Is this possible?

Question

In the first part of the movie "Pirates of the Caribbean: The Curse of the Black Pearl", Captain Jack Sparrow and Will Turner are seen walking along the bottom of a bay underneath an overturned boat in an attempt to board and commandeer a nearby ship. In a couple shots, they are shown with their heads on the inside of the boat, and they are able to breathe and even talk. The problem, though, is that the water is all around them, and even above them for quite some way.

I know that some may explain this similar to an old childhood game, where one would take a cup and submerge it in a large basin of water, and then remove it from the water upside-down, causing the water to exit the cup, but not allowing air to get in the empty space, forming a vacuum. What I am wondering is if this would be possible with a boat as it is shown in the movie, and, if so, how one would be able to breathe in a vacuum. I do realize that the movie is a work of fiction.

Thanks for the help!

-Mr. X

Answer 1

This can also be explained quite easily in your child-hood games. The opposite of the one you tried. (For this, let's assume the bottom of the boat is air-tight).

Take a cup, bowl down, the push it down into the water, trying to keep the open section of the bowl facing downwards and level. After a few moments (allowing some bubbles to leave the area), turn over the cup (still underwater) and watch the captured bubbles float to the surface.

There are 2 factors at work. First, bubbles "float", therefore they can only move upwards in water. If you use a bowl or cup to contain air (the makings of a bubble), then push that underwater, you capture a cup-ful of air inside the cup. It can't move down to get around the lip of the cup to escape.

Finally, air compresses under pressure. Since the pressure of the water is more the lower you go, there will be less and less "volume" of air in the cup, meaning the air bubble would have to travel even farther "down" to get around the lip of the cup. From under the cup, it would look like the cup was getting less full as you went deeper and deeper, though it's really just the air compressing.

Note, there's very little exchange of gases between the air and the water in your example, so once most of the oxygen in the air bubble captured by the boat is used up, the actors would go unconscious and die :).

Answer 2

The specifics of what you are asking is possible. It is not a vaccuum - just air trapped under the boat. Because air is lighter than water, it rises and is unable to escape to the atmosphere. Eventually the oxygen would be conervted to carbon dioxide and they would asphixiate, but it would take longer than is shown in the movie.

The PROBLEM with this scene is that the force the air creates beneath the boat would be too strong for them to hold it down themselves without some leverage. Think of how difficult it is to walk on the bottom of a pool when you are completely submerged - your body is naturally bouyant and wants to float. Now the air in the boat is pushing upward and wants to pull the boat up to the top, making it even harder for you to stay down. This would only be possible if their feet were weighted, or maybe if the hull of the boat were partly above water and the pressure were not as great.

BUT if you still want to really prove it - check out Discovery Channel's website. The show Mythbusters is looking for new myths to put on the show right now and this would be great! :-)

Answer 3

There would not be a vacuum under the boat but an air bubble. Of course if the air bubble was too big the boat would float. In addition the air pressure would be very high. It would be equal to the water pressure at this depth. If you breathed this air and then surfaced, you would be looking at the bends for sure.

Answer 4

First of all, don't forget air is lighter than water and will try to reach the surface.

so more than likely, if you had a overturned boat large enough to hold enough air for your journey with conversation (talking uses up more air), then you wouldn't walk very far beacuse the boat would be trying to float back to the surface. Thus lifting your legs off the bottom of the ocean.

It's just a fantasy scene for movie fun :) Don't try it at home.

Answer 5

I haven't seen the film, but what you've described seems to be possible. It isn't a vacuum. If it were, the water would come in and fill the interior. The reason the water doesn't do that is because it is full of oxygen.