The US Navy has just patented a technique that could make ships harder to find by eradicating the telltale bubbles as soon as they appear. As a ship pushes its way through the sea, pockets of air get trapped in the water flowing around it. These bubbles get caught up in the strong counter-rotating currents created by the propellers. Big bubbles stay in the wake for a relatively short time because they are more buoyant, but smaller bubbles can be caught in the wake for a really long time. It's those small bubbles that make ship wakes easy to spot from the air because they scatter light well.
In a test to remove bubbles, a series of transducers that pumped 1-megahertz acoustic waves into water in a tank were used. These waves interfered with one another, producing a three-dimensional grid of high and low-pressure pockets. It was found that small bubbles - about 0.2 millimetres across drifted into the low-pressure regions where they formed bigger bubbles, about 1.5 millimetres across. This gave them enough buoyancy to float to the surface.
A research group at the University of Michigan in Ann Arbor that has studied ship wakes for the US Navy says bubbles in the wake can be a significant problem - they are as long as a kilometre for some military vessels. Radar systems mounted on satellites can also detect ship wakes, but for pilots hoping to intercept ships, real-time identification is vital.