Question

When a short length of the hull has a lowered deck, how can I keep the margin
line from following it?  By using a negative value?


Answer

The margin value interpolated at a given location is used as a deduction from
freeboard (i.e. a minimum freeboard requirement).  It is applied at each
station where there is a deck edge marking but only on those components which
have margins (may be zero or negative) assigned.  This means that you have
three ways of controlling your "margin line": 1) Use suitable margin values
(perhaps negative as you suggested -- by the way, I noticed that it's
presently negative 2.5 feet on the GF you sent!); 2) Make the raised deck a
separate component with margin and omit the margin on the hull; 3) limit the
deck edge markings to the raised deck portion of the hull.  The third method
is probably the best in this case.  You can employ Model Converter to do it.
First, remove the deck edge markings:

  mc c151dam.gf1 fixup /nocodes 

Then put them back on only for the higher portion of the deck:

  mc c151dam.gf1 fixup /deckedge:-.307, 256.307 /margin:.25 .75 .25