The current cost estimate applied by shipyards in the pre-contract phase is insufficiently valid and accurate for “one-off” ships. Most shipyards use the unit man-hours/metric ton for the estimation of man-hours in steel assembly. This key figure does not take blocks’ properties and the required actions for assembly into account. It is evident that e.g. welding above head by hand or Submerged Arc Welding (SAW) and the amount of lift and turn movements influence the lead-time and number of man-hours. The estimation of man-hours in steel-assembly can be improved by the identification of production patterns and the resulting actions. Therefore, the rationale, the reasoning and decision-making processes of the production engineers needs to be captured. This paper focuses on rationale capturing, codifying and storing for (re)use. First, it is discussed how the rationale is captured through experts-interviews, based on the reactive knowledge capturing method. Thereafter, it is explained how the captured rationale is stored in an Expert System, linking construction plans to a fitting assembly-order. This resulting assembly-order can be used to derive the amount and type of actions required. This enables a better substantiated cost estimate. The developed system in this research is generally applicable and proves the concept. The paper concludes with recommendations for future research, which includes further automation of this Expert System and integration of man-hour calculations.