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


We heard from John Bonn about the discrepancy in last week's COW. He pointed out that while the narrative mentioned a "righting-arm curve," the plot labeled the curve "Righting Moment."

Of course he knew that he could get one from the other since the arm is the moment divided by the displacement weight. But he wondered why the plot showed moments when all the criteria are based on arms.

The reason is that the displacement weight changed when the weight of the bilge water was lost as the cockpit flooded. There is no indication of displacement weight on the plot page, so for the sake of exactness GHS automatically switches to plotting moments if the displacement does not hold steady for the range of angles examined.

Here is a water-on-deck example that shows the weight change more dramatically:

The arm is a convenient representative of moment normalized for vessel size; the moment is the physical reality. Limits that are based on righting arms still work when the displacement changes. (If the arm is good for one displacement, we can assume it is good for another for the purpose of evaluating stability.)

The spilling of the deck water illustrates another feature of the DAMAGED tank type that was not evident in the cockpit example of last week: the point of "damage" lets water out if necessary in order to balance the pressure at that point.

The TYPE DAMAGED tank adds its contents weight to the weight of the vessel while the reference point is above the waterplane; then when the reference point becomes submerged, the tank is open to the sea and it becomes lost buoyancy. Note that for this to work properly the tank must be within a buoyant part of the model. (By the way, the density of the tank's contents need not match the sea water, but that gets into other applications which we will leave for another week and another COW (don't anyone write in and suggest that we use milk in an example)).

The coaming height in this example was exaggerated, which reminds us that some types of vessels can retain a lot of water on deck temporarily and the water-on-deck criteria can become much more complex. GHS offers two additional tank types to address these cases: TYPE DECK and TYPE FLOODED PLUS.

More information on treating water on deck can be found at

The run file that both created the geometry and produced the output shown above is here.

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