Game Development Reference
In-Depth Information
If you're a sailor, then you know how important it is to keep the center of gravity of your
boat low. This helps increase the height of the metacenter above the center of gravity,
and thus helps with stability.
In the case of fully submerged objects, like submarines, the situation is different. The
buoyant force still acts through the geometric centroid of the object, but for stability,
the center of buoyancy must be located above the center of gravity. This way, when the
object rotates, the lines of action of the weight of the object and the buoyant force are
separated and form a moment that tends to restore the object to its upright position. If
it's the other way around, then the object would be unstable, like trying to balance one
bowling ball on top of another. In this case, the slightest disturbance would upset the
balance and the object would flip upside-down such that the center of gravity is located
below the center of buoyancy.
In general, boats protect their stability by compartmentalizing the hull into several wa‐
tertight sections, fittingly called compartments . This way, if the side of a vessel hits an
iceberg, only the compartment damaged will flood with seawater. If enough compart‐
ments are damaged, the vessel will not have enough buoyancy to support its weight and
it will sink. The end with the flooded compartments will sink first, causing a large angle
about the transverse axis. This is what happened to the Titanic . In fact, in that ship's
case, the angle, called trim , was so large that the stern was lifted out of the water. The
hull could not support the weight of the stern section that was no longer being supported
by buoyancy, and the structure ripped in two.
It should be noted that ships can sink in the matter of minutes, or it can take hours. For
instance, the Titanic took about three hours to sink. The Lusitania sank in 18 minutes.
The time it takes depends heavily on the type of damage and the construction of the
vessel. We don't suggest trying to get players to wait three hours for their game to end;
however, it is possible to continue fighting/propelling a vessel that is terminally dam‐
aged. In many cases where terminal damage is suspected, captains endeavor to ground
their vessels to prevent the ship from actually going under.
If side damage occurs, especially in high wind and waves, then it could be that the vessel
can still have enough buoyancy to float, but no longer enough stability to remain upright.
As damage usually occurs only on one side of a vessel, the center of buoyancy will no
longer be on centerline. This means that the restoring moment in one direction is di‐
minished by whatever amount the center has moved to that side. A big wave comes
along and pushes the vessel over to the point where the righting arm is no longer positive.
The vessel will flip 180 degrees with the bottom pointed skyward but will still float
(capsizing). Once rolled over, the remaining compartments will tend to fill with water
as vents or other openings fail over time. In the case of recreational boats, they are usually
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