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August 2014 Issue

The Sewol Korean Ferry Could Have Been Saved
Alexander Gorlov

The sinking of the Sewol South Korean ferry in April en route to Jeju Island from Incheon involved 476 people on board, mostly high-school students. About 300 passengers died in this tragic accident, which happened near Chindo Island. The incident was a national blow and resulted in South Korea's president disbanding the country's coast guard for what she considered its failure to respond effectively. The captain and three senior crew were charged with negligent homicide, and other crew were charged with negligent manslaughter.

It's possible that the ferry could have been saved, had the captain and crew been more attentive to what was happening to the boat in the ocean environment. I am personally familiar with the accident site, which has very strong ocean currents suitable for harnessing kinetic energy, specifically by hybrid (water/wind) helical turbines, which I helped to introduce to South Korea in 1999. This experience made me look closely at the potential effect of the lateral current pressure on the stability of the Sewol ferry when it entered that area. Instability occurred when the ferry approached this area, which happened several hours after the ship left the port of Incheon. Before that, it sailed without any warning signs of tilting, indicating that the ship had been balanced.

What could have happened to destabilize it? We can analyze the external forces that could have caused tilting in the worst-case scenario: Ps as the horizontal lateral current pressure on the underwater part of the ship, Pw as the horizontal wind pressure applied to the ship's superstructure, and W as the vertical cargo weight. We assume that the wind and the ocean current are in opposite directions. It is reasonable to assume that the center of tilt (CT) at the instant of catastrophe was at the intersection of the ship's vertical axis and the ocean surface, while the center of cargo gravity (CG) was located close to the ocean surface.

Lateral pressure to the underwater part of the ship from the current can be calculated as a drag force formula: Ps = 0.5CdqAV2, where Cd = 2.0 is the drag coefficient for a long rectangular vertical barrier, q is the saltwater density, A = 918 m2 is the frontal underwater area of the hull, and V = 2.2 m/s is the water velocity. The result is Ps = 453 metric tons, which should be one of the major force factors causing initial tilting.

The superstructure is loaded by pressure Pw from strong wind gusts, which we assume act opposite to the direction of the current. Pw is calculated by the drag formula. It appears that the resultant force of the wind gust at 40 km/h on the frontal wall of the superstructure might have been about 80 tons.

The forces from currents Ps and wind Pw develop an initial momentum at CT at the ocean surface: M = 1,800 to 1,900 ton-meters. This total momentum is strong enough to force the ship to tilt leftward as the first stage to capsizing. The tilting accelerates under pressure of new forces that come from the cargo, consisting of centrifugal force Fc and vertical downward weight of cargo W = 3,608 tons generated at CG, displaced by initial tilting.

Soon after entering the area of high lateral current velocities, the ship performed a sharp right turn, followed by a single loud 'bang,' which was supposedly caused by part of the cargo breaking loose from holding straps because of centrifugal force. Some people have attributed the entire capsizing to that sharp turn of the ship.

However, we are not sure that centrifugal force Fc = mu2R (mass-angular velocity squared-curve radius) is a substantial component of the twisting momentum M, which forces the vessel to tilt since Fc is perpendicular to the hull and is located at the same level as the CG near the ocean surface. Fc itself cannot develop substantial leverage as an addition to M. Also, the mass of the displaced cargo remains unknown, so we cannot calculate centrifugal force.

The water ballast at the bottom of the ship was partially discharged before starting the voyage to allow for more cargo loading upstairs. This inevitably raised the ship's center of gravity, partially reducing its stability.

The ship, moving at 21.5 knots, needed just a few minutes to enter the zone of high lateral current, creating a barrier for water flow. Thus, we must include the added mass of water volume displaced by the vessel, 22 x 6.26 x 146.31 m3, into the calculation of lateral resultant force on the underwater part of the vessel.

Calculation of the added mass means introducing the shock impulse G = m(dv/dt), or roughly mV/t, where V is the resultant vector velocities of both 2.2 m/s (current) and 11 m/s (vessel).

Considering the shock impulse, the vessel should have been stopped immediately, eliminating the major part of the shock impulse G, namely, the ship velocity V, at the first signs of tilting. Unfortunately, nobody on board knew what was going on. In their confusion, neither captain nor crew could prevent capsizing, except possibly turning the vessel left towards the current. The initial tilting was mostly caused by the underwater lateral current pressure, likely triggering cargo displacement, shifting weight far to the left.

The new location of the displaced cargo's CG accelerated the tilting by a growing momentum from the weight around the CT. The shock impulse completed the capsizing.

The shock impulse can be estimated only qualitatively since the displaced cargo's mass or its CG cannot be quantified. Nevertheless, the drag pull calculated at 453 metric tons was enough for initial tilting of the overloaded ship.

The captain could've restored the ship's balance by very sharply turning left when the ship began tilting, instead of turning right as he did, following the bicycle rule of turning in the direction of falling. The shock impulse and rocking momentum from the displaced cargo made capsizing inevitable without appropriate action by the captain.

Alexander Gorlov is professor emeritus at Northeastern University, Boston, and former vice president and chief technology officer of GCK Technology, Inc. He invented the helical turbine, used for tidal and wind energy, that won him the Thomas Edison Patent Award of the American Society of Mechanical Engineers, as well as honorary citizenship of Chindo County, South Korea.


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