The prediction of wetdeck slam loads on high-speed catamarans

The earliest catamaran ferries were essentially conceived for use in very sheltered waters such as river, estuary and bay crossings and for routes protected by offshore reefs where tour crossings were easily postponed in rough weather. They were therefore designed with relatively small wet deck heights (height of the underside of the central deck above the water line) and thus with a relatively low transverse deck structure at the bow. However, the speed capability and length of these vessels gave rise to length based Froude numbers of 1.0 (Figures 2 and 3) and it became apparent that wave encounter induced significant vessel motions. Therefore if such vessels were operated in significant seas there resulted not only increased passenger discomfort but also increased risk of wave slam impact on the bow and wet deck. To overcome these problems the wave piercing and semi-SWATH (small water plane area twin hull) designs were introduced (Figures 4 and 5) so as to reduce the vessel motions by making the bow region relatively soft with regard to vertical wave forces on the structure. This was to a certain extent successful in reducing passenger motions towards the bow of the vessels, but the softness of the bow region exposed such designs to deep bow entry, slamming impact on the central structure and damage in following seas.