The task of searching for people who fall overboard and, hopefully, rescuing them, falls to the U.S. Coast Guard. In 2009 alone, they have been involved in seven overboards – what they call Person in Water – from cruise ships.

One PIW was found alive clinging to a buoy. One body was found in Alaska. And in the four other cases, no bodies were ever recovered and exhaustive searches were called off, explained Commander Erin MacDonald, with the Office of Search and Rescue at Coast Guard headquarters in Washington, D.C.

"In many cases, we don't find a body," she said. Surviving a fall from a cruise ship, she said, unfortunately, is the exception to the rule.

First, there is the fall. So few bodies have been recovered from cruise ship falls over the years, no one really knows how many survive the initial fall. But, MacDonald said, the data they have from bridge jumpers has shown that the fatality rate is likely to be high.

People who jump from the 245-foot Golden Gate in San Francisco die 98 percent of the time. And even at lower heights, the Coast Guard has estimated that people hit the water at close to 75 mph. At that speed, hitting the water is much like slamming solid concrete.

THE SEARCH

Then, there is the difficulty of actually finding someone in the vastness of the ocean. The likelihood of the Coast Guard pinpointing the location and either rescuing the person in water, or recovering the body depends on a host of factors, she explained, chiefly, how much information they have from the cruise line; how quickly after the incident the Coast Guard is able to get to the scene; and how accurate the information is about where the person went into the water.

To help them, the Coast Guard uses a sophisticated computer software program called Search and Rescue Optimal Planning System, or SAROPS. They feed in information like sea water temperature, wind and water currents, the ship's location and its track, the likely time of the overboard to come up with an area to search.

Then, the ships, planes, helicopters or other resources begin to search in grid patterns, using the human eye, binoculars or night vision goggles, MacDonald said.

"The more information we have, the better. It makes the search planning more accurate. It doesn't mean we'll find them, but the search will be more accurate," she said. "If we don't know the time or the location of the fall, you're dealing with a much bigger piece of ocean. It becomes more challenging. And in many cases, the person was last seen at dinner and then discovered missing in the morning."

SURVIVAL

Beyond the fall and the difficulty of actually locating someone, there is the issue of surviving in the water itself. To aid them in the search, the Coast Guard uses another computer program, the Coldwater Exposure Survival Model, or CESM. They calculate the person's age, gender, height, weight, body characteristics, water temperature and factors such as whether the person has a life vest, whether they are likely to be clinging to debris or are fully immersed, whether they were intoxicated when they went overboard and what kind of clothes were they wearing?

"The model gives you two times," MacDonald explained, "Functional Time, which is an estimate of how long a person can help themselves, or stay above water until help arrives. And Survival Time, meaning, how long they'll survive and not succumb to the conditions. Someone may no longer be functioning, but still alive and able to be rescued."

For instance, the Coast Guard used these hypothetical variables: a 29-year-old male, 185 pounds, 71 inches tall with 15 percent body fat, which is fairly average, who is not fatigued, wearing a heavy sweater, immersed in water up to his neck and soaked, floating in 55 degree water in light seas, dry relative humidity and calm wind. The man's functional time is about 4.4 hours, according to the CESM computer program. The survival time would be about 7.2 hours.

The Coast Guard uses the program as a guide, not as an absolute, MacDonald said, in determining when to call off a search.

"There are some things we can't calculate," she said. "Like someone's willingness to survive."