Medium size light vessels are operated worldwide to perform patrol and rescue activities. The characteristics of such vessels, together with the various environmental conditions they might have to cope with all around the world, lead to significantly different sea-keeping behaviour and structural response when operating in open oceans or in closed seas. A first comparison had been carried out a few years ago [1], stating that, from a static point of view, the structural behaviour of such vessels can be better in open seas, where waves are generally higher and longer, rather than in closed seas, characterised by smaller but shorter waves. Further to that paper, aim of present study is to deepen the subject and try to make the comparison more exhaustive. In fact reliability of the structural behaviour of the vessel has been assessed by means of a comprehensive calculation procedure, which allows determining the stress fields acting on the ship structure and the "Structural Response Operator" (SRO) and to perform an exhaustive fatigue assessment in way of significant structural details.