Will air injection in a concave sailboat or catamaran hull make it faster

Discussion in 'Boat Design' started by jon haig, Nov 19, 2019.

  1. portacruise
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    portacruise Senior Member

    My guess is the slower, shorter and less streamlined the boat is, the greater the % benefits of air injection with a hull designed to retain bubbles. Seems like the bubbles would stay in position longer with less energy expenditure at low speeds. The resistance to motion of an air barrier lubrication would be less than any fluid the way I see it, and it is easier to double a speed of say two knots compared to one of 20 knots? If these ideas are incorrect, what am I missing?
    Hydrofoils have their own issues which don't affect air injection as much or at all. Disadvantages for foils like greater to severe consequences for hitting or tangling debris, issues related to deeper draft, higher speeds needed to fly, Etc.
     
    Last edited: Nov 26, 2019
  2. Eric Lundy
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    Eric Lundy Junior Member

    It seems like making a ghetto hovercraft has advantages. Yet I am not sure if they are applicable in this situation.
     
  3. Ilan Voyager
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    Ilan Voyager Senior Member

    You are missing the laminar layer of the water sticked on the surface of the underwater hull.
    On a slow hull there no advantage of injecting a layer of air bubbles as the eventual advantage will be not measurable or so small that it's not worth to enter in the complication of air pumps and all the plumbing. The bubbles will run immediately to the surface. There will not have enough speed to obtain a Bernoulli effect of suction of air also.
    The Re number is too small. That explains also why small RC planes can use a simple flat board as wing, with little advantage of using sophisticated profiles if there is enough power.
    But on a object moving fast on the surface of the water it's another matter (hyper cavitating torpedoes are a very different subject as they are totally submerged).
    On sailboards, planning very fast on the surface of the water, special waxes are used to forbid to water to stick on the wetted surface of the board and to form a thick laminar layer. It's similar to the farts used on skis which slide on water of the melted snow by the pressure and friction of the skis. The gain in speed is absolutely spectacular.
    Teflon has been used also with some success.
    Air injection by pump or suction works really on fast planing boats, where the stickiness and thickness of the laminar layer of water on the surface of wetted surface is an important factor of drag. The friction by the water on the hull goes up at the square of the speed if I remember well. That means that any significant disminution of the stickiness ie density of the laminar layer closer to the hull will have a huge impact on the global drag of the boat. More are you going fast, longer will be the trajectory of the air bubbles under the hull before going to the surface. Strakes can be used to keep the air running along the hull.
    On slower boats concave bottom hulls are used to create a big surface pocket de air under the hull thus diminishing the surface of hull in contact with water. Air pumps are used to maintain the volume thus the surface of the air pocket. But the surface of the air pocket must be an important fraction of surface of underwater hull to give a significant advantage. On rather thin hulls this air surface of the concave bottom is not enough big to present an advantage.
     
    Last edited: Nov 28, 2019
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  4. portacruise
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    portacruise Senior Member

    Ilan, thanks so much for the detailed reply!

    I am wondering if the hyper cavitating torpedo concept might work with SWATH boats for the submerged sections? But it would seem to me that it would require a tremendous amount of energy to maintain that bubble in front of the submerged sections, especially over long distances.

    Also, I'm wondering how much of a speed increase would be possible with a concave bottom hull set up- on a tiny, one man un- streamlined, six foot boat?
     
  5. Ilan Voyager
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    Ilan Voyager Senior Member

    First sentence NO, and simply it's impossible on a surface boat. That needs to be totally submerged and going very fast to have any interest. It's a military technique, regardless of cost, for a very precise application.
    Second sentence 6 feet boat ???!!!! why a ridiculously small 6 feet??? whatever the kind of boat: rowing, sailing, displacement or planing, the answer is simply NO. Even one person beach canoes are bigger than 6 feet.
     
  6. portacruise
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    portacruise Senior Member

    Ilan, Thank you so much for your considered advice, on what most boat enthusiasts consider some foolish proposals on my part!

    Lots of reasons for the why on both of my proposed examples above.

    I've always been interested in engineering challenges. Lots of potential applications all the way from smaller and faster jet skis, to drone and weapon applications, to emergency Lifeboat and small tenders, to tiny portable powered boats that can be transported in a suitcase, Etc.. I have had some success in different tiny boat craft, not related to above Concepts, in the original posting.

    The record smallest size for transatlantic Sail navigation is well under 6 feet, the "Vera Hugh" and "Father's Day". A search on this forum under "famous small boats" will turn up a discussion on these record Breakers, for anyone that might be interested.
     
  7. Ilan Voyager
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    Ilan Voyager Senior Member

    These sail boats do not demonstrate anything except the personal challenge of crossing the Atlantic in a boat less comfortable and just a bit more seaworthy than a coffin. Their mean speed has been just a bit over simple drifting.
    But you can cross the Atlantic pretty well, in decent comfort and security, at a correct speed on a plywood sail 18 feet. That have done several times on the old Muscadet. That was a boat pretty cheap to build.
    Boats transportable in a suitcase; that have been done with foldable canoes and kayaks which are longer for evident reasons of efficiency than 1.60 m. Their origin is military just before WWII for the commandos.
    Nautiraid (Kayak pliant RAID 540 - Nautiraid https://nautiraid.com/fr/produits/kayak-pliant-raid-540/) is an old specialist of these boats you can take on a plane in a suitcase, this brand works also for the military.
    You have also inflatable canoes and kayaks.
    Origami and inflatable small power boats have been done also. The complete boat with the 2S outboard can be put in a rather heavy suitcase which, with the actual norms in commercial passenger airlines, will give you big troubles and big expenses, their value is purely anecdotic.
    Inflatable emergency lifeboats have been the norm since ages in personal and commercial boats. You can find them in a great variety of sizes. Some can be used with a kite.
    Jet skis must have a minimal size as someone has to seat on. It must have also a minimal volume for efficiency, and some capacity of gas. Thus it's rather difficult and useless to go smaller than the actual racers.
    So a 1.60 m boat with concave bottom and air pocket has no use as an air pump of rather high volume/low pressure capacity will be needed. On such a 1.6 m boat there is no place for all that air pump mechanics, with motor or engine, plus gas or batteries. And as human being is only able to give around 0.30 HP for a few hours, it's seems very difficult for a human to pump the air except for a short moment. And a human will be unable to pump the air and to row in the same time, because the total power needed is above the strength and stamina of 99 % of the people.
    Furthermore the gain will be close to zero and the final result will be extremely disappointing compared to for example an optimized rowing boat, or even a correctly designed motor boar for one person.
    There is an effect scale of complexity versus results. The Occam's razor is the best way for choosing technical solutions.
     
  8. BlueBell
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    BlueBell Ahhhhh...

    I love it, thank you!
    "Ad Hoc Hypothesis" gets a mention in the Wiki definition.
    I wonder if there's any connection...
     

  9. portacruise
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    portacruise Senior Member

    Ilan, thanks for your detailed and helpful comments!

    I don't think any of the conveyances mentioned below have been attemped with air bubble lubrication was the point I forgot to mention. Bubble lubrication is not an all-or-nothing application like occurs in a hovercraft or hydrofoils when being on top, unless my understanding is incorrect? Would that mean that designers might be able to relax on some current length constraints related to drag when using excellent bubble lubrication? I have interjected some more possibly foolish ideas within your post below, as I have found that myself has sometimes been surprised by simple overlooked ideas.


     
    Last edited: Dec 3, 2019 at 5:53 PM
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