There have been a large body of papers on preliminary ship design, both in the Transactions and elsewhere in the ship design literature. However when these are considered, it is generally the case that they are either describing a specific ship design or talking in general about different ways in which ships may be designed. What this paper therefore sets out to provide is a detailed description not so much of a general process, with examples to illustrate the generality or to overly focus on the end point of a particular ship design process, but rather to take a given detailed case study of a design, worked up in some depth. The example chosen is a trimaran option of the US Navy Littoral Combatant Ship produced by the authors as the result of an investigation for the US Navy's Office of Naval Research into the capability of the UCL Design Building Block approach to Preliminary Ship Design and specifically its realisation in the SURFCON module of the Graphics Research Corporation's PARAMARINE ship design tool. The paper is considered to be unique as a published exposition on preliminary ship design in that the intermediate steps in the evolution of the detailed concept study are given in some technical depth along with the presentation of the major design issues as they arose through the concept process. The paper commences with the justification for the presentation of this investigation into the nature of preliminary ship design, including a survey of previous work in the field. The specific DBB approach is briefly outlined, as is the basis of the particular case study, before the main part of the paper presents the main steps in the specific case study, through technical and graphical descriptions. The discussion considers the insights the case study provides, as well as their limitations, before presenting conclusions on the nature of preliminary ship design for complex vessels, typified by the modern multifunctional naval combatant. © 2008 Royal Institution of Naval Architects.