J Lavroff

Experimental analysis of the wet flexural mode response of an NPL 6A hydroelastic segmented model

Abstract This paper investigates the effects of stiffness and mass distribution on the longitudinal flexural natural frequency response of an NPL 6A segmented monohull model in still water at zero speed. Experimental tests were undertaken in order to establish the parameters that affect the whipping frequency at model-scale so as to replicate the vibratory response of a full-scale vessel subject to slamming. The model was cut into two halves at the longitudinal centre of buoyancy and connected by a backbone beam with an elastic hinge joining the two segments. Wet vibration tests conducted on the model showed significant influences on the flexural natural frequency response through variations in stiffness and ballast mass distribution. The whipping frequency was predicted with a two degree of freedom theoretical model using an added mass approximation to provide good correlation with measured experimental data. Damping ratios of the wet transient response are also presented with respect to variations in the condition of the void separating the two model halves.

An experimental investigation of ride control algorithms for high-speed catamarans Part 1: Reduction of ship motions

Ride control systems are essential for comfort and operability of high-speed ships, but it remains an open question what is the optimum ride control method. To investigate the motions of a 112-m high-speed catamaran fitted with a ride control system, a 2.5-m model was tested in a towing tank. The model active control system comprised two transom stern tabs and a central T-Foil beneath the bow. Six ideal motion control feedback algorithms were used to activate the model scale ride control system and surfaces in a closed-loop control system: heave control, local motion control, and pitch control, each in a linear and nonlinear version. The responses were compared with the responses with inactive control surfaces and with no control surfaces fitted. The model was tested in head seas at different wave heights and frequencies and the heave and pitch response amplitude operators (RAOs), response phase operators, and acceleration response were measured. It was found that the passive ride control system reduced the peak heave and pitch motions only slightly. The heave and pitch motions were more strongly reduced by their respective control feedback. This was most evident with nonlinear pitch control, which reduced the maximum pitch RAO by around 50% and the vertical acceleration near the bow by about 40% in 60-mm waves (2.69 m at full scale). These reductions were influenced favorably by phase shifts in the model scale system, which effectively contributed both stiffness and damping in the control action.

The influence of the centre bow and wet-deck geometry on motions of wave-piercing catamarans

The effects of tunnel height and centre bow length on the motions of a 112-m wave-piercer catamaran with an above-water centre bow were investigated through model tests. Five alternative centre bow configurations were considered, and multiple series of model tests were conducted in regular head sea waves. The results showed that both heave and pitch increased over a wide range of wave encounter frequency as the wet-deck height of the catamaran model increased. However, increasing the length of the centre bow showed an increase in the pitch but a decrease in the heave for a limited range of wave encounter frequency near the heave and pitch resonance frequencies of the catamaran model. The positions of minimum vertical displacement were found to be aft of the longitudinal centre of gravity, between 20% and 38% of the overall length from the transom. Increase in the wet-deck height and consequently the archway clearance between the main hulls and centre bow also resulted in an increase in the vertical displacement relative to the undisturbed water surface in the centre bow area. The results also indicated the vulnerability to wet-deck slamming for the different bow and wet-deck designs.

An Experimental Investigation of Ride Control Algorithms for High-Speed Catamarans Part 2: Mitigation of Wave Impact Loads

High‐speed craft frequently experience large wave impact loads due to their large motions and accelerations. One solution to reduce the severity of motion and impact loadings is the installation of ride control systems. Part 1 of this study investigates the influence of control algorithms on the motions of a 112‐m highspeed catamaran using a 2.5‐m model fitted with a ride control system. The present study extends this to investigate the influence of control algorithms on the loads and internal forces acting on a hydroelastic segmented catamaran model. As in Part 1, the model active control system consisted of a center bow T‐Foil and two stern tabs. Six motion control feedback algorithms were used to activate the model‐scale ride control system and surfaces in a closed loop system: local motion, heave, and pitch control, each in a linear and nonlinear application. The loads were further determined with a passive ride control system and without control surfaces fitted for direct comparison. The model was segmented into seven parts, connected by flexible links that replicate the first two natural frequencies and mode shapes of the 112‐m INCAT vessel, enabling isolation and measurement of a center bow force and bending moments at two cross sections along the demi‐hulls. The model was tested in regular head seas at different wave heights and frequencies. From these tests, it was found that the pitch control mode was most effective and in 60‐mm model‐scale waves it significantly reduced the peak slam force by 90% and the average slam‐induced bending moment by 75% when compared with a bare hull without ride controls fitted. This clearly demonstrates the effectiveness of a ride control system in reducing wave impact loads acting on high‐speed catamaran vessels.