Solid metal hovercraft skirts

Discussion in 'Boat Design' started by rubenova, Jun 23, 2017.

  1. DCockey
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    DCockey Senior Member

    Surface Effect Ships are a cross between a catamaran and a hovercraft, with solid side walls and flexible seals at the bow and stern. They are intended for high speed operation. The US Navy studied SES extensively in the 1970's and built two 100 ton prototypes. Norway has built the Skjold class of SES patrol craft.

    What Are Surface Effect Ships? http://www.marineinsight.com/types-of-ships/what-are-surface-effect-ships/
    Skjold-class corvette - Wikipedia https://en.wikipedia.org/wiki/Skjold-class_corvette
     
  2. cmckesson
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    cmckesson Naval Architect

    There have been many attempts to make practical skirt systems out of rigid and semi-rigid materials. In all cases the drag and motions penalties have outweighed any benefits. One of the functions of a flexible skirt is to "move out of the way" in response to water pressures, either from the forward motion of the craft or from ocean waves it encounters. Of course, a steel barge (non-hovercraft) represents the other extreme: "Out of my way, ocean."

    So now if we imagine a marriage of the two, what do we have? The hovercraft-with-steel-skirts will have barge-like drag, except for the elimination of the frictional resistance on the (absent) bottom plating. It will have barge-like motions as well, except slightly worse due to the destabilizing effect of the air cushion.

    In this case, most practitioners have found that the reduction in friction is not big enough to justify the cost of the lift fan system.

    There are, as above noted, hoverbarges. These are used in conditions where the amphibious function is essential, such as in broken ice. All of the hoverbarges that I know of do use fabricate skirts.

    And of course, the completely skirtless solution (e.g. the S.R. N-1) does remain viable, although it demands a larger lift fan system.

    Hope this helps

    Chris McKesson
     
  3. rubenova
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    rubenova Junior Member

    Thanks DCockey, thanks cmckesson,
    I like the surface effect and the S.R. N-1 ideas, and the idea of combining (marriage). What jumps to mind would be to add to the S.R. N-1 concept. A length of pipe cut in half to form a "C" and welded around the flat bottom to capture/slow down the air and lift the craft. Half a 8 inch pipe might get 2-3 inches of lift with enough pressure. This would get my barge further up a drying beach (my primary interest), minimize changing stability at rest, and maybe a little faster travel through the water....?
     
  4. gonzo
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    gonzo Senior Member

    A circular section would be very inefficient. You need a section that would produce a maximum downwards force. I think a parabolic shape would be better.
     
  5. rubenova
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    rubenova Junior Member

    Interesting point gonzo. Maximizing the lifting area. I wondered if the "low-tech-half-pipe" skirt would need to be rethought. What are your thoughts on leading and trailing shapes for the skirt at different speeds? The difference between hovering and lifting slightly in the water is getting good!
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    What's the need to get landward of the littoral zone ?
     
  7. rubenova
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    rubenova Junior Member

    Hello Mr Efficiency,
    I will admit to you (and boatdesign.net) that I would rather be "aground" than anchored. No anchor watch,,, just aground and napping. The barge described earlier would need to lift out of the water to land further up a beach and allow for a nice lay down with no worry of drift.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Put out two anchors ! :) Hovercraft are bothercraft ! And if you snag your rigid skirt on a tree stump hidden in the grass, you will be cursing and kicking yourself.
     
  9. cmckesson
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    cmckesson Naval Architect

    If you intend the vehicle to move across land, be aware of some additional considerations:
    Texture in the land will cause tremendous increases in the lift air flow requirement. For example, if you sit on a trench then the air will flow out of the trench, demanding increased fan flow. And while trenches may be rare, logs aren't and the same situation applies if you set down on a log or a rock. In fact, in the case of amphibious hovercraft this "adapting to the texture" is one of the primary functions of the fabric skirt, and one of the reasons that the skirt has multiple fingers around it's perimeter.
    Second, be advised that driving the craft up a slope takes a lot of force. A hoverbarge may have a thrust to weight ratio of about 1:100, but this will only push you up a slope of less than 1%. To overcome this you may need to rely on a kedge anchor ashore to haul you up onto the beach (and another kedge offshore to haul you off.)
     

  10. kach22i
    Joined: Feb 2005
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    kach22i Architect

    Thanks for posting that video, was new to me.

    The craft mentioned in the video were of narrow side catamaran design, later ones became fatter although not a wide as real cats.

    The US Coast Guard became the customer of later craft, the 100 knot US Navy was not to be.

    USCG Surface Effects Ships https://www.uscg.mil/history/articles/SurfaceEffectShips.asp
     
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