Alleged suction created by a sinking ship

Factual Questions
1

If a huge ship goes under, might the suction created pull nearby things and people down as well? I’ve heard that tale any number of times since childhood, and wonder about it’s accuracy.

Recently I saw a TV documentary about the sinking of the USS Indianapolis after it delivered the atomic bombs to Midway(?) for use on Hiroshima and Nagasaki.

One of the survivors described how he jumped from the ship into the sea. He then put his foot against the side of the ship to push off, but the the ship suddenly gave way, rolled in the other direction and sank. Aware of the suction factor, the sailor swam frantically to avoid being pulled down himself.

Was he needlessly worried? Is this suction stuff an old wives’ tale, or is it legitimate?

2

Well, Jamie didn’t get sucked down when they tried it on Mythbusters.

3

Didn’t see it.

4

Just a thought: A sinking ship, whether or not it creates actual suction, might possibly be releasing quite a volume of air bubbles as it’s sinking. Theoretically, if that happens, might that make the bubbly water much less buoyant?

5

I believe it would make it more bouyant.

6

Maybe not.

There was a TV documentary about this: gases from the ocean bottom, bubbling up to the surface, decreasing the sea’s buoyancy and causing ships to sink.

However, if I remember correctly, the documentary dealt with ships lost in the Bermuda Triangle, so you might well prefer to reject the hypothesis.

Maybe someone knowledgeable will weigh in on this subject.

7

No, water with a lot of gasses disolved in it (like carbonated soda) are less able to support the weight of floating objects than regular old water.

I have a theory that, with ship’s suction, it depends on how the ship is sinking. If the ship still contains a lot of air filled spaces when it goes down, than the suction may be the water rushing to fill the volume just vacated by the ship. But if the ship fills slowly and sinks, than there is much less vloume that needs to be replaced when the ship goes down.

I believe that the sinking suction is very similar to the same forces observed when a ship moves across the surface of the water, and things are draw towards the ship. (Like when the RMS Titanic sailed out of Southampton harbor, the suction created by her large mass displacing the water (and that water filling back in alongside and behind her) caused a ship (S.S. New York?) to snap her lines and swing out into the channel, almost striking the Titanic.)

Unfortunately, I don’t know the technical term for this effect, so I am having a tough time googling the proof…

8

Technically, the water isn’t buoyant, but water with air in it would make the things in the water more buoyant.

Imagine your open palm on the surface of a sink full of water. Now imagine plunging your hand into the water quickly. Water that was once on either side of your hand will rush to replace the volume your hand was recently displacing. The force of this wave will be dependent on how much volume is no longer being displaced at the surface, and on how quickly it happens.

9

Not in my experience. A few years ago, I posted a story about the time I nearly drowned, and it was in aerated hydraulics at the base of a falls. I normally float quite well- in those bubbles, I sank like a stone.

Kayaks will sink in aerated water, as well.

Think about it- what makes things bouyant is that they weigh less than the water they’re displacing. If the water has bubbles in it, it’s going to weigh less, and so won’t be able to support as much weight as water without the bubbles.

10

Here is a clip. It isn’t the best, but mythbusters ‘debunked’ the whole suction myth. Thats not to say that if a large ship sinks fast enough or has large pockets of air trapped and is released quickly, that there might be some suction.

11

Aaah, yes, I see the fault in my reasoning. I was thinking of what Santo Rugger said.

Now, as for “suction” when a ship goes down, I’m thinking, it’s not so much a suction, but you have this massive object displacing a hell of a lot of water, it’s sort of “pushing” the water upward and outward to make room for the ship to go down. There’s sort of a trough created and if you are close enough to the center of this event, like right next to this ship, you will move downward as the water around you is being pused upward. For a brief moment, a few seconds I’m guessing, you are completely enveloped in water. You’re not going to sink all the way down with the ship but in the panic, the confusion, you could, obviously, still drown.

12

Imagine a wind tunnel experiment where air is flowing over a model ship on a gimbal. I can imagine that at certain speeds and certain geometries, the air flowing over the model will form eddies on the ‘back’ side of the model. Change the fluid from air to water and I can picture localized eddies forming behind a large sinking mass - but not in every position behind the ship, and also not permanent. Rather, they would tend to come and go as the ship tumbled or changed its orientation. A large enough eddy could easily ‘trap’ a swimmer, just like eddies behind boulders in a stream or behind a dam can trap swimmers and boaters. However, you’d probably have to already be underwater and pretty close to the sinking vessel for it to work - and you would have to be pretty small compared to the scale of the ship. An eddy might ‘capture’ one person for a few seconds while leaving a nearby person unharmed, but even a few seconds of being tumbled around underwater can end up being fatal.

