Most of us have seen the moment when Jaws is finally vanquished by the no nonsense Chief Brody (if you haven't - the clip is below). As the boat is sinking and all hope seems lost. One lucky bullet strikes the SCUBA cylinder lodged in the sharks mouth and KABOOM! Blood and gore fly everywhere, and Amity Island is safe again (well... until Jaws 2). But could this actually happen? What actually happens when a bullet hits a diving cylinder?
Earlier on on the movie Matt Hooper, a consulting oceanographer, chastises Brody after he knocks over his aluminium cylinders claiming:
"...This is compressed air...you screw around with these tanks and they are going to blow up..."
Hmmm...
In this post we will try to figure it out, and who knows? If you are ever up against a giant man-eating shark, maybe this will save your life!
First things first, would the cylinder explode?
Short answer: no. Fortunately air doesn't combust, even at high pressures or temperatures. Cylinders will only "explode" when there is a flaw in the material leading to an failure. Typical example of this is internal rust thinning the cylinder wall to dangerous levels. A better way of describing this is a cylinder rupturing rather than exploding. Explosions bring to mind huge fireballs and Michael Bay movies, and can only really happen when the gas ignites. In contrast to air, oxygen can explode and so it is important to store it away from sources of heat and avoid over-pressurising (also ensuring your cylinders are O2 cleaned if diving on mixed gas).
Rupturing on the other hand is when there is a failure of the cylinder material. If the cylinder is degraded to a dangerous level you can end up with the material not being able to contain the pressure. As the bullet strikes the cylinder this is effectively what is happening, creating a localised failure point.
Ballooning
The cylinder would depressurise very fast through the hole created by the bullet. Imagine a balloon being blown up and then let go. It whizzes around the room releasing all that potential energy. Now imagine that balloon is made of 12 kg of metal. That would certainly cause some damage to a 25-foot shark! If you are in any doubt check out this video which shows what happens if a valve fails on a cylinder.
So the answer is yes - the shark could have been killed by a SCUBA cylinder, but just not in the dramatic way that Spielberg did it. Can't fault him though, it certainly made for a memorable scene!