Wave sets in the ocean

I USED TO SCOFF at the surfers’ notion that ocean waves arrive in sets of 7 or 9.  They could never explain why that should be. They were adamant, however, and later in life I discovered that there were many people with scientific training who agreed with them, although they had no explanation for it either.

I put the theory to test once on a tropical beach on the island of Fernando de Noronha, off Brazil, where ocean surf was pounding the only beach where we could land in our inflatable dinghy. I stood off, outside the line of breakers, and started counting swells.  I found it difficult to tell whether one breaker was bigger than another, but I certainly wanted to miss the biggest ones because I didn’t have any experience of landing an outboard dinghy on a beach through heavy surf.

But the swells did seem to arrive in sets, as my surfer friends had claimed. After each set there was a calmer patch, and that was the signal to gun it for the shore, riding the back of the last wave ahead.

We did this many times, of course, and the biggest problem seemed to be deciding whether the particular set you were watching comprised seven waves or nine. If it was a nine-wave set, and you started off on the seventh wave, you could be in trouble. There didn’t seem to be any pattern  that I could decipher. Sevens and nines rolled along in a totally random fashion.

Another problem was the variation of the size of individual waves in each set. You could never tell when one wave was going to be smaller than the others, which is what we would have liked to have known. But there were usually one or two that were bigger than the rest, sometimes one after the other, sometimes not. In the end, we mostly crossed our fingers and hoped we had timed it right, between sets.

It’s natural to be fascinated by waves if you sail on an ocean or a decent-sized lake and, indeed, there is an awful lot to be learned about them. One of the first things you learn about waves on the open ocean is that the water in them doesn’t move forward with the wave. The molecules in a deep-water wave merely move up, forward a tiny bit, and then down again.  You can achieve almost the same effect by laying out a line on the ground and snaking a wave through it.

Guy Murchie put it rather nicely when he wrote in The Seven Mysteries of Life:

What’s an ocean wave made of?

At first glance, nothing but salt water;

But keep your eyes on it ten seconds . . . twenty seconds . . .

You’ll notice that the water is roused

Only momentarily by the wave

Which passes it by,

That the wave leaves the molecules and bubbles behind,

That the wave in essence is a kind of ghost

Freed from materiality by the dimension of time,

Made not of substance

But energy.

Today’s Thought

There is no excellent beauty that hath not some strangeness in the proportion. — Francis Bacon

Tailpiece

“How much is a bottle of brandy? It’s my nephew’s birthday and he likes brandy.”

“Well, madam, it depends on the age. Seven-year-old is quite reasonably priced. Ten-year-old costs a bit more. Twelve-year-old can be quite expensive.”

“Gee, that’s terrible. My nephew is 25.”

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