A Contoured Car Deck could cut the risk of a "roll-on roll-off" ferry capsizing, according to researchers at the University of Strathclyde. Their new design has just won a Royal Institution of Naval Architects competition on making ships safer.
Ro-Ro ferries are very successful commercially and there are now about 5000 of them worldwide. Most have an unobstructed car deck running the length and breadth of the ship not far above the water line. If water gets onto the deck, the ship can quickly become unstable and flip over. In 1987, off the Belgian port of Zeebrugge, water began flooding through open bow doors on the British car ferry Herald of Free Enterprise. It turned over 90 seconds later, and 193 people died.
Despite the introduction of tougher safety standards, the Baltic ferry Estonia capsized in 1994 after water reached the car deck during a storm, killing nearly 900 people. Nations bordering the Baltic, the North Sea and the English Channel then introduced still stricter stability standards for ferries using their ports. The key requirement is that by 2002 all ferries will have to be able to survive with up to 0.5 metres of water on the car deck.
Two designs have been put forward for improving a ship's stability while keeping the car deck free of obstructions. Both involve curved decks. In the first design, the deck is 1 metre higher at the bow and stern than in the middle of the ship. Its cross section is convex, with the centre about 20 centimetres higher than the sides. If the vessel is holed close to the water line, there could be two alternative scenarios. One is that the ship would heel towards the damage. In this case the curve of the deck lets water collect only on the side of the hole, preventing more entering.
Alternatively, the ship might heel away from the damage, in which case the water will maintain the list and keep the hole above the waterline. The researchers calculate that, if holed amidships, a ferry with a contoured deck could survive 4-metre-high waves, compared with 2.5 metres for a typical flat-decked ferry.
The second design has a deck that is higher in the middle than at the bow or stern, again with a convex cross section. Any water that enters the deck collects above pressure-operated valves around the edges that dump it back into the sea.
It is expected that the curved decks would cost no more to build than a conventional type, and that their curvature would be imperceptible. (New Scientist, 22 May 1999).
More information: Ship Stability Research Centre - University of Strathclyde