Glitter Ship Waves
April 10, 2006
This image was taken from the 15th floor of an office building in Norfolk, Virginia, on a crisp morning in early January 2006. A barge had just passed through a glittering patch of water below me, and the characteristic wave pattern produced this somewhat unusual picture. In deep water (i.e. when the depth of the water is large compared with the wavelength generated) the speed of a surface gravity wave is proportional to the square root of the wavelength, so longer waves move faster. A ship (or duck!) may be expected to generate a wide range of wavelengths (and hence speeds) as it moves through the water. However, the wake of either is formed by those waves which are able to keep up with it, so it is the waves that are of exactly the right speed and in the right position to reinforce each other that will contribute to the visible wake.
The basic theory of these waves indicates that the apex angle of the "wedge" trailing the ship (or, again, duck) is, in deep water, about 39 degrees of arc. Frequently, other waves are visible that cross the line of the wake at an angle of just over 35 degrees. The remainder of the ship pattern is composed of waves that move behind and in the same direction as the ship; their wavelengths are such that they travel at the speed of the ship, though in this picture they're obscured by the turbulent wake of the barge.
In shallow water (for which the wavelengths are comparable with or less than the depth of the water), all the waves travel with about the same speed, proportional to the square root of the water depth. The wedge angle is in this case is narrower the faster the ship moves.Related Links: