reducing hull friction

Discussion in 'Hydrodynamics and Aerodynamics' started by nimblemotors, May 12, 2011.

  1. nimblemotors
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    nimblemotors Senior Member

    I always try to follow another power boat to run in the calmer water behind them. It also occurred to me that the bubbled water might reduce the drag,
    and a recent mythbusters (my favorite tv show) did something on bubbled water. So some research and indeed this idea has been looked into from many angles, just one such example.

    http://www.mme.wsu.edu/~matveev/concept1.htm

    My thoughts were to use this idea and somehow use the action of the surface waves to create the air so its "free" energy.

    And while I'm asking, are their other ways of reducing the hull friction,
    certain those billion dollar sailboats would use some kind of surface treatment that made them faster. I believe skis use some type of surface treatment. And it makes me think of soapy water.. some biodegradable soap released ahead of the hull.. :)
     
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  2. keysdisease
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    keysdisease Senior Member

  3. nimblemotors
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    nimblemotors Senior Member

    oh yeah! so definitely validates the idea. but of course, the military can expend $1 million dollars each to make it do that..the cost of the bubble maker vs going slower really isn't in the equation. So if you can do even a minimal effect like this using free energy and little added weight, it could be a winner. I'm thinking of wave energy, so like a float in a vertical tube so a wave pushes it up and compresses the air above it. just a million dollar research budget will figure it out.. :)

     
  4. nimblemotors
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    nimblemotors Senior Member

    Ok, so this relates to another idea of using a pulse-jet powered boat.
    A pulse jet is very simple, but very inefficient because it generates tons of wasted heat. So here is the idea, make the torpedo tube the pulse jet body, and as the jet exits the rear, it has heated up the hull and boiled the water in contact with it, creating the same air bubble effect but harnessing the wasted heat. This would be an incredibly simple propulsion system!
    You would need to pump the combusion air into the hull, but possible this could be done using wind power itself. hmmmm.

     
  5. BYDE
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    BYDE Junior Member

    There's quite a lot of research going on about air-ventilated hulls for commercial ships, because their friction resistance is the greatest part of the total. Especially in Japan where they already did some sea trials, but other nations are following them too, as it looks promising. After some years of lab experiments they mounted an air blower on a real ship and tested it. I think the results were generally good, but now I don't remember the details... However the power requirements were reduced, at least in calm waters, also including the energy spent to produce the air flow.

    One of the main problems is that the air flow may reach also the propeller and cause a loss of thrust, other than possible damages in the long term
     
  6. daiquiri
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    daiquiri Engineering and Design

    I remember reading some technical reports about those experiments, and one of the main problems encountered was leakage of air bubbles towards the water surface. In other words, the air bubbles want to escape upwards, rather just wrapping around the hull, so at some distance after the air-injector nozzles there were no more air bubbles left.

    The use of Air Cavity Ship technology partially resolve that problem by giving the bottom of the hull a concave shape, forming a cavity or pocket which allows a stable formation of an air bubble. The walls are still off-limits for this method of friction reduction.

    For more interesting info about air-cavity ships see here: http://docs.hydrofoils.org/SAS03.pdf
    and here: http://www.dkgroup.eu/Default.aspx?ID=64

    By the way, did you know that penguins have arrived to this technology long time before we did: http://www.int-res.com/prepress/m08868.html ? :) ;)

    Cheers!
     
  7. Leo Lazauskas
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    Leo Lazauskas Senior Member

    Patents for these devices and systems have been around for about 100 years, as mentioned in the paper attached to the next thread:
    http://www.boatdesign.net/forums/hy...common-errors-powering-predictions-38062.html

    As Almeter says in that paper: it is up to the promoters claiming benefits to substantiate their claims.
    As far as I have seen so far, they don't stand up to close scrutiny, but I'm happy to be proved wrong. :)

    Leo.
     
