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So, in my story, in a prelude to an epic aquatic fight scene, my main character gets chased by a great white shark. He swims as fast as he can but he can’t escape. Just to clarify, he’s wearing one of these types of underwater gear:

Scuba Gear

He gets an idea that just might work. He takes out his air tank, holds his breath as his helmet fills up with water, and puts his sealed air tank underneath him. He then unseals it and gets propelled straight out of the water. He gets on his boat, gets a harpoon gun and kills the shark.

My question is: would it be physically possible to do that with the air tank?

In most questions, I got a resounding “No” as an answer, so I guess this idea isn’t very plausible. But I still want to make my story action packed, so, in future answers, can you provide some sort of alternative?

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    $\begingroup$ This question is being discussed on meta. $\endgroup$
    – Monica Cellio
    Commented Jun 21, 2018 at 15:40
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    $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monica Cellio
    Commented Jun 21, 2018 at 17:51
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    $\begingroup$ "he’s wearing one of these types of underwater gear" Are you aware that the equipment pictured is a surface-supplied breathing apparatus? There's no tank. $\endgroup$
    – David Scarlett
    Commented Jun 22, 2018 at 5:56
  • $\begingroup$ I have no idea if it would work on a shark but one piece of advice I've heard for dealing with aggressive aquatic creatures is to purge one of your mouth pieces in their face (that is you hold a button on the mouthpiece and air flows through it rapidly). It tends to scare most things off though I have no idea whether big sharks that have decided to savage a person would care... $\endgroup$
    – Chris
    Commented Jun 22, 2018 at 13:59
  • $\begingroup$ Of course, shark attacks are rare. Only four people per annum worldwide die of shark attack. Sharks attacking SCUBA divers are even rarer. Once divers were sure sharks didn't attack divers, but there is at least one diver fatality. While it can happen, it is extremely improbable. Falling down stairs is a greater hazard. I know you're going for drama and that's fine. $\endgroup$
    – a4android
    Commented Jun 24, 2018 at 8:51

12 Answers 12

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Here is a way to bypass the issue

Our hero removes his air tank, points it away from himself and starts fumbling to undo the top. It's not working! He's never opened one of these things himself let alone underwater, let alone uninstructed first time around while being attacked by a shark, and these things probably have safety mechanisms to prevent stupid divers doing it on accident, and why am I doing this, why am I here oh no here he comes!

Just then the shark lunges at our hero and bites over the top end of the tank and somehow wrenches the seal out of the tank. The tank goes flying out of our hero's grasp and into the deep blue. But the blast of decompression spooks the shark and gives the diver time to escape.

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    $\begingroup$ I like this one! Maybe the tank is one of the newfangled carbon-fibre ones instead of a steel tank, so the sharp points of the sharks teeth might punture the tank enough for it to rupture, blowing the shark up Jaws-style. $\endgroup$
    – bukwyrm
    Commented Jun 19, 2018 at 19:39
  • $\begingroup$ I'm not sure how hard it is to pierce the metal / carbon fibre casing part. I imagine the shark grabbing where the tube joins the cannister and pulling that end off. $\endgroup$
    – Daron
    Commented Jun 19, 2018 at 20:14
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    $\begingroup$ In any case there is less to explain since there is no requirement for the characters to know exactly which component of the tank broke. $\endgroup$
    – Daron
    Commented Jun 19, 2018 at 20:15
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My question is, would it be physically possible to do that with the air tank?

No.

Source: Tried it.

We've actually been taught to breathe off a tank without a regulator. You can do so by feathering the valve. But even when you fully open it, the thrust is simply not enough to propel you at a speed comparable with even relaxed finning.

Much worse than the limited thrust is the presence of torque. It feels like all of the modest thrust that you get goes towards spinning the tank, not moving it laterally. The torque is really strong, it's virtually impossible to hold. Even if you open a tank strongly attached to yourself - yes, I've done that - the thrust still goes towards spinning you, rather than propelling you.

A tank could be modified into an effective propulsion device using the Venturi effect, similar to aircraft evacuation slide inflators, to use water as added working mass, and to push it along the tank's axis, not to the side. But that's not something you could jury-rig underwater.

