Going Down with the Ship: It’s Aeration, Not Suction

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.

14 thoughts on “Going Down with the Ship: It’s Aeration, Not Suction

  1. The scenario you describe with the propeller is not cavitation. It’s aeration.

    Cavitation is a completely different phenomenon and occurs in liquids absent of any air ingress, it is the vapourisation of the liquid due to very low pressure, followed by a sudden recollapse, which causes a tiny implosion capable of causing serious damage over time.

    Photographs of aeration and cavitation can have a similar appearance but they’re very different states.

    Your theory on the sinking sailors makes a lot of sense though.

  2. It’s a good distinction. We called it cavitation when I was sailing, but it was not really an accurate description. Thanks for stopping by.

  3. “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…” – It’s OK, I’m going to give you a minute to think that statement over – go ahead, I’ll wait – and let you realize it’s the complete opposite of that. You’re welcome.

  4. Well yes, both are happening. The ship’s hull is displacing water as it sinks, and water is filling in above it as it descends. Either way, it still makes more sense to me that the thing that causes people problems swimming in the vicinity of a sinking vessel is aeration, not suction. Thanks for stopping by.

  5. the bubbles would make indeed make it harder to swim, but the current created by those bubbles rushing upwards would push you outwards/away from the sinking ship i would think.

  6. “the bubbles would make indeed make it harder to swim, but the current created by those bubbles rushing upwards would push you outwards/away from the sinking ship i would think.”

    This doesn’t make any sense to me. Why would air push you away while you are in the water? I could see that if it was water shooting up, but the less dense water is more likely to make you “fall” than to push you up. Just think of how much air pressure it takes on those carnival rides that keep you afloat in the air. It is massive compared to aeration of a ship.

  7. Interesting topic; fluid, water in this case, is incompressible, therefore the idea of a vacuum being created above the sinking vessel is not possible IMO. I prefer the less dense two phase fluid theory. It’s been used to describe ships sinking in the Bermuda Triangle from the release of large stores of frozen hydrates on the sea floor. They rise to the surface, and sink ships quickly in the reduced buoyancy of said 2 phase fluid.

  8. What I find interesting isn’t so much the scientific aspects of a sinking ship but that of the analogy of the sinking ship and the result of sucking everything surrounding it down with it. I am currently working some place which has been “struggling to stay afloat” for a long time, in truth, it’s been sinking a long time and we’ve been swirling in this vortex and getting pulled under. Instead of us foolishly bailing water we should have found life boats and distanced ourselves from the sinking ship long ago!

  9. Thanks for the Clarification on the term. All I remember was that I was drawn towards the sinking ship I had been on. And was able to swim away from her to a safe distance. The bottom line if you are ever in that situation ,God for bid get away from the ship or boat. Even if you don’t have life jacket there will be debris that floats. Above all stay calm easier, said than done.

  10. Compare sinking ship to riding bike after big truct. Air stays behind this truck, so I guess sinking ship get stuck with water behind it – so it would pull you down. But that would be on very limited range. I also thing that when you get little away then you would still go down thanks to water aerotion – water is not enough dense, so you could push it away to swim up. Life jacket would help here much either – you would drawn before it would eventualy pull you up. Anyway it is all mater of luck.

  11. This type of question comes up in wastewater treatment plants where aeration basins are used. The air is bubbled through diffusers on the bottom of a tank. Some thought you would sink in the less dense water. In the 70’s test were made and it was found that you only lose 1 – 2% buoyancy and could swim just fine the problem was the current near the sides of the tank circulate down to replace the water flowing up with the air bubbles. It was almost impossible to stay on the surface near the walls.
    I believe that the air has little effect but the currents generated by the sinking ship will cause the downward pull like an eddy behind a rock in a river.

  12. Yes, but the volume of air being bubbled through the water at a waste treatment facility is comparatively low. There are also many studies of the effects of aeration and cyclical current at hydraulic drop structures such as coffer dams. In any case it is probably a combination of all these factors. Thanks for stopping by.

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