veryone knows the story of a ship captain's worst nightmare: the "unsinkable" Titanic was sent to the bottom by a collision with a massive iceberg. But most ships lost at sea -- other than those involved in collisions with other ships -- fall prey to the more ordinary but equally savage forces of wind, waves, and water. Although rescue at sea is possible up to a point, the best hope of a ship in peril is its built-in capacity to ride out a vicious storm. It is then that man and metal are tested to their limits, with the lives of the crew and their ship hanging in the balance.
Fundamentally, what holds a ship up in good or bad weather is its buoyancy. A ship floats because the volume of water that it displaces equals or is greater than its weight. Or, in other words, the "hole" a ship makes in the water occupies a certain volume, and if the ship weighs less than the water it would take to fill the hole, it will float. Similarly, a hot-air balloon floats because it weighs less than the volume of air it takes up. When water seeps or floods into a ship's interior, the vessel is slowly robbed of buoyancy and, eventually, it will be dragged under by its own weight unless the leak can be plugged.
Ship designers build different amounts of buoyancy into a ship, depending on its expected operating conditions. The buoyancy of a ship can be judged in terms of its "freeboard," which is essentially the distance the ship rises above the waterline. The more buoyancy a ship has in proportion to its weight, the greater its freeboard and the higher it floats in the water. Most important, freeboard enables a ship to recover after being slammed by a monster wave. "You could call it reserve buoyancy," says William Garzke, a naval architect with Gibbs & Cox, Inc., of Arlington, Virginia. "In rough conditions, the more freeboard you have, the better off you're going to be."
|Seafaring vessels must be built to withstand severe conditions in the water.
Garzke has designed ships for the Navy for 40 years. Currently, he chairs a panel of experts affiliated with the Society of Naval Architects and Marine Engineers. They meet periodically to study the causes of shipwrecks and try to draw lessons from them useful to ship designers and operators. These "forensic naval architects" are like medical examiners, except they conduct their autopsies on lost ships instead of people.
The panel's investigations of various shipwrecks show that just making a ship that can float is not nearly enough to protect it from disaster. A ship must also be stable. The savage seas exert powerful forces on a vessel, especially in rough weather. A seaworthy ship must be able to withstand those forces without capsizing or thrashing so violently in a storm that a captain loses control of the vessel.
Simply put, well-designed, stable ships stay upright in rough seas. In a storm, ships roll from side to side and pitch up and down like a see-saw. In a stable ship, the portion of the hull below the waterline, like a weighted pendulum, keeps the keel down and the deck up. In fact, the keel fins of sailboats are outfitted with heavy weights to counteract the force of the wind pushing sideways on the sails.
But if this balancing act is compromised, ships can be lost. One dramatic example of this is the spectacular capsizing and sinking of the Italian passenger liner Andrea Doria.
To keep its massive hull in balance, the crew of the Andrea Doria had to pump seawater into its fuel tanks as the fuel was burned up on trans-Atlantic voyages. Approaching the coast of the United States off Nantucket on July 25, 1956, the Andrea Doria's tanks were full of this "dirty ballast." But rather than pay a premium to have it pumped into barges at the dockside in New York Harbor, the crew decided to pump the ballast out into the open ocean before they reached port.
This proved to be a fatal decision. The Andrea Doria collided with a smaller cruise ship, the Stockholm, and began to take on water on her starboard side. This made the ship list, and, lacking proper ballast, this normally survivable tilt turned deadly. The ship capsized and sank, although all but 54 of her more than 1,600 passengers and crew were rescued.
But many sinkings are not quite so dramatic as the Andrea Doria's. Most often, ships sink because they spring a leak in rough seas. A 30-foot wave moving at 30 miles per hour exerts a force of one million foot-pounds. That's often enough to tear cargo hatches open and allow water to pour in.
|Massive waves battered the forward hatch cover of the Derbyshire.
Such was apparently the fate of the MV Derbyshire, a 200,000-ton ore carrier that went down with all hands in 1980 in a Pacific typhoon. Cruising 400 miles off the coast of Okinawa, the ship was battered by waves up to 20 to 30 meters high. "Now, those are big waves," Garzke says. "When they break over the ship, they hit you hard." The ship took on some water in the forward hold, which nosed it down into the water. According to the official explanation, the waves pounded through the hatch covers in the forward cargo hold, causing the ship to nose down into the water. Further pounding caused more hatch covers to fail. The ship flooded and sank with 44 people aboard.
In designing their vessels, shipbuilders are likely to keep the following in mind:
aval architects have some tricks up their sleeves to make a vessel more shipwreck-proof. One of the best methods is to build watertight compartments into the hull. The ship is divided into a series of compartments sealed from each other by steel walls, or bulkheads. Some ships have watertight compartments that run the length of the ship. If the hull is breached, or a hatch breaks free, the flooding can be contained to a relatively small area.
nother way to shipwreck-proof a vessel is to give it a "double hull." These ships have an inner hull and an outer hull, with hollow space between. This guards against long tears in the side of a hull like the kind inflicted on the hull of the Titanic by an iceberg. One of the Titanic's sister ships, the Britannic, was retrofitted with a double hull to protect it from just such "raking" damage. But ironically, this may have actually contributed to the surprisingly fast sinking of this ship in 1916 in the Aegean Sea. (See sidebar.)
owadays, engineers can choose from a larger palette of shipwreck-proofing features. A small number of passenger ships have been outfitted with large, spinning flywheels that counteract some of the rolling and pitching of a ship in heavy seas. Some ships have computer-controlled fins protruding from the lower hull that nudge the hull and correct for wave motion. Some are equipped with "flume tanks," chambers filled with a sloshing fluid. The size and shape of the tanks are "tuned" to counter ship motions. When the hull rolls to the left, the fluid in the flume tanks sloshes to the right, and vice versa. This helps steady the ship, somewhat like the metal bar that tightrope walkers grip to keep their balance.
The Human Factor
o doubt shipbuilding has advanced greatly since the first trans-Atlantic mariners ventured onto the savage seas. Ship-to-shore radios allow captains to call for help when they need it, and satellites circle overhead and relay weather data to the ground that allows ships to navigate around instead of through dangerous storms.
But even with modern technology, one thing has stayed constant: Good seamanship is what ultimately saves most ships. "Let's just be honest about this," Garzke says. "There is no such thing as an absolute safety factor on any ship we design." Overloading a fishing boat and steaming into heavy seas, for instance, can mean certain death. "In other words," Garzke says, "you can't idiot-proof a ship."
-- By Daniel Pendick