In addition to the bubbles, the sinking vessel may be vomiting out a lot of fuel oil which would also decrease the local density. An oil slick only a couple inches thick may make it impossible for you to keep your face above the surface.

My objection to the Mythbusters episode was that the boat that they sank was not all that large. Even if they had sunk 100 battleships, though, they might not have been able to recreate just the right eddy conditions to trap a swimmer.

13

small nitpick
Barn Owl

If I remember Robert Shaw’s speech from “Jaws”, he said they had just gone to Tinian (Island) to deliver the bomb … the Hiroshima bomb.
It definitely was Tinian Island and it seems they only delivered the one bomb.

14

Agreed.

I don’t think so. Water with bubbles in it is essentially a less dense fluid than water without - your floating object needs to displace a greater volume of it to equal its own weight, so it floats lower in the bubbly water, or if it was only just floating in plain water, it sinks.

Unless you’re talking about an object that can trap the rising air bubbles and use it for supplemental buoyancy, but I don’t think you are.

15

Bubbles would make it sink. There are theories that the Bermuda Triangle has intermittent releases of bubbles due to underwater activity, causing ships and boats to vanish without a trace over and over again. The bubbles create air in the water, where there is air, there is no water and where there is no water there is no buoyancy.

16

That’s a good nitpick, wolf_meister and I thank you.

I couldn’t remember where and how many bombs were delivered.

But damn! Everyone’s focussed on bubbles right now, and no one’s on the OP. :smiley: :smiley: :smiley:

17

Adam was the one getting sucked under – Jaime was just there to give him some air if he did get sucked down.

Despite the fact that there was no suction from the ship, they did show earlier in the episode that there was suction when Adam sat on top of a big weight that they sunk in a swimming pool. And they didn’t really go into much about why it happened with the weight in the swimming pool and not with the actual ship. So maybe under some circumstances, if the ship sinks like the pool-weight did …

18

Er, Jamie. My wife’s cousin’s name is Jaime, but pronounced like Jamie, and I always end up spelling it that way these days.

Anyway, back to your regularly scheduled thread.

19

OK. I don’t know about suction, but I could easily imagine that the turbulence caused by a sinking ship could make it impossible for a swimmer to remain safe - you don’t necessarily have to get sucked a long distance (even assuming that’s possible) under the water to drown - a few inches will do it. People drown in turbulent water just because it’s hard to keep their heads above the surface.

20

There seems to be a lot of odd notions and misconceptions about physics and science in this thread.

Buoyancy forces probably wouldn’t come into play here. But if there were a bunch of bubbles you’d have the effect of bubbles moving up and water buoyancy forcing you up, and the bubbles might impede that a little. But it’s nothing compared to being “in” a moving volume of water, which is what is really happening.

Due to boundary layer effects of an object in a fluid, the fluid will be affected. You see this in the reduction of pressure and turbulence behind a racecar, allowing for drafting behind another driver. One can see this in a wind tunnel experiment. It is real and observable. So there is a water the ship is displacing, and other water comes into that volume near the surface to fill it in, and yes, it will move down too. It’s a fluid.

So while there is not a suction, there is fluid movement (there is no suction force by the way…there are differences in pressure, and higher pressure regions like to reach equilibrium with lower pressure regions).

It would seem obvious that a small boat sinking would create this turbulence and ‘drag’ some fluid with it, around it. But a small boat/object would create small amounts of turbulence. A large object (cruise ship) would create large turbulence. So the experiment with a guy on a small boat would not feel much. Jacque Couseau’s son strapped himself to the bridge of a 100 foot vessel which was being purposefully sunk for artificial reaf, and maybe he felt something. But of course he had scuba gear on, so he went all the way down with it.

But there are many other factors, including weight of the vessel, shape, cavities, the way it sinks, speed etc. All these would contribute to the way the water flows around it as it sinks.

So it seems reasonable that some people observe being “sucked” down in some cases, and others don’t.

The Myth busters experiment was only valid for their case.