Axis Shift and Waves dialog

This dialog is used to set up wave and axis rotation data and is accessed from:

• Hydrostatics

• Cross Curves,

• Loading Conditions and

• Deterministic Stability.

It enables you to change the settings for individual calculations.

Axis Shift:

The axis parameters which consist of the pivot point's location (defined by co-ordinates) and the swing angle are primarily of concern when considering calculations for offshore structures

For ships, these values are usually all zero except X, which should be at amidships, that means, LBP/2.

• X:

  The longitudinal location measured from the AP, in meters.

• Y:

  The transverse location measured from the centreline, positive to starboard, in meters.

• Z:

  The vertical location measured above the baseline, in meters.

The Swing Angle (degs) is the angle through which the x-axis is rotated to starboard about the pivot point. Entering a swing angle changes the planes in which heel and trim occur, affecting values such as KMT and therefore stability. However the internal reference axis of the vessel (usually defined at the AP) maintains its relative location and orientation within the vessel; thus items measured using this axis system such as LCB and TCB, remain unaltered.

Note:
The values of MCTC in the hydrostatics are incorrect when a swing angle is entered as the LBP is used for the calculation and not the notional length in the altered x direction.

Wave Type

For still water calculations, you should select None. The two wave type options available are Sinusoidal and Trochoidal.

If either of these are selected the other parameters are required to describe the shape and location of the waves. These are Height (equal to twice the amplitude), Length, distance of Crest from Pivot point (units of all 3 parameters: meters or feet), Direction angle to centreline (in degrees) and Tolerance (in meters). The tolerance is the maximum error or distance from the true sinusoidal or trochoidal wave, allowed for any point on the wave profile.

This wave data can be subsequently used for assessing the vertical bending moments and shear forces on the hull girder with the vessel statically balanced on either one or two waves, thus producing both hogging and sagging effects, and at a user-specified heading angle, as an addition to the normal still water condition.