On the Lateral Accelerations of Multihulls compared against the Cruise Performance requirements of the HSC Code Annex 9 - a Safety Issue

The author has conducted seven series of measurements in accordance with Annex 9 of the HSC Code in which the horizontal accelerations are measured in two different sea states and extrapolated to define the worst intended conditions. Four of the trials were on catamarans of various sizes and three on trimarans. One catamaran and one trimaran were extensively model tested at all headings and speeds using self-propelled models. The results of both trials and model tests confirmed that mathematical simulation using strip theory gave highly dependable results, which were then used to explore more accurate methods of defining the worst intended conditions. The deficiencies of the HSC Code methodology are explored and alternatives suggested. characteristics. 1.0 BACKGROUND All High-Speed Craft intended for service on international routes are certified in accordance with the International Maritime Organisation's (IMO) International Code of Safety for High-Speed Craft, 2000 (the HSC Code or simply "the Code"). In practice, many countries lack national safety regulation for high-speed craft and therefore adopt the HSC Code even though this was written only for international service. Annex 9 Section 3 of the HSC Code defines a methodology to determine the Cruise Performance and accelerations experienced during cruise modes, with no passenger load or cargo load, during two conditions; namely the normal operating conditions as defined (with any automatic control system in operation) and the worst intended conditions as defined in the Code. This is to be applied to the first craft of a new design. Operational levels are established and documented by full- scale tests in at least two relevant sea conditions and in head, beam and following seas. Presumably the relevant sea conditions relate to the "normal operating condition" and the "worst operating conditions". The worst operating conditions for a vessel are stated in the craft "Permit to Operate". Recognising that it is completely impractical to obtain specific sea conditions for sea trials. a method is defined in the Code that permits two arbitrary sea conditions to be adopted for trials, and then to assume a linear relationship between wave height and acceleration, up to a maximum wave height of 150% of the more severe of the two measured sea conditions. Annex 3 of the HSC Code also defines the maximum permitted horizontal accelerations corresponding to the two operating conditions, namely: Normal operation Safety Level 1 0.20g maximum Worst Intended Safety Level 2 0.35g maximum