April 18, 2010: No Harm Done

By Dallas Murphy

We’re disappointed to learn that the rendezvous with the Dutch sailing vessel Clipper Stad isn’t happening, at least not today. The wind shifted onto their bow, and they had to fall off well to the south of our position. As I stressed, wind determines the shape and condition of life on any ship at sea, but for a sailing vessel, unlike Knorr, it determines where—and if—you go. Too bad, a square-rigged ship hoving up over the horizon and sailing close aboard would have been a spectacular sight. We were all up for the break in routine and for the photo ops. Maybe tomorrow. Or maybe in Cape Town.

We had a potentially serious overnight incident with the CTD, illustrating the vulnerability of dependent systems and the risk inherent to this business. The CTD crashed into and was dragged along the bottom.

The problem originated with the ripping current on the “surface,” down to about 1,500 meters. This carried the package away from the ship, “kiting,” in the parlance. Beneath that fast current, lay colder, heavier water that was either moving counter to the Agulhas or not at all (the jury’s still out on that, pending ADCP data). So when the package passed below 1,500 meters, out of the influence of the current, it slid back toward the ship. The wire, however, still remaining in the current, bowed significantly. Therefore, more wire was being unspooled then the depth of the package, and this condition obtained as the cast continued.

An altimeter—looking downward—is mounted on the package. Instead of measuring altitude in the sky, this one measures distance above the bottom. But typically altimeters don’t kick in and begin informing the operator of that distance until the package is about 250 meters from the bottom. This altimeter has not been behaving reliably since the first cast, so that was an additional factor. Of course if the altimeter is not pointing at the bottom, but out at an angle, not down, into the water column, as would be the case if the package were still kiting, then the device would report misleading readings.

As the package was nearing the bottom, the bridge watch changed, and the new officer, recognizing that the wire was again streaming outboard, sensibly moved the ship toward the wire, which in effect straightened it. And that, the third factor, put the package on the bottom.

Still, no one knew that it had struck, until the CTD watch brought it to the surface. Ordinarily, if the package hits the bottom, the sensors would clog with mud or sand and send crazy readings back to the CTD console, thus alerting the operator. Also, the winch driver, in that little house one deck above the main deck, has a device that measures and reports the weight on the wire and the length of wire spooled overboard. So when a package hits the bottom, the weight on the wire goes to zero or at least gives erratic readings. None of that happened. There was no indication that anything untoward had occurred until they brought the thing back aboard—and found mud on the bottom and up the side of the package.

Luckily, surprisingly, not a bit of damage was done. The pumps worked fine, so did the sensors and the ADCPs; none of the bottles was damaged. There were kinks in the wire; they had to be cut away, the wire respliced, but that was nothing compared to the degree of damage that might have been inflicted.

These people all care about their work and recognize the magnitude of their responsibilities to their shipmates, the expensive gear they’re handling, and to the ship. Each member of the team was going around saying, “It was my fault. I should have seen—“ “No, no, it was my fault. I should have noticed that the—“ The watch was still shaken, discussing it when I cam down from the bridge about 0900.

It doesn’t matter whose fault it was, probably no one’s. Still the incident vividly illustrates that old maxim of physical oceanography: Once you put a thing in or under the water, you no longer own it. It has become the ocean’s property, and you should consider yourself fortunate if the ocean gives it back. This trip has been remarkably free of serious incidents, losses, rotten weather, or any of the incidents that become mess-room sea stories on subsequent cruises. If I said that out loud, which I probably wouldn’t, people would be looking for wood to knock. So with only several days left on the study line before we begin the steam back to Cape Town, we can at least say that thus far this has been the kind of cruise chief scientists hope for—uneventful.

However, the weather is supposed to take a swerve for the worse in the next 48 hours. The last time heavy weather was predicted, we didn’t take it very seriously; there just didn’t seem to be enough isobars on the map. This time conditions seem more conducive to strife, 40 knots of southwesterly if the low slides north over our latitude, as forecast. That means the wind will be blowing bang against the current, and that’s highly undesirable, particularly in the Agulhas Current.