I think the most promising submarine detection is electromagnetic radiation and magnetic fields but water is a good insulator and it’s hard to see them coming up with much. In WW III the subs will be the last survivors.
Well, a bit more buoyancy than air - (contained)vacuum is buoyant in air
Given that the biggest issue here seems to be that the outer and inner hulls would have to be significantly connected (to support the vacuum itself, and also at any point where there is a hatch or other opening), wouldn’t it be equally effective to just use regular sound-deadening methods such as foams, etc?
I have trouble visualizing this concept.
On a small scale: put a jar inside a bigger jar- create a vacuum in the bigger jar. How are you preventing the smaller jar touching the bigger jar?
I also have trouble imaging the pressure vessel, high pressure| hull | vacuum | hull | normal pressure?
What is preventing the outside hull of wrapping tighly around the inner hull??
I would think normal acoustic principles apply: hard surfaces reflect/resonate, heavy, floppy surfaces dampen. how does creating a second hard hull dampen sound? (given that the 2 surfaces have to be connected at some point)
That seems to be exactly the problem(s) - you can more or less do this with a small object (a Thermos flask only has the outer and inner layers connected at the neck), but it just doesn’t scale up - and also, even Thermos flasks are sort of known for their need of careful handling.
I don’t think changing the speed of sound affects its transmission. High-frequency sounds would move through SF6 just as well as they would through air - they (along with other frequencies) just take a little longer to arrive.
Nuclear subs can reportedly go as low as 1000 feet (or at least that’s as much as they’ll publicly admit to). Adding another 14.7 psi of pressure differential is equivalent to being 33 feet deeper, which is not a huge ask.
As the rest of the thread points out though, it’s still not worth it.
While evaluating SF6’s emissions footprint, I’ve often seen it mentioned that SF6 is commonly used to soundproof windows.
Well dayum, ignorance fought. Relevant PDF here, with intro quoted below:
Attenuation in gases results from several mechanisms that transfer the translational energy of the acoustic wave into other forms of energy. These mechanisms include classical effects related to viscosity and thermal conductivity, losses due to diffusion in gas mixtures, and losses due to excitation and relaxation of vibrational or rotational molecular energy levels in the gas.1 Classical acoustic attenuation results from irreversible losses of acoustic energy to heat due to shear viscosity and thermal conductivity across temperature gradients related to the compression and rarefaction of the acoustic wave. Classical attenuation is well understood and can be predicted for ideal gases, both pure and in mixtures, as long as the viscosity and conductivity of the mixture are known. Attenuation due to diffusion occurs in mixtures when light gas molecules diffuse faster than heavier ones locally changing the mixture composition as the acoustic wave passes. This loss of entropy in the arrangement of molecules results in a reduction in the energy in the acoustic wave. Attenuation related to diffusion is largest for mixtures of gases having much different masses ~e.g., hydrogen/nitrogen mixtures. The attenuation from diffusion can be calculated for gas mixtures, if the appropriate constants are known.2 Exciting the energy states of the gas molecules and the relaxation of these states also causes attenuation. The transfer of translational energy to internal modes occurs with a relaxation time that depends on the collisional dynamics of the internal vibrational modes available and is different than the time required to equilibrate the translational energy. The collisional dynamics in some gases result in relaxation times that correspond to relaxation frequencies ranging from a few Hz up to 10 MHz, where a significant increase in attenuation can be observed. On the other hand, rotational modes in the molecules typically have much shorter relaxation times and consequently usually affect the attenuation only at much higher frequencies.
They didn’t address SF6 specifically, but they talk about acoustic energy (molecules being pushed to and fro by pressure waves) being diverted into the vibration of atomic bonds within each molecule, and into the rotation of individual molecules. And waddyaknow, an N2 or O2 molecule has only one bond that can take up vibrational energy, whereas an SF6 molecule has six such bonds to work with - and an SF6 molecule also has far more rotational inertia than diatomic oxygen or nitrogen. Not sure whether the latter helps or hinders attenuation, but I can see now that the geometry and complexity (i.e. # of bonds) are relevant to acoustic attenuation, and that SF6 is pretty different from air in that regard.
The degrees of freedom of a more complex molecule certainly allow it to absorb more acoustic energy and thermalize it (i.e. convert it into random motion); hence why complex molecules deviated so much further from ideal gas behavior. This does translate into more acoustic damping, particularly in the higher frequencies at which molecules will absorb energy. An obvious example are the sound suppression systems used with large rocket launch vehicles which spray water into the exhaust, attenuating the sound and disrupting shock waves. How much of a difference this would make in silencing an already very quiet submarine, however, is questionable. For the most part, the high frequency noise will be lost in the background and it is the lower frequency mechanical noise of pumps, tools being dropped, hatches being slammed shut, et cetera, that will be transmitted through the water.
Stranger
Obviously I know that. But I’m saying in this context it’s the same to within 0.1%. It’s the mass of displaced water that creates buoyancy.
Thought experiment here. You have a box with a vacuum inside. No sound will penetrate the vacuum. How do you enclose the vacuum with material that won’t transfer sound?
To illustrate, I put a microphone on a desk in front of a speaker. Even if the speaker says nothing at all, when he hits his hand on the desk or shuffles papers, the sound will travel by contact into the mic stand, mic body, and mic diaphragm. And it will be LOUD.
Now, how you abate the desk slapping is with something called a “shock mount”. Which is basically two concentric rings. The outer ring goes on your mic stand. The inner ring goes on your microphone. The two rings are connected to each other with a series of elastics or bungies or springs.
When you slap the desk, the two rings are free to vibrate separately, and ultimately, little to none of the vibration gets to your microphone.
So if we go back to my original triple-hull vacuum suggestion to the OP, am I suggesting connecting the vacuum hull to the pressure hull with bungie cords?
YES, YES I AM. LOTS OF BUNGIE CORDS. Because that’s brilliant drunkard’s logic which befits this as a shouty pub chat.
I think we should build one that also can fly as a vacuum blimp.
Oh no! not the vacuum blimp again. Last time I remember we went for pages and pages about vacuum blimps / balloons and the proponents just Would. Not. Get. It.
No, because a vacuum means an absence of air, and thus the absence of buoyancy.
That’s not how buoyancy works. Overall density is all that matters – a vacuum filled void would reduce the overall density of the submarine, and thus increase the buoyancy.
I’m stealing this for the “Made-Up Band Names” game. 
Regardless, submarines have ballast tanks and you’d just need to make them a little big bigger. The effect of the vacuum on the submarine’s buoyancy is an utter non-sequitor here, a problem whose solution is utterly trivial.
Good call. However, we’re talking of maybe 3 inches between the inner and outer hulls. That’s almost nothing. Density may be reduced, but the effect wouldn’t nearly be as dramatic as predicted in the posts I’ve read.
What’s described would be a massive increase in buoyancy, and it would require massively larger static ballast to make up for it.