This study treats the calculation of longitudinal forces on a ship in a lock chamber due to the levelling procedure. The focus will be on the force resulting from the filling jet through the levelling door in the lock, with the main point of interest being the interaction between the ship and the flow resulting from the jet. To understand more about this interaction the following methods are used: the one-dimensional software model Lockfill, computational fluid dynamics (CFD) and scale model tests. The study was initiated in the context of improving the accuracy and applicability of the software Lockfill. Lockfill is based on the conservation laws of mass and momentum, while schematising the flow using a two-dimensional parameterization of the filling jet. A two dimensional situation was created for the scale model, neglecting variations over the width. The scaled situation was tested using a CFD model to prepare for the scale model measurements. This preparation revealed that the jet attaches to the bottom for the chosen geometry. The flow measurements consist of multiple point measurements and particle image velocimetry (PIV) measurements just downstream of the gate. The longitudinal forces on the ship were measured for variations in: keel clearance, downstream position of the ship with respect to the gate and water level difference over the gate. The presence of the ship reduces the deflection of the jet towards the bottom. The reason for the reduced deflection is the interference of the ship’s bow with the upper eddy limiting the fluid motion, resulting in a reduction of pressure and therefore a deflection of the jet in the upward direction. This means that the momentum flux in the jet for the situation without ship decreases faster because it attaches to the bottom closer to the gate. The measured forces were compared to the forces Lockfill would predict for the same situation. Both CFD and PIV predict the forces with a small relative error. Lockfill determines the force on the ship accurately as long as the momentum flux can be determined successfully. Determining the momentum flux comes down to a correct schematisation of the filling jet. Variations in keel clearance and discharge are covered quite well by Lockfill. However an increase in distance from the gate reveals big differences compared to the measurements. This is caused by an overestimation of momentum flux dissipation in the Lockfill jet schematisation. The reason for this is that in reality the jet stays more attached to the bottom, therefore maintaining a higher momentum flux. Taking into account the results from this study, the improvement of Lockfill should start with the schematisation of the filling jet. A start can be made by implementing the tendency of the jet to attach to a boundary.