For a diver not planning on this situation, using the air for breathing and fins for propulsion will give considerably more speed. Nowhere near as much as a shark, of course...

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    $\begingroup$ Wild suggestion: Could you use the spinning thrust to round-kick the shark to oblivion? $\endgroup$
    – xDaizu
    Commented Jun 20, 2018 at 11:35
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    $\begingroup$ You could definitely hurt a shark with this. Not kill it, except in a very freak case, but sharks aren't some insane bloodthirsty maniacs. They're generally very cautious; add to this that humans aren't their target prey, only occasionally get mistaken for it. $\endgroup$
    – Therac
    Commented Jun 20, 2018 at 13:23
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    $\begingroup$ Personal experience doing something the majority of us wouldn't even dream of doing... +1 $\endgroup$
    – JBH
    Commented Jun 20, 2018 at 13:40
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    $\begingroup$ @Therac Great! Just think of the awesome factor of jet round kicking a shark. Bonus points for being actually feasible in real life! (I'll take your word for it!) $\endgroup$
    – xDaizu
    Commented Jun 20, 2018 at 14:15
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    $\begingroup$ +1 for personal experience! Question: Even without the regulator is there some component that prevents the gas leaving the tank as fast as it wants to? Could the gas leave more explosively if you removes/broke the piece below the regulator? $\endgroup$
    – Daron
    Commented Jun 20, 2018 at 17:08
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First of all, holding your breath while emerging from a deep dive is a really bad idea: the air in your lungs will expand as you approach the surface and will turn you into a balloon, probably killing you faster than gaseous embolism would do.*

Then, coming to your real question, to get propulsion from a rocket in air we use a Venturi tube to accelerate the flow of hot gas at supersonic velocities, thus achieving the needed thrust. In your case we have a cold gas which can hardly be accelerated to any decent velocity, but for sure not supersonic.

I would say that it cannot work.

People who have studied propulsion by compressed air have used it to move a propeller.

The possibility of application of pneumatic propulsion method in tourist submarine is analysed. Results illustrate that: on the premise of lightened quality of the propulsion system, pneumatic propulsion method fulfills the required underwater speed and it's duration.

Gaseous embolism is what happens when gases dissolved in the blood circle get back to the gaseous state, forming bubbles which can obstruct the blood flow.

And the image below shows what happens to the swim bladder of a fish when it is pulled up on a boat from the bottom of the sea.

Warning: graphic image

Photo of a dead fish with its swim bladder swollen out of its mouth

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    $\begingroup$ True I completely forgot Boyle's law, that would be messy. $\endgroup$
    – Ash
    Commented Jun 19, 2018 at 19:07
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    $\begingroup$ Humans don´t have a swim-bladder and the fish shown surely came from way deeper than anyone in scuba-gear would dive. While you can die from a decompression event, it does not look very spectacular ... $\endgroup$
    – Daniel
    Commented Jun 20, 2018 at 15:07
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    $\begingroup$ The common term for gaseous embolism is "the bends". $\endgroup$
    – Acccumulation
    Commented Jun 20, 2018 at 17:22
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    $\begingroup$ @Daniel, I have seen that very thing on fishes I have fished from a bare 10-15 meters $\endgroup$
    – L.Dutch ♦
    Commented Jun 20, 2018 at 18:04
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    $\begingroup$ @L.Dutch I agree that spoiler formatting is warranted for the benefit of those with weak stomaches, but it's usefulness is reduced somewhat by the lack of any prior explanation as to why the image is spoiled. It is not immediately clear to me that "what happens to the swim bladder of a fish when it is pulled up on a boat from the bottom of the sea" is going to be visually disturbing. A clear warning to that effect would be appropriate. $\endgroup$
    – jmbpiano
    Commented Jun 20, 2018 at 22:43
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Alternate idea: your diver already has a propulsion device.

scuba propulsion motor https://blog.mares.com/understanding-scuba-divers-code-1082.html

It is just not fast enough to get him away from the shark. Which clearly is interested but not that interested or it would have bitten him already.

The idea is taken from a scheme of how to get away from an angry bull. Your diver strips off his suit and sends it off with the propulsion device, empty suit flapping behind. The shark follows that and the diver gets away.

