Semi-Submersable hull dynamics

Discussion in 'Hydrodynamics and Aerodynamics' started by rubenova, Apr 14, 2013.

  1. rubenova
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    rubenova Junior Member

    I'm curious, what happens when a vessel is designed to run partialy under water? Wavemaking, prismatic coefficiant, power requirements etc. Are there any benifits that the weekend boater or passagemaker would be interested in? Or is it just stealth for smugglers.;)
    Thanks in advance
     
  2. Ad Hoc
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    Ad Hoc Naval Architect

    All boats run "partially" underwater.

    Any "benefits" depend upon your requirements. Like everything, there is no such thing as a free lunch. Ergo, its all about trade offs and compromises and whether one is willing to accept them, to obtain the usually "singular" objective.
     
  3. rubenova
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    rubenova Junior Member

    Sorry, I should have used "mostly" instead of "partially" under water. Let me try again. Is a hull designed to be more submerged than not, need more or less power at passagemaking speeds? The closest thing to an example I can think of is compairing surface and submerged speeds/power of something like a nuclear sub that has the same power available above and below.
     
  4. philSweet
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    philSweet Senior Member

    Displacement goes way up. That generally means significantly more power, more wetted surface, higher drag. Either run deep, or run on the surface. Near surface is the worst condition for drag.

    And I think any semisubmersible will just be assumed to be a drug boat. You'd get taken out by the competion, most likely.

    You can search for info on SWATH hulls here and elswhere.

    one example- http://www.yachtsilvercloud.com/SSC/specifications.htm
     
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  5. rubenova
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    rubenova Junior Member

    That generally means significantly more power, more wetted surface, higher drag. Either run deep, or run on the surface. Near surface is the worst condition for drag.

    This is what I was looking for, thanks.
     
  6. Dayu
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    Dayu Junior Member

    At high speed stage, hull drag is dominated by ship-induced waves. To set hull far from free surface is to reduce ship-induced waves, consequently, to reduce the drag. Another benifit is to reduce seaload and wave induced motions. This is the design concept of semisubmersible rigs.
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    You're forgetting the obvious.

    When far from the free surface, if the hull is still a "surface vessel", i.e. NOT a submarine, then there will still be "some parts of the hull" that extends from the submerged buoyant hull, far under the free surface, up to the free surface level. In other words lots and lots of WSA = very high drag!
     
  8. Squidly-Diddly
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    Squidly-Diddly Senior Member

  9. Mike Graham
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    Mike Graham Junior Member

    Semi-submersibles┬╣ have favorable seakeeping and stability characteristics, but can have poor resistance and maneuvering characteristics. (Resistance performance is not as bad as one might initially assume, because semi-submersibles can have very little wavemaking drag.) Their favorable stability characteristics come from their extreme draft, which can make them less suitable for many ports.

    Because of all these things, semi-submersibles are extremely popular...for permanent and semi-permanent offshore installations.

    I suspect semi-submersibles have little to offer the weekend boater. They are likely very heavy and expensive for the amount of habitable volume and difficult to maintain. Apart from platforms and special-purpose vessels, it's hard to imagine them having much of a role.

    ┬╣It has been pointed out this is an ill-defined term. I've chosen to ignore this fact.
     
  10. Dayu
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    Dayu Junior Member

    Hi!
    This is one picture of resistance components of one monohull and one catamaran.
    [​IMG]

    For the detail, please see http://www.sciencedirect.com/science/article/pii/S0029801812002053
     
  11. jonr
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    jonr Senior Member

    > Because of all these things, semi-submersibles are extremely popular...for permanent and semi-permanent offshore installations.

    Also with some drug runners.

    My understanding is that submarines have less drag when running fully submerged.
     
  12. samana
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    samana New Member

    A submarine may have less drag than an 'equivalent' surface vessel, it really depends on how it's designed and to what criteria. Naturally, the whole question descends into farce quite quickly once you try to work out the defining characteristics of 'equivalent' surface vessels and submarines.
     

  13. Wolfgang Zhang
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    Wolfgang Zhang Junior Member

    Semi-submersibles is still close to the free surface, maybe the wave making resistance is smaller but, what about form drag? Much bigger than the ordinary ship with the same load capability.
    Seakeeping is good, although it's not "that" good...
    I think this type of ships are good for sightseeing in the shallow sea, such as great barrier reef, or some place like this...of course drug dealer and smugglers would love it...
     
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