Wind Heeling Moment Calculations
(Updated 06/1990, Applies to GHS, BHS, and BHS/YACHT)

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 | /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.)

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