It is not always easy to decipher the news being reported on the tragic sinking of the Korean ferry Sewol , which capsized in the Yellow Sea on April 16, leaving more than 300 dead or missing.  There have been numerous reports that the ferry was overloaded when she sank, yet by all indications, the likely problem was that the ship had inadequate stability. The ferry sank while under the command of a relief captain.  Last week the captain who was regularly in command of the ferry told Korean investigators that he had warned the ferry’s owners of “serious stability problems,” but that his warnings had been ignored.

How was the captain able to judge the stability of the ferry?  It is actually reasonably easy to estimate the initial stability of a ship by its roll period. There is a even a free iPhone app to do just that. (The app uses the phone’s accelerometer and gyroscope.) You don’t need an iPhone, however.  Any watch will do. I have seen a ship’s captain look at his watch as his ship neared the sea buoy, counting the average number of seconds the ship took to roll.  The captain knew reasonably accurately how much stability his ship had based on the roll period.  If the ship was not stable enough, he could ballast, or in the most extreme case bring the ship back to port for restowage.  The longer rolling period, the lower the ship’s initial stability.  The shorter the rolling period, the higher the initial stability.

At this point we should look at how the ship’s officers determine initial stability. My apologies for getting wonky, but please bear with me. The calculation of the initial stability of a ferry or any other ship is relatively straightforward. The center of gravity of the ship, the cargo, the passengers, fuel and ballast are all used to calculate the final vertical center of gravity of the ship. The center of gravity (G) is them compared to the ship’s “metacenter” (M),  which is the point about a ship rolls at small angles.  The distance between the metacenter, M, and the center of gravity, G, and is referred to as the GM.  As long as G is below M, the ship is stable.  Each ferry will have a required minimum GM, set by IMO or local regulations.  If the GM when the ship is loaded is inadequate, then either more ballast should be taken on or cargo removed to bring the center of gravity down to meet the requirements.

For passenger ferries this is a delicate balance. They are required to meet minimum stability requirements, because too little stability, as measured by GM, increases the chance of capsize.  On the other hand, more stability means that the ship will roll faster, which can be uncomfortable for the passengers.

In the case of the Sewol, those loading the ship appeared not be paying much attention to the required stability regulations.  Doing so probably allowed them to carry more cargo and less salt water ballast than they would have if they met the regulations. Paradoxically, until everything went wrong, the lower stability may have contributed to a more comfortable ride for their passengers.  Tragically, when the worst did happen, a large roll appears to have shifted the cargo and the ferry ultimately capsized.  The ongoing investigation will, with luck, determine precisely why and how the ferry capsized. Thus far, however, it appears that poor cargo stowage and lack of attention to the basic operational rules cost the more than 300 passengers and crew their lives.