Violent seakeeping tests of a fast catamaran

With the main goal of increasing the cruise speed of the ships, designers have proposed a wide variety of arrangements. In the most of them, the weight of the vessel can be supported by submerged hulls, hydrofoils, air cushion effects, or combinations of them. Two different geometries, the mono-hulls and catamarans are the commonly used. A mono-hull with the same displacement of a catamaran is characterized by a lower wave induced vertical acceleration since its larger length is beneficial from this point of view. The beam-to-draught ratio B/D of high-speed mono-hulls, typically around 5, can also exceed 7 in some cases; large B/D values limit the accelerations in heave and pitch motions, Faltinsen (2005). However the roll motions of mono-hulls need special attention and may matter for ship stability. Among multi- hull ships, Small Waterplane Area Twin Hull (SWATH) has higher heave and pitch natural periods and generally lower vertical excitation loads than a similarly sized catamaran. On the other hand, if control surfaces are not introduced, the SWATH is dynamically unstable in the vertical plane, beyond a threshold Froude number. When operating in head-sea conditions, its seakeeping behaviour is better than the one of a corresponding catamaran. However, if the sea state, speed and heading cause resonant vertical motions, the SWATH may not have a good seakeeping behaviour. Surface Effect Ships (SES) use an air cushion mechanism to obtain the desired cruise velocity and performances. The excess pressure in the air cushion between the two SES hulls lifts the vessel and carries about 80% of its weight. On the other hand, it reduces the metacentric height and causes wave generation and additional wave resistance. As a consequence, it can suffer more speed loss in waves than for instance a catamaran. Still, the total calm water resistance is smaller than the one of a catamaran of similar dimensions.