In fact, a much smaller wave could put such a boat in danger. A 24-foot powerboat hit on the beam by an eight-foot breaking wave will almost certainly capsize. The wave height danger zone would begin at a smaller relative size for an unballasted powerboat. But if those same waves were taken over the stern or over the bow, most hull forms remained upright. When struck beam on by a breaking wave of a height equal to or greater than 35 percent of the boat's length overall (LOA), every model tested was rolled to 130 degrees or more - well past the horizontal. Boats are very vulnerable to being rolled or capsized when taking waves on the beam. Larger boats can better handle larger waves. Breaking waves should be avoided if at all possible. Breaking waves often occur where large waves enter shoaling water or the wind blows against a strong current, but they can also occur in other situations as when the wave grows too high to support itself, thus toppling over. At that point, the wave collapses down its front, crashing the entire weight of all that water and the force of its motion onto a boat caught in the wrong place.
Waves break when they become too steep - think surfing waves rolling onto a beach (see Figure 1). While any large waves relative to the size of the boat can be problematic, breaking waves are always dangerous. Here are some facts learned from the Fastnet tragedy:
This study emphasized monohull sailboats. The 60-plus-knot winds created giant waves that capsized dozens of boats, calling into question the safety of yacht design at the time and launching a massive study including extensive tank testing, the results of which changed the way we look at boat design today. In August of 1979, a fleet of 306 boats was hit by a massive, low-pressure system during the Fastnet Race along the south coast of Ireland 136 people were rescued from 24 boats, 15 people died, and five ballasted sailboats were lost.
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