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


Because GHS models allow one component to deduct volume from another, it is possible to have the deducting volume slightly exceed the positive volume in some region of the part.

This happened when the tank was almost empty and its "waterplane" passed through a deduction. The net volume should have been zero, but the deducting volume was slightly greater than the positive volume, causing the net volume to come out negative.

The top of the thruster tunnel forms part of the bottom of this tank. When the geometry was built, the thruster was fitted as a deduction from the overall shape. Normally when this is done the net volume below the top of the thruster would be zero, but it can happen, as it did here, that the permeability of the deduction was set incorrectly, leaving a slightly negative volume.

At this point the PARTS /CHECK command can be issued. It will check for "negative waterplane" at the current heel and trim angles and report its findings. This information will help identify the place where a correction is needed in the geometry.

After a geometry model is built, it is not a bad idea to check all parts in this manner:

PARTS *  `← selects all parts in the model

This does not guarantee that at all angles of attitude a negative waterplane will never occur. These errors are best avoided by careful model building and inspecting the model using DISPLAY or SECTION EDITOR, where the new Visualize command can be used to look at a tank from all sides. Any deduction that extends outside of the overall envelope will be obvious.

Also make sure that the permeability of deductions is not greater than the permeability of the volume being deducted from. For example, if a tank's permeability is 0.95, deductions should also have 0.95 in their permeability or "effectiveness" settings. This is the default in Part Maker. It must be admitted that it took an extra PERMEABILITY statement in the Part Maker CREATE command to force the error for this demonstration.

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