rowed 300 miles with the flat blades and 1,700 miles with the curved blades.
    It's not science, but I get the feeling (using GPS data) that I get 1/2 mph better performance with the curved blades. :)
    [​IMG]
     
  2. gonzo
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    gonzo Senior Member

    If you look are racing boats, they all use curved blades. However, in rough conditions, flat blades tend to hook less, so they are easier to row with.
     
  3. TANSL
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    TANSL Senior Member

    I guess that, after rowing for about 2,000 miles, SailorDon knows that. The question to be answered is: "Has anyone done the vector analysis of oar blade forces for curved blades vs. flat blades?"
    I have not done the decomposition of forces in both cases but I think, if curved blades have more submerged surface than flat ones, less slippage of the blade will occur for the same force applied by the rower.
    It is an interesting topic, hopefully to read the opinion of experts. Here the experience with snipes will not help.
     
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    The flat water racing shells now have oars curved in the vertical, but the various coastal rowing competitions seem to have quiet a variety. In waves I don't think it would be easy to do a theoretical analysis, but looking at the various ocean rowing competitions around the world you might get a sense of what works best in one situation versus another and how much of a difference it makes. Technique still makes a huge difference, and also has to vary with conditions. How shallow or deep you have to dip your oars for example.

    The interesting thing to get your head around, with oars and with paddles, is that once the boat is moving well there is relatively low velocity through the water so the up and down and outward and inward velocity of the blade relative to the water can be more than the backwards motion of the blade. This means the direction of the blade as it enters the water during the catch, travels through the water in a dipping arc or outward and inward sweep, can be at a considerable angle, so it can generate a considerable lift component in its total thrust rather than drag. This lift takes a little time to develop so the oarsman or paddler learns to hesitate slightly after the catch before applying full force on the blade.

    To figure out what these velocity vectors look like at any point in your stroke, add the velocity vector of the boat through the water to the velocity vector of the blade relative to the boat. Another way is if you look down from a bridge as a rower rows past and concentrate on just one blade and not the boat, you will see that what the blade does after the catch is it sort of scoops our sideways as it turns and scoops its way back. In more of a dory stroke for possible waves in open water the dory blade is also diving down and then back up, going down and up in a fairly sharp vee and maintaining a favourable angle of attack both down and up. So the coefficient of thrust can be quite high, and the curve can either increase the lift or reduce the end plate effect or both. There is also sometimes a bit of a cant to the blade to maintain stability so you don't have to fight to control the blade with your wrist. Good oarlocks make a huge difference also. Facinating stuff.

    Here is a video of a flat water rowing viewed from above...
    remember to move your eyes backwards with the water, not with the boat going forward.
    https://www.youtube.com/watch?v=kREO1sRb01w
    also
    https://www.youtube.com/watch?v=0XfL_2EzEHI

    Here is the Navy rowing tank, and again you need to move your eyes with the water.
    https://www.youtube.com/watch?v=E2l-7cleACw

    Dory rowing. Excellent technique. Flat water.
    https://www.youtube.com/watch?v=_M-qpsIg-6c
     
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  5. SailorDon
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    SailorDon Senior Member

    As TANSL replied in his post, after 2,000 miles of rowing my boat, it didn't take me long to figure out that when it gets choppy, I store the 8 foot Shaw & Tenney curved blade oars and row with the 7 1/2 foot flat blades.
    I don't think the Thames Rowing Skiff design was made for rough conditions. It was probably designed for getting around on the River Thames 200 years ago! :D
    Here is an example of a "flat oar blade day".
    [​IMG]

    My quest for an analysis of curved blade vs. flat blade oar is strictly for flat water, no wind and with respect to cruising type rowing skiffs (no racing shells that knife through the water, and no hard chine skiffs that have a lot of resistance to moving through the water.
     
  6. Jamie Kennedy
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    Jamie Kennedy Senior Member

    You look to be handling that boat rather well. Great looking skiff also. I think there are some technical papers around on the old style blades compared to the new style blades, but it might be harder to find curved blades versus flat blades.

    Here is a practical test you might try. You could see how fast you can cover a certain distance at the same heart rate with one set of oars and then the other. Assuming you are putting out the same power in each case, and the drag on the hull increases with speed squared, and power required with speed cubed; your curved blades would have to be roughly 15% more efficient in order to provide a 5% increase in speed, and 30% for a 10% increase in speed. 30% seems a bit high for flat versus curved, but the longer blades might also be making up part of the difference. Longer blades will not just give you better angles but also more dip, so more lift and less slip.

    You could also look at some outriggers if your boat is quite narrow, but for cruising speeds you might be just as well without a sliding seat and rigging. So what is the width from pin to pin? I am looking to put oars in my Yngling keelboat for auxilliarly power. It is 6 feet wide so width is not an issue. I might also be able to row standing up and facing forward as well as seated and facing back. It's going to be fun to work things out. It is a 1400 pound boat and I would be more than happy with a cruising speed of 2 knots to get me home if the wind dies.
     
  7. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Here is a technical paper you might have already seen. It has more to do with angles, but also has some good stuff on "Macon Blade" versus "flat blade" and so on...
    http://espace.library.uq.edu.au/view/UQ:134269/MER2008_03.pdf

    The introduction is very interesting. Never thought about this, but the blade is actually going forwards and sideways at the beginning and end of the stroke, yet it still generates a backwards push.
     
  8. SailorDon
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    SailorDon Senior Member

    When I watched the video of the racing 8's, it looked like their catch was 40 degrees from the fore/aft centerline of the boat, and the release was 50 degrees from the centerline. Perhaps I am missing something, but for the time the oar blade is in the water, you would like the blade to be as close to 90 degrees as possible for the maximum resultant driving force from the blade. At 40 degrees (at the catch) you only get 0.643 times the resultant blade force to drive the boat forward. At 90 degrees you get 1.000 times (100%) the resultant blade force to drive the boat. The angle of the blade with respect to the boat is constantly changing throughout the stroke, so each team has to determine its optimum performance with respect to catch and release angles.

    It is a fact of trigonometry that once you get past 30 degrees, the force in the direction of motion is going to fall off fast. If you could ignore inertial effects, a short rapid stroke would convert more oar blade resultant force to driving the boat forward. How about a stroke of 20 degrees each side of mid stroke at 5 strokes per second? :D
    Might work in theory, but never in reality.

    Given the fact that the duration of most power strokes causes them to swing through a 90 degree arc, commonly 45 degrees each side of mid-stroke, the question is, does a curved blade provide greater efficiency? It would not make any difference a mid-stroke since the blade is 90 degrees to the motion of the boat. But how does the curved blade effect the first 20 degrees of the stroke and the last 20 degrees? This is where the sideways components of the resultant blade force are "lost" to pushing water perpendicular to the direction of motion of the boat.

    Does a curved oar blade help to minimize this inefficiency?

    Even with a simple flat blade oar, the analysis is somewhat complex since the resultant force of the oar blade on the water changes direction not only with the angle of the oar with respect to the boat, but also with the speed of the boat through the water. Also, the speed of the boat and speed of the oar stoke change during the power stroke. Slow at catch, fast at release.

    That is more analytical trigonometry than my brain can handle. :eek:
     
  9. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I think you have that right. There is a trade off in oar length and weight and rowing angles. From that paper that is mostly concerned with losses due to angles they seem to get around 80% efficiency, and they don't talk more about slippage. They included flat blades in some of the results but not the efficiency unfortunately. I gather that the most important thing is to choose inboard and outboard distances and stroke rates and blade area to reduce slippage to something reasonable, but there is still a trade off with angles.

    For cruising speeds, where you don't have to exert as much power, well because you can't, it is interesting to compare forces and angles and speeds and so forth. Consider a cruising skiff at cruising speeds compared to a racing shell at racing speeds. The cruising skiff despite being somewhat heavier and wider, might still require only one quarter the power to go have as fast. That might be a comfortable output for out cruising rower. Also the end of the blades would only have to go half as fast relative to the boat, so for the same stroke rate the inboard and outboard lengths can also be half as much and the oars could be considerably lighter, being half the length and shorter levers. At the same stroke rate and angles the blades would spend the same amount of time in the water, but because the drag on the hull is less, they can actually be smaller in area. In practice, I think the recreational cruising rower might go 3-4 knots at 100-200 watts output, but would want a slower stroke rate than the sprint rower, especially if you were still using a sliding seat. I think in practice it is better to go to a fixed seat when cruising unless you want to use your legs for fitness purposes. By using a fixed seat for cruising speeds you can use arms and back only and still generate enough power to keep your heart and lungs as the limiting factor, and by using arms and back only you can keep the inboard and outboard dimensions and oar length dimensions shorter and still have reasonable angles and a slow enough stroke rate. You might want the blade size to be just as big though. That is something that you might look at also is the size of the blades as well as the curvature. But I think you can still reduce slippage with the right technique. Short hard strokes with very little blade time in the water and very little slip for a count of 1, and then perhaps a count of 2 or 3 or even four on the recovery before the next catch. I think for longer and longer distances the oar can become increasingly smaller and lighter, the stroke rate slower with longer recovery, and it becomes less and less important to use your entire body. Everything should become more relaxed. I think a super lightweight oar makes a lot of sense for cruising speeds. You still need to put your back into it now and then, and it can bend a little for that work, but it might be better to have a light set and a heavier set for rowing long distances. How much do your flatties weigh compared to your curved blades?

