3D motion model for the freefall lifeboat during its launching from a moving ship

Studying a lifeboat's launch motion mechanism is essential for safe marine evacuation. Motion modeling and simulation offer an effective approach for analyzing the motion characteristics of lifeboat launches under different environmental conditions. However, existing models of the complete launch process for a freefall lifeboat (FFLB) do not capture the latitudinal motion of both the FFLB and its parent body, resulting in models with reduced accuracy and general applicability. To address this issue, we propose a 3D motion model for FFLB launch from a moving ship using Kane's method. The launch process is divided into the sliding phase and the water entry phase. To account for the impact of ship motion on the lifeboat during the sliding phase, we utilize the mathematical model of Manoeuvring Modelling Group (MMG) to simulate the ship's motion in waves. Contact forces between the FFLB and the skid are calculated using the node-to-segment method based on elasticity and friction theory. During the water entry phase, hydrodynamic forces are calculated using the strip method, and the slamming force is captured by employing the asymmetric water entry of a finite wedge. Flow separation from the lifeboat's body is calculated by introducing a fictitious body surface. Our experiments verify the accuracy of the model, with acceleration curves matching well, and a 20% maximum relative error of acceleration extremes during the water entry phase. The simulation results indicate that ship latitudinal motion leads to a similar motion of the FFLB. The ship's longitudinal motion affects the maximum acceleration of the FFLB and may cause it to setback after water entry. Additionally, the occupants experience less impact when the FFLB enters the water near the wave crest.