Reasonable efficiency estimate for a pedal drive

Discussion in 'Boat Design' started by kerosene, Nov 19, 2021.

  1. kerosene
    Joined: Jul 2006
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    kerosene Senior Member

    good call on the pontoons and padlle clearance. Padlle will be a backup anyway an needed for harbour maneuvers so ability operate without obstruction is important.

    I have read the 1:6 ratio many times but all race vessels are way narrower - including Greg's and Rick's boats. Same goes for rowing shells etc.
    My guess is that the returns start to diminish at 1:6 point so it is a good target.

    here is Greg's 24h boat. Impressive but not a practical all around boat.
     

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  2. Tiny Turnip
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    Tiny Turnip Senior Member

    Your goals are spot on, I think. Its worth noting that Rick's most full on efficent craft were optimised at around 7 metres, but later boats he scaled back to around 5, as, I suspect, the gains from the extra two metres were fairly negligable. FWIW, my open sailing tri is 5m x .8m, and is a very comfortable camping cruiser for two adults. The 80cm beam even allows two mediumish children to sit side by side. And its plenty swift. Long amas with paddling comfortable between the akas.

    I'm very familiar with the 'not getting things done' problem. Paul McCready was a believer in 'quick and dirty', and built the first Gossamer Condor in 10 days. When I built my pedal cat, I was recently widowed, with 2 small children and a full time job. I didn't have the time and energy to build hulls and drive, hence the use of ready made items.

    Now I am early retired, (still find it very hard to use the R word!) I am contemplating a rebuild of the pedal cat, same drives, just wanting a lighter boat as it is fiendishly heavy. The sailing tri is light enough for two people to carry fully rigged, and I find that a big advantage.

    I'm actually wondering about a monohull with outriggers, but maintaining the side by side seating, as it is so companionable, but even with careful attention to balance, I suspect the amas would have to have much larger volume, and I might as well stick with the cat arrangement.

    And I'd recommend prioritising comfortable seats, too.

    In terms of simplifying to help getting things done, how much advantage in efficiency does a 5 panel chine give you over 3? parallel sided, with pointy ends?
     
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  3. alan craig
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    alan craig Senior Member

    This is how small you can go with the stabilisers; a single 3 metre length of 68mm dia. PVC downpipe cut in half and fitted with an extruded polystyrene bullet in the front and a sealed plywood plug at the back. Two threaded rods go into wood blocks inside. Stabilisers about 1.8 metres apart.
    Tiny Turnip, I second the dislike of the R word. It means no longer of use to the world!
     
  4. Andrew Kirk
    Joined: Jul 2021
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    Andrew Kirk Junior Member

    We're I to go the prop. driven route on a future build I'd be tempted to DIY the mechanism. Here's one.

    Plenty of bodgers have made drives using hand drills.
     
  5. Tiny Turnip
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    Tiny Turnip Senior Member

    Alan, we used your method for outriggers on a raft race build a while back. (We used 100mm downpipe, and closed the ends with 2 litre pop bottles (gaffer tape and squirty foam may have been harmed in the process)

    camping kid build for raft race https://www.boatdesign.net/threads/camping-kid-build-for-raft-race.28311/#post-295404

    I've been trying to come up with an acceptable alternative. 'No longer ground down by the b*st*rds' doesn't quite trip off the tongue!
     
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  6. Heimfried
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    Heimfried Senior Member

    Andrew, especially at full speed your paddlewheel is operating in a wake. That means your boat drags some water with it, which has a certain velocity.
    According to this source
    Wake fraction coefficient https://www.wartsila.com/encyclopedia/term/wake-fraction-coefficient
    its velocity could be a considerable part of a ships velocity (I don't think that figures for boats will be very different in this regard).

    That would explain your "extraordinary efficiency".
     
  7. Andrew Kirk
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    Andrew Kirk Junior Member

    Yes, I asked a question about this on this forum because it was the only explanation I could think of. Maybe my inefficient flat transom is actually an advantage in my unusual application.
     
