Crazy Video: Super-Hydrophobic Substances

Interesting.

Possible hull coating/marine application?

Pipe lining?

Current subs use air-flow to simulate the laminar flow of air over an airframe, which reduces turbulence, and quiets the sub's passage through the sea. Seems like this might be a much less complex and expensive method. Just coat the sub (paint it?) with the same hydrophobic treatment as that small sheet of metal had, to trap air in the skin.

It may also increase efficiency and/or speed.

I know that studies where done on marine mammals concerning conformal shaping of the skin/fat layers and fluid dynamics for application to sub hulls.

Maybe "fuzzy hulls" are the next step?

Seems weird, doesn't it? Make a surface rougher to make its passage through a fluid medium smoother, and less turbulent? Who'd a thunk it?

Maybe the sharks have something going on with the sandpaper hide?

Its a thought. How quietly does a shark travel through water? I don't think anyone has researched it, and I suspect the US Navy has some info on that, but it is probably classified, as is most of the really detailed stuff available.

Begs the question (for me, anyhow):

Can the wetted surface of a displacement hull coated with this stuff maintain buoyancy? IE, will a ship stay afloat?

The reason I ask is that there have been experiments showing that ships can sink due to gaseous eruptions under them from the sea floor.

Hooker

Yeah, but the problem with the seafloor eruption is that it has to be large enough to create a hull-swallowing bubble, wherein the hull itself is smaller than, and thus heavier than, the GAS it has to displace. It will only sink to the bottom of the cavity created momentarily by the bursting bubble, and then, my understanding is that the inrush of water to fill the cavity ALSO fills the hull, swamping it. Its a very momentary confluence of events, and requires that enough water rush in fast enough to overrun the gunwales of the boat. My guess is that an enclosed boat (such as an open ocean lifeboat, or the semi-submersible "rescue boats" used in places like Puget Sound) would just bob up through the would-be swamping wave, out of the cavity, and ride the surface as normal. It could only sink a boat that would allow the water INTO the hull (through deck opening, scuppers, etc.) to fill the hull deep enough to overcome its natural displacement buoyancy.

So, the air trapped on a "furry" hull should actually make the hull float with greater buoyancy (just as MORE air in a sub's hull causes its buoyancy to increase, and it to float closer to, if not actually at, the surface.

I see exactly where you're coming from, and, intellectually, fully agree. Except for the "hull-swallowing bubble" thing. Actually, the experiments I saw involved gazillions of bubbles to lower the effective density of the water around the boat. And I am knowledgeable of the mechanics of submarine ballast tanks.

What provoked my thoughts was the view of the "skin", pointed up (against gravity), encapsulating all that air, effectively isolating the "hull" from the water. I'm still trying to get my head around the vector forces that would result from hull to air to water and how air compressibility would factor into the mix.

One thought I had for the practicality of the surface finish that can encapsulate air; would it make it possible for submarine vehicles (or space craft) to visit higher pressure environments than possible with an untreated hull? IE, would the compressibility factor of water (or other medium) take a chunk of the pressure before it acts upon a hull?

Hooker

Hmm. Interesting thought. Sort of a "pressure buffer" zone? But wouldn't the aggregate total pressure still be the same. Afterall, press in on the "skin" to relieve some of the direct pressure on the hull, sure, but then you have to press THROUGH the skin to put the residual pressure on the hull, which still transfers the static pressure on the skin to to the hull. I think what the fur could do would be to minimize impact spikes, sort of slow the pressure spike on its way toward the hull, like a heavy coat can protect your body from bruising blows better than a thin shirt can. And the fur could spread the pressure to avoid smaller, higher pressure spots on the hull (can't think offhand of anything that could cause those in the first place, though) just like the coat would on your body.

But all in all, the same total pressure would still come to bear, I think.

And I read about the cavitation affects of a mass of bubbles, also, but that description of the swamping due to inrush came from the same source. Maybe I misunderstood. But one thing that never made sense to me was that unless the "bubble mass" with its reduced lifting capacity, extended deep enough, and persisted long enough, it didn't seem to me that the ship (I know subs are called boats, but I'm differentiating between anything big enough it SHOULD be in water that deep, and something too small to be there in the first place) would be able to sink deep enough fast enough to be destroyed by the loss of lifting capacity alone, before the bubble mass would rise to the surface, dissipate, and be replaced by a much denser body of water again.

Don't know. Just didn't seem to be a complete picture of events.

As long as the boat displaces it's weight in water it will remain afloat: look up Archimedes.

"As long as the boat displaces it's weight in water it will remain afloat"

That does appear to be the bottom (Pun intended! ) line, doesn't it?

Oh, yeah, as a former displacement hull sailboat owner and racer and a current aviation designer, I understand Mr. Archimedes' principals.

I was just mulling over the possibilities/mysteries of introducing a captured compressible fluid (air) between an incompressible fluid (water) and a hull, and what may be advantages and/or disadvantages.

For example, I was once involved in an experiment of blowing air out of minute holes in the skin of airfoil surfaces in an effort to lower drag at the boundary layer. It did lower drag but the cost of installing and maintaining the system was prohibitive. Could this super-hydrophobic surface perform the same function at a greatly reduced cost?

Hooker

Well, that was indeed the question, wasn't it? But I suspect, as I noted earlier, that if the US Navy (Or any other interested in quiet submarines. Would that by ALL other?) thinks this has enough merit to research, we'll never hear about it again, at least from the military side.

But is IS fairly common knowledge that at least some subs (Typhoon Class Soviet, I believe) did try using blown air. With what degree of success I couldn't say.