High Speed Craft Wash

Due to the ability of fast ferries to operate in shallow water with high speeds the influence of the ratio of water depth to ship velocity becomes the major parameter in analyzing the wash. This parameter is described in common as the Depth delimited Froude number, defined as:

Fr = velocity / (g * water depth) ^0.5

Depending on the Froude number resp. the ratio fo ship speed to water depth the different speeds are called

• sub critical for a Froude number below 0.8 (deep water, conventional ship speed)
• near critical for a Froude number between 0.8 and 1.1
• super critical for a Froude number above 1.1 (shallow to very shallow)

Sub critical speeds

The following picture shows a wave pattern of a ship in deep water, in general called the Kelvin wedge after Lord Kelvin or sub critical wave pattern.

Super critical speeds

For higher ship speeds the ship looses the long period waves, as they are no longer able to travel with the ship. The wave pattern is called super critical as it occurs for all ship velocities with a Froude number >1. The crests and troughs are long continuously curved lines in contrast to the short crests and troughs in the sub critical wave pattern.

Aerial photograph taken of Seacat operated by Seacontainers leaving Belfast Harbour navigation channel at super critical speed.