51,000 tons above the sea: find out how the world's largest cruise ship stays afloat.

Imagine a city where you can order pizza day or night, bustle through a shopping mall, and squeal with glee while plunging down a 60-meter 200-foot) water slide.

Now picture this city floating on the ocean and you've got the Carnival Destiny - the world's largest cruise ship. This latest record-breaker set sail on its maiden voyage last month.

Nearly three football fields long, the Carnival Destiny weighs in at a whopping 51,000 tons. But what's really significant is the ship's volume. It's the first passenger ship to top the 100,000-gross-registered-ton[*] (10,000,000-cubic-foot) mark for capacity. That's the figure that earned the ship its record. About 375,000 household refrigerators could fit inside.

For years, cruise-ship companies have been trying to outdo each other, building ever-larger ships. Before Carnival Destiny, the 77,000-gross-registered-ton (7,700,000-cu.-ft.) Sun Princess, completed in 1995, held the record. And there's no sign that shipbuilders will put on the brakes in the size race any time soon. In two years, World City America plans to launch a megaship more than twice the size of Carnival Destiny.

How do these gargantuan ships stay afloat?

SOMETHING IN THE WATER

Believe it or not, the same forces that keep a toy boat afloat are at work when a cruise ship sets sail.

When you lower a ship into the ocean, the ship displaces, or pushes aside, a certain volume of water, says Lt. James Houston, a naval engineer and assistant professor at the University of California at Berkeley. The water, in turn, pushes back on the ship with an amount of force equal to the weight of the displaced water. Scientists call this push the buoyant force.

As long as this buoyant force is equal to or greater than the weight of the ship, it will counteract the downward push of the ship's weight and keep the vessel afloat. That means the heavier a ship gets, the more water it must displace to stay afloat.

But sometimes a heavy object won't displace enough water because of its shape. If you melted the Carnival Destiny and molded it into a 51,000-ton solid-steel cube, it would sink. Why? Because the volume of water displaced by the cube would weigh far less than the cube of steel. The water's upward push would not be enough to hold up the cube.

But if you spread out the same 51,000 tons of steel to form a ship shape, the steel would push away a lot more water. The buoyant force created by this displaced water would be more than enough to support the weight of the ship.

In fact, says Koichi Masubuchi, a professor of oceanic engineering at the Massachusetts Institute of Technology, there is no real limit to how big a ship can be - as long as it fits through the waterways it sails or the harbors it enters. The Carnival Destiny, for example, is too large to fit through the Panama Canal.

Of course, the larger a ship gets, the more passengers and cargo it can carry. Does this additional weight put the ship in danger of sinking?

Not really. The extra weight pushes the ship deeper into the water, which displaces more water and increases the buoyant force.

But say the ship settles so deep that water starts to flow over the sides and into the ship. In that case, the ship would no longer displace enough water to support its weight. Glub, glub, glub....

WATERED DOWN

Most of the time, extra weight actually helps a ship remain stable. The weight gives the ship a lower center of gravity - the point around which the weight of an object is concentrated. That means the pull of gravity is greater near the base than the top, so the ship is less likely to topple over.

To stabilize a ship, shipbuilders place the vessel's heaviest machinery and equipment, like the engines and fuel, at the lowest level, below the waterline. As the ship's load lightens - from using up fuel, for instance - the ship takes in ocean water for ballast, or added stability.

But the water needs to be in the right place on the ship. Too much water in the wrong place will make a ship sink," explains Capt. Tom Thomason of Carnival Cruise Iines.

Take the case of the Normandie, an ocean liner built in the early 1900s. While the ship was docked in New York Harbor in 1942, a fire broke out Firefighters sprayed so much water on the ship's top deck that the ship became top-heavy. The Normandie rolled over and sank, right at the dock.

Even water entering a ship's lowest level can be disastrous - if the water fills up only one end of the ship. That's what sank the Titanic in 1912 (see box, p. 13).

SMOOTH SAILING

But don't let this talk about sinking ships get you down. Shipbuilders take many precautions to prevent disasters.

A passenger ship usually has a double bottom - a second steel "skin" that provides extra protection in case a rock or iceberg punctures the outer hull (see miniposter, p. 12). If water does get through a hole in the ship, the crew can seal off watertight compartments to isolate the flooded compartment. On the Carnival Destiny, two out of 18 compartments can flood without sinking the ship.

As a final resort, passengers and crew can board lifeboats and float to safety. The key, of course, is to have enough lifeboats for everyone. The Titanic didn't. Carnival Destiny and all modern cruise ships do.

So if you ever get a chance to sail on the world's largest cruis ship, be assured that laws of physics and shipbuilders' experience will keep you and the ship safe. Then, sail away for fun m the sun.

WATER PUSH-UPS How does shape affect an object's ability to float? To find out, try this experiment.

WHAT YOU NEED:

fist-size lump of plasticine clay * clear plastic container half-filled with water * marker * metric ruler * stack of pennies * paper towel

WHAT TO DO:

1. Mark the water level on the side of the tub. Measure the distance from the bottom of the tub to the top of the water.

2. Roll the clay into a ball and place it in the water. Does the clay float or sink? What happens to the water level? Measure the water level again. Record your observations.

3. Take the clay ball out and dry it. Think: What shape would displace more water? Mold the clay into a different shape and repeat the float/sink and water-level tests.

4. Repeat Step 3 until you find a shape that floats.

CONCLUSION:

Which shapes sink? Which float? Do you notice a relationship between how much water an object displaces and whether or not it will float? Explain.

DON'T STOP NOW!

What would happen if you added weight to your clay shape? Add pennies one at a time. How many pennies can the shape hold before it sinks? Does it make a difference where you place the pennies in your clay shape? How can you arrange the pennies so the shape holds more? Experiment to find out!