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

    When starting, the oar blade motion obviously moves backward with respect to the surrounding water (as expected).

    [​IMG]

    When rowing at speed about 6 mph, the oar blade does not move forward with respect to the water, but stays close to "fixed position" with respect to the water.

    [​IMG]

    The video clips from which the above screen captures were taken can be viewed on youTube:



    Due to the urgency for me collecting the data before the onslaught of tropical storm Bill, I had to ignore the effects of gravitational lensing as I approached "c" (the speed of light) on my trial runs. This may have introduced some inaccuracies. [​IMG]

    GPS track:
    [​IMG]

    GPS Performance data:
    [​IMG]

    Conclusion: My rowing skiff does not have the racing performance required to get the reported lift on the catch and release portions of the oar stroke.

    This concept of lift on the catch part of the stroke is an interesting sidebar to my primary quest for an analytical analysis of curved vs. flat oar blades. That issue still remains unresolved.
     
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  2. markdrela
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    markdrela Senior Member

    Your oars are showing much more slip than what I see on racing oars. In effect, your oars are always "stalled" and acting like parachutes, so better leading edge shapes won't help much. The main thing I'd try is increasing the blade area so that you actually have significant lift phases of the oar motion.
     
  3. Rurudyne
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    Rurudyne Senior Member

    Nice visual capture of slip in real life.

    Since your GPS data is time logged could you use a heart monitor with time logging capability to try to collect and overlay some data. That way you could, over a number of trials, have some indication of the effort on your part for the speed attained with a given set of oars. This may not be exactly what you're looking for but it could be a useful reference to check any analysis you do come up with.
     
  4. Jamie Kennedy
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    Jamie Kennedy Senior Member

    Thanks for the video. Good work. I really love the boat.

    I think you assessment is bang on. In your case there is none of or at least very little of the forward and outward and downward 'swimming' motion of the blade. I think this might be because your catch angle is not as extreme as it could be. One of the advantages of adding a little more extreme angle at the beginning and end of the stroke is that it 'uses' different water than the blade in the middle of the stroke, effectively increasing the blade area, rather like when you are slightly broad reaching rather than running your sail is moving sideways to the wind and getting some fresh new wind. In the case of the oar blade it moved outward and generates a big lift eddy spinning one way, and then slips back a bit in the middle of the stroke while stalled with small eddies around all sides, and then at the finish it generates a big lift eddy spinning the other way. These two eddies supposedly cancel each other out in some way but I don't really get that. I think it has more to do with the fact that you get less slip from a combination of all three of these motions than if you just had one or two, because you change the water you are using and the way you are using it. By going back against itself on the finish the blade is going against some water coming towards it with momentum from the beginning of the stroke, recovering some of that slip energy. It isn't so much that anything cancels itself out after the blade leaves the water, but the blade is able to recover some of the kinetic energy from the beginning of the stroke with the finish of the stroke. Does that make sense?

    So how might you get a little more angle out of your stroke. Like me, your middle aged gut gets in the way a little, but unlike me, the miles you have put in has made it less of an issue in your case. Good work, and keep rowing. Without adding a sliding seat, which I am not sure is as neccessary for long distance cruising as for sprinting, you might try moving your feet (stretchers) a little further aft, and just stretching yourself a little further at the catch and finish. To compensate for this extra effort during the stroke, give yourself a little more time during the recover. One count for the stroke. Three counts for the recovery. That is for when you want the most out of your stroke. When rowing more relaxed just keep doing what you are doing, which is fine form. With more practice with the more complete range of motion it should get easier, but when practicing it still doesn't have to be an all out effort. It can even be slower. Just a good long stroke with good effort, and then lots of time on the recovery phase. I am not an expert but I hung around a rowing club as a boat man for a couple of summers. They seemed to really stress the importants of that long recovery phase, especially when training for form and endurance base. Cheers.
     
  5. NoEyeDeer
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    NoEyeDeer Senior Member

    Utterly impractical and impossible to row with. It doesn't matter how much better it is in theory if you can't row with it.
     
