This paper describes a research programme to construct a Model-Based Systems Engineering (MBSE) methodology that supports acquiring organisations in the early stages of Off-the-Shelf (OTS) naval vessel acquisitions. A structured approach to design and requirements definition activities has been incorporated into the methodology to provide an easily implemented, reusable approach that supports defensible acquisition of OTS naval vessels through traceability of decisions. The methodology comprises two main parts. Firstly, a design space is developed from the capability needs using Set-Based Design principles, Model-Based Conceptual Design, and Design Patterns. A key idea is to employ Concept and Requirements Exploration to trim the design space to the region of OTS designs most likely to meet the needs. This region can be used to specify Request for Tender (RFT) requirements. Secondly, the methodology supports trades-off between the OTS design options proposed in the RFT responses using a multi-criteria decision making method. The paper includes an example implementation of the methodology for an indicative Offshore Patrol Vessel capability.