The diver is not Ahab. He returns to the boat and proceeds to drink beer and listen to music and watch the sunset and rejoice that he is alive.

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    $\begingroup$ Stripping off a wetsuit while 30ft underwater and trying to evade a hungry shark is something I would never want to even consider trying to do. They're difficult to put on, and just as difficult to take off, if fitted properly. $\endgroup$
    – Rayanth
    Commented Jun 20, 2018 at 5:15
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    $\begingroup$ Agreed, speaking from experience, a wetsuit is very close-fitting, it might be fractionally easier to take off underwater than when wet on the surface, but I can't imagine any scenario where stripping off would be a practical option that would make sense to me. especially not in frigid waters where the wetsuit is one of the few things keeping me from hypothermia! $\endgroup$
    – Ruadhan
    Commented Jun 20, 2018 at 9:10
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    $\begingroup$ Such a scenario exists and it's called (don't google it) Warhammer maneuver. Don't look it up; if you ever need it, you'll know what to do. $\endgroup$
    – Therac
    Commented Jun 20, 2018 at 9:22
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    $\begingroup$ @Ruadhan2300: Not to mention that you would first have to take off your tank, and hold it, keeping the mouthpiece in your mouth, while taking off the suit. Maybe Bond is cool enough under pressure to do this, but I'm not. Then too, in order to take off the suit, you'd pretty well have to take off your weight belt first, and you have to hold on to that, otherwise you're heading for the surface quite quickly. (Had that happen to a friend: one second she was swimming beside me, next thing I knew she was on the surface.) $\endgroup$
    – jamesqf
    Commented Jun 20, 2018 at 18:14
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    $\begingroup$ Taking off your buoyancy compensator and sending it off as a decoy would not be easy, but certainly easier than a wetsuit - you would after all have to take it off first to take the wetsuit off anyway. And without it, you might need to drop your weight belt, so let's hope you aren't too deep for an uncontrolled ascent. $\endgroup$
    – armb
    Commented Jun 22, 2018 at 9:41
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Well... kinda... maybe... Oh, alright... No.

Have you ever tried to hold on to a fire hose? If not, there's a reason firefighters tend to use multiple people to hold them. The force behind the release of pressure is greater than the human hand/arm can compensate for.

I can easily imagine that the pressure in an oxy tank will produce the same problem. Yes, you could hold onto the harness, but the force may still rip the tank out of your hands and/or harness (which isn't designed to withstand the force).

Even if you could hold onto it, how do you direct it? You'd either be riding the tank somewhat like Slim Pickens riding the nuke in Dr. Strangelove (and have about as much control) and the most likely outcome is turning reverse somersaults because your body is creating serious drag above the plane of energy release (think "pinwheel"); or you'd be hanging onto the harnes (with all the discharge in your face...) and going wherever the tank wants to take you.

This would make a great James Bond moment, the kind where you smile and slap your forehead with your hand because you know it's entirely impossible — but it's great fun to watch.

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  • $\begingroup$ Hey but what if he had a harness that was tougher and more restistant than normal? $\endgroup$
    – MindX
    Commented Jun 19, 2018 at 19:09
  • $\begingroup$ Hey, can you provide a possible alternative I could use in my story. I would really help. $\endgroup$
    – MindX
    Commented Jun 19, 2018 at 19:22
  • $\begingroup$ Now come on, riding a nuclear rocket can't be that hard youtube.com/watch?v=B3wpnXxBt8o $\endgroup$
    – Cephalopod
    Commented Jun 20, 2018 at 8:55
  • $\begingroup$ well, the force is mass times acceleration. Firehose shots water, which has considerable mass while airtank has gas. I don't know how much air there is in a typical scuba tank (in terms of mass), but I doubt it has much more than the weight of the diver total. Firehose, on the other hands, delivers up to 1000l/minute. That's about mass of 1 human every 5 seconds - this is why a single fireman is not enough. $\endgroup$
    – Agent_L
    Commented Jun 20, 2018 at 13:24
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    $\begingroup$ @JBH The oxygen tank at welding classes wasn't scary at all. It was just a hissing noise. $\endgroup$
    – Agent_L
    Commented Jun 20, 2018 at 15:18
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While it might be theoretically possible to do this, it would not be practical. Your hero would have to take off his tank (which is carried on a backpack-like harness), bring it around to where he can get his hands on it, close the tank valve, unscrew the regulator, and open the valve again. While he's doing this, the shark has plenty of time to catch and eat him.

