The following paper presents findings from a qualitative study conducted on board of two merchant vessels. Interviews and observations have been used to obtain insights in how safety is defined and promoted by the personnel working on board. The merchant vessel, the crew and the single mariner are identified to be part of a sociotechnical system displaying three levels of system aggregation; personcentred, crew-centred, and vessel-centred. The common ground of a crew, an overlap of the individual mariners’ experience and knowledge, is identified as a basis for trust and predictability of action on board, which is a necessity to be able to conduct work safely. Furthermore, the results also show how storytelling is used to transform individual and organisational experiences into knowledge that can guide safety-related work on board. The stories told among the crew often exemplify how mariners, both on an individual, but also on a crew-centred level of system aggregation, balance safety and efficiency in the light of increasing production demands.