Paddle vs. Pedal?

Discussion in 'Boat Design' started by KalleA, Mar 28, 2009.

  1. markdrela
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    markdrela Senior Member

    A major advantage of circular pedaling is that it is a forced motion, in which the kinematics or motion paths of all the major mechanical and body masses are fully constrained by the mechanism. Such forced motion conserves kinetic energy, so that nearly all muscle force power ends up in the drive shaft, the only losses being mechanical friction in the gears and bearings.

    In contrast, paddling, sculling, rowing, etc. are free motion, where the mass motions are determined by the athlete. In this case the athlete must dissipate some kinetic energy on each stroke in order to reverse the motion to start a new stroke cycle. The braking is typically done by the muscles in "brake" mode, where they exert a contractile force while extending.

    One can directly compare the power loss of free and forced motion having roughly the same kinematics:
    1) Free motion: Make rapid pedaling circles "in air" while lying on your back, or sitting on a stationary bike with your feet ahead of or behind the pedals. It's quite tiring, because your muscles are constantly exerting positive and negative work to accelerate and decelerate each leg over each pedal cycle. But the negative work is 100% lost as heat, since muscles do not have a regenerative braking system.
    2) Forced motion: Now hook your feet to the stationary bike pedals without a chain. Pedaling at the same RPM is now almost effortless, because the cranks are doing all the motion reversals, not your muscles.

    The siding-seat rowing or sculling system is a sort of halfway solution where only some of the motion is forced, but not all. The sliding seat also reduces the mass which is oscillated, which is always better with free motion (with forced motion, how much mass is being oscillated doesn't really matter).

    The sliding seat could be made nearly 100% forced by incorporating a flywheel which would would absorb and provide the power to do the motion reversals. I think such a system would blow away all current rowing records, but it would most likely be quickly banned to prevent a complexity and $ arms race.
     
  2. KalleA
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    KalleA Junior Member

    Sounds right. Wish I had paid more attention in mechanics class.

    Thinking of rotating pedals vs. top pivoted swing arms (which would both be forced motions, right?): is there a relevance of continous motion (pedalling the crank) vs. accellerating/decellerating motion (pushing the swing arm in a pendulum motion)?

    Cheers
     
  3. markdrela
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    markdrela Senior Member

    Oscillating motion in itself isn't necessarily bad. For example, the thighs oscillate during pedaling. It's only bad if the oscillating motion must be enforced by "eccentric" (or negative) muscle work.

    There's a simple mental test to determine whether a power motion is forced or free:
    Imagine that the athlete goes completely limp for a few seconds, and the mechanism becomes frictionless and subjected to vacuum so there are no aero/hydro loads.
    If the mechanisms keeps running in the same cyclic way (as pedaling would), then it's of the forced-motion type.
    If the mechanism bangs against a stop or flails about in some way (as rowing, paddling, running, etc all would), then the mechanism is of the free-motion type.

    As I mentioned before, there are partially free/forced systems. Also, some systems are "forced" by gravity or spring forces, even though they appear to be mechanically "free". For example, a classical cross-country skier must reverse his arm and leg motions repeatedly (free motion -- bad). But by letting the legs and arms swing like pendulums, some of this reversal is done by gravity rather than by muscle brake forces. This gravity-forced motion produces a rather large power savings for the skier. If the oscillating motion is close to the natural pendulum frequency, nearly all the muscle braking forces can be eliminated.

    I don't see such gravity forcing being usable for paddling or rowing. However, one could imagine putting a spring behind the rower so that his upper body motion is stopped and reversed by the spring rather than by his muscles. Another spring or springs could do the same for the oar, perhaps installed in the oarlock. By tuning the stiffness of these springs, the power savings might be quite substantial.
     
  4. KalleA
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    KalleA Junior Member

    OK, let me just check that I get this:

    - for a pendulum swing arm setup, top pivoting must be the way to go since it uses gravity for some of the reversal?

    - augumenting a top pivoting setup with return springs would further improve the efficiency, potentially resulting in a fully forced motion?

    - the FrontRower discussed above might actually have some real biodynamic advantages over conventional rowing with its fixed seat, top pivoting swing arms and spring assisted return stroke?

