Prediction of crabbing in the early design stage

Crabbing is the ability of ships to move sideways without having a forward speed. Crabbing can be induced by the use of a combination of main propellers, rudders and lateral thrusters. The crabbing ability is a useful operating mode and a ship owner can have considerable profit from this ability because of a reduction of the time and costs in the harbour. Consequently, the crabbing ability is a design criterion, and needs to be assessed in early design stage. Ship yards need to know the power and amount of lateral thrusters that are to be installed in the ship. Some used criteria for crabbing are given. When the crabbing operating mode is chosen, a complex amount of circulation and fluid flows occur around the ship, which is effected considerably by the environment (quay and water depth). Using an example ship, it is shown that simple calculations are not sufficiently adequate to estimate the crabbing capability, so that advanced calculations or model tests are needed.