Another detail is that the air is going to leave the tank at right angles to the tank, due to the way the valve is built: https://www.leisurepro.com/blog/scuba-gear/a-beginners-guide-to-scuba-valves/

Also, while I've never tried it, I really doubt that the escaping compressed air is going to supply enough force to propell your hero very far, certainly not straight out of the water. If he wants to ascend quickly from a mere 30 ft, he'd probably do better just to drop his weight belt, but even that is going to give the shark plenty of time to snatch a bite. That's the real problem here: sharks swim fast, scuba divers don't.

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  • $\begingroup$ Yeah, I knew scuba divers aren’t as fast as sharks, that’s why he uses the air tank $\endgroup$
    – MindX
    Commented Jun 19, 2018 at 19:18
  • $\begingroup$ +1 The only answer mentioning that the output is going sideways, not rocket-like. I've never ever seen the latter and I've seen many various diving tanks. $\endgroup$
    – kubanczyk
    Commented Jun 20, 2018 at 12:02
  • $\begingroup$ @kubanczyk I wouldn't say it's the only one. Therac's answer devotes a whole paragraph to the spinning effect. $\endgroup$
    – xDaizu
    Commented Jun 20, 2018 at 14:11
  • $\begingroup$ @MindX: But my point there is twofold. First, the speed difference is such that no sensible scuba diver is going to try to outswim a shark in the first place. Second, given the limited visibility underwater, your hero's not going to have time. $\endgroup$
    – jamesqf
    Commented Jun 20, 2018 at 18:00
  • $\begingroup$ There is plenty of energy to launch him out of the water; however, it would be next to impossible to 1) break off the valve by hand underwater, 2) harness the energy properly too give vector propulsion. See this video: youtube.com/watch?v=ejEJGNLTo84 $\endgroup$
    – MrWonderful
    Commented Jun 21, 2018 at 21:19
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The best answer is to not put yourself in such positions =)

However, assuming you do find yourself face to face with a shark, these tanks can produce an enormous amount of force.

The big question will be how much energy it has. Fortunately, storing energy as compressed air is a thing, so we have a wikipedia page with the required calculations!

$W=P_bV_b\ln\frac{p_a}{p_b} + (p_b-p_a)V_b$

In this case:

  • $V_b$, the volume of the tank = $0.011 m^3$, a typical diving tank
  • $P_b$, the pressure of the tank = $20 MPa$, roughly 200 atmospheres, or 3000psi
  • $P_a$, the atmospheric pressure, $0.1MPa$

Plug these in, and we find roughly 370kJ of energy. That's a lot of energy. It's on par with the kinetic energy of a 55mph car! Respect these tanks.

Now there are two problems to solve here. First is that air tanks are not efficient propulsive devices. They don't have a nozzle. That's why they only blasted through a concrete wall for Mythbusters, instead of doing even more damage. Your character may do well to fashion a nozzle, though I am not sure what from.

The second issue is that the designers of these tanks had a vested interest in this event never ever ever ever ever ever ever ever happening. Scuba divers are not interested in how well their life-supporting tank can turn into a rocket when they're 80ft below the surface. Great effort is taken to prevent this from happening. If you look at all of the videos I've linked showing the damage air cylinders can cause, they all involve the neck breaking. If you just opened the valve up, the amount of thrust you would get is negligible, by design. You really need the neck to break.

You might consider the story involving the main character choosing to go scubadiving with some horribly horribly ill maintained gear. No scuba diver worth their salt would ever use such tanks (and no dive master worth their salt would be willing to fill such a monstrosity with air). But maybe your diver is "special."

Have the shark take a bite at the main character while the main character is inverted, and have the main character dodge to cause the shark to strike the neck of the tank, and crack it. Don't sever it... that'd be too much force on the straps holding the tank to the main character's back. Now no scuba tank in an acceptable state of repair would break this way, but the main character's Macgyvered tank might fall prey to a good shark bite.

