|
|
General HydroStatics
Ship Stability Software |
|
Command of the Week
(New or interesting aspects of GHS that you may not know about) |
|
ADD /GYRADIUS (review)
|
|
For seakeeping calculations, total gyradii about the vessel's three axes must be provided. Typical loading conditions include fixed weight items, which are specified using the aptly named ADD command. However, defining only the magnitude and location of these weights does not provide a complete representation of the distribution of the weight in three-dimensional space. For example, the following commands establish a lightship weight and create a fixed weight item:
WEIGHT 250, 0, 0, 5 ADD "Midbody Extension" 50, 11.5a, 0, 5 In the hydrostatic sense, the result is accurate, but in this "Midbody Extension," all of the 50 LT weight is concentrated at the point 11.5a, 0, 5. The consequence of this interpretation is most evident since the weight is coincident with the vessel's longitudinal CG axis: |
|
|
Notice that even though "Midbody Extension" is a relatively large weight, it imparts no influence on the roll gyradius. The transverse and vertical inertia are the result of the offset of the weight relative to the transverse and vertical axes. Obviously this is not an accurate representation of the weight. ADD /GYRADIUS accounts for this by letting one specify the gyradii of each individual fixed weight item. The gyradii are specified about the longitudinal, transverse, and vertical axes through the weight's center of gravity. For example:
ADD "Midbody Extension" 50, 11.5a, 0, 5 /GYRADIUS:10, 0, 5 |
|
|
Now the influence of "Midbody Extension" is correctly accounted for in the total gyradii, as evidenced by the status report. Both the distribution of "Midbody Extension" about its own axes, as well as its location with respect to the vessel's axes, are included in the total inertia calculation.
In a similar manner, /GYRADIUS may be specified for distributed weights. This includes WEIGHT /GYRADIUS which should be standard practice because as noted in last week's COW, adjustments to light ship via the waterplane will affect only the WEIGHT item and not ADDed weight items.
|