Caught speeding on camera?
By Hamish Johnston
The renowned Victorian physicist Lord Kelvin spent a good deal of his life at sea and had a great interest in all things maritime. So it’s not surprising that the wedge-like pattern that follows in the wake of a slow moving ship is named in his honour.
Kelvin worked out that the pattern arises because of the interference of two distinct types of waves that are created when an object moves through the water. One type of wave diverges away from the ship, while the other follows the ship.
These two waves sum to create a distinctive wedge-shape wake, outside of which there are no significant waves created by the ship.
Apparently, these “Kelvin wedges” can be easily seen in Google Earth images and two physicists in Brazil claim that the images can be analysed to give velocity of the ship.
The paper is very brief and I’m no wave expert — but it seems to me that the trick is to spot evanescent waves, which do manage to propagate a little way beyond the Kelvin wedge before petering out.
The wavelength and direction of these waves can be extracted from the satellite image and a simple equation can be used to give the speed of the ship.
The team tested their theory on an image of a ferry boat that is known to cruise at 33 km/h — and clocked it at 31 km/h.
You can read a preprint of their paper here.
The paper is less than three pages long so I’m guessing that their analysis is highly-simplified (the aim of the paper is to encourage students to analyse boats operating near to their school). I would have guessed, for example that the shape of the boat would have some effect on the wake?