6.2 Co-Ordinate Systems
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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 6 – Human Tolerance To Acceleration Forces - 6.2 Co-Ordinate Systems

6.2 Co-Ordinate Systems

  6.2.1 Two co-ordinate systems commonly 'are used when discussing acceleration forces in free-fall lifeboats. The first co-ordinate system is associated with the lifeboat. The X axis of the lifeboat is the longitudinal axis. It is parallel to the keel and is positive toward the bow. The Y axis is the lateral axis of the lifeboat. It is positive to the port side. The vertical axis the Z axis which is positive upward. These axes are shown in Figure 6.1. Acceleration forces on the hull of the lifeboat are measured in this co-ordinate system.

Figure 6.1 Lifeboat and Seat Coordinate Axes

  6.2.2 The second co-ordinate system is associated with the seats in the lifeboat. The x axis of the seat is perpendicular to the chest and is positive to the front. The y axis is the lateral axis. It is parallel to the shoulders and is positive to the left. The seat z axis is parallel to the spine and is positive upward. These axes, and common notation for referring to them, are presented in Figure 6 2. The co-ordinate system associated with the seats in the lifeboat is the co-ordinate system commonly used to discuss acceleration forces in the Hybrid III dummy.

Figure 6.2 Coordinate Axes and Notation for Accelerations Acting on the Body

  6.2.3 An injury potential analysis considers acceleration forces acting in the axes of the lifeboat seat because it is in these axes the passengers experience the forces. The seats in a free-fall lifeboat usually are reclined relative to the axes of the lifeboat and are often rearward facing. Acceleration forces in the axes of the lifeboat at a seat location can be transformed into the axes of the seat at that location using the following transformation:

The quantities AX, Ay and AZ are the acceleration forces in the axes of the lifeboat. The quantities AX, Ay and Az are the acceleration forces in the axes of the seat. The angle θ is the angle through which the seat has been reclined as shown on Figure 6.1. The constant k in Equation 6.1 is equal to unity if the seat is forward facing and is equal to -1 if the seat is rearward facing. The subscript i indicates the step in the time-history at which the transformation is being performed; the transformation is performed at all steps in the acceleration force time-history.

Parent topic: Section 6 – Human Tolerance To Acceleration Forces
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