Slam Loads and Kinematics of Wave-Piercing Catamarans During Bow Entry Events in Head Seas

Abstract

Centre bow (CB) design for wave-piercing catamarans (WPCs) is a critical compromise between minimization of slamming and protection against deck diving. To inform the design process, this article investigates the slam loads and kinematics during bow entry events in regular head seas for a 112-m WPC with systematic variations to its CB and wet-deck geometry. Model tests using a 2.5-m hydroelastic segmented catamaran considered five different CB configurations, designated as the parent, high, low, long, and short CB. The results indicated that changes in the CB length had little effect on the general kinematic trends obtained for the pitch, heave, and bow vertical displacement at the instant of slamming, but increasing the wet-deck height resulted in an increase in heave (but not pitch) at slamming. Two new design ratios are proposed. The CB immersion depth to arch height ratio showed slamming occurring in the range of 0.3–0.6 depending on the wave encounter frequency and the CB configuration. The CB buoyancy in the encountered waves was estimated by considering both immersion depth and area along the CB in waves through relative motion analyses. It was found that the buoyancy to slam force ratio increased with increasing wet-deck height but not with increasing CB length. This suggests that an optimal CB configuration could be achieved by first modifying the arched cross-structure to reduce the arch filling effect on slamming severity and then maximizing the CB buoyancy to slam force ratio by increasing either the wet-deck height or the CB length. 1. Introduction An above-water centre bow (CB) for improving seakeeping is a feature of modern wave-piercing catamarans (WPCs) (Soars 1993; Boulton 1998; Fang & Chan 2007; Dubrovsky 2014). Figure 1 shows a 112-m Incat WPC with the CB located between the two demihulls. There are several important factors to be considered when designing such a central bow. First, the reserve buoyancy offered by the CB is the primary design factor for providing a pitchrestoring moment and eliminating deck diving in the following waves (Davis & Whelan 2007). Second, the CB configuration can influence the slamming loads in WPCs during bow entry in waves (Lavroff et al. 2013). This is due to the complex fluid-structure interaction in the CB area. When the CB enters waves, the water gradually fills the spaces between the CB and demihulls, referred to here as archways, and may result in complete closure of the archways and slamming in excessive pitch conditions. Finally, the frequency of slamming occurrence is, to some extent, related to the CB design as it contributes to lateral jet flow during the CB entry. As a result, slamming may occur in even partial water entrapment below the arch wet-deck cross-structure, which could be the case in small pitch motions (Lavroff & Davis 2015).