Coordination is a key activity in complex organizations with distributed actors. In this paper, we focus on conversations about coordination in a specific safety-critical domain: steering a large container ship in and out of a harbour. Based on a thorough ethnographic study of work activities in this domain we have identified communication problems and modelled how conversation about coordination is supported by formalized communication. From this conversation model, we have designed a mobile computer system that makes the conversation persistent by facilitating exchange of pre-defined text messages. We have validated the conversation model and the design of the computer system through development and qualitative evaluation of a functional prototype. The evaluation demonstrated that computer mediation to make conversations persistent is highly relevant to the work task considered. The evaluation also uncovered fundamental limitations in the conversation model, so we discuss how it can be extended.