Can a Single Gas Bubble Sink a Ship?

Yes, according to an experimental and theoretical analysis performed by researchers at Monash University in Australia (David May and Joseph Monaghan). The ocean floor contains vast quantities of methane gas hydrates, ice-like crystals of methane surrounded by cages of water molecules. If disturbed, these methane gas hydrates can erupt from the floor and rise to the surface as gas bubbles, some of which can be very large. Copious amounts of methane hydrates exist in the North Sea, which lies in between the United Kingdom and continental Europe. At a large eruption site in the North Sea known as the Witches Hole off the coast of Aberdeen, a sonar survey recently uncovered the presence of a sunken vessel, but the cause of the wreck remains undetermined. Simple experiments have previously shown that many small bubbles rising to the surface could sink a cylinder of water (and conceivably a ship), by causing a loss of buoyancy (Denardo et al., American Journal of Physics, October 2001). But could a single large gas bubble do the trick? The Monash researchers investigated this possibility in a simple, roughly two-dimensional system. Trapping water between a pair of vertical glass plates, and launching single gas bubbles from the bottom, they used a video camera to observe a single large bubble's effect on a small piece of acrylic shaped like the hull of a boat. Along with numerical simulations of this scenario, the experiments showed that the bubble could sink the ship, if the bubble's radius was comparable to or greater than the ship's hull. Sinking would occur because a mound of water formed above the bubble as it approached the surface. As the bubble reached the surface, it would temporarily lift the ship. However, water in the mound would then flow off the sides of the bubble, forming deep troughs at either side, and the water flow would carry the boat to one of the troughs. In addition, the eventual rupture of the bubble would create high-velocity jets of fluid that moved into the troughs, creating vortices that further pulled down the boat. The researchers say that their numerical simulations could test other scenarios, including those involving multiple large bubbles, more realistic boats, and ultimately a full three-dimensional simulation. (American Journal of Physics, September 2003).