Question:

Are there any special considerations in calculating stability for a
load of timber?

Answer:

If your timber is loaded on deck so that it contributes to the
buoyancy when heeled, you may want to model the timber load as if
it were superstructure.  You can use a "permeability" factor to account
for the air spaces.  For example,

  CREATE LUMBER
    CLASS SUPER
    PERM 0.75
    ENDS 0, 100
    TOP 20
    BOT 10
    OUT 20
    SPACING 4
  /