This paper discusses the way in which the SWATH concept was applied to a 21 tonne fishing vessel making use of various advantages over a monohull of similar size; low responses to waves, large deck area, good stability characteristics, outstanding maneuverability at low speeds, working efficiency as well as safety and lower speed loss in waves. The hydrodynamic characteristics have been rigorously examined by tank testing a one seventh scale model and comparing the results with computational results based on several computer programs. A 21 tonne SWATH boat fishery of shellfish by traps on the North West coast of Scotland was designed and constructed. The full scale measurements of resistance and powering, stability trials and rough sea trials were performed and reported in this paper.