October 20, 2015

Watch out for the crest

I SUPPOSE it’s pretty natural for sailors to ask themselves just how seaworthy their boats are, compared with others. One thing they worry about in this connection is stability, but I wonder how many of them know the difference between static and dynamic stability.

In other words, I wonder if they know the difference between how stable their boats might be (that is, how resistant to capsize) in calm water (static) and how unstable they 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 tendency 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 momentarily vanishes completely as she floats over the crest. At that moment 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 waves, 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. Alternatively, they try to aim the boat off 20 degrees or so to one side or the other, to avoid surfing dead downwind, and to get the rudder out of the foam, where it can’t work properly. 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 I listen and ponder; the sea thinks, and every boom of the wave repeats my prayer.
— Richard Jefferies, The Story of My Heart

“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.”

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)


Marco Pinho said...

Great tips... will probably be needing them soon. I am set to purchase a run-down yet sturdy Cape Dory 28, and I'm sure she qualifies as "heavy displacement." I will definitely think back to this post once I get into my first storm with her!

As a sidenote, I just want to say I love reading your blog. I'm 26 years old and since last year have been on a journey to find and refit a small bluewater sailboat which I plan on taking wherever the currents / winds will take me. To that end, I've been doing a lot of reading and studying, and you are one of my favorite authors on the matter (mostly because you introduced me to the point of view of "Go young, go small, and go now.")
Please never stop writing. And you mentioned in another post that no one would buy a book made out of these blog posts, but know that I would!

John Vigor said...

Thanks for the kind words, Marco, and congratulations on a wise choice of boat. She is indeed a sturdy little bluewater cruiser, one of Alberg's best. Of course, I may be a little biased because I owned a Cape Dory 27 for many years, and a Cape Dory 25D before that. Be sure to join the Cape Dory bulletin board and get more useful tips from fellow owners.
Fair winds and calm seas,
John V.