    Cheers and thanx for the lessons
     
  5. Guest625101138

    Guest625101138 Previous Member

    The swing arm system that I proposed to Warren Loomis, and he supplied the parts, has two rollers meaning it is being pulsed almost constantly. There is a small period during the arm reversal when it freewheels. Initial testing that Warren did showed it was slightly more efficient than his sliding rigger set up with the long power stroke and long coast.

    There are pictures and video of my test frame that might give you a better idea of the reeving for the swing arms in the Pedal boat thread.

    Clearly someone thought enough about the swing arm system to try it on a bike. I do not know anyone who has tried this bike but I could imagine gear changing being a disadvantage to a standard system. Greg did not believe it was more efficient than cycling and he has looked into some of these things.

    The critical part of the design is to link the two arms via the pull cord so you get an enforced motion or simple harmonic motion. I did a dynamic model of the legs and swing arms to optimise the geometry. It is a very pleasant motion for a relaxed pace and I am reasonably confident can be set up to be more efficient than cycling.

    One of the reasons I stopped testing my system was the lack of reverse and I had a bad experience with weed fouling the prop. Took me about 15 minutes to get to shore to clean the weed from the prop.

    Rick W
     
  6. jehardiman
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    jehardiman Senior Member

    FWIW, SubHuman III was built with a "stair-stepper" Bi-directional drive that used over-running clutches to drive a counter-rotating prop set. This drive type was selected for enevlope considerations because it has significant advantages over a rotory crank set. SubHuman II used an elipitical sproket set for the dead space problem, but needed a 30" diameter , 3000# hydrodynamic mass hull to fit it in, SubHuman III was able to get the hull down to 26" and about 1300# hydrodynamic mass.

    I'll see if I can find some pictures. Additional, I have pictures of almost all HPV submarines from the first 3 ('89, 91, & '93) international sub races.
     
  7. sigurd
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    sigurd Pompuous Pangolin

    Would love to see all those pics.
     
  8. Village_Idiot
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    Village_Idiot Senior Member

    This makes me wonder about the drag of a paddle or oar when being pulled through the water. You know, the great void left on the lee side of the paddle that tries to suck the paddle as well as water back in to fill the void?

    Have there been any design concepts on paddles or oars to minimize this drag? Perhaps a teardrop-shaped bubble on the backside of the paddle? Of course, this would turn it into a "one-way" or unidirectional paddle, which may not be a good design in applicability.
     
  9. KalleA
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    KalleA Junior Member

    It would be interesting to see some pictures or drawings of this. Did you make any estimates regarding biodynamical and mechanical efficiencies of this setup?

    Cheers
     
  10. Guest625101138

    Guest625101138 Previous Member

    Kayak paddles are cupped. The aim of any paddle is to grip the water so the relative velocity through the water is very low.

    Rick W
     
  11. Village_Idiot
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    Village_Idiot Senior Member

    True about the low velocity, once you are up to speed. However, I was also thinking in terms of paddles on a paddlewheeler...
     
  12. jehardiman
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    jehardiman Senior Member

    High efficiency paddle wheel blades are cupped also. In order for a "paddle" blade (or oar or anything to else for that matter) to generate "force" it must be 1) attempting move at some relative velocity to the water, 2) have a significant area perpendicular to the relative direction of motion, and 3) change the speed and direction of the water. A prop or a paddle that is not "slipping" relative to the water generates no thrust. A "cupped" propeller, oar, or paddle generates more thrust because the apparent velocity off the edge generates a larger relative velocity change (i.e. 1-0=1 but 1-(-1)=2).

    See the pelton wheel diagram below:

    [​IMG]
     

  13. markdrela
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    markdrela Senior Member

    True. A propeller also has some "slip" in the water. This is also known as "induced velocity" or "self-induction" velocity. In each case, this slip implies a maximum possible efficiency that the device can achieve, known as Froude efficiency:
    eff_Froude = 1 / (1 + v_slip/V)
    The attached plot compares this for an ideal prop and an ideal paddle (for CD=1). The prop is vastly superior for the same disk area. Both curves asymptote to 1, but the paddle asymptotes extremely slowly, which means it must be huge to be efficient.
    The paddle can be improved by increasing its CD by cupping, but that's just the same as increasing the area a bit.

    The prop has an additional loss from blade-airfoil profile drag that the paddle does not, but even with this it's still better than any paddle of reasonable size.
     

    Attached Files:

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