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

shift weight (update)


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,

SOLVE LCG

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,

SOLVE DEPTH, LCG

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

WEIGHT REPORT /PROFILE

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 that runs through all the added weights, shifting the L values. For example, this command sequence will add 50 to the LCG of each weight item:

 MACRO SHIFTADD
  i:=i+1
  ADD ({i}) s /ACCESS
  IF "{s}"="" THEN EXIT
  ADD "{s}", *, *+%1 /NOWARN
  EXIT SHIFTADD %1
 /
 variable (str) s
 variable i=1
 .SHIFTADD 50

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.




Questions, comments, or requests?
Contact Creative Systems, Inc.

support@ghsport.com

USA phone: 360-385-6212 Fax: 360-385-6213
Office hours: 7:00 am - 4:00 pm Pacific Time, Monday - Friday

Mailing address:
PO Box 1910
Port Townsend, WA 98368 USA

www.ghsport.com

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