Pedal Powered Boats

Discussion in 'Boat Design' started by Guest625101138, Jul 14, 2008.

  1. Guest625101138

    Guest625101138 Previous Member

  2. Dave Gudeman
    Joined: Nov 2009
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    Dave Gudeman Senior Member

    So much for all my speculation about boat performance :)
     
  3. Guest625101138

    Guest625101138 Previous Member

    Well not entirely out of place. Weight does affect performance but it is not as critical on a boat as say a bike on a hill. There are many factors involved with a boat and for any given displacement and power level there will be a lowest drag shape for calm water. How it translates into performance in wind and waves is another level of complexity.

    If you have a single person pedal boat that takes 15 minutes to get out of its storage location with two people to load it on a car and likewise to launch it then it will be less likely to be used regularly than something that can be easily handled by one person. Getting the weight down to achieve this is not easy so it is something that I spend a lot of time thinking about.

    Rick W
     
  4. OldNick
    Joined: Dec 2009
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    OldNick Junior Member

    Complete noob bursting with questions

    This is a fascinating thread.

    I have sailed, motored, paddled and rowed, but never pedalled (except once on one of those touristy paddle-wheel things)

    But I really like the idea of the power that your legs can provide.

    I am interested in driving a catamaran (Hobie 18, which I already own), probably with two pedal stations, linked to a common prop (I have read enough of this thread to think that props are still the most efficient). I want the common prop (or props driven by linked shaft) so that if the two pedallers are unequal, even though one is not providing equal power, the speeds are the same on the prop(s).

    I realise the cat has extra drag, especially one as big as the 18. I want a cat, for the purposes of stability and room.

    Question 1: Will it be too big. The pedallers will be moderately cycle-fit 50+. But I would like to use only one pedaller if needs be. I would look at using an electric trolling motor as supplement. The usage would be mostly up-river stuff, with the river maybe 50-100 metres wide. So you can get strong winds (20 knots sea breeze) but not many waves beside boat wakes. (8 Knot speed limit). Mostly the worst breezes will be dead ahead, because there is heavy riverside bush in most places.

    I have looked at a lot of posts here about various linkages and drive shafts. Is the flexible / bent shaft drive a solid bar, or one of those cables that are used to hang off drills to get round curves?

    That will do, while I ask. "Should I post a lot of questions at once, or post a few posts?"

    thanks for any help.

    Nick
     
  5. Guest625101138

    Guest625101138 Previous Member

    Nick
    Probably best if you just piece it together bit by bit.

    The curved shaft is preferably spring steel or some other very high strength steel. Fibreglass has also been used. Carbon fibre is really too stiff.

    The reason for the curved shaft is that it allows you to run high aspect blades without getting unbalanced forces. You can use universal joints to do the same thing but they do not feel as smooth as a solid shaft.

    Getting hold of spring steel can be a challenge so something to check before you get locked into a path.

    The attached photo shows a concept with the riders sitting transversely driving a shaft in each hull. A possibility is transverse seating but driving a single prop. Sitting across the hull avoids the expense of a right angle box or the complexity of a severely twisted chain.

    It is possible to do performance predictions. The nearest to your hulls already on the thread is Fangle that Tiny Turnip put together. Two up on it do about 5mph at all day type pace on it. The Hobie might do better because it is a bit longer.

    Close to Perfection, attached photo, hit 11kts with two athletic riders.


    Rick W
     

    Attached Files:

  6. OldNick
    Joined: Dec 2009
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    OldNick Junior Member

    Wow! Fast answer. You must be interested in this stuff or something :)

    Ok I will ask a bit at a time. Thanks. I will also be taking my time before rushing in to the build! I have built a few boats and repaired a few more and there is nothing like skimping and rushing to make you pay later.

    Ok. Yeah transverse makes a lot of sense! It would also intrude less into the deck of the craft.

    I do have to say that given that pedalling a boat is already different, this adds to it!

    Good for conversation, not good if you are not talking to the other paddler. You know how it can get on boats with "loved ones"?

    Flexible shafts. I am assuming that the shaft does not whip because of its springiness (I imagine it sort of flipping direction), and this would also decide how many mounting points you need? I think I remember you mentioning also that it allows the prop to be taken from the water.
     
  7. Guest625101138

    Guest625101138 Previous Member

    The transverse seating is not my preferred but it is a cheaper way to start. A gearbox will set you back about USD250 and it will need transfer shaft for central mounting. On a large cat like a Hobie I would probably build a drive frame for each hull with is own prop on the inside of each hull. But then I am unlikely to use something like a Hobie. I like stabilised monohulls as you can see from the various ones in previous posts. I have made a few cats but they end up too heavy for me to cartop easily.

    The shaft only needs one point of support - at the gearbox in my case. A pushing prop is self stabilising so there is no need for a shaft strut but I usually have one to prevent the prop from hitting the hull as it swings about during turns and over waves. There is no vibration as the water dampens out any resonance as you see in air. I have never worked out what size shaft would give problems but the 6mm and 8mm shafts you need to transmit human power is not much different to a guitar string and a guitar string will not vibrate under water for very long.

    Rick W
     
  8. OldNick
    Joined: Dec 2009
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    OldNick Junior Member

    Just to add I am not into breaking any records. I definitely looking at the "all day" speed thing. 4-5mph would be heaps.

