Quantifying the Uncertainty of the Crabbing Capability

Crabbing is a critical manoeuvre that requires all of the ship’s propulsors to counteract environmental forces. It is a
complex hydrodynamic scenario in which it is difficult to assess the factors, nonetheless the crabbing capability is often calculated by means of simple static calculations which consider only means values. The ship designer or master should instead be aware of how much the crabbing capability can change when the propulsors are less efficient than expected or the wind loads are higher.
The present paper details the application of sensitivity and uncertainty methods to the crabbing manoeuvre of a ship. The objective is to estimate the sensitivity of the main parameters involved in a crabbing calculation and express the results in terms of uncertainty range. To this purpose, first a sensitivity analysis is performed, showing how the maximum sustainable wind speed of a test-case ship varies. The curves derived from this analysis are then used to estimate numerically the relation between wind speed and a variation in each parameter. Typical values of uncertainty of these parameters are presented, and finally the crabbing capability calculated taking into account the uncertainties.