In an ocean storm, large waves as such are not a threat to a small boat. Even a very steep wave which can roll the boat violently will not capsize or damage a small sailing yacht. The period of such ocean waves is much longer than the natural roll period of a small yacht or fishing vessel, and rolling motion will damp between waves. Also, there is evidence that with all sail off the wind forces are not a very important factor. Most of the time the vessel is in the lee of large waves. The blast which strikes the boat as it passes over the crest is of short duration and in the Fastnet storm sailors reported no serious problems associated with the wind except, of course, the effect of the wind on the waves.
In many storms most of the waves are irregular and unstable; that is, no single wave maintains its shape for very long. Sailors report a wave as having suddenly humped up out of a flat spot and forming a crest which cascaded down three sides of the wave. A variety of unusual shapes can be seen in photographs of storms at sea. Irregular waves as such do not pose a particular threat. It is the breaking wave which is dangerous.
The characteristic of a breaking wave which can cause capsize is the fact that a mass of water on the crest of the wave or tumbling down the face of the wave is moving at approximately the speed of the wave. For a wave with a wave length of 300 feet, this water can strike the boat at a speed of 20 knots or more. A small boat lying ahull in non-breaking waves of any shape moves more or less with the surface water. It will not be struck by a large mass of moving water and therefore will not capsize, whereas a boat struck by a breaking wave can be violently thrown into the trough and capsized.
Breaking waves can have many forms. The wave height and wave speed can differ for each storm. Of particular importance with regard to capsize potential are the shape of the front f ace of the wave and the momentum of the water in the breaking crest. Momentum is defined as the mass of moving water times its velocity. The force imparted to a boat struck by a breaking wave is largely determined by this quantity. The most familiar breaking wave is the cup-shaped breaker seen on a beach. This wave forms as a fast-moving ocean swell and is slowed down by the shelving bottom. In an ocean storm the waves are not slowed down but are actually accelerated by the wind. The pressure forces of the wind on the wave surface causes the wave to steepen until a small portion of the crest breaks and forms a whitecap. A whitecap is a mild form of breaking wave in that some of the water has been accelerated up to wave speed. A whitecap on a very large ocean storm wave may have enough force to capsize a life raft. When two or more storm waves intersect they may combine to form a larger wave which in some circumstances may then become a dangerous breaking wave. From photographs of storms at sea and from observations of smaller wind-driven waves, it appears that most of the breaking waves in an ocean storm do not resemble a slow-moving cup-shaped plunging breaker but might be described as an enlarged whitecap, that is, a large, high-speed wave with only the top breaking. However, waves of many different shapes have been reported from time to time.
Breaking waves formed in a towing tank or formed by the wake of a powerboat, as described later in this report, do not represent the complete spectrum of wave types which might be encountered in an ocean storm. However, it is believed that such simulated storm waves can be used to evaluate survival gear such as drogues provided that the testing is supported by an adequate theoretical framework.