As an added bonus, you can now directly reference this event in the story as jumping the shark, which is really what this sort of stunt is.

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  • $\begingroup$ Will the high standard of repair be upheld in all localities and communities where diving happens, especially in developing countries and among poor professionals that need to rely on whatever gear works? $\endgroup$
    – rackandboneman
    Commented Jun 21, 2018 at 21:13
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Definitely, not possible. For a multitude of SCUBA medical factors and practical factors. Taking your tank off underwater, is not something that can be done quickly or easily. The idea of "shooting up" out of the water quickly, is asking for decompression sickness or baro trauma, meaning your character will likely not survive.

Better idea might be, to take off the tank (the whole scuba jacket) and hit the shark with it.

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    $\begingroup$ Having SCUBA dived myself, the idea of taking off your air tank fills me with terror. Rather than taking off the tank and hitting the shark I'd suggest taking off the weight belt and wacking the shark with that. $\endgroup$
    – a4android
    Commented Jun 24, 2018 at 8:29
  • $\begingroup$ It's really not that hard to take off your tank underwater, given you can't let it get too far away... but it would be much easier to use it to keep the shark away from you. The weight belt would almost immediately fall to the ocean floor when you took it off, heavy but useless as a weapon. And I think it would be nice to have a bigger spacer between you and the shark. ALSO, the diver would most likely shoot up once he lost his weight belt, bringing back the decompression sickness issues. I am a SCUBA diver and have completed all these "skills". $\endgroup$
    – diver0427
    Commented Jul 6, 2018 at 14:12
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Maybe, if you have a grip capable of denting steel, the biggest problem is going to be staying attached to a very smooth tank that is accelerating away from you quite rapidly. You also have an issue in that the valve tends to be at right angles to the long axis of the tank making it even more unwieldy as a propulsion device.

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As many have pointed out on here, tanks don't work that way without serious ruptures.

However, Divers already have two methods for a quick escape (both highly dangerous at anything below 30 feet do to risk of bends which requires a visit a specialized hospital visit... thankfully these exist in most coastal cities):

  1. Cold Water Divers use a dry suit, which can inflate their suit, this makes them buoyant and they shoot up. The more they inflate, the faster they shoot up. And due to air expansion, they accelerate as they go up, like a rocket until they break surface or quick-release their suit valves.
  2. All divers wear a weight belt or quick-release weights. Releasing these makes them more buoyant and they shoot up.
  3. Do both and the diver will shoot up blindingly fast... and likely die if much below 30 feet. Should be fine if above 30 feet though, and might make a cool visual bursting out of the water.
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TL;DR: Lots of goriness and pieces of bloated shark

The main issue would be pressure change so an alternative to that would be to do the opposite thing, your guy makes the shark go up really really fast from a depth of under 300 feet, and then watch your shark go into decompression sickness, the blood inside him will start to boil from dissolved gases like nitrogen, this thing its so freaking dangerous that lots of scuba divers have been kill just by getting up too fast just swimming and that's also why all the abyssal fish are bloated, they kinda explode on their way up.

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  • $\begingroup$ While this doesn't really answer the question, it does raise the rather important issue that if you did somehow use your airtank to rocket to the surface, you'd likely be crippled by the pressure-changes $\endgroup$
    – Ruadhan
    Commented Jun 21, 2018 at 8:12
  • $\begingroup$ Can sharks get the bends? $\endgroup$
    – Daron
    Commented Jul 1, 2018 at 15:36
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If you were to take off your mask and hold your breath deep underwater, while trying to go to the surface, the pressure inside your lungs would force you to breathe out, or cause damage to them.

Not only that, the valve is at a right angle, so you would have some trouble there and would have to take it off for easy travel. Most people can hold their breath for 90 seconds or less, so you would have to either go really fast, or be close to the surface.

Finally, it would be very hard to control the pressure and prevent it from all depressurising in a short time to let you travel at a steady rate.

At the end of all this, you would have to be close to some air or the surface because your tank is depressurised. (unless you managed a way to control the air and even then you couldn't or it would be very hard to get the air from the tank because you opened the tank and it's very hard to close it underwater)

So in conclusion, the chances of all this happening is low, so most likely the answer is: no, it will not be able to happen in real life.

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