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


PRINT PREVIEW is often used to examine a report before the run file has been fully developed. "It would be nice," someone said, "if I could refer to the preview while editing my run file."

Thus /SPAWN became an optional parameter of the PRINT PREVIEW command. It causes the preview window to stay open for as long as you need it.

If you're familiar with the MAXVCG command, you will notice that the stability LIMITs appeared seemingly out of nowhere.

That STAB EN1 command made the limits for the first of the energy criteria. It did nothing else because no weight had been assigned to the ship; it came back with a nonfatal error message saying "Not enough weight."

This is exactly what we wanted; we wanted only the LIMITs, not the righting-arm curve.

STAB EN1 was placed before the report was opened so the error message would not come out in the report -- to keep the report preview clean for this demonstration. But if you did not care about that, you could issue the STABILITY command again for another criterion without having to close the report.

Yes, this is a shameless plug for STABILITY (which requires the Advanced Features module).

Sharp eyes will notice that no zero margin was met for the highest displacement. Evidently GHS found it difficult to evaluate Limit 5, which must find the angle at the peak of the righting-arm curve. This calls for an investigation.

Below is a plot of the max VCG curves.

The curves show the reduced stability at the higher drafts. The last one, at 5 meters, leaves little freeboard. The straight downward trend suggests that the slightly conservative margin on Limit 5 did not affect the VCG significantly.

In order to find out why Limit 5 was not being nailed down, we set up a righting-arm curve for the first trim at the 5-meter draft:

trim -1
draft 5 @ lcf
solve weight
vcg 4.644
solve lcg
ra /lim

It can be seen that this condition matches: the margins are the same as in the MAXVCG report. Note that the curve does not have a definite angle of maximum.

If you like PRINT PREVIEW /SPAWN, you might want to assign it to a function or control key. For example,
v(st) s="print preview /spawn"
key 3, "{s}|"

This both brings up the preview in its independent window and defines the function key (in case you want to repeat the preview after having closed it too soon).

Questions, comments, or requests?
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