January 27, 2010

Cresting and capsizing

EVERY TIME I HEAR some sailor boasting about the alleged seaworthiness of his dear sailboat I wonder if he knows the difference between static and dynamic stability.

In other words, I wonder if he knows the difference between how stable his boat might be in calm water (static) and how unstable it might be at sea in big waves (dynamic).

It’s an established fact that no amount of static testing will reveal how much more vulnerable a boat is to capsize when it is weaving its way through heavy swells.

This phenomenon was investigated in the late 1800s by William Froude, an eminent British naval engineer who was well versed in fluid dynamics. Froude did many experiments for the British navy, including his most famous, which determined the amount of force that water exerts on a body passing through it. But the experiment that should concern all small-boat sailors dealt with the inclination of a sailboat to capsize on the crest of a wave.

As you have probably noticed, you get a strange feeling in the pit of your stomach when your boat heaves upward suddenly on the face of a steep wave and then drops off suddenly. Froude discovered that at the top of the heave your boat experiences a degree of weightlessness.

At that stage, the boat is virtually in free fall. And thus, Froude found, a boat’s stability vanishes completely as she floats over the crest. There is no resistance from the water to stop her from being blown over by the wind.

This rather scary theory is well borne out in practice. The phenomenon of ocean-going sailboats and small racing dinghies capsizing on the crests of even non-breaking waves is well documented. The degree of danger depends, among other things, on the height and steepness of the swells as well as the design of your boat.

Froude also found that the presence of a wave crest near amidships resulted in a decreased righting moment. On the other hand, a wave trough amidships increased the righting moment, compared with the static stability.

If this all seems highly scientific to you, be aware that good sailors know intuitively that when they’re running in heavy seas in a displacement hull they shouldn’t spend too much time on the crest of a wave. That’s why they try to slow the boat with a drogue, to let the wave crest pass underneath quickly. Sitting on top of a wave, especially a breaking wave, is never where you want to be.

Today’s Thought
The sea thinks for me as a listen and ponder; the sea thinks, and every boom of the wave repeats my prayer.
— Richard Jefferies, The Story of My Heart

Boaters’ Rules of Thumb #7
Statistics from cruisers in Mexico, the Caribbean, and the South Pacific show that the average yacht spends 10 percent of the time at sea, 5 percent tied to docks, and 85 percent at anchor. This emphasizes the importance of good and easily handled ground tackle, and an efficient dinghy.

“I see that restaurant on Main Street is hiring a gypsy band from Romania, and waiters dressed as bandits.”
“That’ll make a nice change. Last time I was there they had bandits dressed as waiters.”

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