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

MARGIN (part 1)

In many cases, it is desireable to mark the deck edge of your Geometry File. Marking the deck edge is a necessary operation when computing freeboard, which shows up in many operations. Some examples include LIMIT commands and computing floodable lengths via FL. Marking the deck edge is typically easy and may be accomplished in several ways:

You may choose to use Model Converter:

Or, you may use Part Maker:

Or, you may add the deck edge in Section Editor by using the keyboard shortcut "CTRL-D". (The deck edge may also be removed using "CTRL-Z".)

Notice that the marked deck edge is displayed as a brown line in Section Editor. If the changes are written to the Geometry File, the deck edge will be indicated in the file by a "DK" code at the corresponding offset.

There are benefits to each of the above methods. Using Section Editor is quick, but it cannot be added to run files. Model Converter and Part Maker also offer additional options when it comes to setting the deck edge. In both cases, it is possible to set an additional MARGIN. Setting MARGIN defines an allowable freeboard margin relative to the deck edge. This is often referred to as a margin "line", but in GHS, the margin is not a fixed line, but truly a value used to reduce the freeboard.

In Part Maker, you may add a MARGIN, and set the deck edge at the same time, using:

   MARGIN = 0.25,0.5,0.25
This command applies to a single component, which in this case is specified as "HULL\HULL.C". In this example, we've specified three MARGIN parameters, which means the margin will be set relative to the deck edge using a parabolic curve. If only two parameters were used, the margin would vary linearly between the two values. And if only one value were given, the margin would be constant relative to the deck edge over the length of the component.

Using Model Converter to set the same margin:

Note the syntactical differences between the two methods. Whichever your method of choice, you can verify MARGIN at each station by using:

Notice that both the freeboard to the deck edge and required freeboard (defined by MARGIN) are given. You will also notice the parabolic variation in the margin values. If you are only interested in the minimum freeboard value for the current condition, you may leave off the ALL subparameter.

Questions, comments, or requests?
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