| 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 invoked:
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 ³ /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 depth.
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 the wind.
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.) |
