General HydroStatics
Ship Stability Software
Command of the Week
(New or interesting aspects of GHS that you may not know about)

(Requires GHS version 16.28 or later)

Custom polar plots give users the option to plot, well, just about anything on a polar plot. In some cases, a polar plot is just the natural way to show the information. For example, when evaluating rig stability, it is important to consider wind heeling moments at different wind headings. This is done using the AXIS and WIND (and HMMT) commands, as discussed here and here. Wouldn't it be nice if we could plot the wind heeling moment for each AXIS on a polar plot? How might we accomplish that?

We write two macros, of course.

macro vary_axis
    set axis_angle={axis_angle} PLUS {axis_inc}
    axis {axis_angle}
    hmmt wind /band /quiet
    me {axis_angle} {hmmt}

macro polar_plot
    vari plot_title="Heeling Moment, Wind Speed: {wind:1} knots" me plotstart "{plot_title}" /polar:-1
    me plotstyle "L"
    me plotend

The first macro called vary_axis varies the AXIS angle and then computes the wind heeling moment for the new angle. The AXIS angle and HMMT are then sent to a custom polar plot using the ME command.

The second macro called polar_plot initializes a custom polar plot using ME PLOTSTART /POLAR:-1. The "-1" means the radial plot axis is rotated CCW by 90 degrees to match the convention used by the AXIS command. This macro also calls the vary_axis macro multiple times.

After reading the geometry file and setting a loading condition, all we have to do is set the wind speed using WIND and call the polar_plot macro:

vari axis_inc=5 `degrees
vari axis_angle=0 `degrees
vari n
set n=360 DIVIDE {axis_inc} PLUS 1

... 'read geometry and loading condition

wind (SP) 30
report /preview

The result is breathtaking:

If you're really excited about polar plots and rig stability, you'll probably want to try plotting the GM at each AXIS angle as well. An example run file that will plot wind force, wind heeling moment, heel angle, and/or GM can be found here. This run file includes essentially the same macros discussed above, but introduces a parameter that lets you select the type of information you want to plot.

Questions, comments, or requests?
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