Wind
and Rolling Calculations 
In the
presence of wind pressure, a vessel may need to have enough righting
energy available to absorb the kinetic energy due to a roll from a
point to the windward of its equilibrium heel. GHS provides a mechanism
for setting such a windward heel angle so that these energies can
be compared.
With a heeling moment in effect (see the HMMT command), the equilibrium
heel angle can be found simply by issuing the SOLVE command (without
parameters). If the roll angle to windward of equilibrium is known,
the heel angle can then be decremented simply by giving the command
HEEL *r
where r is the desired roll angle.
A more powerful way of handling the roll angle is provided by the
ROLL command. For example,
ROLL r
HEEL *ROLL
does about the same thing as the HEEL *r command, except
that it always moves the heel angle in the direction opposite to the
initial heel angle. Further, if the actual value of r is to
be calculated from the present condition according to the IMO rolling
formula, then the ROLL command may be given once in the form
ROLL IMO
and the actual value of r will be calculated when it is needed
from the thenpresent condition.
A typical sequence of commands to check the energy available to resist
an IMO roll would be:
ROLL IMO
WIND 53.4
HMMT WIND /CONST
SOLVE
HEEL *ROLL
RA /AREA
If an increase in the heeling moment (due to a gust of wind) occurs
at the rolled angle, it can be modeled by using a /GUST parameter
on the HMMT command. The issuance of the HEEL *ROLL command then
automatically causes the heeling moments to be multiplied by the gust
factor from then on  until another HMMT command is issued. For example,
...
HMMT WIND /CONST /GUST: 1.5
SOLVE
HEEL *ROLL `< The gust factor does not take effect until after
this.
RA /AREA
2
When using the MAXVCG process, the roll angle and gust factor are
automatically applied in the proper sequence. Thus
ROLL IMO
WIND 53.4
HMMT WIND /CONST /GUST: 1.5
SOLVE MAXVCG
is all that is necessary.
Further, the MAXVCG command automatically recalculates the wind moments
at each new draft when they are based on the lateral plane. Hence
ROLL IMO
HMMT WIND /CONST /GUST: 1.5
MAXVCG ...
produces maximum VCG curves for a stability criterion involving the
ROLL angle.
The LIMIT command, used for establishing stability criteria, accepts
the keyword ROLL as an angle, in which case it actually refers to
the *ROLL angle. For example,
LIMIT RESIDUAL RATIO FROM ROLL TO ABS 50 OR FLD > 1
expresses the IMO criterion for severe wind and rolling. 

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