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

Check tank volume with LS
(Requires GHS with Longitudinal Strength)

GHS calculations rely on the geometry model being an accurate representation of the vessel. In order for a given volume to act as a tank and also provide buoyancy, it must be referenced by two different parts: a displacer and a container. Displacer class parts float but cannot be loaded or flooded. Container class parts can be filled with any contents you like, but don't contribute to buoyancy.

One way to check a GHS vessel model is to compare the total tank volume against the total displacement volume using COMPONENT /VOLUME. If the model has all the internal volume modeled as tank parts without any overlaps, then the tank volume should equal the displacer volume. You can target the COMP /VOL report to a particular region of the vessel by first using CHOP in Part Maker to prepare a temporary geometry file.

However, what if I told you there was a way to find the sectional area curves of all the displacers and containers in the model? The Longitudinal Strength (LS) module needs to prepare a weight curve and a buoyancy curve in order to determine the load curve by subtracting the two. By setting up a very specific condition and removing the portions of the report that aren't needed, we can use LS to tabulate and plot the density of the vessel displacement and the tank weight at each station. Pretty nifty, huh? Here's how:

PERM (*) 1
LOAD (*) 1
DEPTH 25         `deep enough to entirely submerge the vessel
ADD DELTA 0 0 0  `so weight equals buoyancy

In order to report the full hull volume, the entire vessel must be submerged. Likewise, to equate the weight curve to the tank section area, we must fill all tanks with fresh water and set their permeabilities to 1.

At this point, it is very likely that the weight will not equal the buoyancy. We don't want the hull to float up to the surface (or sink) so we need to use LS /NOSOLVE. But when solving is skipped, LS automatically scales the buoyancy curve to match the total weight. ADD DELTA creates a point weight sized to bring about weight/displacement equilibrium thus eliminating any scaling.

The result below shows two curves revealing how internal tank volume and total displaced volume vary longitudinally in the model.

The blue line is the hull displacement density, and the red line is tank weight density; both are proportional to sectional area. This graph makes it easy to identify regions where tank volume is either missing or duplicated!

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