3.3 Rotation Phase
Clasification Society 2024 - Version 9.40
Statutory Documents - IMO Publications and Documents - Circulars - Maritime Safety Committee - MSC/Circular.616 – Evaluation of Free-Fall Lifeboat Launch Performance – (22 June 1993) - Annex – Evaluation of Free-Fall Lifeboat Launch Performance - Section 3 – Launch Behavior of Free-Fall Lifeboats - 3.3 Rotation Phase

3.3 Rotation Phase

  3.3.1 The geometry of a free-fall lifeboat as it rotates at the end of the launch ramp is shown in figure 3.6. After the CG has moved past the end of the launch ramp, the lifeboat begins to rotate. The primary factors affecting rotational behavior are the velocity of the lifeboat, the mass and mass distribution of the lifeboat, and the distance from the CG to the end of launch rail. The equations of motion describing the behavior of the lifeboat during the rotation phase are (Nelson, et. al., 1991):

where Φ is the friction angle which is equal to tan-1μ. In addition to these three equations, an equation for the compatibility of displacements is required. This equation is:

Figure 3.6 Geometry of a Free-Fall Lifeboat As It Rotates Off the End of the Launch Ramp

  3.3.2 As implied in Equation 3.7, rotation is caused by a couple formed by the reaction force between the ramp and the lifeboat and the weight of the lifeboat. This couple imparts angular momentum to the lifeboat. The momentum increases during the rotation phase and then remains constant during the free-fall phase.

  3.3.3 Presented in figure 3.7 is the angular momentum of a lifeboat versus ramp length for three launch angles (Nelson, 1992). The lifeboat for which these data were computed is about 10 meters long and weighs about 125,000 N. The end of the launch rail is at the stern and the CG is 3.85 meters forward of the stern.

Figure 3.7 Variation of Angular Momentum at End of Rotation Phase

  3.3.4 The angular momentum imparted to the lifeboat during rotation decreases as the distance L increases. This occurs because the velocity of the lifeboat at the beginning of the rotation phase increases as the distance to the end of the ramp increases. As such, the time during which it rotates, the time during which the couple acts, decreases as L increases. Because the time of rotation is reduced, the time during which the forces act, and therefore the angular momentum imparted to the lifeboat, is reduced. Likewise, the duration of the rotation, and therefore the angular momentum, increases as the distance to the end of the launch rail increases. The angular momentum increases until the time at which the lifeboat is no longer in contact with the launch ramp. After leaving the launch ramp, the lifeboat continues to rotate at constant angular velocity until it impacts the water.

  3.3.5 The differences in the time during which the lifeboat rotates are evident from the force data presented in figure 3.8 (Nelson, et. al., 1992). The relative values of Land D were changed by moving the CG forward and aft. The data, which were computed for a typical free-fall lifeboat that was launched from a ramp at an angle of 30 degrees, were normalized by dividing by the boat weight. Until the rotation phase begins, the force between the boat and the ramp is constant. This force, which is about 87 per cent of the lifeboat weight in this case, decreases during the rotation phase and becomes zero at the end of the rotation.

Figure 3.8 Typical Forces Acting on a Free-Fall Lifeboat During Ramp and Rotation Phases for Three Locations of the CG

  3.3.6 For the three locations of the CG presented, the rotation phase ends at approximately the same time. The time at which rotation begins, however, changes with the location of the CG. When the CG is in the forward location, the boat begins to rotate about 1.75 seconds after it is released whereas rotation begins about 1.95 seconds after release when the CG is moved aft. Although this difference in time is small; the difference in the resulting rate of rotation during free-fall is significant. Presented in figure 3.9 is the angular velocity from a time just before the boat begins to rotate until shortly after the free-fall phase begins. As can be seen from this figure, the rate of rotation increases during the rotation phase. At any time during the rotation phase, the rate of rotation is higher when the CG is located forward than it is when the CG is located aft. The constant angular velocity at the right side of the curves is the rate at which lifeboat is rotating as it is falling through the air. For the lifeboat represented by these data, there is about a 20 per cent change in the angular velocity during free-fall as the CG is moved from an aft location to a forward location.

Figure 3.9 Angular Velocity During Rotation for Three Locations of the CG

Parent topic: Section 3 – Launch Behavior of Free-Fall Lifeboats
Copyright 2022 Clasifications Register Group Limited, International Maritime Organization, International Labour Organization or Maritime and Coastguard Agency. All rights reserved. Clasifications Register Group Limited, its affiliates and subsidiaries and their respective officers, employees or agents are, individually and collectively, referred to in this clause as 'Clasifications Register'. Clasifications Register assumes no responsibility and shall not be liable to any person for any loss, damage or expense caused by reliance on the information or advice in this document or howsoever provided, unless that person has signed a contract with the relevant Clasifications Register entity for the provision of this information or advice and in that case any responsibility or liability is exclusively on the terms and conditions set out in that contract.