Pedal Power or Oar Power - which is more efficient?

Discussion in 'Boat Design' started by SailorDon, Jun 6, 2015.

  1. SailorDon
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    SailorDon Senior Member

    I have rowed my Thames Rowing Skiff for over 2,000 miles. It is recreational exercise.
    I have never had the opportunity to try a pedal powered boat. There are many pedal powered boat designs on the internet, many with absurd designs, unsubstantiated performance claims, and broken internet links.

    My performance benchmark is to achieve and maintain a speed of 4.5 mph for 2 or more hours at a time in calm water with negligible wind. I can maintain 4.0 mph with my Thames Rowing Skiff.
    [​IMG]

    If I stay with oar power, I think I could improve my rowing performance with a Heritage 18 from Little River Marine.

    I searched performance pedal boats on the internet, and found Cadence from Open Water Cycling.
    http://www.openwatercycling.com/owcwhats_new.html

    I find their performance claims exceed what I would expect for that design. 5 to 6 mph (cruising speed) all day seems to be over optimistic.
    [​IMG]

    Does anyone have GPS data on the performance of the Cadence pedal boat?
    I don't think you can take a passenger on the Cadence. It looks like a solo vessel.

    IMHO the friction drag of the water on the propeller and rudder, combined with the mechanical losses in the chain drive and gearbox, would make the pedal powered boat a less efficient design, but I can't prove it.
     
  2. daiquiri
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    daiquiri Engineering and Design

    Hi Don,

    Not so long ago I was convinced that oars were inefficient and clumsy, a thing of past. But in the recent years I have seen a number of research papers which seem to prove the contrary. One of the best IMO being this one: http://ruina.tam.cornell.edu/research/topics/locomotion_and_robotics/oar_efficiency.pdf
    This thesis work done at the Cornell University (NY) seem to take into account all the nuances of the rowing mechanics, and demonstrates that oars can be up to 84-85% efficient. That's a number at least equal to (if not even better than) what the most carefully designed propellers can achieve.

    That being said, I agree with you that, everything else being equal, due to various appendages a boat pushed by a propeller is more draggy than the same boat pushed by the oars. However, is everything else really equal in these two cases? The answer is - not really.

    A big difference is given by the fact that oars are operated by arms and muscles of the torso. If the boat has a sliding seat, then leg muscles contribute too.

    Viceversa, a pedal-powered boat is moved almost exclusively by legs. Legs are very powerful and energy-efficient movers. Nature has made us to walk and run on legs, and has optimized them for that job. While pedaling, contractions of leg muscles act as an additional pump which helps the circulation of blood and takes a part of the burden from the heart. Ultimately, it is the fitness of your heart and your lungs which determine the long-distance power output of your body (through the parameter called "aerobic capacity"). So any external help can improve the energy efficiency and endurance of your body. Hence, from the biomechanical point of view, pedaling has this significant advantage.

    So it's not only a matter of the boat. The whole machine - the boat plus your body - have to be considered.

    With all this in mind, the OWC claim could easily be true. Even if more draggy, their boat might indeed have the potential of reaching higher speed and endurance, due to the fact that its motor (the human body) works in a more favorable condition.

    Though this reply does not answer your question in a definitive way, I do hope it will give you some food for thought. ;)

    Cheers
     
  3. Rurudyne
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    Rurudyne Senior Member

    Yep.

    I would also like to say it's easier, or at least potentially safer given possible reactions, to try impress ladies with your broad shoulders than your powerful thighs and buttocks.
     
  4. alan white
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    alan white Senior Member

    Is it not true that props have a theoretical max efficiency of about 59%? Whereas a paddle or an oar could potentially be much more efficient.
     
  5. philSweet
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    philSweet Senior Member

    Regarding biometrics, to be efficient at pedaling, you really need to bias your body mass towards your legs. If you are 6', 137lbs, 13" neck, 10" biceps, 37" chest, 25" waist, 23" thighs, 17" calves, and have a VO2 of 88 or better, cycling is pretty efficient (but it's really hard to find pants that fit). If you are 6' and look good in an off-the-rack 42 regular suit, then stick with rowing.

    When I planned to do a triathlon, I could gain 10 pounds in my arms and chest in
    2 weeks by swimming 2 hrs per day. This turned out to be too much. 7 pounds was about right.
     