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  8. BYDE
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    BYDE Junior Member

    wow, that was interesting. geniuses animals...

    oh but that's true for many research works! :)

    For how I see it, if the shipbuilding industry invested some money to do sea trials with a modified ship for such experiments then it means that they must have seen some possible gain.
    The CFD numerical simulations showed some lower resistance too, but we know that CFD is all about interpretation of the results :)
    I think that the Japanese tested it in a relatively 'slow' commercial ship (I mean, not an high-speed craft), so it should be easier to control the air flow and to control the results of the experiments too. However if I remember well with a bit of waves and ship motions the advantages were already nearly zero. That's the main practical problem.
     
  9. BYDE
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    BYDE Junior Member

    To add more to the initial question...
    A typical example of a system to reduce friction is to release polymers (some specific types, long chains, but I don't remember the name) along the hull. Highly polluting stuff anyway.
    It was used in the America's Cup for example, then it was banned by new rules. However in the last AC that was raced according to the original protocol, due to the legal actions following the 32nd AC, BMW Oracle did build such system again. Though they didn't need to use it eventually.
    Then, also from the AC, there's also the famous ''dolphin skin'' and other coatings to reduce frictions.
     
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  10. nimblemotors
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    nimblemotors Senior Member

    Thanks for the links! Interesting about the penguins, evolution can engineer some pretty amazing stuff, just need to give it enough time..
    What seems obvious is using an engine exhaust output for this air injection.

    From researching the heated hull idea, the viscosity of water is cut in half when you heat it from 30c to 70c, http://en.wikipedia.org/wiki/Viscosity,
    so at the very least I'd suspect the heated exhaust air would be better.
    The efficiency of internal combustion engines are very poor, as about half the energy is wasted as heat, if this heat can be used to reduce hull friction,
    it would be a huge win.

     
  11. Stumble
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    Stumble Senior Member

    BYDE,

    There was a conversation on the board a while back about 3M friction reducing film. There is no question that the stuff worked on day 1, the problem is that so far the possible products have one of two problems.

    1) The nano-structure surfaces have to be reapplied every 3-5 days of operation. This is for sloughing skin membranes, and while they do reduce friction significantly with the exception of unlimited budged race boats they just aren't affordable.

    2) The skins exibit very quick growth of marine life (Barnacles and algea) that reduces the friction reduction effect in a matter of weeks.

    Basically the things that work are not practical for commercial boats, and the racing rules prohibit them from racing boats (in most classes), plus they are insanely expensive.
     
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  12. lobsterman
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    lobsterman Junior Member

    There has been much military research done on a type of surface effect ship design called CAB (captured air bubble), it is not a hovercraft !!!, but i have not seen anything practicle come form it in the commercial sector, as would be expected with any successful technological advances.
    But that does not mean that the principle of it is wrong, simply that they have not gotten the right hull shape yet to gain the full benefit of reduced friction by use of the bubbles. ( remember... stepped planing hulls use sort of the same principle of reducing drag ).
    Think of a skipping stone, the best ones for skipping (although they look smooth) are actually covered in tiny dimples. I was wondering if a hull design that was covered with a surface area that had thousands of micro dimples incorporated into the hull's wetted surface areas would be beneficial ???, or would it actually create drag ???. Any thoughts ???.
    Obviously the prop (or jet drive intake ) for any vessel must be kept below the cavitation or air bubble turbulance.
     
  13. Boston

    Boston Previous Member

    Sounds like boundary layer effects kinda like what the Tesla turbine uses. By roughening the surface you actually help separate the boundary/stationary layer from the surrounding fluid. A smoother surface creates a more closely knit boundary layer. Counter intuitive but it works, so the golf ball skin idea lives on. I believe ice burgs also develop this texture over time, path of least resistance and all.

    Could be interesting to apply the same texture to a boat and see how it works out.
     
  14. lobsterman
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    lobsterman Junior Member

    Ya and you could name the craft .... Rumpled Dimpleskin ... LOL
     

  15. lobsterman
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    lobsterman Junior Member

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