How does it work?! (Pirates of the Carribbean fans)?

Question

In the first POTC movie, when Jack Sparrow and Will Turner are running after Jack is helped out of prison: the scene where the two are walking underwater with what seems like an air bubble under the boat... How does that work?!?!

Answer 1

The air is simply trapped under the boat with no where to go. It is lighter than water so it does not sink down under the lip of the boat.

Answer 2

The previous answer is absolutely correct. Before the invention of scuba equipment and submarines, there was an invention called the diving bell that worked on the same principle. Air would be trapped inside the bell while underwater. A person could stand inside the bell and breathe the trapped air. Of course, the bell would need to be brought to the surface periodically to replenish the air.

Answer 3

The answer: it doesn't work!

Well, to clarify, part of it is scientifically plausible and part isn't. First, a gas is far less dense than water. Put them both together in a container and the gas will rise to the top. However, if the gas is trapped under something solid like an overturned boat, and that boat is submerged, the air bubble will try to rise but will be held under water by the boat. If you can get your head in there, you can breathe inside the bubble (at least until you use up all the oxygen).

However, the part that isn't plausible is that they were able to hold the boat submerged. The boat with the trapped air bubble will displace a volume of water that weighs much more than the boat and two men combined. This would mean that a buoyancy force would push it back up to the surface and they wouldn't be able to hold it back. So that scene in the movie is not possible. In fact, this principle is why boats can float on the surface. If you want more information in this, look up Archimedes.