  6. philSweet
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    philSweet Senior Member

    No Alan, that sounds like the old Betz limit for a turbine, which is based on some pretty crude assumptions anyway, and uses a different metric of efficiency than that used for props. Props can very efficient, but they do have to contend with surface friction, so not 100% in any practical device.

    http://en.wikipedia.org/wiki/Betz's_law
     
  7. daiquiri
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    daiquiri Engineering and Design

    As philSweet said.
    The best airplane propellers can achieve up to approximately 0.90-0.91 efficiency.
    The best ship propellers are in the ballpark of the previously cited oar efficiency (0.84-0.85), as far as I know.
    Cheers
     
  8. SailorDon
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    SailorDon Senior Member

    After scanning the Cornell research paper on rowing, I now know why my application to their faculty of engineering was rejected decades ago. :D

    Aside from the involved mathematical models, there are two things that don't make sense.
    #1
    No consideration is given to the efficiency of the curved oar blade as shown in Fig. 2. The force balance seems to treat it as a (inefficient) flat blade. The curved blade would decrease the effective theta angle, increasing the resultant force in the direction of boat motion.
    [​IMG]

    When I first bought my rowing skiff, it came with flat blade oars. I rowed it 300 miles with the flat blades before replacing with Shaw & Tenney wide curved blade oars (same 8 foot length). I have no scientific data collection, but after another 1,700 miles of rowing (with the curved blades), I feel that my oars are more efficient, maybe contributing an extra 1/2 mph to my normal cruising speed of 4 mph. This would be attributed to less slippage of the oar blade on the power stroke.

    #2
    In Fig. 3 of the referenced research paper, they present a graph of force on the oar blade vs. time (of the stroke). The duration of the power stroke is about 1 second which is close to my power stroke duration (as measured from a video clip where I was rowing 5 mph). The difference is they show a linear increase to maximum force at mid-stroke which then sharply drops off linearly to zero force at the end of the power stroke. I have no scientific measuring instruments, but it is my feeling that when I hit the "catch" I am putting full force on the oar blade and holding that force relatively constant throughout the power stroke. When I hit the "release" position, the force on the the oar blade drops to zero very fast (much faster than the 1/2 second on the Fig. 3 graph).
    Maybe I am doing it wrong and should not try for full force during the entire power stroke.
    Without any analysis, it is my opinion that when the oar blade is in the water, it needs to be pulling at full force for maximum efficiency.

    In section 4.1 of the paper, they state that oars are more efficient than propellers since the blades are out of the water on the recovery stroke. That seems to make sense to me.
     
  9. Tiny Turnip
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    Tiny Turnip Senior Member

    Hi Don

    I've followed the pedal powered boats thread from its inception. There has been a fair amount of discussion about comparative human powered boat efficiencies. IIRC, given an ideal set up for each of paddling, rowing and pedalling, pedalling does work out a percentage point or two more efficient than rowing or paddling overall, but it is pretty marginal, and there may well be other concerns (in a notional statement of requirements) that would eclipse achieving maximum efficiency and speed.

    Rick Willoughby's V series boats

    will, I believe, be the most efficient designs you are likely to find, (check out the V11)

    If you want to buy off the peg, I believe the Cadence is well regarded.

    For reference, my pedal boat, which is a heavy affair, a dart 18 with a ply deck, 2 recumbent seats, 2 seacycle drive units, and is intended for fishing, picnicing and general mucking about, achieves 4 mph comfortably as an all day cruise speed. I wasn't the slightest bit pernicketty about efficiencies when I put it together. Long and skinny hulls, and low windage is key.
     
  10. portacruise
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    portacruise Senior Member

    There are just so many variables besides the efficiency of just the prop vs oar, as I see it, (per Daiquiri). Ergonomics, differing muscle fatigue/aerobic capacity, strength and experience of the human motor, etc. may be greater considerations. Rick builds mostly for racing boats, which does not necessarily translate to longer endurance situations, but he gives free advice on custom build and might custom build an endurance prop for a fee . The Cadence prop is not particularly efficient compared to a larger 2 blade, and the setup is built to appeal to a wide gamut of customers around an average. Even so, the Cadence is probably capable of your target speed at perhaps the typical 100 watts sustained input. Another consideration is how difficult it is to clear the prop in weedy waters, which is not much of a factor with oars. Some of Rick's best weed free designs put a folding prop within instantaneous reach by hand, for fast clearing.

