The wakes produced at the sea surface by various types of oceangoing vessels have
been of interest to scientists and sailors for more than a century. They continue to be,
especially now that they can be observed, photographed, and imaged by remote
sensors from space platforms.
A moving vessel expends energy to overcome water resistance, creating eddies
in the turbulent wake and generating a wave wake. Most of the power of a ship's engines is consumed in
wave generation (Peregrine, 1971).
The wake formed from the bow of a moving ship is a superposition of two
families of waves. One family of waves, the longitudinal waves, propagates along parallel lateral lines. These
lateral lines form an angle at the ship's bow of approximately 39 degrees. This angle (the Kelvin-wake
angle) remains constant, depending neither on the speed nor on the shape of the vessel in deep water. It is
determined by the principle that the group velocity of waves in deep water is equal to one-half the phase velocity.
The second family of waves, the transverse waves, forms immediately behind
the ship and at a right angle to the direction the ship is traveling. Both families of waves are stable relative to
the ship, but the transverse waves are modified by the turbulent wake behind the ship. There, the stern wake
is modified by a large amount of foam and bubbles created when waves break at the ship's bow and by
cavitation from the propellers.
Another interesting phenomenon is the narrow V-shaped bow waves imaged
by the SEASAT synthetic aperture radar (SAR). Various analyses have defined the backscattering conditions
that produced the appearance of a less-than-39-degree bow wake when a moving ship is remotely sensed in
microwave frequencies (Vesecky and Stewart, 1982).
From visual observations and photography, astronauts aboard the space shuttle
provided information on ship wakes that differed from that obtained from a spaceborne SAR. Stern wakes
extending behind a ship for more than 200 kilometers were observed routinely. Perhaps the most surprising,
and intriguing, however, were the "nested-V" bow waves photographed during shuttle missions in 1984 and
1985 (Munk et al., 1987). Their geometry differs completely from those imaged by a SAR, requiring a new
theoretical approach to the response of ship wakes to the local sea.