Air bubble lubrication successful trials

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

  1. jakeeeef
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    jakeeeef Senior Member

    Hi,
    I've been searching on here for any info on success stories with air bubble lubrication on small boats, kayaks etc.

    Surprised to see there's not much on here, or have I been searching the wrong terms?

    I used to be a rower and remember reading in FISAs 'definitions of rowing' ( their overarching rules) document that hull air lubrication, channels etc were banned, along with shark skins, polymer injection, planing hulls, multihulls, propellers, hidden motors etc! so have wondered whether anyone had ever tried it. Of course the air wouldn't stay underneath a conventional u profile rowing boat hull for long- it would need a flat bottom with little walls along the rails to keep the air in. So a very different hull type.

    Also at one of Watercraft Magazine's fantastic cordless canoe challenges. ( Now sadly defunct) there was a guy with a hull with a garden leaf blower hooked up to a little plenum that led to a sheet of the channelled hollow acrylic sheet that you sometimes see our conservatory roofs covered with here in the UK. The sheet exited under the flat hull just behind the bow and farted and spat bubbles out of the stern quite pleasingly when the boat was at standstill. The hull was about 2.5 m long and didn't plane so in that respect (it was a race... which I won with a much less ambitious design ), it failed, but I did like the idea of somebody doing it.

    Does anyone know of anyone who's actually got this sort of technology working at a small, DIY scale?

    On the one hand I'm keen to try it, but wonder how sensitive it is to the air pressure, bubble size etc. And therefore whether it is realistically achievable by the keen amateur boatbuilder in his shed.

    On the other hand it's an appealing technology to test in that it's a flick of a switch to turn the air on or off and see if it does anything.

    If anyone's got any experience of doing it at small scale, I'd love to know some basics- how much air for speed/ displacement, what pressure, plenum size, bubble size and how generated.

    Jake
     
  2. portacruise
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    portacruise Senior Member

    Seems like the passive air cavity design would be simpler to implement than using bubbles, nozzles, blower, Etc. So I'm thinking maybe towing a weighted down elongated raft tube shape might be an easy way to test the concept, trying it upside down and comparing it to right side up, as a start. But it may require pumping air in or a stepped hull cavity of some sorts, which would require more time and effort to explore. Would that be a good approach? Link to a 3 meter concept boat, to be tested.

    A Practical Application of Air Lubrication On a Small High Speed Boat - ScienceDirect https://www.sciencedirect.com/science/article/pii/B9780080439501500156

    Click on next chapter at the bottom, for some hydrofoil info.
     
    Last edited: Oct 22, 2021
  3. DogCavalry
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    DogCavalry I aim to misbehave.

    Well, there's plenty of information available about the effectiveness of bubble lubrication, and in the speed regime you're talking about, drag is mostly viscous drag, so should be worthwhile. But the added paraphernalia and hull form compromises might completely eclipse the gains.
     
  4. Ilan Voyager
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    Ilan Voyager Senior Member

    Air lubrication was a dead end. Plenty of people mainly military navies (some with very big budgets, specially Russian) have tried it without interesting results. Beneteau sold one model of small motor yacht, another shipyard in Norway also, that looked like marketing tricks. The true results are not marvelous. Plenty of NA, including the French top cream designers of the oceanic race multihulls, have worked on all the variants active and passive and forgot it.
    There is an equation of complexity and gains. The big questions were : Does it get tangible results ? Is it worth ?
    On ships ? No. Even on fast military boats designed and built regardless of cost.
    On planing fast boats ? Sometimes, but it's not really extraordinary. In my opinion the complication and cost is not worth.
    On small, slow boats ? A useless complication for unmeasurable results. So no.
    It has been tried on RC models, where following the rules of scaling down water becomes pretty sticky, so a lubrication would do miracles. Nada, nothing, the intersideral vacuum of any measurable result.
    You can imagine when there are no results, or just failure, nobody publishes any paper with details. So you'll get a few descriptive articles in internet but even in the very specialized NA and NE mags there is nothing interesting.
    So working on a dead technical solution is a loss of time and efforts.
    Air lubrication has been treated on a few threads. The most recent to my knowledge. It contains some answers.
    Will air injection in a concave sailboat or catamaran hull make it faster https://www.boatdesign.net/threads/will-air-injection-in-a-concave-sailboat-or-catamaran-hull-make-it-faster.63104/page-1
     
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  5. Milehog
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    Milehog Clever Quip

    Random thought.
    Aerated water has less lift. Just ask a white water kayaker.
    Displacement of equal weight and all that.
     
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  6. BMcF
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    BMcF Senior Member

    We played around with a number of air-lubricated/pressurized-air cavity craft over the last 35 years or. Key results:
    • Air lubrication schemes always seem to work best at small scale in tow tank tests. In every one I worked with or knew anything about, the full-scale results were pretty dismal. That was true for single-cavity designs, dual-cavity (catamaran) designs, and V-step air injection designs. I did observe the successful implementation of a V-step air injection system on a planing crew boat. Quite a bit of power was installed to provide the air. The owner/operator of that vessel was quite pleased with the added speed and didn't care about the added fuel consumption because he was providing a passenger ferry service and operational speed was the dominant requirement for the contracts he was getting.
    • The former SES-Europe company in Norway does have one or two operational demonstration craft that have achieved better results than most.
     
  7. BMcF
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    BMcF Senior Member

    That reminded me that it was a Russian guy that was pedaling licenses to add air-lubrication to various hull types. I've since forgotten his name, but he had some impressive demonstrations in tow tanks of how well it would work. One of the yards that he sold the "technology" in to, installed his air lubrication cavity design on a new catamaran passenger ferry (around 40 meters loa as best I recall at the moment). The results were quite sad....not the slightest reduction in hull drag was noted. They quickly proceeded to remove the air supply system, plate over the bottom cavity, and sell it as a conventional cat ferry like the previous ones they'd done.
     
  8. DogCavalry
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    DogCavalry I aim to misbehave.

    Works well in the lab... if only the real world was as controllable as the lab. So the obvious problem is a problem. The bubbles only provide lubrication when they are on the hull, when they don't escape, when there are enough, when they are the right size.
    Not much to ask? Just make a hull that's a terrible shape, so they hold the bubbles, and add enough equipment that simultaneously steals power and adds weight and voila! A vessel that behaves poorly, costs a lot, and is so inefficient that the gains from air lubrication are completely buried under all the losses.

    Not to say there aren't gains. I've read a bunch of papers, and the reduction in friction is real, and meaningful. And completely not worth it.
     
  9. wet feet
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    wet feet Senior Member

    Exactly this:the lower density must surely lead to the hull sitting a bit deeper in a fluid that has had it's density reduced by the bubbles.A zero sum game that might seem attractive if all your data comes from having a test model rigidly attached to a towing rig and taking no account of the reduced density of the aerated water.
     
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  10. DogCavalry
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    DogCavalry I aim to misbehave.

    Well, a floating vessel is supported by static pressure gradient. The presence or absence of bubbles isn't going to effect that to a meaningful degree. It's a different story when you're talking planing. Then it's exchange of momentum, and reduced density matters.
    Additionally tests have shown worthwhile results with bubbles well under 4% of water volume around ship. Not enough to matter.
     
