General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)


It is sometimes necessary to calculate the total gyradii of a vessel about its three axes for a given loading condition. Aside from lightship and tank loads, many loading conditions are composed of additional fixed weight items, which are specified using the aptly named ADD command. However, defining only the magnitude and location of these fixed 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 GHS interprets the Midbody Extension quite literally as a point weight. All of the 50 LT weight is concentrated at the point 11.5a, 0, 5. In this case, 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. Unless the fixed weight really is the size of a pinhead, this is obviously 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 the Midbody Extension is correctly accounted for in the total gyradii, as evidenced by the status report. Both the distribution of the 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. /GYRADIUS may also be specified for distributed fixed weights in a similar manner. (Lightship inertia is missing in this example, which may be likewise specified using WEIGHT /GYRADIUS.)

Next time you are preparing a loading condition with ADD, don't forget /GYRADIUS. You may just thank us when it comes time to run a seakeeping analysis.
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