Measurements of the internal wave wake of a ship in a highly stratified sea loch

An internal wave experiment was conducted in Loch Linnhe, Scotland, in which three ships were used to generate internal wave wakes. Synthetic aperture radar images of the wakes confirmed the applicability of the kinematic theory of Keller and Munk for the geometry of the wave pattern. The waves were measured in situ using three-axis current meters and other instrumentation. The waves were quite well represented by linear propagation theory, although the dependence of amplitude on depth did not agree precisely. The time-varying frequency of the waves was determined from the data in two different ways, including a Hubert transform method, and agreed well with that predicted by the theory. The subsurface data were used to estimate the surface current and its gradient (the strain rate). Any dependence of wave amplitude on ship speed was weak enough to be obscured by variations due to changes in the stratification. Amplitude estimates of waves from the 1400- and 5300-tonne ships were similar. Those from the 29,000-tonne ship were 2–3 times greater.