    I would go mostly on trial and error, using a heart rate monitor to determine which setup and technique provides the best performance for the same effort. All the best.
     
  10. Rurudyne
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    Rurudyne Senior Member

    Sailor Don, your OP got me thinking a bit.

    It seems to me that part of the intent of a curved oar, relative to a flat oar, is to move some of the oar's effective area from one end of the stroke to the other. At the point you've got the most force to give, at the start of the stroke, the blade is presenting slightly more effective area than a flat blade would, but as the stroke is running out and leverage decreasing it has effectively less area.

    Or at least that's what I would suspect. Especially if rowing on a sliding seat.

    Edit: has anyone tried to make a feathering oar? The mechanism could be based on two parallel bars pivoting about the oar lock, though ergonomic concerns is likely to limit how effective it would be. To keep it light enough it might easily turn into an expensive piece of kit (carbon fiber maybe) as well.
     
  11. SailorDon
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    SailorDon Senior Member

    I don't think the Fig. 8 analysis of blade motion with respect to the water is correct.

    [​IMG]

    If the catch is inner edge of blade at distance zero and the release is at inner edge of blade at distance 1, the blade is shown at mid-stroke as having moved 0.5 (relative to the water) in the direction of the boat's forward motion. That would be negative slippage of the oar blade relative to the water. WOW! That would be efficient! :D
    Or maybe I am reading their Fig. 8 incorrectly.
    I would settle for zero slippage, as if the oar blade was set against a fixed piling. If you could have the pilings set exactly the distance you travel in one stroke, you could stroke from piling to piling with no loss of efficiency due to oar blade slippage. Almost like poling a raft down the river. No slippage there. :)
     
  12. Jamie Kennedy
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    Jamie Kennedy Senior Member

    That graph is a hard thing to get my head around for sure, and I had to really think about it. The blade really is going forward through the water, but because it is also moving sideways it is generating lift as well as drag and so there is a net thrust toward the back of the boat. There is slippage in the sense that some water gets pushed aft, which is what you are trying to minimize, the the blade is always moving forward through the water, at least when things are going as they should. Cool eh.

    p.s. Actually in the mid part of the stroke it is going backwards, but in the beginning and end it is going forwards, but still pushing backwards because of lift. It is explained in the Introduction of that paper. Same thing happens with a canoe paddle when you reach forward and strike the water with the paddle. You are stabbing forward, but also sweeping down, and at the end of the stroke you are sweeping up, and so even though the blade is moving forward relative to the water at the beginning and end of the stroke it is also moving down, and then up, and to it is still pushing backwards by generating lift for net thrust.
     
  13. SailorDon
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    SailorDon Senior Member

    The problem with trial and error is subjectivity. If I could program myself to pull equally hard on both oars, I would put the curved blade on one side and the flat blade on the other side. Whichever way the boat turns is the inefficient oar. (In the extreme case, if you stop rowing with one oar, it's obvious that the stroking oar is providing more driving force.)
    Both curved and flat blade oars weigh the same in my case.

    [​IMG]

    [​IMG]

    Also interesting to note that curved oars don't work very well in reverse. Just another way of saying that reverse curved oars are very inefficient.

    I think the determination of the resultant force on a curved blade is a complex task, especially when you need to determine it for every angle of blade movement from catch to release. Perhaps the complexity of the problem is why there is no published analysis of curved vs. flat blade oar performance.

    After analyzing the oar blade reaction, the geometric analysis of the human body interface could be added to find if the curved blade assists at the point where the body develops maximum pull at the oar handle. That would have to be analyzed for both fixed and sliding seat configurations.
     
  14. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Same thing happens with a canoe paddle when you reach forward and strike the water with the paddle. You are stabbing forward, but also sweeping down, and at the end of the stroke you are sweeping up, and so even though the blade is moving forward relative to the water at the beginning and end of the stroke it is also moving down, and then up, and to it is still pushing backwards by generating lift for net thrust.
     

  15. NoEyeDeer
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    NoEyeDeer Senior Member

    It's not that simple. You're just taking basic geometry and ignoring everything else.


    Again, it's not that simple. Sculls use catch angles around 70 degrees and release angle around 45. This is actually more efficient than using longer oars with lower angles.

    Have a read of this stuff: http://www.atkinsopht.com/row/rowrpage.htm
     
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