Ship waves - a type of wave on the surface of a liquid created by a body moving quite fast on the surface. A distinctive feature of ship waves is the characteristic arrangement of the crests of the waves when viewed from above (see illustration).
Three features of ship waves should be noted:
- the crests of the waves do not go at all parallel to the train of diverging waves, but seem to go out
- the ridges themselves are not in the form of straight segments, but in arcs curving outward.
- the direction of the wave train is approximately a constant angle (19.5 °) with the direction of movement of the vessel
This behavior of ship waves is explained by the fact that gravitational waves on water (which are sufficiently large waves) have the dispersion law , from which it follows that the phase velocity of such waves is two times greater than their group velocity . A body moving in a medium faster than the wave propagation velocity in this medium concentratedly emits waves in the Mach cone , and the angle of the cone's solution is the smaller, the larger the Mach number (the ratio of the body velocity to the wave propagation velocity). As a result, it turns out that the angle between the crests of the waves and the direction of motion of the ship (which is determined by the phase velocity of the waves) will be larger than the angle of the solution of the train of waves diverging from the ship.
To explain the second property of ship waves, we recall that the law of dispersion for gravitational waves on water also implies that long waves propagate faster than short ones. Therefore, the larger the surface is already occupied by the waves, the faster the waves run further. When the speed of wave propagation reaches the speed of the ship, the waves will leave the Mach cone.