Air bubble lubrication successful trials

Because this is about displacement, you don't have to be moving ( keeping the air there once moving is a whole other issue).
So for those interested isn't this a simple experiment involving two wooden boxes ( one with cavity, one without- otherwise identical), a hinged arm, an air pump, a ruler, a bath?

Quicker to set up than write all these posts I'd say!

 

Nope. Displacement of water is not how things float. Archimedes noticed that an object displaced a volume of water that could be weighed, so he could calculate an exact volume for a random shaped object. And for a couple thousand years since, everyone has misunderstood, and confused themselves and everyone else. Try floating something like a cup inside a similar cup. With only a tiny amount of water between them, the inner cup floats. It floats in water (if the cups are well matched) in water that weighs a very small percentage of it's own weight.

 

This thread invites confusion because it has included discussions of both air lubrication and air lift and they are two different things. Air lubrication is (or is hoped to be) a clever way to detach the hull skin from the water in the boundary layer region, or at least some of it. That's a very "thin" region and still the air must be injected at much higher pressure than the local static pressure on the hull or no air will be injected.

Static air lift, on the other hand, involves a true cavity of some sort, on the underside of, or between, the hulls, that can contain the pressurized air injected in to it. In the case of static air lift, the water surface in the pressurized cavity will be pushed down by the pressurized air, and be lower than the external waterline. Take an SES with 500mmWC air pressure supplying the air cushion; the average draft inside the air cushion will be roughly 500mmWC lower than outside the cushion. (The actual shape of inside and outside water surface levels is a bit more complicated than that, and varies with speed, but you get the general idea).

 

Pressure in a fluid is the same everywhere at the same depth. So bubbles in water are squeezed down to the same pressure, and exert the exact same pressure on anything they touch as the water beside them.

 

Yes, my OP was about bubble lubrication, and it has been hijacked by air lift discussions.

It's all interesting though.

 

Right. Bubble lubrication works just fine, in the right applications. Time for google searches, and reading a bunch of research paper pdf's. Wärtsilä has a nice overview.

 

This thread is really encouraging me to use my imagination to understand what might be going on with air bubbles.Could it be possible that as the bubbles are carried aft by the motion of the vessel,the energy that has been used to compress the air is returning to reduce the resistance of the hull by expanding the size of the bubble as it travels along an approximation to the buttock line?Something like squeezing a watermelon pip between a couple of fingers?

 

A nice thought, but research, darn I can't find the pdf, showed there was a very strong connection between bubble size and gains. When the bubbles were very small, gains were around 6%. When they were big enough to deform under the shear load imposed by water displaced by the moving vessel, drag reduction could be as high as 40%.

 

Had not considered that, It definitely thickens the plot.

 

Makes perfect since.
Slightly deformed bubbles would be the max "dry" to "wet" surface area ratio.

Is there any data about size of bubbles and distance between bubbles?

 

The game is afoot! I'm back to the banal, but if you find anything, let us know.

 

That would be convenient. I'd imagine larger bubbles would be cheaper ( power wise) to create, less compression of the air required? Lower pressure/ higher volume compressor? More like your airbed/ dinghy pump than your garage compressor?

Tempted to drill some backwards facing tubular holes through an old windsurf board near the front then connect up my 36 volt leaf blower and a big plenum. Have inserts in the tubing through the board to make larger or smaller bubbles. Tow with spring balance. Not tempted enough to actually do it, but tempted.

With technologies that haven't been done or proved either way yet, there's a lot to be said for putting away the computer and getting out the epoxy.

This experimentation is on my 'to do' list. But with everything else on it I'll be commencing it 2035.

I think I speak for a few people here though when I say, please find this PDF!

 

The other thing of course (incitement to commit environmental crime coming up)... How much soap would you need to ease things along a bit? Probably only a few drops a second. Much less needed in fresh water.

Anything involving sliding solid objects and water ( and the making of bubbles) is rather more impressive with a bit of soap in the mix.

Biodegradable soaps (Like ecover) are available, but they must still lather. Don't know how long they take to biodegrade though. Just a thought.

Probably won't have been tested. No towing tank in the world would let you inject soap into it!

 

https://www.sspa.se/ship-design-and-hydrodynamics/hull-air-lubrication-future-and-challenges

An interesting read. This process does two things, reduces draft as it acts a bit as a hovercraft and reduces skin friction. Be sure to look at the SES technology link

https://ww2.eagle.org/content/dam/eagle/advisories-and-debriefs/Air Lubrication Technology.pdf
and another with many references, but larger vessels only

 

Soap acts as a surfactant. Makes the bubbles much smaller. No soap.