Crew Log 192 – Eight-knot Hum

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February 14, 2010 – Puerto Montt, Chile
By Herb McCormick

Aboard Ocean Watch, when the boat’s under full sail and really starting to roll, we don’t need to look at the instruments to know we’ve hit 8-knots or better. For precisely at that moment, the 64-footer begins humming. And we don’t mean in the figurative sense, either. Many moons ago, David Thoreson was the first to notice this regular, recurring phenomenon, and he even gave it a name: “the 8-knot hum.”

It’s no coincidence that sailors “tune” a rig in the same way a musician “tunes” his guitar. As with the latter, you tune a rig by applying tension to the “strings” of wire or rod that keep a mast upright, rigid and in column. Sailors call such stays and supports the “standing rigging.” (The “running rigging” on a sailboat are the halyards and sheets – the lines of rope – that adjust the trim and set of the sails, both up and down, and in and out.)

The source of the 8-knot hum is a bit hard to pinpoint. My original theory was that it had something to do with the rig, or perhaps even the keel, but I’m not so sure anymore. By whatever name, Ocean Watch uncannily commences to resonate right at 8-knots. As the speed increases, so too does the volume. At 10-knots it’s good and loud, at eleven it really starts to whine, and at twelve and above it’s so noisy you can practically feel it.

And today on Ocean Watch, by golly, we’re feeling it.

As if to celebrate her return to the blue Pacific Ocean, this afternoon the rangy steel boat was absolutely soaring up the coast of Chile, reaching hard before a steady 30-knot southerly breeze and routinely registering speeds of 10-12 knots or better. As the day has lengthened, so have the waves stacking up in long sets, and Ocean Watch has caught her fair share of them, routinely surfing down the face of steep 10-footers. All too often, in our long sail around South America, we’ve been bashing and crashing our way into similar seaways, and it’s exceedingly satisfying to now be running before them.

“This boat is as comfortable going this way in a blow,” said first mate Dave Logan, pointing north, our course for our next port of call in Valparaiso, “as it is uncomfortable going that way in one.” With that he gestured astern, with a shudder: we’ve seen that movie before, and are happy not to be in the midst of a sequel.

As mentioned in previous logs, we’ve enjoyed the excellent First Yachtsman’s Navigator Guide to the Chilean canals and used it religiously since arriving in Patagonia. Author Alberto Mantellero lives right on one of those canals, at a place called Puerto Elvira on the north end of Isla Chiloe. On page 39 of his book, writing about his home waters, Alberto extends the following invitation to passing sailors: “If you see a flag on top, at the front of the hut, it means you are invited to have a sparkling gin & tonic.”

Yesterday, Ocean Watch set sail from Puerto Montt and by late afternoon had closed in on Puerto Elvira, where the flag in front of Albert’s place was indeed flying. In fact, he was expecting us, having visited the boat earlier in the week and extending an invitation to the crew for dinner ashore. Captain Mark Schrader picks up the tale, and provides an update on our travels, in the latest installment of his skipper’s log:

“Dinner ashore at Puerto Elvira with Alberto’s family was exactly what a beach cabin dinner should be—steamed local mussels, baked salmon, home-made bread and a selection of fine Chilean wines—all served up and garnished with interesting stories and shared experiences.  The short, dark, after dinner dinghy ride from the beach to the boat was followed by an even shorter deep sleep aboard Ocean Watch. In other words, morning came quickly.

“The tide runs at a fast clip through the Canal Chacao with the ebb flowing west, emptying into the Pacific Ocean, the highway connecting us to all points north, and eventually, home.  At first light we hoisted the anchor, pointed OW west and caught the fast morning tide.  Very shortly we were out of the Canal and able to make our turn to the north, with a single-reefed main and full jib setting nicely on a broad reach.  Blue skies, sparkling blue water, a pod of whales blowing and keeping pace with us just a short distance off of our starboard beam; what a great welcome into the Pacific.

“The Chilean weather forecast, kindly forwarded to us early this morning via an Armada VHF station, called for southerly wind in the 20-30 knot range with gusts to 40.  As the afternoon has progressed the wind gradually increased from the high ‘teens’ to the mid twenties and is now a solid 30 and still climbing.  We’ve tied in the second reef, rolled up some jib and are comfortable as OW makes ten to eleven knots through the water, occasionally catching just the right angle on a wave and ‘surfing’ along at a faster pace.  We’re also now getting a little positive boost (+/-1 kt) from the north-flowing Peru Current, a nice little payback for the weeks of miserable adverse current we didn’t enjoy along the coast of Brazil.

“Sometime close to midnight tonight we’ll leave the Roaring Forties, ironically on the 40^th day since entering that latitude in the South Atlantic Ocean.  I’d say we’re leaving relatively unscathed after 2,852 n.m. and many interesting experiences, both on land and on the water. The barometer continues to fall, slowly, and the wind continues to rise.  These conditions suit Ocean Watch and crew very nicely.”

They certainly do, as that sweet hum attests.

Before reaching Patagonia, onboard scientist Warren Buck was more than a little intrigued by the hum, and to say Warren looks at the world a little bit differently than the rest of us would be an understatement. Warren’s also a sailor, and he understands boats, so much so that he wondered what frequency the hum might be registering. Once back in Seattle, he sent us this note:

“The ‘8-knot’ hum is definitely caused by the hull vibrating air inside…somewhere. My question is: Is it louder astern than forward? The air column cannot be the entire inside of the hull but rather a portion of the air column. The best way to discover its frequency is to measure it directly! It has to be between 1 and 5 kHz to hear it. I was originally thinking a multiple of 8 from the characteristic length of the boat; with that the likely answer is 4 kHz but that really needs to be measured to get a better answer.”

Right. We’ll get on that, Warren. To everyone else, we hope that clears it up.

In the meantime, Ocean Watch keeps humming along, tracking downwind as if on rails. We may not know the frequency, but make no mistake about it, the rhythm of miles flying by is music to our ears.

-Herb McCormick and Mark Schrader with photographs by David Thoreson

*This crew log submitted by Iridium OpenPort and Stratos

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