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Theoretically, if I were to launch something faster than the speed of sound in water (around 5 times that of air), what would happen?

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  • $\begingroup$ Google supersonic torpedoes. $\endgroup$ – Mike Dunlavey Jul 26 '15 at 12:18
  • $\begingroup$ en.m.wikipedia.org/wiki/Sonic_boom scroll down for pistol shrimp $\endgroup$ – user81619 Jul 26 '15 at 12:55
  • $\begingroup$ Please do not put tags in the question title. See this meta post on question titles for useful advice on writing good titles. Also, the proper noun "I" is capitalized in English. I edited this for you, but please do pay attention to spelling and capitalization in the future. $\endgroup$ – DanielSank Jul 26 '15 at 17:49
  • $\begingroup$ I'm voting to close this question as off-topic because it is a What if _______ happened question. $\endgroup$ – Kyle Kanos Jul 26 '15 at 19:35
  • $\begingroup$ @KyleKanos I take your point, but I think, if you believe certain websites cited in the answers below, that is more than a what if, the US navy may have got somewhere with it. $\endgroup$ – user81619 Jul 26 '15 at 19:48
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The reason that the speed of sound is a well-defined quantity is that, for small pertubations, the equations which govern the fluid dynamics can be linearised. In that linearised form, the solution boils down to a simple wave ansatz with linear dispersion relation, i.e. constant velocity.
Those are the sound waves.

It so happens that in air, this linear solution holds up pretty well even for rather big pertubations. Only, when something moves faster than the speed of sound, the linear wave mode obviously can't be used anymore to transport away energy, hence any disturbance (which is inevitable when something's trying to move through the air) is “trapped”. The pertubation gets ever stronger, until the dynamics are completely nonlinear and you get a shock wave.
This is the reason the sound barrier is such a crucial limit for aircraft.

In incompressible fluids like water, this doesn't necessarily work out the same way. In water, the dynamics tend to be far more violent, even well below the speed of sound. In particular, you will readily end up with cavitation bubbles. As already said by Acid Jazz, this allows for a rather remarkable mode of underwater motion, which is completely unlike anything you get in air.

tl;dr, the sound barrier isn't really relevant under water, because stranger effects turn up before you ever get close to it.

However, it would still be relevant if you could manage to keep the pressure pertubations small enough even close to the speed of sound. Actually, this is relevant for any supersonic motion under water, if you look at it on a big enough scale. For instance, an asteroid impact into the ocean does no doubt cause a sonic cone quite analogous to the one generated by supersonic aircraft.

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  • $\begingroup$ "an asteroid impact into the ocean" -- for that matter an asteroid impact on land can result in it (briefly) travelling through the solid faster than the speed of sound in the solid. With likewise catastrophic results for both the projectile and the medium! $\endgroup$ – Steve Jessop Jul 26 '15 at 22:43
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Speed of sound in water at 20 degrees Celsius is 1482 m/s., (2881 knots), just for comparison to current claimed achievable speeds.

Small related fact: The pistol shrimp can create sonoluminescent cavitation bubbles that reach up to 5,000 K (4,700 °C) which are as loud as 218 decibels, breaking the sound barrier in water. Says Wikipedia

YouTube video of Pistol Shrimp In Action

Water, at say ocean temperatures, is pretty much incompressible, so it's a tall order. The moving object would be moving through a cloud of steam , essentially, rather than touching the water directly.

In designing the profile and shape of the object, you would need to consider the most efficient method of turning liquid water into steam.

If you could do it, (comparable to moving through a block of solid oak wood, or something even denser) the water would turn to steam, caused by boiling, at the leading edge of the moving object. As the steam bubble expands, the object begins moving through the void created by the expanding vapor.

The moving object is then surrounded by the bubble of steam created at it's leading edge since it is moving through the expanding bubble faster than the surrounding water can cool the steam and return it to a liquid state. That allows the moving object to travel, in effect, through steam, rather than liquid water.

See Carl's comment below on possible problems achieving this effect.

Supersonic torpedo diagram

From Wikipedia: Supercavitation

Several challenges remain for the supercavitating torpedo, including how it will be steered underwater. Water-tunnel tests have already proven that speed can be achieved: In 1997, the Navy tested a supercavitating projectile (illustration above) that reached 5,082 feet per second, becoming the first underwater projectile to exceed Mach 1.

Mach 1 Torpedo

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    $\begingroup$ Well, maybe. Consider a bounding case of a 1-dimensional object made of some material with near-zero friction coefficient. My suspicion (not based on any analysis) is that a hypersonic object would cause significant cavitation but that the leading edge/point probably moves past the water it's compressing before the water actually boil. $\endgroup$ – Carl Witthoft Jul 26 '15 at 11:57
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    $\begingroup$ @CarlWitthoft Maybe is exactly the right word. I thought that if anybody was interested in this, it would the military, but searching only turns up rumours and suggestions. Not my field, but I don't think it's a viable proposition, especially for the reason in your comment. $\endgroup$ – user81619 Jul 26 '15 at 12:05
  • $\begingroup$ en.wikipedia.org/wiki/Wikipedia:DAW $\endgroup$ – Stop Harming Monica Jul 26 '15 at 20:44
  • $\begingroup$ @OrangeDog seriously, i take your point and you are correct, but would you consider posting this on meta for larger circulation? Google tried to stop people using "Google It", never worked tho. regards $\endgroup$ – user81619 Jul 26 '15 at 20:54
  • $\begingroup$ This is the opposite of Google's problem. $\endgroup$ – Stop Harming Monica Jul 27 '15 at 7:08
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Oddly enough, supercavitation can help increase max speed. According to the not-necessarily-correct PopSci, the Navy has achieved Mach 1:

supercavitating torpedo . A link in wikipedia claims the Germans have achieved 800 km/h .

Other than the Soviet Shkval, I can't find any updates on such torpedos, so either the Navy programs have gone deep black or they've given up.

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  • $\begingroup$ 800 km/h (222 m/s) is nowhere near the speed of sound in liquid water, so how can that be considered Mach 1? $\endgroup$ – Time4Tea Jul 17 at 13:16
  • $\begingroup$ @Time4Tea Blame PopSci for using "Mach 1" as the air-speed Mach number. $\endgroup$ – Carl Witthoft Jul 17 at 15:40

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