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

(Requires GHS version 16.38 or later with SK)

Owner and regulatory requirements often guide stability and strength evaluations in GHS. The long-standing LIMIT and LSLIMIT commands are well known among GHS users for this purpose. These commands see a lot of use partly because of their generalized syntax. Yes, those complicated limits can still be tricky, but once you are familiar with the methods it is possible to write custom limit statements for just about any requirement imaginable.

It is natural that our users expect equivalent functionality for their dynamic analyses. The new SEA LIMIT command provides this capability. The syntax is also rather simple, but the power of this command is none-the-less appreciable. SEA LIMIT gives users the option to write limits for any absolute or derived SEAKEEPING response, so it is possible to apply limits to any mode, any response, or any derived response (such as MSI). Because SEAKEEPING is very effective at running many, many cases, SEA LIMIT makes it easy to evaluate dynamic limits on each case during the run.

Many dynamic limits are given as Root-Mean-Square limit values of a particular response. For example, the vertical acceleration and roll angle amplitudes are often given specific design or operability values. In this example, let us assume the RMS vertical acceleration must be less than 0.2g and the RMS roll angle must not exceed 4.0 degrees. Working with these example requirements, we can write SEA LIMIT statements for each criterion as shown:


When the SEAKEEPING command is subsequently issued, all existing SEA LIMITs will be automatically evaluated. By default, the limits are assessed independently at each valid Critical Point for each case. After all cases are evaluated, limit plots are automatically appended to the open report file. This makes it easy to evaluate only specific points on a vessel against a certain limit. However, it is also possible to combine limits to create additional composite limit statements:


Limit 3 will then return an additional limit plot, but the limit is now the composite of limits 1 and 2. One can always view all current SEA LIMITs by typing SEA LIMIT without any additional parameters.

To finish this hypothetical example, let us assume port-starboard symmetry and a speed range of 0 to 12 knots. We will be working with a most probable Ochi-Hubble spectrum with a significant wave height of 3.2 feet. To simplify things, we will only evaluate the limits at a single Critical Point located on the main deck forward of midship.

CRTPT (1) "Limit Point" 56.7f,0,9.4

WAVE (SPE) O1 3.2

SEA /HEAD:0,15,...,180 /SPEED:0,2,...,12 /CRT:1

The result of this admittedly simplified example is a case limit plot that shows which cases pass or fail each limit. If a case fails the limit, the case will be indicated by an unfilled red point. If the case passes the limit, it will be indicated by a filled green point. Here we only show the combined limit 3:

Because limits 1 and 2 are still evaluated individually, it is easy to check to be sure the combined result is correct. This can also help determine which responses or modes govern at each case.

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