Pedal Powered Boats

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

  1. I57
    Joined: Feb 2008
    Posts: 172
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    Location: Melbourne, Australia

    I57 Senior Member

    Skeg

    Fitted a skeg to the flexible shaft, purpose is to not lose the prop if the shaft breaks. Also allows me to raise the shaft in shallow water and might be a way to clear weed. Skeg is a aluminium flat bar 25mm x 3mm, skeg can slide up and down the slot allowing the shaft to be under less stress. Having a bit of weed on the skeg is better than losing a prop, also it will enable me to reverse.

    Ian
     

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  2. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    Some ideas on how to minimize weed on the skeg FWIW. Use a feed screw spiral type attachment on the flex shaft that discourages surface weeds from following the shaft down to the skeg area. I am finding this requires a very coarse pitch spinning in a direction opposite of travel direction. The other thing that seems to help with surface weeds is to increase the angle of the shaft to be more vertical. Straight up vertical would collect the fewest weeds, but of course, then the thrust drops to zero. In my case, operating without a skeg, I simply adust the angle downward so the prop runs much deeper below the surface when I have to clear patches of weed. This slows me down only a little, but not as much as having to stop to clear weeds periodically. Probably by calibrating the longitudinal torsion, one could run with the shaft curved where it enters the water and still have the prop running deep below almost parallel to the water surface.

    Hope this helps

    Porta


     
  3. Guest625101138

    Guest625101138 Previous Member

    Vic
    Have you got a photo of the auger or screw flights you are using?

    Rick
     
  4. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    Rick, don't have a pic but will try to get one in the next few days. The shaft is all black so it is hard to see the ridges which don't protrude much from a normal photo. Will try painting the ridge tips so they show up better.

    What I have is not my final, successful product by any means, but it is better than a plain shaft that is not augered. I will try a higher ridge with maybe a 4 turn spiral over the 4 ' length next time.

    Vic

    Porta


     
  5. Dennis A
    Joined: Aug 2009
    Posts: 40
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    Location: Amersham bucks uk

    Dennis A Junior Member

    Out-Rigger Shape

    I realise that under ideal conditions the outriggers on pedal boats should be just clear of the water.
    Due to waves and boat wabble , these can and are often touching the water but what shape produces the least drag.
    Should they be flat bottomed or have a vee so that as they rise the contact surface reduces.
    Should the front end be pointed or flat like a ski.
    If they are ski shaped surely with the very light surface loads, they will tend to plane or semi-plane.
    Should the stern be pointed or flat.

    Dennis
     
  6. Guest625101138

    Guest625101138 Previous Member

    Dennis
    The things to aim for with an ama:
    1. Enough buoyancy so that in a static condition you are not going to roll the boat by moving your body weight to do the things you want to do. I find volume of 10 litres at 1m to be tolerable but it is possible to roll with this even in calm water. For my weight of 72kg I find 20l at 1m to be enough. Dimensions 2m long 100mm wide and 150mm high with tapered ends 500mm long and ski bow works well.

    2. The weight of the ama is really important. It is really a safeguard that only has a tiny fraction of its capacity in use most of the time. However the weight has to carried by the main hull all the time. An ama that weighs more than 1kg is overbuilt.

    3. It needs to give good roll compensation with small angles of roll. They are essentially in planing mode so I find ski nose and flat bottom the best as this offers good lift with little immersion. I have a pointed stern so when trimmed stern down the narrow portion is just kissing the water and offering a small degree of initial stability.

    4. The supporting beam needs to be set high enough so it is not clipping wave tops.

    5. Some slight adjustment vertically and in pitch so it can be set to suit the conditions and pilot weight.

    6. They are ideally set with the sterns roughly in line with the stern of the main hull so the boat does not flop roll when suspended between waves crests.

    7. The drag from windage is more than the water drag so aerodynamic shape is also important. The supporting beam can be foil shaped to reduce drag.

    Rick
     
  7. Dave Gudeman
    Joined: Nov 2009
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    Location: San Francisco, CA, USA

    Dave Gudeman Senior Member

    Rick, you emphasize the importance of weight quite a bit. I'm wondering if you can explain the mechanism for how it causes problems. It isn't obvious to me why weight matters so much at constant-speed on flat water. Does it have to do with getting more of the hull out of the water?
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Dave,

    It has to do with moving a larger volume (mass) of water out of the way for the hull to proceed forward. The faster you go, the faster you have to displace this mass, the more work per time has to be done, so more energy.

    -Tom
     
  9. Dave Gudeman
    Joined: Nov 2009
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    Location: San Francisco, CA, USA

    Dave Gudeman Senior Member

    Tom, I know that moving more water is going to take more energy, but how is that connected to the boat's weight? Well, I know there is a connection between the boat's weight, the boat's displacement, and the amount of water it moves because a boat sitting deeper in the water will have to displace more water when it moves, but is that the only factor here?

