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THE LAST WORD Sucked under During coverage in January of the Costa Concordia cruise liner disaster off the coast of Italy, I heard some survivors voice concerns about being “sucked under” if the boat sank. In what conditions would this be likely? How long does the downward force last, and would wearing a life jacket help? n There has been much amateurish debunking and misunderstanding of this phenomenon. First-hand evidence from people who have been sucked down is hard to come by, because few survive, but any survivors’ accounts make sense in light of the discussion below. To understand the process, put small, slightly buoyant objects on large weights, let them sink through fluids and observe their behaviour. Start with a pillow or a slab of wood held in the air (air is, of course, a fluid). Scatter slips of paper on top. Most swirl away as the pillow or wood falls, but one or two in the middle will fall with the weighty object. You can see similar effects with a brick covered in twigs as it sinks in clear water, or try it in slow motion by dropping large ball bearings in a jar of clear detergent containing a scattering of small bubbles. Objects slightly out of line simply swirl, but those caught directly in the wake follow the falling weights like a cyclist slipstreaming a truck. As a ship goes down, the passengers most at risk are those on the top. Water in a hurry does not query the size of your life jacket; it grips you and down you go. Your best bet, whether in suction or in a rip current, is to swim to one side. You won’t have far to go, and then your life jacket can get to work. Jon Richfield Somerset West, South Africa n It is quite possible for passengers of a foundering ship to experience the sensation of being sucked under. However, unless the ship is big and sinking quickly – creating a lot of turbulence and releasing a lot of trapped air on its descent – the forces involved are most likely to be small and transient, allowing passengers to swim to safety. Air escaping from submerged compartments could bubble up through the column of water above the sinking ship. Aeration of water decreases its density and, according to Archimedes’s principle, passengers would sink if their weight exceeded the reduced weight of water they displaced. This is why swimmers are less buoyant in the “white water” of the surf than in the “blue water” outside the breakers, and why small boats should avoid passing through the white water wake of big ships. It is thought that bubbles caused by the release of methane gas from methane hydrate deposits beneath the sea floor can sink ships. In 2003, Joseph Monaghan of Monash University in Australia argued that a trawler discovered in a large methane pockmark known as Witch’s Hole, about 150 kilometres off the east coast of Scotland, was sunk by a bubble at least as big as the vessel. Bruce Denardo of the Naval Postgraduate School in Monterey, California, tried to disprove the theory by floating a set of small spheres on the surface of a tank of water while feeding bubbling air into the bottom of the tank. The spheres sank. If bubbles can sink ships, the same could happen to people, although people might be able to swim clear of trouble. While a sinking ship is still just below the surface, passengers could be dragged along in water currents flooding in to displace the escaping air and this helps explain why passengers on different parts of the same sinking ship can have very different experiences. Charles Lightoller, second officer of the Titanic, was twice “sucked under”, carried by water flooding down through ventilators and air shafts. In contrast, chief baker Charles Joughin claimed that he did not even get his hair wet as he stepped off the stern of the Titanic while it sank beneath him. There are other good reasons to stay clear of a sinking ship. For example, when the hospital ship HMHS Britannic sank off the coast of Greece in the first world war, a lifeboat full of passengers was caught in the turning propeller as it rose out of the water. And for passengers left in the water, there is the danger of being struck from below by buoyant objects that break loose from the submerged ship. Mike Follows Willenhall, West Midlands, UK This week’s questions WELL SPOTTED What causes freckles? And why do some people have them while others don’t? Maeve Halloran Dublin, Ireland PLAY IT AGAIN BRAIN Why do we sometimes get a tune or refrain stuck in our heads and play it over and over again even though it’s driving us crazy? Angelina Phipps Bromley, Kent, UK BIG FISH IN A… Is it true that goldfish (or other captive fish) grow in proportion to the size of the tank they live in? I have a small goldfish in a tank at home but have seen much larger goldfish in ponds and believed they were a different species. But a friend insists they are the same, with the size difference being caused only by the volume of the body of water in which they live. Is my friend right? And if so why? Barry Dwyer Truro, Cornwall, UK “Charles Joughin claimed he did not even get his hair wet as he stepped off the stern of the Titanic” Last words past and present, plus questions, at last-word.com A new collection: the usual insight, ingenuity and wit – this time with full colour photographs Available from booksellers and at newscientist.com/orangutans Why are orangutans orange?