  6. NoEyeDeer
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    NoEyeDeer Senior Member

    Catch angles are important too. Your catch angle appears to be very short when compared to sculls, something like 30 degrees on your boat compared to around 70 degrees on a scull. By the time a scull's oars are at 30 degrees they are well into the drag phase.

    You may be right that your top speed is not enough to get any lift, but to know for sure you'd have to compare over equal stroke angles.
     
  7. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I also think it has more to do with catch angle than top speed. As you noted when starting out the catch angle is less important and 2 or 3 quick half strokes are more effective, but once you are up to speed, even at 4-6 knots versus 10-12 knots, I think it is the catch and finish angles that matter.

    Like you mentioned in your earlier post it seems counter-intuitive for efficiency because of the transmission angle, but just as will machinery the transmission angle does not effect efficiency directly, only indirectly because of things like friction. The high angles at catch and finish are what allows the blade to travel through different water, first close to the boat, and then away from the boat, and then close to the boat again recovering some of the momentum from the water from the outward motion. You may be going half the speed, but your oars are also half the length. I think the angles and the motion can still work the same way, just as a sail or keel or propeller works pretty much the same way at half speed as at full speed. But you need the angles in order to get the sideways motion at the beginning and end of the stroke.

    I know you are going to improve your time now, with all our help. Just be careful out there in that hurricane. ;-)

    p.s. This is all new to me too by the way. Thanks for the thread.
     
  8. SailorDon
    Joined: Apr 2013
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    SailorDon Senior Member

    I bought my rowing skiff a little more than 2 years ago. I was looking for a performance rowing skiff where I could take a passenger out for a cruise on the lake. (Or row doubles.)

    The first design that came to my my attention was the fiberglass Heritage 18 from Little River Marine (Gainesville, Florida).
    [​IMG]
    It has two rowing performance features that I don't have on my Thames Rowing Skiff.
    1. Outriggers for oarlocks.
    2. Sliding seats.
    The Heritage 18 is about 20 pounds lighter than my wooden Thames Rowing Skiff, so it would be the better performance rowing skiff for me.
    But the Heritage 18 won't win any trophies at wooden boat shows!

    I was looking for performance, but it turned out that I enjoyed showing (and rowing) my wooden rowing skiff at wooden boat shows.
    The builder of my boat has a
    First in Class from Keels and Wheels, Seabrook, TX.
    I have a
    Second in Class from Pirate's Cove, Josephine, Alabama, and a
    First in Class from Madisonville Wooden Boat Show, Madisonville, Louisiana.
    [​IMG]

    If I want to get serious about improving my rowing performance, I need to sacrifice some "show boat" features and go for "row boat" go-fast features, but no racing shells. I need the stability and passenger/cargo capability of a rowing skiff.
    .
     
  9. NoEyeDeer
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    NoEyeDeer Senior Member

    If all the extra stuff just drops into place and is easily removable, you don't have to worry about it restricting performance. Varnish doesn't weigh much. Wood is good for structural stuff. No problem. :)

    Mine came in at hull + main thwart + footrest + oars = 33.9 kg (74 lbs 11 oz) for a boat which is 19 feet long and 4 feet wide. The bare hull is exactly 28 kg (61 lbs 11 oz).

    Note that the scantlings are adequate for use but not immune to abuse. However, it is possible to build a usable boat this light. I think I could get it down to 70 lbs rigged.
     
  10. Jamie Kennedy
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    Jamie Kennedy Senior Member

    That really is a beautiful boat, beautifully finished. I love the rudder.
     
  11. Jamie Kennedy
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    Jamie Kennedy Senior Member

    The wonderful thing about boats is the speed^3 thing.
    When running, to run twice as far you need to slow down by 10%.
    When rowing, when you slow down by 10% you can row 8 times as far.

