Shallow Water

Shallow water has a significant influence on the handling of ships. The most obvious effect is the changing shape of waves in shallow water. Due to the different wave propagation and wave group velocities of deep and shallow water at the same wave length, the interaction between the different wave systems of a ship changes. This is manifested, among other things, through strong changes in KELVIN angle.

To illustrate the shallow water effects the Froude depth number is generally used; wherein the driving regimes are subvided in a subcritical (Fr_h < 0.9), critical (0.9 < Fr_h < 1.1) and a supercritical (Fr_h > 1.1) ranges. Normally, ships operate in the subcritical range. For critical Froude depth numbers, a strong increase in resistance and a large change in the dynamic flotation position can be expected depending on the ship type since, in this region, the transverse waves move at the speed of the vessel. As a special case, soliton waves can occur in this area. In the case of supercritical Froude depth number, the ship is faster than the maximum wave speed and the transverse waves disappear in the secondary wave system.

Numerical methods have extensive applications in the calculations of shallow water effects.

• Calculation of the resistance and the flotation position at varying depths, velocities and ground topologies
• Calculation of waves / wave heights on banks and shores.

The SVA uses ANSYS CFD for this.

Fast moving ships are more affected by shallow water effects than slow moving ships. The areas of the Baltic Sea are represented in the image below for three different speeds where the vessel would be running in a critical Froude depth number (0.9 <Fr_h<1.1) speed range. Near the coast the boat is moving in supercritical range, further out on the Baltic Sea in the subcritical Froude depth number speed range.