This paper presents an advanced and accurate integrated system for the design and performance optimisation of fibre reinforced sails, commonly named string sails, developed by SMAR Azure. This integrated design system allows sail designers not only to design sail shapes and the reinforcing fibre paths, but also to validate the performance of the flying sail shape and have accurate production details including the overall sail weight, material used, which means costs, and length of the fibre paths, which means production time. The SMAR Azure design and analysis method, extensively validated and used to optimise several racing and super-yacht sailing plans, includes a computationally efficient structural analysis method coupled with a modified vortex lattice method, with wake relaxation, to enable a proper aeroelastic simulation of sails in upwind conditions. The structural analysis method takes into account the geometric non-linearity and wrinkling behaviour of membrane structures, such as sails, the fibre layout, the influence of battens, trimming loads and interaction with rigging elements, e.g. luff sag calculation on a headstay, in a timely manner. Specifically, this paper presents an optimisation of a real fibre reinforced membrane sailplan of an aluminium super yacht, carried out in collaboration with Paolo Semeraro (Banks Sails Europe). The optimisation process of the fibre layouts led to a sensible reduction in maximum stress, strain and displacement compared to the initial designs, keeping the same fibre weight or slightly increasing it. The results have been confirmed in the sailing tests, although no exact measurements have been performed.