  8. Heimfried
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    Heimfried Senior Member

    Every solid moved through water will cause some kind of wake because of the viscosity of water. It looks like your flat transom is partly immersed in water in operation - this will cause turbulence.
    I suspect this will weaken the wake rather than strenghten it - not shure about it.
     
  9. portacruise
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    portacruise Senior Member

    The question I wonder about, is whether a Waterwheel while spinning under propulsion counteracts the drag and turbulence associated with a square transom, (and does it even get to break even) when compared to a tapered transom, as in a kayak?

    Specially designed water wheels and air props can be pretty fast, but I've never seen any of them legitimately in the record books for speed, or efficiency. Decavitator exploited a rule that allowed a 3 mph wind boost, and later lost in a rematch with a prop driven craft.

    The water speed record that's surprisingly hard to break https://www.bbc.com/future/article/20170921-how-fast-can-a-watercraft-powered-by-humans-go
     
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  10. tane
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    tane Junior Member

    I wonder about the efficiency-loss due to the recumbent position. A critical dimension of a street/mountain-bike is the seattube-angle, i.e. the vertical position of the rider over the crank. Lore has it the front of the kneecap has to be vertically above the pedal axe, crank in horizontal forward position, for maximum effort being possible. Of course wind resistance is high in this position, but there has to be a loss due to worse ergonomy in the recumbent position. I wonder how much...
    they seem fast:


    !!!
    & recumbent:
    & also:
     
  11. DogCavalry
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    DogCavalry I aim to misbehave.

    There doesn't have to be a loss @tane . Correct position of standing has no ergonomic relationship to correct recumbent position.
     
  12. tane
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    tane Junior Member

    cyclists, particularly competitive ones go to great length to position the saddle correctly in a for-&-aft position for maximum efficiency, and suddenly tilting the whole thing 90° bckwards doen't matter?
     
  13. DogCavalry
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    DogCavalry I aim to misbehave.

    Different positioning considerations, not irrelevant positioning considerations. Ergonomically massive improvement, although peak energy is lessened. The reason recumbent bikes aren't allowed to race with the others in most races is because they make a mockery of the race. Regular bikes can't compete except on steep climbs, where they pull out ahead. But they are much less efficient on level ground and downhill, so unless the race is just a hill climb, recumbents have extremely unfair advantages. Incidentally, massively easier on the body, and don't cause the nerve damage in the junk area.
     
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  14. jehardiman
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    jehardiman Senior Member

    Not really. In powering tests for the SUBHUMAN human powered submarines, we found that more thrust could be achieved in the prone and supine than could be achieved on a test bike. It is really simple if you think about it, in a "normal" cycling position the maximum force that can be brought to bear on the pedals is the cyclists own weight. When constrained, either waist or shoulders, the maximum force was up to twice the "normal" force.
     

  15. tane
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    tane Junior Member

    the big advantage of recumbent bikes on the road is the air resistance, which at speed is massively bigger (A.cw.v²/2 if I remember correctly) on a "normal" bike. Climbing this becomes irrelevant & the recumbent loses. When one stands up on the pedals the force that can be created is much bigger than the cyclist's weight, as one pulls quite strongly on the handlebars. I wonder if the extra amount of thrust, that can be created through "restraining" is sustainable for any length of time & how you make up for the power of the gluteus maximus & the hamstrings lost through the "upright" position if recumbent. Through bending in the waist these muscles contribute strongly in a normal riding position. Watch yourself climbing on a normal bike: try a "Holland position" & then bend at the waist. I cycle & mountainbike quite a lot & the contribution of hamstring & gluteus declines sharply as one straightens the waist/sits more upright: you can feel that the load is mainly take by the quadriceps & then, leaning forward, you start feeling hamstrings & gluteus.
    On the water of course air restistance is not irrelevant if there is any wind blowing or if one goes sufficiently fast...
     
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