Do two ships close alongside each other in a choppy sea tend to be attracted together?

I have read conflicting reports of this phenomenon. One of them, saying that they do, gave a thorough but highly ... synoptic I think might be the right word ... or top down maybe ... explanation in terms of the re-radiation of the wave energy by the pair, & the state of the waves between them, & how a sort of pressure is a property of a plenum of waves that would be expected to be greater beyond the ships than between them, rather than "volume δV of water acts with force δF on area δA of the hull", or suchkind of more bottom up argument. I can't reproduce the argument in detail I'm afraid, but it was thorough, with each ship bobbing up & down represented as a monopole and the two ships creating an energy-field mediated by the waves, and arguing that the energy-field would have such-&-such a shape & that kind of thing. But I would have thought that a definitive answer to this question would be easier to come by than it seems to be, what with the maritime art being such an ancient & noble & widely practised one. If anyone's curious about this I'll see what I can find about it, as I'm recalling from memory something I read in a fluid mechanics textbook years ago. But I have seen mentions of the existence of the phenomenon in a fair few places.

Got a bit more detail already: some say it's an analogue of the Casimir effect.

  • $\begingroup$ Think of a surfer, the wave pushes her forward due to gravity due to the slope of the wave, like a skier on a hill. So if one ship is lower than the other they will collide. If both ships are facing into the wave there is no danger but in a rough sea waves come from all directions. $\endgroup$ Nov 2, 2018 at 19:03
  • $\begingroup$ Google "squat maritime" to see an analogous effect between a ship and a shallow bottom... $\endgroup$
    – DJohnM
    Nov 2, 2018 at 19:56
  • $\begingroup$ @PhysicsDave - - I accidentally wrote my answer to you as an answsr to this post! $\endgroup$ Nov 3, 2018 at 7:47
  • $\begingroup$ @DJohnM -- I certainly will have a look at that. Thankyou. $\endgroup$ Nov 3, 2018 at 7:48
  • $\begingroup$ @DJohnM -- Ah yes! The hydrodynamic (& ærodynamic) effect of reduction in pressure Bernoulli-theorem -wise, due to the increase in speed of fluid through a constriction. It also applies to vessels moving in parallel. Both these are thoroughly undisputed effects. But that's not really the sort of phenomenon that is being queried in this post - rather it's the tending to 'conspire' of the forces on ships being buffeted about in such a way as that the forces on the pair (of ships) inward tend to preponderate. But that account of a ship being gotten under a bridge ... $\endgroup$ Nov 3, 2018 at 7:59

1 Answer 1


@PhysicsDave - - The scenario is that of the ships being generally buffeted about by the waves, rather than being systematically tipped in some particular direction. I found some stuff about this. There is some grounds for considering it plausible that in a choppy sea the random buffeting might end up conspiring in such a way as to cause the ships to tend to drift together. In the wikepdedia article on the Casimir effect, there is a short clip of two parallel plates in a small tank of water drifting together when a transducer excites the water with sonic vibrations ... so analogues of the Casimir effect are plausible.

I know! I ought to have put it under comments. I thought I was doing!

  • $\begingroup$ zenodo.org/records/883891 This is probably the Casimir effect analogue you are referring to. Could you please add a bit more details on the paper in your answer? $\endgroup$
    – Jono94
    Nov 11, 2023 at 14:38

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