Since the 1980s, the nautical bottom in the outer harbour of Zeebrugge is determined by the 1150kg.m -3 density level, as due to siltation conventional bottom survey techniques are not adequate. As a consequence of modifications of the mud layer characteristics, a revision of this criterion was required. A research project, involving comprehensive captive model testing in a manoeuvring tank with a bottom covered with mud simulating materials, mathematical modelling and real-time full mission bridge simulator runs, resulted in new criteria, based on the ship behaviour and controllability in muddy navigation areas. The application of the results of this research project increases the efficiency of maintenance dredging works and contributes to the safety of shipping traffic, as the pilots involved in the manoeuvres obtain a more profound insight into specific aspects of ship behaviour.