Over the coming decades the worldwide fleet of ships will have to change to reduce emissions. Hybridisation of the propulsion system is a stepping stone to full electrification, which is required to reduce emissions of the maritime sector to zero. In hybrid propulsion systems, clutches are used to switch between drive engines when transitioning between operational modes. The operation of clutches is indicated to generate significant disturbances in propulsion systems, however, this impact is overlooked in hybrid propulsion simulation studies in the scientific literature. The identified research gap is addressed in this thesis by simulating propulsion mode transition in a hybrid propulsion system and analysing the impact of clutch operation, using a model developed for this purpose. A hybrid propulsion system is defined by introducing multi disk wet friction clutches in a reference propulsion system consisting of a 9.1 MW diesel engine in parallel with a 3 MW electric motor. A broad scope of impact measures, that includes the electric load on the power system and the temperature development of the clutch, is defined and the system is thoroughly modelled to enable evaluation of these measures. The transmission system is modelled with a higher fidelity than typically applied in propulsion system simulation studies to enable assessment of the basic vibratory behaviour of the transmission system. The considered propulsion modes transitions are between diesel and electric propulsion. To enable these transitions, three transition control approaches with an increasing degree of sophistication are implemented. The developed model is used to perform simulations, from which it is concluded that torsional vibrations, gear hammer and a drop in propeller speed characterise the significant impact of clutch operation on hybrid propulsion systems in propulsion mode transitions. It is also concluded that the impact of clutch operation can be limited by implementing an appropriate propulsion control approach for the transition. A torque controlled handover of drive torque between engines is identified as a suitable approach. However, the induction machine switching from torque to speed control introduces power peaks in the load profile that have the potential to severely disrupt the stability of the ships power system. It is finally concluded that the temperature development of the clutch during the propulsion mode transitions is not significant. It is shown in this thesis that there can be a significant impact on the propulsion system as a result of the operation of clutches in propulsion mode transitions. Therefore, this impact has to be taken into consideration when designing hybrid propulsion systems. The model described in this thesis can be used to predict the impact of clutch operation in the early stages of the design process. Furthermore, the model can be used to develop and evaluate propulsion mode transition control approaches.