Human powered surface drive?

Discussion in 'Boat Design' started by clmanges, Aug 26, 2008.

  1. clmanges
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    clmanges Senior Member

    I was looking at the propulsion forum, trying to get an idea of how surface drives worked, and had a thought -- could this be adapted to muscle power?

    I'm thinking of a large diameter prop, possibly with many blades, working at low speed. It would function sort of like a continuous series of sculling strokes, all in the same direction, with perhaps only a small portion of the blades immersed.

    Any merit in this idea?

    Curtis
     
  2. CDK
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    CDK retired engineer

    Splendid idea! The immersed blades cause a lot of friction though; it will be easier if none of the blades touch the water.
    Wasn't it Otto von Lieliental who used this principle and flew with it?
     
  3. clmanges
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    clmanges Senior Member

    Uh, if my memory serves me right, Lillienthal built the first hang-glider . . . what I had in mind was a shallow immersion, to avoid weed fouling and work in shallow water where you couldn't go with an ordinary prop.
     
  4. diwebb
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    diwebb Senior Member

    Hi,
    interesting ides. The guy who seems to have thought of all the permutations and combinations is Rick Willoughby, I hope that he comments on this thread.
    I would think that the idea has a distinct drawback in that the prop would generate considerable sideways force and the boat would go around in circles, however if two props were fitted rotating in opposite directions then it could be a viable suggestion. This may create more friction losses and make it less efficient than a single fully immersed prop but the reason for using them is shallow draft so this may not matter. It would in effect be a sort of paddlewheel.

    Best of luck with the idea.
    David
     
  5. Guest625101138

    Guest625101138 Previous Member

    I well designed surface prop can get efficiency around 70%. They are a water deflector not a foil. Lower efficiency than a paddlewheel. The blades are like cleavers with shape to enter and exit cleanly. The EAR is 1 in most cases from what I have seen. The aim is to hurl water backwards so the diameter is not very big and the rotational speed is high.

    The air prop works well if you want high efficiency at speed. It is superior to a water prop at high speed. Efficiency of 90% is possible.

    I did some experimenting with surface prop. I managed about 30% efficiency with the configuration pictured. You can find many examples of more efficient surface props but they are poor by comparison with a submerged prop. Their advantage is reduced appendage drag which becomes significant at 40+kts.
     

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  6. diwebb
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    diwebb Senior Member

    Hi Rick,
    rather than a high speed surface prop we are talking of a low speed prop which needs a very different approach. Have you thought of an arrangement with two props, similar in appearance to the water pump windmill wheels seen in the midwest 100 years ago, set close together and rotating in opposite directions. I would anticipate that with the multiple blades and the opposing sideways thrust from the two props, this may increase the efficiency of the whole arrangement, and make it suitable for a narrow single hull boat. I would anticipate a prop diameter of between 18" and 24" for this sort of arrangement. Also as surface props all of the drive mechanism would be above the waterline. What do you think?
    David.
     
  7. clmanges
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    clmanges Senior Member

    That's about what I envisioned, only I was thinking of one big one. I hadn't guessed that there would be much of a sideways force. Two smaller ones could be okay, though.

    The bonus with this, as I see it, is that a headwind would actually help drive the prop for forward motion.
     
  8. Guest625101138

    Guest625101138 Previous Member

    The physics constrains performance. What you are describing is a smaller diameter version of what I made but using multiple blades.

    One of the features with the high speed prop is that the non-working surface operates in a partial vacuum so very low drag on that side. From there it is a matter of optimising the blade angle for any given radius to maximise thrust while minimising swirl. There are some photos here:
    http://www.twindiscpropulsion.com/uk/ASDRange.htm
    Blades need to be as thin as required to take the forces. They have sharp entry and exit (they are not foils) designed to minimise splashing.

    Generally it is just as efficient to use control surfaces to counter any unwanted reactions rather than having the complexity and added losses of twin props. Usually twin prop set ups are to give redundancy rather than efficiency.

    I am confident you could design a high speed paddlewheel to be more efficient than a surface prop but it does not have the macho appeal of a prop. There are some very fast tracked and wheeled craft:
    http://www.youtube.com/watch?v=wGSfJbXAIRs
    No substitute for power. Don't try that at home!

    Rick W.
     
  9. diwebb
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    diwebb Senior Member

    Hi Rick,
    yes all well and good for "high speed craft", but does the physics still hold good at the low power/ low speed end of the equation that we are talking about with a human powered arrangement? It could be that there are differences similar to those between displacement speed and planing, that would make a difference? I dont know enough about the numbers or physics to make a definitive assumption but with your knowledge I am assuming you have done the research?
    David.
     
  10. clmanges
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    clmanges Senior Member

    Like the man said, "This is going to require some serious commitment!"
    Yikes!
     
  11. Guest625101138

    Guest625101138 Previous Member

    The factors that make them suitable to high speed craft tend to be more difficult to achieve in a low speed application.

    It is harder to achieve the partial vacuum on the unloaded face in a low power application. The entrained air tends to bubble off as the blade rotates and there is simply not enough pressure differential to pull a vacuum.

    Unless the boat is planing the water level in the vicinity of the prop is all over the place. High speed surface props are slicing into a sheet of water. In low speed the surface can be anywhere. It would take a very powerful pilot to push a human powered craft at planing speed using a surface prop.

    The most effective feature of my prop was engine cooling. Fortunately I tested it in summer.

    If I worked at I believe I could get a human powered surface prop to get better than 50% but I can get small submerged props on flexible shafts to 80+% so why bother. A squid type thruster can give close to 50% and a squegee type can possibly get better than 70%. Both of these are likely to be more tolerant of weed and obstacles than any prop.

    Rick W
     
  12. clmanges
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    clmanges Senior Member

    Okay, I think a squid must be like a jet propulsion, but what's a squeegee?
     
  13. Guest625101138

    Guest625101138 Previous Member

    The limiting efficiency with a squid drive where you suck in water and expel it at velocity is 50%. This is because the water has to be brought up to boat speed before expelling.

    The squeegee thruster has water entering a tube at the front and it is pressurised by rollers running along the tube like a peristaltic pump and expelled at higher velocity than it enters. For best results the water should be expelled above the surface and the tube/ducting kept short. A practical system could better 70% but some engineering challenges involved:
    http://en.wikipedia.org/wiki/Peristaltic_pump
    Efficiency would be similar to a jet but you avoid any impeller that catches foreign objects.

    Rick W.
     
  14. clmanges
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    clmanges Senior Member

    Rick,
    After that last photo you posted, I have to ask: is there any type of drive system you haven't tried attaching a pedal crank to?

    Just curious,

    Curtis
     

  15. Guest625101138

    Guest625101138 Previous Member

    There are many I have not tried but there may be more than a hundred different things I have tried. I have made maybe 30 or more model boats and something like 15 boats I can pedal. Each of these typically have had numerous drive systems or slight variation.

    I do genuinely believe that CP2 shown here:
    http://www.adventuresofgreg.com/HPB/2008/08/cp2-goes-on-diet.html
    Is currently the most efficient boat on the planet for long distance travel using human power. It embodies all the lessons I have learnt over the last 5 years.

    I have assessed many more drive systems than I have actually tried. In some cases I draw on the experience of others and back calculate.

    I am always open to ideas. The big area untapped as far as I am concerned is viscous drag. I have done virtually no experimenting with methods to reduce this and it is the most significant component of drag on my boats.

    Rick W
     
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