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I read reports of people drowned swimming in rivers near medium-sized boats or flat-bottom barges. The idea is that an anchored ship stands still, and the surface current (which goes exactly along it side) creates lower pressure under the ship (or call it draft), which pulls the swimmer underneath the ship.

Is this true or fake, and how does this effect work?

A recent news report says:

A student has sunk last weekend at diesel ship "Moscow".

...

The man dived from the board of a river cruiser into the water to swim. For a while he swam, but then he was pulled by the current under the ship, and sunk.

The ship is quite small and has kiel 1.14 meters deep.

image from wikimedia commons by user Andshel

(Not clear if the ship was standing still, or moving. River current here is 1 m/s in the middle, while ships go at up to 6.5 m/s.)

The question is not if one can resist the river current (it's also much slower next to the shores). I know it's almost impossible. But the current goes along the ship or barge side. So my question is if there is such an effect that creates lateral draft, that pulls swimmers underneath.

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  • $\begingroup$ It is instructive to compute the swimming speed of medal winning athletes so that you have something to compare against when trying to figure out what "fast" is. $\endgroup$
    – dmckee --- ex-moderator kitten
    Commented Jul 20, 2015 at 23:45
  • $\begingroup$ @dmckee I've seen those figures. I know one can't swim against the current. But the current goes along the ship board, not underneath, and the question is if there's an effect that creates lateral draft. $\endgroup$
    – culebrón
    Commented Jul 21, 2015 at 2:00
  • $\begingroup$ Good question. I assume the ship is anchored and aligned with the current. People often swim near large boats that are anchored, but maybe not in a river with current. $\endgroup$
    – Mike Dunlavey
    Commented Jul 21, 2015 at 12:43

1 Answer 1

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Basically the 1 m/s makes a drop in pressure with v^2/2g, so it's not much; 5 cm pressure difference when described in water depth, or 50 kg/m2 But this means that the river depth is infinite, Or at least 2 times the Width of the boat. If the river depth is less than the width of the boat, the thing goes worse.

You say the boat's keel is 1.14 m deep, and its flat bottom. If we assume that the boat is 2.3 m wide, and the river is also 2.3 m deep, then under the boat, in the middle the flow velocity must be doubled, which means that the velocity is 2 m/s and the pressure difference in water depth is 20 cm; 200 kg/m2.

Because of incompressibility there isn't any flow though, which would "suck" you under. But as you go near the boat you are also sucked against the boat as the water flows with even higher velocity between your body and the boat; as they both causes this acceleration, if you are hanging on the boat. So once you are in there, it's difficult to get out. The only way out is to go with flow. And the only safe place to climb in, is the back of the boat. The same thing which slams the door close when you have windows open through your house. The narrower the gap, the greater the suck.

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