    The reason that I'm questioning this is because it seems that, broadly speaking, the weight of water displaced is proportional to the cube of the depth while the cross section of the hull below water --that area of water that has to be "pushed"-- is only proportional to the square. If that supposition is right, then adding weight will increase the depth only until it has displaced enough water to equal it, which is very small because the weight displaced is proportional to the cube of the depth. And then the effect on the cross section is only the square of that small number.

    I hope I'm making myself clear. I'm mixing terminology from my field (which I know well) and mechanics (which I know less well) so this may be incoherent...

    By the way, I'm not saying that you are wrong. In fact I'm sure that you and Rick are right. I'm just trying to understand why.
     
  10. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    I see your point, Dave. There must be a trade off at some point, because it would appear that a heavier boat with very long hull is still more efficient than a very light boat that has a shorter hull. Are "Hull speed" limits the biggest factor?

    Porta

     
  11. Submarine Tom

    Submarine Tom Previous Member

    Okay, I think I see your point.

    Remember, the weight of the boat = the weight (mass) of water it displaces.

    Water has inertia and every time you move that hull in the water you have to move that water out of the way.

    Yes, it is a lot easier to move it forward than sideways (the boat) but you still have to displace the volume of water.

    So, the heavier the boat, the more water to move.

    I'm sure Rick will have something to add.

    -Tom
     
  12. Tiny Turnip
    Joined: Mar 2008
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    Location: Huddersfield, UK

    Tiny Turnip Senior Member

    My understanding is that because the available (human) power is so limited, small improvements in efficiency are well worth going for, and all add up. The other effect of an increase in draught is an increase in wetted surface area and the drag associated with that. The benefits of length come into play when the boat starts to move fast enough to generate a significant wave, and the (much larger) wave drag requires vast amounts of extra power for a small increase in speed. The speed that this happens at is the so called 'hull speed,' and within reason, the longer the hull, the higher the hull speed. Obviously a longer hull will also have more wetted surface.

    Rick has some great graphs which illustrate this.
     
  13. Dave Gudeman
    Joined: Nov 2009
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    Location: San Francisco, CA, USA

    Dave Gudeman Senior Member

    I think I've just answered my own question. My assumption that the weight of the displaced water is proportional to the cube of the depth assumes a hull shape where the width and length are a linear function of the height. Now that I think of it, that is probably not true in general. In particular, if the length and width are constant in the area near the water line then change in depth would be linear with change in weight of displaced water. That is, lowering the hull one inch displaces and extra n square inches of water, and lowering it 2 inches displaces an extra 2n square inches of water. In this case, reducing the weight could have a linear effect on the hull depth.

    It is still unintuitive to me, though. Rick talks about shaving a pound hear or there on a system that probably weights more than 200 lb over all. It's hard to see how such a small percentage change could have a significant effect.
     
  14. Guest625101138

    Guest625101138 Previous Member

    Dave
    I am approaching 60yo and my ability to lift weight above shoulder height to place on a car or even to carry something weighing 30kg say 50m to water becomes a struggle.

    The utility of the boat, as measured by the ease of transport and simplicity to launch, is linked to weight.

    This really hits home when you see a kayaker hop out of his kayak at the waters edge with paddle in one hand and boat in the other and simply saunter up the beach. If you have an 8kg kayak it is a cinch. Doing same with a pedal boat that weighs less than 20kg is tolerable. More than this and it gets challenging.

    The Hobie flappers have quite a neat jockey wheel set up that doubles as the seat back but it is still fiddling about compared with the kayaker and his 8kg boat.

    My target weight is always under 20kg for this reason - utility of use. The only concern I have had following this approach is making boats that can get blown over in strong wind. Side area is a consideration.

    For a displacement boat there is an advantage to heavier boat for a constant power to weight. This is why there are heavyweight and lightweight sculls. A heavier person may find over 20kg tolerable but remember the boat is 6+m long, being placed onto a car in a bit of breeze. This is somewhat different to lifting a 20kg barbell to head height.

    Pedal boats have one disadvantage to paddled boats and that is that they are inevitably heavier. You cannot make a pedal drive system lighter than a paddle. If the weight gets out of hand it is easy for all the advantages of pedal power to be lost. It is always sobering to compete with other boats so you know where you rank. A pedal boat is the most efficient form of human powered craft but only if the weight of the boat does not become a significant portion of the overall displacement.

    Rick W
     

  15. portacruise
    Joined: Jun 2009
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    Location: USA

    portacruise Senior Member

    I'm 62 myself, and know what you mean by the weight factor. Have gone to using light weight, stow aboard wheels when I have to go any distance to water.

    Have you seen the Hobie inflatable peddler:

    http://www.hobiecat.com/kayaking/models_i12s.html

    But, not quite as lightweight as I would have expected.

    This company continues to surprise. It has a group of loyal followers, and should be prospering, even in these hard times.

    Vic

    Porta
     
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