A 6 degrees of freedom manoeuvring model of an estuary container vessel was implemented in the inland navigation simulator Lara at Flanders Hydraulics Research (FHR). The container vessel, with dimensions 110 m × 17.5 m × 4.5 m, is equipped with two Z-drives, each one consisting of two contra rotating propellers. Both Z-drives have a 360° azimuth angle of operation during manoeuvring. The mathematical manoeuvring model was built to cover all degrees of operation. This required the execution of a significant number of captive model tests in the shallow water towing tank of FHR (in co-operation with Ghent University). Based on these tests a new mathematical manoeuvring model was built to cover all effects of operation, including the interaction effects between the two Z-drives. The new mathematical model was implemented in the simulator and both fast time and real time manoeuvring simulations have been carried out. A paper discussing the validation of the manoeuvring model has been presented during MARSIM 2015 (Vos, Delefortrie, and Van Hoydonck in Validation of the manoeuvring behaviour on an estuary vessel. MARSIM, Newcastle, 2015). The present paper discusses the details of the manoeuvring model, with emphasis on the interaction effects between the Z-drives and the influence on the hydrodynamic forces.