The 2002 IMO regulations regarding the turning, course keeping and stopping ability for vessels with a length greater than 100 m do not cover the presence of waves, wind and current. But their effects may significantly reduce the manoeuvring performance of ships, especially of smaller vessel types in shallow and restricted waters. Since January 2013, an additional IMO regulation is in force, covering the energy efficiency of ships by defining an Energy Efficiency Design Index (EEDI) that must not exceed a specified reference line for any new-built or converted vessel. The reference line to be met will successively be lowered in three steps. One way to meet the EEDI is a reduction of the installed power, which reduces the powering margin and may lead to significant safety issues for some ship types like smaller general cargo vessels since manoeuvring capabilities in adverse conditions might not be sufficient anymore. Due to the unpredictability of waves, a performance assessment is certainly not feasible in full scale and systematic model test series are time consuming and expensive. It is therefore of utmost importance to develop reliable and efficient software tools that are capable to simulate and predict the seakeeping and manoeuvring behaviour of a vessel at the design stage. In this paper, MARINTEK’s combined seakeeping/manoeuvring simulator VeSim is presented, calibrated and successfully validated by model tests with a general cargo vessel. In this software, the vessel hydrodynamics are solved taking care of both the seakeeping and maneuvering problems simultaneously. External forces result from waves, current and wind as well as from e.g. the propulsion system and mooring lines. The pre-calculated hydrodynamic properties of the vessel include speed-dependent resistance, maneuvering forces (mainly viscous), mass and restoring properties, damping and added mass properties (represented as retardation functions) and viscous roll damping. The maneuvering forces are calculated using current and wave particle velocities as input in addition to the ships velocities. A simulation study with VeSim is performed to find the minimum required power for advancing in head seas as a function of wave period and wave height for a general cargo vessel. In addition, two IMO standard manoeuvres — turning circles and 10°/10° zig-zag tests — are simulated in calm water as well as one regular wave condition in order to exemplify the capabilities of VeSim.