Fast ferries and deadly waves
High-speed ferries are an increasingly common sight along heavily trafficked routes on both sides of the Atlantic. Their high speeds caused an uproar here in North America, but recent events in England suggest that additional troubles may lie ahead. According to recently completed research by British Marine Technology, high-speed ferries can produce unusual "solitary waves" capable of causing considerable damage, even death.
In one such accident off the southeast coast of England this summer, one fisherman was killed and another injured when their 36-foot boat was swamped by a 15-foot solitary wave produced by a ferry. This was no wake; the surviving fisherman reported that the single wave "came out of nowhere" 10 minutes after the ferry had passed.
Even the casual beach-goer is familiar with the waves one finds in the sea, whether wind waves or those from the wake of a ship. These waves come in long trains of crests and troughs, one following the other to the horizon. Except when they are breaking, these waves don't actually move water. The water circles around a bit, but no net movement of water accompanies the passage of a common wave. A solitary wave is a different beast altogether: it is just a single crest of moving water. No trough, no endless train of waves out to the horizon, just undisturbed water, a crest, and then more undisturbed water. One can appreciate why those English fishermen were taken by surprise by such a wave. Solitary waves are possible only because the speed of waves is not constant.
Wave speed in shallow water is determined by water depth aloneif you know the depth, you can compute the speed of a wave. As the accompanying figure shows, the wave speed is pretty highso high that until the advent of high-speed ferries, few vessels approached the critical speed for solitary wave production. But now it is routine to find vessels traveling at, for example, 40 knots in 20 fathoms, just the speed waves naturally travel in this depth of water. As this critical speed is approached (yellow band), solitary waves begin to radiate off the bow of the vessel (any vessel, not just ferries). These waves are small, perhaps only a few inches tall, but they radiate out in a circular pattern from the vessel's bow and continue to spread for large distances. They become potentially dangerous only when they reach shallower water. Although the mechanisms are still poorly known, it appears that the solitary wave shortens in wavelength and grows taller as the water shoals, potentially rising to a dangerous height before crashing on the shore (or unlucky fisherman). Indeed, Some English beaches near high-speed ferry routes now carry signs warning bathers of waves erupting from an otherwise quiet sea!
In the open ocean, where water is deep and boats are few, solitary waves are clearly not dangerous. However, the increasing number and speed of high-speed vessels operating in shallow harbors will produce solitary waves that have the potential to cause damage. Further research is clearly needed to understand more fully the production, evolution, and ultimate fate of solitary waves generated by high-speed vessels. While BMT is no longer pursuing such research, the controversy surrounding high-speed ferries is certain to lead some one to conduct more. Stay tuned.