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Chapter 2 Using Multiframe
Rotation Angle ()
It is possible to model the structure heeled in any direction, or combination of directions,
by entering values for Rotation Angle. In reality the structure is not rotated but the
subsequent accelerations are applied at that angle.
Linear Acceleration (a)
Combinations of surge, heave and sway can be applied be entering values for Linear
Acceleration. Surge is applied along the x-axis with forwards positive, heave is applied
along the y-axis with upwards positive and sway is applied along the z-axis with to
starboard positive.
Angular Acceleration ()
Angular accelerations can be applied. These act around the centre of motion of the
vessel, the location of which is defined by the Motion Axes, see below.
Roll is applied about the x-axis with starboard side down positive, yaw is applied about
the y-axis with bow to starboard positive and pitch is applied about the z-axis with bow
up positive.
Motion Axes
The location of the motion axes with respect to the origin is defined by the Motion Axes.
The x direction is positive if the model’s origin is forward of the centre of motion, the y
direction is positive if above the centre of motion and the z direction positive if it is to
the starboard of the centre of motion.
Self Weight
It is possible to include the Self Weight load case within the Sea Motion case. The reason
for doing this, instead of creating a combined load case made up of a Sea Motion and a
Self Weight load case, is because linear accelerations due to gravity will be adjusted for
any Rotation Angles entered.
To include the self weight of the structure in the Sea Motion calculation tick the Include
Self Weight check box and enter the gravitational acceleration. You will more than likely
usually use the default setting of –g in the y direction.
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