Ok, this will seem a little off the wall. Many of you who know me know that I used to be a professional sailor years ago. I worked on everything from small oyster boats to tug-n-barge combos running 600 feet in length. Sailors are great exchangers of tales, and no tale is more horrible and morbidly fascinating than that of a ship sinking. Such tales often feature, in one way or another, the idea that people who aren’t able to swim far enough away from the vessel risk being “sucked under” as it goes down.
That idea never struck me as very plausible. A ship going down creates a void in the water where its mass used to be, and water will rush in to fill that void, but the idea that some sort of suction could be created that would literally pull you down with the ship never made sense to me. I don’t have the technical chops to say exactly why, but it just struck me as wrong. While watching video of a sinking fishing vessel yesterday I thought of an alternative explanation that seems much more reasonable.
As most people who mess around with boats know, a prop that breaks the surface can no longer effectively propel the vessel. The reason for this is a phenomenon known as cavitation. When the prop breaks the surface it pulls air down and aerates the water around it. Aerated water does not have the mass of non-aerated water, and the prop can’t push against it effectively. For the same reason you cannot swim in aerated water. If I put you into a tank of water and bubble air up from the bottom you will sink, however mightily you flail.
Which brings me to sinking ships. They have a lot of air inside them, and when they go down that air comes bubbling up from all the various openings through which it can escape. You can see that effect pretty clearly in this two-minute video of a small fishing vessel sinking. A much larger ship means a lot more air, which in the process of escaping turns the water above into a aerated froth. And as I said above, you can’t swim in froth. So, I think the reality is that when a ship sinks and you are in the unfortunate position of treading water right above it, you don’t get sucked down. You fall.