    Here's an older link on 24 world records which has a table of comparisons (scroll down) for amusement purposes:

    http://www.adventuresofgreg.com/PredictContest.html

    Hope this helps.

    PC
     
    Last edited: Jun 7, 2015
  11. philSweet
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    philSweet Senior Member

    Propellers don't have a recovery stroke. They provide constant thrust instead of surge-and-drift. This has several implications regarding efficiency, all of them in the prop's favor. The knock against props is the draggy, exposed, parasitic bits like shafts and hubs and struts; and in having to pass the thrust through a relatively fast moving bearing.
     
  12. messabout
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    messabout Senior Member

    A while back we had a discussion about oar efficiency. I had noticed that Olympic teams are using flat oars not cupped or curvy ones. They are slightly offset in most cases, but flat. Some of the forum respondents vowed that flat blades are more efficient or perhaps more powerful... maybe both.

    I'll go out on a limb and presume that Olympic oarsmen and women have tested the various oar designs thoroughly. I was disappointed with those observations because I have some sexy cupped oars and some expensive kayak paddles that are curvy. Anecdote; When I was an active kayaker I noticed that my paddling buddy, a female who was not as strong or as big as me, stubbornly used a Greenland style paddle while I used fancy carbon fiber ones. On the long haul she almost always beat me with her damned ugly slab sided wooden paddle. We had equivalent boats.
     
  13. Jamie Kennedy
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    Jamie Kennedy Senior Member

    I think both can be as efficient, or as inefficient, as the other. It is all about getting everything set up right for the speed and power of the person and the speed and drag characteristics of the boat. With oars, or paddles for that matter, the key is to have the oar or paddle plant itself in such a way to provide lift/drag with as little slip as possible in order for the boat to lever itself past the blade as though the blade was levering off a post stuck in the mud. Similarly the key with a propeller is to be screwing the boat through the water with minimal pushing of water backwards past the boat. Generally that means large diameter and lower revolutions. Of course compromises always have to be made to minimize other losses.
     
  14. Petros
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    Petros Senior Member

    messabout,

    greenland type paddles are very efficient for long distance low speed travel, though it would not fair as well in a short high speed sprint. I have studied fluid mechanics professionally at one point in my engineering career, do not assume that "curvy" commercially made paddles are well designed or efficient. many aspects of fluid mechanics are not obvious, and most of the paddle and oar makers are completely clueless. There are many who have done excellent research on the topic, but that information seldom makes it to commercial production. many of the mathematical models on analyzing a paddle or oar stroke have very faulty, or at best, unwarranted assumptions, so those types of models seldom produces better designs. Since there is no big prize or money in designing efficient paddles (an obsolete form of transportation, reduced to use in sports or recreation today), that will not likely change anytime soon. There is however big money to be saved with more efficient props on commercial vessels, so the tools to develop more efficient prop designs are available.

    In the past I had consulted and assisted in developing designs for use in Olympic cycling (used in the 1984 Olympics), I am somewhat familiar with the efficiency of the human "machine". As pointed out, the legs are far more suited for long distance effort, and likely more efficient in terms of energy in vs. power output. But for a water craft, that means a much more complex device, gear box, shaft, etc to apply that power to the water to achieve forward motion. A simple high aspect ratio blade in the water at the end of a long pole is a much less complex device, a lot fewer places to lose energy (less friction and mechanical loss through flex, trubulance, etc).

    So it is difficult to say, you have 1) more efficient power out put with the legs, but less efficient use of that power through a drive train and prop. Vs. 2) a simple and efficient oar configuration, with the less desirable upper body to power it. hard to say what is better, with lots of testing it likely can be determined which approach is better overall. But it is kind of a moot point, in competition, the rules determine which one you are using, and for purely recreation, people buy based on their own desires, not by which one is more efficient.

    I always thought it would be kind of fun to make a simple catamaran type float system that can be towed behind a bicycle, and when at the water you clamp the bike into it and the rear wheel drives a prop system for crossing a lake.

    As f
     

  15. daiquiri
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    daiquiri Engineering and Design

    How about carrying the catamaran in a backpack and then inflating it and mounting he bike on it when necessary: http://www.gizmag.com/go/2505/ :)
     
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