upside-down Casserole Dish as large air supported hull?

Discussion in 'Boat Design' started by Squidly-Diddly, Oct 16, 2011.

  1. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Way I figure it, any wetted hull area creates drag, and any solid hull takes a lot of force from waves, so why not only have solid hull on the perimeter to trap air, thus eliminate nearly all friction(aside from perimeters) and even allow the bulk of a wave's energy to pass harmlessly?


    Not a high energy Hovercraft, but a LowPressureAirPocket large ship.

    I think this might start working when the ship gets pretty big, and the initial loss from the odd hull design and problematic perimeters and the transition become a smaller percentage of overall equation in relation to much larger area supported by the Air Pocket.

    Hasn't this been tried before?

    I remember some trials of pumping bubbles along the keel to supposedly reduce drag, but thats all.

    What does the water do if on a conventional large ship there is a big hole of pressurized air at the level of the bottom of the hull(maybe starting where the bow ends and the flat, straight hull begins), and the ship is moving at normal speed(15 Knots)?

    [​IMG]
     
  2. DCockey
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    DCockey Senior Member

    If your idea is a vessel which would be supported entirely by air pressure then that's how the idea of a hovercraft started. The reason hovercraft became more complicated with the addition of the skirt with higher pressure air is stability. A simple open cavity will be unstable and tend to roll over.

    If you are thinking a vessel with a trapped air buble on the bottom under the surface but supported by displacing water then there have been several recent tests of various configuartions with mixed results once the power needed to replace the air in the cavity is taken into account.
     
  3. kach22i
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    kach22i Architect

    DCockey's answer is a good one.

    You may also wish to review the topic of SES (Surface Effect Ship).

    People are always trying to reinvent the wheel, or hovercraft. I found this today by doing a quick Google search.

    Alternative fuels drive eco-craft developments
    http://www.motorship.com/features101/fuels-and-oils/alternative-fuels-drive-eco-craft-developments


     

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  4. Squidly-Diddly
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    Squidly-Diddly Senior Member

    I was "feeling" the stability issue. Maybe use AirCavity but have

    a droop towards the middle where heavy machinery or cargo could be stored, and only the tops of the inner-hull perimeter wave swells would "kiss" the drooped middle section.

    "replace air in cavity" would tend to be less as size increases, but the ability of air to support a boat, while letting wave energy pass by would be far more important in a small vessel.

    The exit and enter points on that upside down hull are about as I figured, but I wasn't even dreaming about a hull that small, or that fast.....I WAS thinking about how a DISPLACEMENT hull at near "hull speed" might benefit from letting waves(and bow waves, and pressures) just "do their thing" up in the Air Cavity without somehow sucking energy.


    How about using Air Cavity on big cat or tri, to negate the "sitting on a beach ball" stability issue?
     
  5. philSweet
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    philSweet Senior Member

    A stepped hull is basically the same thing without the aft edge coming back down to the original hull line. A catamaran is the same thing without either a front or back edge. Sometimes less is more. Controlling the bubble in the cutout area would seem to be a challenge. Lots of makeup air required. Since power for makeup air would be proportional to draft, shallow draft only need apply. I looked at a mid engine canoe with a bubble box to trap engine exhaust but never built it. If you could improve a centerboard sailboat by making the slot bigger I think someone would have figured that out by now. Crap- Air injected centerboard cruiser- I smell a patent. PAR, waddaya think? A second thought- use it to control trim. That might justify it. I've been painting all day- maybe it's the fumes.:D
     
  6. gonzo
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    gonzo Senior Member

    Loose slots in centerboards create turbulence and make the boat slower. Hence, the centerboard gaskets for a tight fit.
     
  7. philSweet
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    philSweet Senior Member

    See, we get rid of the gaskets, pressurized air eliminates the need. Heck, we could tack the centerboard with the same air pressure and use it to raise and lower the board. What else can we do with it? Everything except reduce drag, I fear.
     
  8. gonzo
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    gonzo Senior Member

    Where do you get the air pressure and flow from? A compressor is heavy and needs an engine and fuel to run it; more weight and less speed.
     
  9. MatthewDS
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    MatthewDS Senior Member

    I remember something similar a few years ago in Marine Engineer magazine. I believe that they were undergoing scale model testing on a tanker. Essentially the idea is that the bottom of the tanker hull had a depression along the full length and width, air was injected into this area. I believe the idea was that the tanker was riding along on an air cushion, thus reducing skin friction.

    I would assume that this would only work on very large, very stable vessels, and I never read the results of the model trials, so it may not have worked.
     
  10. Jeremy Harris
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    Jeremy Harris Senior Member

    The problem is the energy needed to replace the lost air. Often, the power needed to do this will come close to being of the same order as the power saving from reduced viscous drag.

    Air will always be bleeding out of the cavity, because its under pressure and because it will get dragged out aft by friction with the water surface, so there has to be a way to pump replacement air in at the static pressure under the hull. This can be by using fans or perhaps by using dynamic pressure from forward motion (in a high speed craft), but both absorb significant power.

    ACVs try to limit air leakage with deformable skirts, because the power needed to maintain the cushion is proportional to the rate of air loss. A rigid hull either has to have the edges deeply immersed to retain the air (OK at the sides, not really practical at the bow and stern) or accept the high power demand of maintaining the air cushion despite the high leakage rate, be it from a shallow depression in the hull or a full blown plenum (a bit like the casserole dish).

    The additional power needed may just fall the right side of the overall power requirement, when compared to a conventional boat, or may not. There is a significant scaling factor, as power required to maintain the cushion is proportional to peripheral area, so larger, broader, vessels need less power in proportion to their displacement than smaller, narrower, vessels.
     
  11. SomeGuyFromOly
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    SomeGuyFromOly Junior Member

    lets say it gets a little rough, and the front of the boat comes out of the water like they do........and exposes your air pocket

    I bet that would be the fastest sinking boat ever designed(?)
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    Not really, the re-immersion would see the air pocket restored quickly enough.
     
  13. Squidly-Diddly
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    Squidly-Diddly Senior Member

    where are you losing all this air? once the vessel is bigger than

    the toy boat size, and the draft is 10' or more, and it is only going 15knots, how much air is going to be lost?

    Some air is going to be gained, from accidental bow wave fallout.
     

  14. BMcF
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    BMcF Senior Member

    Exactly. Numerous variations on the same 'captured air bubble support' theme have been tried since the earliest one I know of was tank tested by Thornycroft over 100 years ago. (And NO, I was not there at the time. ;-))

    To date, only those that make use of flexible bow and stern 'seal' structures to capture the air have worked, and not even all of those have worked well enough to justify the added cost and complexity. The concepts that employ fixed rigid structures fully around the bublle periphery (i.e. those that most closely resemble the description in the OP) have universally failed to meet performance expectations when built at anything approaching full scale sizes.
     
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