    Put another way, even if you only have half the power as the Olympic sprinter, and you and your boat are twice as heavy with twice the wetted surface, and you want to be capable of rowing all day even in choppy water as opposed to 6 minutes in flat water, you can still go half as fast and cover about 50 times the distance, and your boat only needs to be about 70% as long, or about 16-17 feet as opposed to 26 feet. You can also add some nice details, and pack a lunch, and bring a friend. So there are compromises, but not as much as with running or cycling. With boats, the benefits always seem to outweigh the compromises. :)
     
  12. SailorDon
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    SailorDon Senior Member

    That is the same analysis on which I based my decision to go with a rowing skiff design.

    Competitive rowing shells race over a 2,000 meter (1.24 miles) course. At the end of the race, they have burned up all their energy reserves and have to rest before they can take another stroke. I won't even consider putting my boat in the water to row only 1.24 miles. Typically, I row 3 to 4 miles when I go out on the lake. I don't aspire to be a competitive rower. I want to go out for an hour or so and get some aerobic exercise while I "zone out" on the rowing experience.

    I have no plans to alter my Thames Rowing Skiff in a quest for an extra 1/2 mph. No sliding seats, no outriggers, no carbon fiber oars, etc.
    .
     
  13. Jamie Kennedy
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    Jamie Kennedy Senior Member

    It's a beautiful boat for sure. I wouldn't change a thing. As a final suggestion though I really think you should get a heart rate monitor. Even though as you say your heart rate will vary with barking seagulls and bikini girls passing on the jet skis, when you are rowing in the aerobic zone of 60-80% your average heart rate is a pretty good indication of how much oxygen you have burned. What I like it for is not so much for setting personal bests or for my harder training days, but for my long slow distance days. This was for running, kayaking, and cross-country skiing and not rowing, but it still applies. So what I would do is say ok, I am going to average 140 bpm today for my hour run or two hour row, or whatever it would be for you for an easy day, and then go out and just focus on form and efficiency and see how fast you can row at a steady speed while maintaining your average heart rate in that zone. This is probably the best way to work on your form and efficiency, and build and maintain a really solid aerobic base. You can experiment with different setups and angles but I think once you have selected your equipment your body will quite naturally find the most efficient form and rhythm, especially on longer rows, like 2 hours or more as you are doing. You've go me thinking now I need another boat. You don't need to wear the HRM all the time either. It is even more important to listen to your boat and your body as you are doing. Once a week or even just once a month is enough. If you like numbers it will give you another excuse to get out on the water. 

    Isaiah 40:30-31 ;-)

    https://www.youtube.com/watch?v=L14oWp_j2ZU
     
  14. NoEyeDeer
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    NoEyeDeer Senior Member

    I'll point out that the choice of very light scantlings for my boat wasn't only in search of extra speed. I transport the boat on top of racks on my truck, and as well as getting it on and off the racks I often have to get it up and down (and I do mean up and down) a rough dirt track 50 metres long to get to the water at one of my favourite launching spots.

    Having the boat as light as possible greatly eases handling out of the water. This was the main concern. If it was 50% heavier it would be a lot more than 50% harder to handle on land. I very much appreciate the light weight for the way I use the boat. I never find myself wishing the effing thing was heavier. :D

    A lesser advantage is that less material equals less material cost, although that's not really significant in the long run.

    If you don't use your boat this way, and if you're not concerned about top speed, there can be some advantage to a bit of weight. At low speeds wave making isn't significant, and skin friction doesn't increase as fast as displacement. This means that at low speeds a heavier boat can have more carry between strokes. It'll still have more resistance* than a lighter boat, but it can still feel nice to row. If you're only worried about it feeling nice to row at moderate speeds, no problem.

    *Assuming equal speeds.
     

  15. SailorDon
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    SailorDon Senior Member

    Cartopping my Thames Rowing Skiff is out of the question. The boat weighs about 110 pounds empty. I cannot lift that onto a rooftop carrier.

    Almost all of my rowing is on Lake Livingston (Texas).
    I own a boathouse with boat lift. I can launch and retrieve my boat with the push of a button (electric winch on boat lift).
    [​IMG]

    When I go to other lakes, I trailer my boat and launch at boat launch ramps.

    [​IMG]
     
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