How does the GHS command differ from the HS command?


Question:

I am using the following run file for hydrostatic properties.  The tables were
ok, but there were no graphics.  


  Macro calc

   HS 4,4.1,...,14

  /

  Trim 0

  .Calc

  Trim 1

  .Calc

  Trim 15

  .Calc


Later I discovered the command GHS and I tried to use it the same way
substituting GHS for HS in the above run file.  When the table reached draft
6.2 the following message appeared: Error: Trim too great - cannot set draft 
at LCF. 


I tried many times and the same message always repeated.  Later I realized
that the results are different when you calculated using HS or GHS.  Could you
explain why I can't calculate GHS for trim > 2 degrees?


Answer:


In answer to your questions:

There are some significant differences between the HS and GHS commands.  The
HS command produces "pure" hydrostatic properties without any reference to the
center of gravity.  The GHS command involves the center of gravity in the
Moment to Trim -- hence the "G" for "Gravity" -- and also in the GM when the
present waterplane is used.  Other differences with which you are concerned
are:

1) The GHS command produces graphics but HS does not.

2) The HS command takes Origin Depths as a parameter.  The GHS command takes
Drafts as a parameter.  If the trim is zero, the two will usually be the same.
However, if trim is not zero, Draft and Origin Depth will be different for the
same waterplane.  This is why you get different results from HS and GHS: they
are using different waterplanes.

You can get the same results from HS and GHS by using parameters which give the same
waterplane.  For example,

  HEEL = 0                                                                                        
  TRIM = 10                                                                                       
  HS 4                                                                                            
  GHS DRAFT @ 0 = 4.0617                                                                          

Here we have the same waterplane since 4.0 / cos(10) = 4.0617.

3) Please note that a Draft always refers to a longitudinal location at which
the draft is measured.  If you do not specify the location, the location of
the LCF is assumed.  However, this presents a difficulty since the location of
the LCF depends on the draft and the draft depends on the location of the LCF!
Therefore, the program has to get the LCF before it can set the draft
accurately.  This involves an iterative process which may not converge
rapidly when the trim is too large.  Therefore, it limits the trim to 2
degrees.  If you want a greater angle of trim, you can specify the location of
the draft.  For example,

  GHS DRAFT @ 0 = 4, 4.1, ..., 14