By Tom Neale
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. 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. This study emphasized monohull sailboats. Here are some facts learned from the Fastnet tragedy:
- Breaking waves matter. While any large waves relative to the size of the boat can be problematic, breaking waves are always dangerous. Waves break when they become too steep — think surfing waves rolling onto a beach (see Figure 1). 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. 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. This can occur even in very deep water. Breaking waves should be avoided if at all possible.
- Size matters. Larger boats can better handle larger waves.
- Direction matters. Boats are very vulnerable to being rolled or capsized when taking waves on the beam. 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. But if those same waves were taken over the stern or over the bow, most hull forms remained upright.
Learning To Work With Waves
The wave height danger zone would begin at a smaller relative size for an unballasted powerboat. A 24-foot powerboat hit on the beam by an eight-foot breaking wave will almost certainly capsize. In fact, a much smaller wave could put such a boat in danger.
With any boat — power or sail — the first and best tactic is to stay out of large waves, with "large" being relative to the boat's size, shape, power, ballast, and structure. Tactics to avoid large waves include staying in the lee of a windward shore for as long as possible, traveling with wind and current running together, timing the entrance and exit to inlets and rivers so that the current is running with the wind and waves, waiting untill slack tide before navigating strong inlets or rivers, or simply staying in port until conditions improve.
Second, don't take waves on the beam. If possible take them on the bow, or it may sometimes be better to take them directly astern or at an angle to the stern rather than the beam. Usually, when heading into waves, it's better to meet them at an angle off the bow to minimize pounding, hobby horsing, and burying the bow. If taking waves astern, it's extremely important to avoid losing directional control as the wave overtakes you. This may require a high level of seamanship skills. If you must change course, watch the waves carefully; time the move when you see a group of smaller waves or a long trough that you can turn in before the next wave comes.
Third, don't get caught in breaking waves. Breaking waves can occur when the wind is opposing a strong current, when waves are passing over a shallow bar, when they are ricocheting off a shore or rocks, when they reach a height too tall to sustain themselves and when they are leaving deep water and meeting shallow ground. Do everything you can to avoid areas where breaking waves might form.
Beyond these general guidelines, there's no single set of tactics applicable to all situations. Here are some tactics that you might want to experiment with, when it's safe to do so, depending on your boat and situation.
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