The most important optimisation goals in ship routing include minimising the overall travel time, fuel consumption and CO₂ emissions. Throughout its operation, a ship must also meet minimum safety and operability requirements, which can be treated as constraints or additional optimisation criteria. Strategic decision support systems, such as weather routing systems, incorporate seakeeping assessments to determine whether voluntary speed reductions or course adjustments are required. This is particularly important in heavy seas when the captain believes that maintaining the current speed or course could endanger the vessel, cargo or crew. The decision to reduce speed is inherently subjective and is influenced by the captain’s experience and judgement. This paper investigates the variability of thresholds for certain criteria that can be used in a weather routing system as the point at which the captain intentionally reduces the ship’s speed, as well as the influence of these thresholds on the attainable ship speed in numerical simulations.