Wind Heeling Moment Calculations
(Updated 06/1990, Applies to GHS, BHS, and BHS/YACHT)
See also HMMT WIND BAND compared with other wind heeling moment options
GHS offers several methods of modeling the effects of wind pressure, ranging
from a constant heeling moment directly supplied by the user to a variable
heeling moment automatically computed from wind pressure on the lateral plane.
Regardless of the method chosen, the HMMT command is the channel through which
heeling moments are specified (even when the heeling moments arise from sources
other than wind pressure).
Independently-calculated heeling moments are specified directly through the
HMMT command -- no other commands are required. Internally-computed wind
heeling moments require two preliminary steps before the HMMT command is
1) A representation of the vessel's lateral plane must be present in the
geometrical model. While all buoyant parts are automatically included in the
lateral plane, there may be other elements of the vessel which do not have
significant buoyancy but which do have significant areas subject to wind
pressure or lateral water pressure. These may be included in the model as
parts with Class = SAIL (even if they are under water, such as a plate keel).
SAIL parts are ignored when computing buoyancy, but their lateral projections
are included with the lateral projections of the other parts of the model
(tanks, of course, are ignored when computing the lateral plane).
2) The WIND command, which specifies the wind pressure, must be executed.
This command takes either a nominal wind speed (referred to 10 meters above the
water surface), or a pressure vs. height function given by means of sample
pressures at given heights. In the case where the wind speed is given, a drag
coefficient may also be given in order to match the resulting wind pressure
profile to a desired standard.
After these preliminary operations, the HMMT command may be issued in the form
HMMT WIND [/C2 | /CONST]
(the parameters in brackets being optional and mutually exclusive). With
neither optional parameter, the heeling moment calculation is deferred until a
time when a heeling moment is actually needed at which time it is calculated
from the lateral projection at the then-present depth, heel and trim.
The presence of either the /C2 or the /CONST parameter causes a heeling moment
to be calculated at zero heel and trim, using the current weight to obtain the
Once the HMMT command has been issued, establishing a heeling moment function,
there are three things which may done:
1) The SOLVE command may be issued to find the heel angle where heeling and
righting moments are equal -- ie. the equilibrium heel angle in the presence of
2) The RA command may be issued to compute a residual righting arm curve.
3) The MAXVCG command may be used to find maximum VCG values relative to a
suitable stability criterion. (Not available in BHS and BHS/YACHT.)
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