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  11. Dolfiman
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    Dolfiman Senior Member

    The only example I have in mind of a commercial use are the 2 air cushion catamarans "San Pawl" and "San Frangisk" in commercial ferry activity in Adriatic Sea, from Venice to Piran :
    Catamaran San Frangisk - Ferry to Venice by ships and catamarans from Piran, Umag, Poreč, Rovinj and Pula https://venice.si/ships/catamaran-san-frangisk/
    2019 Video :
    These 2 boats derived from a technology developed by Norway in mid 80's to mid 90's, the main issues are :
    * the flexible fore and aft seals rapid wear >> numerous costly maintenance
    * to avoid air intake within the water intake of the hydro jets (air and propellers don't go well together)
    * cobblestone effect in some cases : when the rapid crossing of small waves, by varying the air volume leads to an air pressure vibration.
    * the noise of the fan and circuits, to isolate
    * the global efficiency : the slot for real gain in fuel consumption is quite narrow, let 'say in the 30-40 Knots range on flat enough water
     
  12. jakeeeef
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    jakeeeef Senior Member

    Thanks for those excellent replies.
    I have to say I did wonder about how a small scale design would cope with even small ripples without losing the pressurised air or area of hull lubricated by bubbles.

    I wonder if a just aft of CG small foil would help? So the foil does what the 'cheating' tow tank test rig does- lifts the hull out of its 'settling due to reduced displacement' regime.

    Foil not quite big enough to lift the hull up so it loses its air cushion ( talking about air cushion now not bubble injection). Quite a balancing act but I could see it being of some interest in GLASS CALM water.
     
  13. Mr Efficiency
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    Mr Efficiency Senior Member

    So if introducing bubbles of air has been a disappointment, what about pressurized air cavities ? Any joy there ?
     
  14. jakeeeef
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    jakeeeef Senior Member

    Yes, the above responses are talking about mixtures of both technologies. They are quite different from each other.

    SES/ sidewall hovercraft have been around a while. And their effectiveness is still extensively debated.

    The usual way to do this is a catamaran pressurising the area between the hulls, with flexible flaps fore and aft to keep the pressure in. And it's clearly free air in there so is clearly understood as an SES, like ESNA in Norway for example are doing.

    But if you try to create an air cavity, and inject free air behind a step on a monohull, it's quite a similar technology in appearance to how you would do an air bubble injection on a monohull. So in a monohull the ideas all blend into one another somewhat.

    Ie, if you inject pressurised air behind a step in a monohull, to reduce skin drag, you are simply trying to get a proportion of the hull to slide on something less dense and sticky than water.
    Whether that something is pure air (cavity), microbubbles, foam etc.

    I would imagine the difference between the design of a monohull running on a big air cavity, versus a carpet of bubbles, is the treatment at the sides and transom.
    The big air cavity needs walls of some sort to keep the pressurised air in there, but with microbubbles they are constantly produced at high pressure and are allowed to constantly dissappear behind.
     

  15. Ilan Voyager
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    Ilan Voyager Senior Member

    Jakeef, air cavities of all shapes with all kinds of hull shapes have been done without interesting results. Complicated shape, need of air pumps, lots of plumbing (air hoses) as the air in the cavity is sucked by the water and also needs some pressure. All that for marginal results and a lot of drawbacks.
    The Russians have worked on that for years. It appears that works slightly on high-speed boats of certain size in a narrow range of speeds, in other words some fast military ships, with the con that out of the speeds, the air cavity is a pain.
    The final results have been rather disappointing as the ratio gain over complexity and cost is not worth. If it had worked you could see dozens of fast boats using this technique in the Russian navy.
    As I wrote in my former post none variant has truly worked, although lots of people have tried at great expense, and brain juice since more than 40 years.
    If the air lubrication (whatever the variant) had a significant success, you would see in a few years an armada of boats and ships using it.
    Better to work on good hydrodynamic shapes...

    A good technique is always immediately adopted by the NA and their clients, so within a few years you see plenty of boats and ships using this technique.
    Ferro-cement has been a dead end in spite of the trials and hype about it in the 1970. Now it's marginal. Nobody builds boats in ferro cement except a few barges.
    The diesel-electric powering systems for the ferries and cruise ships. You find it now from small yachts to 350 m cruise ships, including warships, icebreakers, roro ships, fishing boats, ferries, servitude boats etc.
     
    Last edited: Oct 26, 2021
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