Air bubble lubrication successful trials

Discussion in 'Boat Design' started by jakeeeef, Oct 22, 2021.

  1. Ilan Voyager
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    Ilan Voyager Senior Member

  2. BMcF
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    BMcF Senior Member

    Speaking as someone who has a few SES designs to his credit; the effectiveness/efficiency of the SES has never been debated since the 1960s when the concept was first really evaluated at any significant scale. What is "debated" are the sad number of marginal or failed SES projects, undertaken by people who either did not understand all the nuances of designing one, or because the construction of the vessels did not include robust weight control. Or sometimes both. With those failures in full public view, some concluded incorrectly that the underlying technical premise was flawed.

    But it's no fluke that the fastest naval combatant vessel in the world, over 100 tons displacement, is the Norwegian Navy "Skjold" class.

    All that aside, a surface effect ship air cushion is not "air lubrication" in the sense that it's been used in this thread.
     
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  3. BMcF
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    BMcF Senior Member

    We launched/commissioned 15 of that Cirr120P SES series, in Noway, from the mid 80s though about 1992. All went in to passenger service and, last I knew, three still were operating and possibly more.

    Every item on your list was true in the early days but have all been rectified in subsequent vessel designs to a large degree. The stern seals, in particular, have been dramatically improved, with the current designs much more stable exceeding 8000 operating hours between overhauls. In those early 1980s designs, we were lucky to get 1500 hours out of a stern seal.

    Again I'll point to the Norwegian Skjold class corvettes. Low noise fan designs. Stable long-life stern seals and replaceable bow seal finger tips. Hull design that is very effective at limiting waterjet air ingestion. We also designed and commissioned three 40-meter SES passenger ferries in South Korea, beginning around 1993, that were successful designs and I think all three are still in service.
     
    Last edited: Oct 26, 2021
  4. wet feet
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    wet feet Senior Member

    I suppose to get an answer that applies to the OP we would have to ask what the cost of producing such a boat would be and whether it would be justified for any gain that might be realised.I would be interested to learn what the pressure inside the bubbles might be as the hull is moving,but I don't really understand how you would go about ascertaining it.I suppose there is no escaping the reality of pressure=hρg and to boost it from atmospheric demands some energy.
     
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  5. DogCavalry
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    DogCavalry I aim to misbehave.

    My information is out of date. It seems there have been a number of successful installations in the last few years. Typically around 6% savings at around 14 knots.
     
  6. BMcF
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    BMcF Senior Member

    I'd be interested to know more about the successful examples you are referring to. Always good to see the successful ones.
     
  7. DogCavalry
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    DogCavalry I aim to misbehave.

    Wärtsilä has a good article on the current situation.
     
  8. Barry
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    Barry Senior Member

    I disagree that the air bubble causes the density of water/air mixture to affect the depth of the boat sinkage.
    The general consensus throughout many threads is that the vertical lift provided by an immersed object in water is equal to the volume of the water displaced. While the lift number is correct, the
    lift force is equal to the (vertical) integration of the pressure caused by depth acting vertically at each point of a hull

    Nothing new here, but presented to support the following case.
    Simplified
    Take a 1 foot x 1 foot x 1 foot cube "hull" that weighs 31.2 pounds. (half the volumetric density of 1 cubic foot of water) It will sit half way down in the water. Inject a bubble of air into the center of the lower surface. So long as the pressure of the air at the injection point is above the pressure due to depth of the water, it will be injected into the area below the lower surface. If it is injected at a higher pressure, the bubble will grow/ ADJUST to the size necessary to equal the pressure due to the effect of the water depth. Ie the water bubble will not be at a lower pressure or higher pressure than its surrounding water.

    Ie the bubble will provide the same lift to the hull as though there were no bubble.

    So therefore no further sinkage in the depth of the hull.
     
    Last edited: Oct 27, 2021
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  9. clmanges
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    clmanges Senior Member

    I think you're forgetting that gases are compressible and liquids are not. And good luck trying to make the bubble stay underneath the cube.
     
  10. Barry
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    Barry Senior Member

    I have not forgotten that gasses are compressible. They will compress or expand and assume the pressure of the ambient pressure surrounding the bubble.
    My comment from above, "If it is injected at a higher pressure, the bubble will grow to the size necessary to equal the pressure due to the effect of the water depth.

    I did not state that the bubble would stay under the plate but whether it is moving or not, the bubble will provide the same "lift" as the solid water.

    The subject at hand is "will a mixture of air and water reduce skin friction?" to enhance a hulls performance.
     
    Last edited: Oct 27, 2021
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  11. portacruise
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    portacruise Senior Member

    Doesn't the bubble essentially stay under the cube, because as soon as it moves away, another one takes its place, since the bubbling is continuous? Also a concave surface could be used which would tend to retain bubbles.
     
  12. BMcF
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    BMcF Senior Member

    You've described, more or less, what was attempted on a few catamaran hull designs and at least one mono hull. All were failures in terms of any drag reduction.
     
  13. Heimfried
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    Heimfried Senior Member

    I agree with Barry's thoughts in #23. But this ideas are restricted to the field governed by the laws of hydrostatics. If you inject a lot of pressurized air in a small area of a pool (spot of "boiling") you will notice the surface of this range is raising above the surface of the calm water surrounding this area. I presume the draft of a boat in this area is increasing if you measure it against the elevated surface and the draft is a tiny bit decreasing if you measure it against the surface of the undisturbed water.
     
  14. DogCavalry
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    DogCavalry I aim to misbehave.

    Barry is entirely correct. There is no loss of buoyancy.
     
    Last edited: Oct 27, 2021

  15. Milehog
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    Milehog Clever Quip

    It seems to me the mass (weight) of the displaced water has to equal the mass of the boat. Bubbly water is less dense therefore a greater volume to equal the same mass will be required. The only way around this is if air is trapped under the hull.
    Just my 2¢.
     
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