    Is the pedalling anything like a bike? A bike tends to be smoother, because you have a bit of momentum, although MTB and fast spinning can be tough.

    What sort of cadence are you aiming for (not the 140 that Chinese one gave!) for comfortable pedalling? I remember the wheeler I pedalled was really bad, because you were like a train on a slippery track: very jerky and not comfortable at all.

    Asking this because I need to start looking into prop diameter, pitch, gearing etc, I guess.
     
  9. OldNick
    Joined: Dec 2009
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    OldNick Junior Member

    Well I was not looking at car-topping the Hobie 18! :)

    Why do you not favour transverse seating? Apart from trying to do that in a monohull of course. I would have thought that it's not just cost, but any gear or direction change is inefficient. That was what struck me about transverse.

    I need to go over the flexible shaft thing very carefully, to get a grip on what you have just said....I know I did see a video of a flex shaft going over rocks and simply bumping across them, and I was puzzled.
     
  10. Guest625101138

    Guest625101138 Previous Member

    The torsional compliance in the drive train is a factor affecting the feel.

    The cadence will depend on how well trained you are. Peleton cyclists sit on about 90rpm. My most efficient is 68 to 70. I can work at 75 but it feels to light at first and then I find it difficult to keep up the spin. If I set to start at 70 then I drop to around 65 after 4 or 5 hours as muscles start to tire.

    Gearing around 1:4 works out for a reasonable size prop. This will give about 15"; maybe a bit bigger with two people on a Hobie.

    A 6mm shaft at 1:4 gearing feels jerky. An 8mm shaft feels quite nice. It hardly reduces the peak power compared with a bike. A 6mm shaft with 1:7 step is OK.

    The most efficient props need to be made or you can get some model plane props that do OK if you gear them up.

    I normally suggest to people setting out on a design to spend a bit of time on a recumbent trainer in a gym. You can set the load and monitor your heart rate, cadence and power level. Do this for about one hour so you body is accustomed to the level of work with steady heart rate. Then spend 10 minutes at the level you find sustainable for power and cadence. These can be used as design power and cadence for the boat.

    My all day power level is 120 to 130W. Others I have designed for work at levels up to 250W but you need to be in good shape and reasonable big for this. Some are down around 100W.

    There are other ways to determine your power level. Tall buildings with accessible staircases can be used.

    Rick W
     
  11. Guest625101138

    Guest625101138 Previous Member

    The precision boxes get around 97% efficiency if selected for the job. Even with the transverse set up you will need to offset the plane of the sprockets and once you do this the efficiency starts to drop off. Could drop from 99% to 97% and that loss translates to tooth and chain wear. The alternative is to use two universals or have a curved shaft in an "S".

    My main problem with transverse seating is getting a sore neck. I like to see what is ahead of me.

    The boat in this clip uses a 1/4" spring steel shaft with no support at the prop:
    http://www.youtube.com/watch?v=OTITPVv_Rac&feature=player_embedded
    You can see how well the power is applied. He would do better with an 8mm shaft but he is well trained and spins well.

    Rick W
     
  12. OldNick
    Joined: Dec 2009
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    OldNick Junior Member

    Reply out of politeness. I have a lot to absorb before I get intelligible.

    I am loving this!

    Yep I could see the need for an S shaft. That adds inefficiency although you would have room for a gentle set of curves. But there is simplicity.

    Surely a 97% efficient gearbox ain't going to be $250.

    Re the sore neck. As an ardent sailor, I am used to sitting athwart and looking forward.

    So the prop just hangs there on a flexy shaft? You talk of it hitting the hull etc during turns

    That video hurts. It's very dramatic. But aiming for 15KPH over a kilo is really something

    Nick
     
  13. Guest625101138

    Guest625101138 Previous Member

    The inefficiency in the "S" shaft would come about by the need for an intermediate bearing and its lower torsional rigidity at high power. At low power it would be OK. It also would need to be very long to get the required radius twice.

    Most people baulk at the idea of a USD250 gearbox. Something like this might do for two:
    http://www.mitrpak.com/product_datasheet.php?product_id=65
    If they are both strong riders it may need to be the next size up.

    I do not use an unsupported shaft for normal use. It is the way to get lowest drag. However it means you cannot go reverse with any power because the shaft dives. Also the prop hits the hull unless it is set deep. It does not suffer this problem if mounted on the centreline of a catamaran. However it does flop about and hangs deep when the prop is not turning. It is not really practical.

    Rick W
     
  14. portacruise
    Joined: Jun 2009
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    portacruise Senior Member

    Rick, I built a low power applications (under 50 watts electric drive) carbon fiber shaft that works quite well. It can be even about 40% thinner than steel if need be, shaving some weight. The main problem has not been the stiffness, but torsional strength. I used a thin, solid pultruded unidirectional kite spar and dig get some cracks in the shaft when my prop tangled and stalled full stop on a tree root at full torque. The twisting strength issue can be alleviated with tubes made with braided fibers combined with longitudinal. Haven't been able to find any shaft thin enough like that for my low power applications. Some larger starting at approx. 1/4" braided/longitudinal carbon shaft are available but only a shorter lengths and high $ at the present. You get a stealth mode with thin diameter shaft which is almost invisible beyond several feet, leaving people wondering what the source of power driving you is!


     

  15. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

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