Half a croissant, on a plate, with a sign in front of it saying '50c'
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air-bottomed boat

Like a half sunk hovercraft
  [vote for,

Make a boat with air trapped underneath like a hovercraft. The skirt goes well under water so it's rare that any air escapes, and to allow enough displacement. This provides a large area of low friction support, as only the skirt around the edge has significant friction.

The craft has a sealed deck, with an airlock in case you want to go below. If it's large enough, there can be walkways below deck above the water.

To provide stability and prevent capsizing, the main craft is connected by long arms about five or ten times its size, to two small floats

caspian, Sep 25 2016

Bubbles sinking a boat. https://www.youtube...watch?v=MSmAXp_BHcQ
[doctorremulac3, Sep 25 2016]

Shkval https://en.wikipedi.../wiki/VA-111_Shkval
Risky [8th of 7, Sep 25 2016]

hull riding on bubbles http://newatlas.com...stem/21196/pictures
[not_morrison_rm, Sep 25 2016]

Undersea volcano https://en.wikipedi...ki/Kick_%27em_Jenny
We were encouraged to avoid this area, as rising bubbles would reduce hull buoyancy... [whatrock, Sep 26 2016]

The first paragraph is baked. https://www.google....with+air+under+hull
[notexactly, Sep 26 2016]


       // only the skirt around the edge has significant friction //   

       At anything other than very low speeds, there's going to be turbulence at the air/water boundary; the air will tend to get entrained and pulled out of the back, into the wake. That's going to cause more turbulence, and drag.   

       Therefore the air pad will require continuous replenishment. That will need energy to pump the air in.   

       Much better to work with a well designed liquid/solid interface than a chaotic liquid/gas/solid system.
8th of 7, Sep 25 2016

       I'm sure this has come up before, but how about a boat with gazillions of small holes in the hull, through which air is pumped to create a constantly- renewed skin of air?
MaxwellBuchanan, Sep 25 2016

       It would sink. See link.
doctorremulac3, Sep 25 2016

       Well ... there's supercavitation, used by some torpedoes to produce a near-frictionless "skin" of gas bubbles around the projectile, and that does work, although with a huge potential to All Go Horribly Wrong <links>.   

       But that relies on a short-burn pyrotechnic gas generator - impractical for anything other than a single use FAF weapon.
8th of 7, Sep 25 2016

       //It would sink. See link.//   

       Ah. Good point, [doc]. That would offset many of the potential speed advantages.
MaxwellBuchanan, Sep 25 2016

       //gazillions of small holes in the hull,   

       I remember one design like that back in the 1990's for speedboats...one Mitsubishi part-bubbles design linky
not_morrison_rm, Sep 25 2016

       I like this idea. Think descending weighted baffles, sort of jelly-fish like, that would keep air centrally contained without pumps since any tippage acts as a closed valve.   

       We don't need super cavitation to get aircraft speeds on or under the water.
Cavitation is bad.

       // how about a boat with gazillions of small holes in the hull, through which air is pumped to create a constantly- renewed skin of air?//   

       It would depend on the surface area of the gas emitted and the speed of the craft.
My own take on it used a perforated hull with an outer mesh and blasted CO2 to create an entirely sublimating hull.


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