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.

24 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.

  13. Just want to add, that Mythbusters actual tested this “myth” about a sinking ship will drag you down.
    ( they came to the conclution you will not get sucked down )
    You can view the video here..:


    Ps. This is a short clip, there is a longer episode where they test if the air escaping as bubbles would effect stuff in the surface and other stuff related to the above debate.

  14. Regardless of what it’s called, or if you sink or fall.. Point I got is get as far away from the ship as possible because you will go towards the bottom and Jason Voorhees kills you..

  15. Ok! So I have read all comments above and I do have questions: If air bubbles cause boyancy problems then why is it that fish do just fine in an aquarium?
    In the case if say the Titanic, I think the sheer size would suck anything down that stayed near it. Life vest or not the rapid descent/time/pressure/diss orientation /cold ratio would most likely not be survivable. Aeration could possibly have a low percentage effect also.
    Just saying!

  16. Hi, Patrick. Well this post was a long time ago and I haven’t thought much about it in awhile. Bottom line I think there is a really big quantitative difference in the amount of air being injected into an aquarium vs. air escaping from the submerged hull of a ship. Our ability to swim/float depends on the density of the medium, and water that is essentially foam due to aeration is impossible to swim or float in. Thanks for stopping by.

  17. You would likely get both effects. Suction and aeration. If a previously sealed window were to break at water level, water would rush in creating suction. Anyone close enough would be in a dire situation. Same goes for the remaining escaping air as the ship finally goes under. I think a lot is due to circumstance location how the ship sinks etc.

    But you would have a greater chance of being caught in a vortex from empty space filling with water than from the air escaping.

  18. “A ship going down creates a void in the water where its mass used to be” – Um, no it doesn’t, because its mass is STILL THERE. The ship’s mass doesn’t suddenly disappear and leave a “hole” in the water just because it’s sinking. Water rushes in to fill the parts of the ship that were previously filled with air, and this is what causes it to sink (because it is no longer displacing a mass of water greater than its own mass, and therefore stops being supported by the buoyant force), but it does not “create a void in the water”.

    Also, with respect to large volumes of air escaping from sinking ships: most this air would have to escape from the ship at or near the surface because otherwise the ship would NOT sink. It’s the replacement of air with water that causes the ship to sink, so a ship with large volumes of air still inside it is not likely to sink below the surface before releasing the vast majority of this trapped air.

  19. Water pouring into into the ship is an important aspect that I do not see mentioned here or on MythBusters. Which surprises me because it is undoubtedly a reason, if not the primary reason, people cannot escape and why some do escape are pulled back. Obviously the most dangerous area is inside the ship and below waterline but innumerable factors determine the risk to any particular person.

    Indulge me if you will…

    Imagine you are in an interior state room on the 8th deck of a modern cruise-ship which has been slowly sinking for the last hour. Flooding from 7 finally reaches your deck so you inflate your floaties before slipping one on each arm and
    grab your RayBan Wayfarers before stepping into the passageway and sitting down in the warm ankle deep water. If you do not let yourself get trapped then you might have a pleasant time and swim to the lido for a pina colada before floating out through the open roof.

    In this case, there was plenty of time for water seep under doors, up drainage pipes, and leak through vents. Each deck was fully flooded before water reached the next deck up and any remaining air pockets were slowly compressed and equalized. You were never more than a few inches below the water line; basically you were floating in the ocean the ship sank around you. Replay that scenario again, except this time, the ship is sinking with you and going down in a matter of minutes…

    Water surges into the lower decks and quickly fills every open area. Water sprays into cabins from gaps around doors as passageways channel the torrent through the ship. An eruption of compressed air precedes the water jet expelled from every pipe and duct while the unstoppable flood reaches another deck.

    Water starts leaking under your door and your floaties are already on. The water is almost knee deep by time you grab your glasses and reach the door. You turn the nob and the door violently swings inward and pushes you back. Your initial struggle to leave the room is futile against the surge rushing in but the force lessons as your room fills. You remove your floaties to avoid being pinned to the ceiling and make another attempt. Diving down you reach for the handle and pull yourself forward the flooded passageway. You are not even halfway through the when the current slams you against the doorway, drags you along the wall, bangs you into mezzanine rail where you are briefly pinned before it carries your painfully mangled body over the rail and drops you into a rising pool. You might have had a chance to make it out from there but the waterline crested the upper deck and water is rushing in from above hastening the inevitable.

    Ok, so neither of are on topic but this one is; same deal, fast sinker but you have an outside balcony…

    Your floaties are on, screw the glasses, you are outside but scared to jump from the 8th deck so you are just chillaxing with a blended margarita in a salted glass. The ship is going down fast and your moment of faith is seconds away. You know tossing a perfectly good drink is a sin but you are only 5 feet up so you do the rational thing and chug it. Moments later you are stricken with a brain freeze as a surge of water carries you along as it fills your room. The pain in your head dissipates giving you time to observe and reflect on the situation…for a few moments as you descend into darkness… “Where did I make my mistake, was it the floaties?”

    To which we all know the answer…

    “No stupid, you forgot to close the door!”

    But any how, yeah, currents of varying sizes, strength, and duration inevitable whenever water moves from one place to another.

    You may find Dave Nitzer’s answer informative:

  20. Hi, Interesting discussion. Could someone tell me why dolphins and cachalots have been found dead after sinking of a 150m long container ship?I understand the suction created when water enters the ship while it sinks and the aeration above. Would these disturb the flocks of mammals swimming within 100m radius as they lose notion of direction and suffocate?

  21. Great post (yes, I know it was years and years ago), and moderately fascinating discussion of the physics of the situation, but to me they all missed the most delightful aspect of your post: It’s marvelous construction. Those final words, “You fall,” are a sudden, unexpected, logical, and terrifying turn in an article of this kind. It’s no easy thing to get to such a sharp fright in as few words as you managed it. For what it’s worth: Well done!

  22. Actually, a propeller does not need to break the surface to cavitate. As most submarines before the 60’s. Maybe sooner.

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