Air entrainment induced by plunging jets at ship locks is a widely observed phenomenon. Due to the lack of suitable investigation methods, the consequences of the air intermixing were unclear heretofore. In this study a new numerical approach for the modelling of multiphase flows with various interface scales is used to investigate the effects of air entrainment on the ship forces during the filling of a lock. The results show that the air entrainment has a significant influence on the flow regime in the lock chamber and can produce critical ship forces.