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


Shifting the Origin

If you have to relocate the origin of the coordinate system, the biggest part of the task might be shifting a complex weight distribution. Part Maker makes it easy to shift the origin of the geometry using the SHIFT command, but there remains that multi-line WEIGHT command for the light ship distribution that has to be dealt with.

In case you haven't discovered this on your own,


does not reduce light ship to a point at the new LCG; rather it shifts all the elements of your light ship distribution (not including ADD + items).

Here is how it works:

At a known attitude (heel, trim) as loaded, issue the command,


Then to view the light ship distribution graph (in your active report):


Before doing that, fixed-weight items must be relocated, which requires editing the ADD commands.

Here is an idea to make that task easier: Write a macro named ADD that issues the ADD command in the special form that shifts the L value. For example,

  ADD "%1", *, *+{LSHIFT} /NOWARN

Then place a dot before each ADD command to turn it into a macro call.

Since GHS version 15.52C this special form of the ADD command properly shifts weight distributions. (Formerly the distribution became a point.)

Any other commands that reference the longitudinal axis of the coordinate system must be edited independently (GROUND, PULL, THRUST, REF, CRTPT, LWL, LBP).

What follows is an example.

The status before the origin shift:

After the geometry has been shifted, the weight curve is still in the old location:

After SOLVE LCG corrects the weight curve it looks like this:

And the Status matches except the Longitudinal numbers are all shifted by 50 meters.

Survey question: Would it be helpful to have a more convenient way to shift all the weights? Or is this a solution looking for a problem? Let us know.

Click here to see the run file that produced the output shown above. It automates the conversion of the ADD commands to macro calls, so the whole thing runs without user intervention.

Questions, comments, or requests?
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