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January 1, 2005 -- In the week since the Indian Ocean tsunami crashed ashore wiping out villages and claiming more than 100,000 lives, a number of cruisers have wondered what would happen if a tsunami racing across the ocean were to strike a cruise ship.

The short answer is that a tsunami probably does not pose much of a risk to a cruise ship, and might even go unnoticed as it raced past a cruise ship in the open ocean.

The explanation lies in the fact that a tsunami does not race across the ocean as a wall of water, but as a shock wave traveling at speeds of up to 500 miles an hour beneath the sea. It is only when it gets to shallow water that is compressed into a huge wave, like the ones that battered coastal villeges in Sri Lanka, Indonesia, Thailand and India.

Even ships docked in a port hit by a tsunami probably would only be knocked against the pier, and suffer only a limited amount of damage.

The biggest risk would probably be if a tsunami were to hit at exactly the moment a cruise ship were entering or leaving a port in a narrow channel through otherwise shallow coastal waters.

Despite the current focus on tsunamis, the greater danger to cruise ships probably lies in rogue waves like the 90-foot wave that struck the Queen Elizabeth II during an Atlantic crossing in September 1995.

Captain Warwick later reported that when the wave was sighted directly ahead looming out of the darkness, "it looked as though the ship was heading straight for the white cliffs of Dover.

"The wave seemed to take ages to arrive but it was probably less than a minute before it broke with tremendous force over the bow. An incredible shudder went through the ship, followed a few minutes later by two smaller shudders," he reported in the Marine Observer.

"There seemed to be two waves in succession as the ship fell into the 'hole' behind the first one. The second wave of (approximately 90 feet), whilst breaking, crashed over the foredeck, carrying away the forward whistle mast."

Because the Queen Elizabeth II took the wave virtually head on, it survived the onslaught with minor damage and no passengers or crew members were injured. But were one of these freak waves to strike a smaller cruise liner broadside, the outcome might be more worrisome.