In the modern world, environmental concerns arise due to an increase in the green house gas (GHG) emissions of the shipping industry; in 2015, 2,5% of global GHG emissions came from the shipping industry alone. Measures must be set in place to reduce the future emissions. One way to reduce emissions is by imposing the Energy Efficiency Design Index (EEDI). This is a measure to define the ratio between the CO2-production and the cargo capacity and speed of a ship. In other words, this ratio defines the environmental cost over societal benefit of a ship. The International Maritime Organisation wants to have a set requirement on the EEDI of new ships and preferably reduce this for existing ships. One way to reduce the EEDI is by reducing the installed engine power of the ship. One risk is that the ship might then become underpowered when it sails in adverse weather. The underpowering of ships could lead to safety hazards for the passengers and crew. For that matter, this thesis will investigate whether the ocean-going chemical tanker 'Castillo de Tebra' is underpowered when sailing in large waves. Specifically, the effect of the waves on the inflow velocity of the propeller will be looked at and how this affects the propulsive performance of the vessel. An extensive literature review was carried out in order to find out what the state-of-the-art is within the determination of ship propulsion in waves. This has been divided into three separate subjects, namely wave excitation, propeller and engine operating in waves and finally manoeuvring and seakeeping. When determining the forces that occur on a ship when it advances in waves, most methods use potential flow as the results are reliable and have low time effort. Determining propeller performance when the full wake distribution is known can also be done using potential flow methods. Basic open-water characteristics are a valid alternative as well. CFD can be used for both wave excitation and propeller performance calculations, but this is time consuming and computationally complex. When simulating manoeuvring and seakeeping behaviour of ships, two-time scale methods or unified methods are mainly used. For the research part of this thesis, numerical simulations were performed to determine the change of inflow velocity at the propeller plane for a large range of wave frequencies and wave directions, for a vessel having a forward speed. This velocity change was used to determine what the thrust and torque generation of the propeller is in waves. The engine operating envelope and ship thrust envelope were used to check whether the engine has enough power to provide the torque that the propeller generates and whether the ship has enough thrust availability. Frequency-domain results were transformed into time records for irregular waves using the JONSWAP wave spectrum and a cosine funtion. It was concluded that the ship was underpowered in adverse weather conditions, when an instant control system was assumed that inject enough fuel instantly to keep the engine at the rated speed when it needs to deliver more power. It was also assumed that the ship speed was kept at the design speed. In reality, voluntary and involuntary speed loss will occur when a ship sails in bad weather. Voluntary in the form of manual speed decrease by the captain and involuntary in the form of added resistance causing the ship to reduce in speed. A number of solutions are finally presented in this thesis to omit engine overloading in adverse weather.