Seeing underwater

I read a very short news snippet in Tuesday’s NZ Herald that said that a British nuclear submarine had collided with a French counterpart. Admittedly, the source quoted is ‘The Sun’ newspaper in the UK – which is not best known for its accuracy in reporting – but leaving that issue aside, you have got to ask the question as to how two submarines, with well-trained crew and possibly carrying rather precious loads (nuclear weapons) are able to come into contact.

 

It is probably easier than you might think. ‘Seeing’ underwater is a pretty difficult undertaking. Vision is no good – not much light gets down well below the surface, and water can be pretty murky stuff. It’s no surprise that dolphins and the like make use of sonar – they send out pulses of sound that reflect off underwater objects such as the sea floor and their lunch, and the dolphin listens for the echos.

Sound travels very, very well underwater, which makes sonar an excellent tool for fishing boats and also submarines.  Leonardo da Vinci, in 1490, said: "If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you."

However, this propery of sound, as well as allowing sonar to work, is also its failing. If you emit a pulse of sound sufficiently loud in order for you to hear the echo, it is pretty certain that someone close by will be able to hear you too. And that’s a problem if you don’t want anyone else knowing that you are there. My guess is that submarine captains are not exactly fond of sending out sonar pulses.

So a collision between two submarines is not, perhaps, as stupid as it sounds. They may have been so quiet that neither could hear each other coming.  (I am of course utterly speculating as to the circumstances of this crash – assuming that there was one. They could be very different, but the Herald failed to elaborate)

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