Flapping Foil on a real ship

Discussion in 'Boat Design' started by alan craig, Jul 18, 2017.

  1. alan craig
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    alan craig Senior Member

    Here is a YT video promoting a flapping foil propulsion system. I thought it was just computer graphics, but near the end you can see it installed on a real ship. Only one comment below video, which asks how does it reverse. Obviously the direction of rotation of the operating mechanism is reversed, in which case the foil is less efficient - just like reversing a propeller.



    I've just realised that the mechanism might be good for human power boats because it can be direct drive.
     
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  2. jorgepease
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    jorgepease Senior Member

    Don't see why this couldn't be used for smaller sailing vessels as well.
     
  3. IronPrice
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    IronPrice Senior Member

    Not a long way away from the mirage drive hobie kayaks can use, which is modeled off penguins flippers.

    I have to say that O-foil looks highly vulnerable to impact damage
     
  4. PAR
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    PAR Yacht Designer/Builder

    That's a lot of gear and reciprocating bits dangling back there. I'd imagine maintenance would offset any savings.
     
  5. alan craig
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    alan craig Senior Member

  6. tom28571
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    tom28571 Senior Member

    The main difference between this and a propeller is that the whole working surface has an identical foil chord and can operate at an optimum angle or pitch and speed. Each point along the radius of a propeller is operating at a different speed and cannot be operating optimally. It is not much different from the fairly common Voith Schneider propeller in which each point on the foil operates at the same speed. A great advantage of a Voith Schneider propeller is the ability to instantly change the thrust to any of 360 degrees direction.

    No doubt it works but there is always lots of energy lost in changing direction of a large weighty mass in each cycle. For river barge traffic it can be optimized for a narrow low speed range. Can anyone see it working well at high speed? As others hav mentioned, the concept is not at all new.

    Water based creatures are forced to use some kind of flapping motion because nature does not provide for rotating joints. Nature comes close to rotational motion with the hummingbird and a few small sea creatures with fins on their sides but its still flapping.
     
  7. jorgepease
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    jorgepease Senior Member

    ... dolphins flap at pretty high speed )) and probably way more efficiently than any man made contraption.
     
  8. tom28571
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    tom28571 Senior Member

    Dolphins swim at pretty high speed and apparently at high efficiency, but they do not flap at a high rate and there is no sudden reversal of all the mass. Like other creatures, their whole body participates in the propulsive force.

    Has a dolphin's efficiency been compared to efficiency of the better man made waterborne vehicles? Many animals have extremely high efficiency in fuel/work compared to man made means of locomotion.
     
  9. IronPrice
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    IronPrice Senior Member

    Dolphins ate efficient for a whole bunch of reasons.

    One is the narrowness of the join between the tail and body - called the peduncle. The same form is seen in tuna, billfish and other fast swimming fish.

    Dolphins also use their whole body in a sinusoidal wave to accelerate and their pectoral fins as foils for control and a bit of lift at the front end.

    The overall shape is very streamlined as is the texture of their skin.
     
  10. tom28571
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    tom28571 Senior Member

    Yes. I did not know what a peduncle is but watching them swimming as I and many others have, shows the whole body working in cyclic motion. Not much like the flaps in the OP post.
     
  11. jorgepease
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    jorgepease Senior Member

    I was simply giving an example of fast motion. Students at MIT have already studied and created models of the motion, more to this application would be a whale probably.
     
  12. alan craig
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    alan craig Senior Member

    Tom28571,
    You say that this foil has an identical foil chord (do you mean a symmetrical section?), it would be more efficient if it could change its camber with each reversal of direction.
    A propeller blade is twisted because at each point of its radius it is optimised for some particular operating point.
    A hummingbird wing, when hovering is twisted just like a propeller blade but it is presumably a fairly efficient motion if it can keep the bird flying long enough to feed itself.

    I think the two casings which house the foil operating mechanism on the ship might add a lot of parasitic drag, but they do claim a reduction in fuel consumption.

    A bit off subject this: I have a fantastic book called "The simple science of flight" by Henk Tennekes who was a professor at MIT. Amongst all the interesting animal/machine flight data is the fact that humans and birds, and probably other animals, have about 25% thermal efficiency - the rate of turning food energy into work.
     
    Last edited: Jul 22, 2017
  13. jorgepease
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    jorgepease Senior Member

    25% sounds really low, wonder how that is measured
     
  14. Torben
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    Torben New Member

    probably by estimating the daily food intake and its caloric value
    then dividing that by weight*travelled distance

    Thanks for sharing the video by the way!
     

  15. IronPrice
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    IronPrice Senior Member

    People do before/after weighing of migrating birds and convert weight loss in flight to calories then calculate the work done. That's little crude but for more precision you can do things like put a mouse in a chamber, have it run on a little treadmill and measure the amount of heat released or oxygen used vs work done.

    Biological systems are generally effective but inefficient (especially warm blooded animals).

    An average adult human male will burn about 1,600 calories doing nothing - just maintaining bodily function and core temperature. That's about 1lb of fat worth of energy. Any work done (physical or mental) requires additional energy.
     
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