This is especially of concern for military and/or rescue operations and means to reduce or mitigate against motion exposures are required to protect the

occupants. In this paper a detailed background describing high speed marine craft motion effects and the hazards of whole body vibration and repeated shock is presented. This is followed by a numerical analysis of the motion mitigation provided by various ‘flexible’ Lumacaftor chemical structure hull designs, including a suspended hull design, an elastomer coated hull and a reduced stiffness aluminium hull, during a slam event. The motions of high speed marine craft, travelling at speed in waves, are characterised by non-linear motion responses with numerous slam events and shock motions (peak magnitudes ⪢⪢ r.m.s) (Coats and Stark, 2008 and Townsend et al., 2008). With an increase in speed from stationary to planing speeds the motions are generally found to increase in magnitude and principal frequency. At greater speeds the motions become non-linear as the hydrodynamic forces find protocol outweigh the hydrostatic forces (Rosen, 2004). At lower speeds vertical oscillatory motion within the frequency range 0.1–0.5 Hz are likely and sea sickness incidences can be expected (Lewis, 1986 and British

Standards Institution, 1987). Although the predominant motion responses occur in the vertical Z-axis direction, X-axis and Y-axis accelerations can also be relatively large and may contribute to the undesirable motion effects. Fig. 1 shows the magnitude and principal vibration frequencies recorded in a high speed RIB craft. The motion responses of high speed marine craft to waves of increasing height, for given conditions, principally results in an exaggeration of the motion responses (Townsend

et al., 2009). In general, with increasing wave height motion responses become non-linear and exhibit additional frequency responses to those of the wave encounter (Townsend et al., 2008). Although Grant and Wilson (2004) and Townsend et al. (2008) comment that the acceleration responses may attenuate with increasing GNA12 wave height and that the relative wave profile (the relative wave height/slope compared to the craft length) may be of greater importance than the absolute wave height when discussing the motion responses of high speed marine craft. The motion responses of high speed marine craft vary with wave encounter frequency. However, with high speed marine craft occasionally jumping over waves and missing wave encounters, the practical use of encounter frequency to describe the motions may be limited. Although, cumulative and therefore potentially dangerous effects may occur at certain encounter frequencies. For example Townsend et al.