Brown Streaks on a Blue Ocean

Voyage 11 of Kalunamoo concluded in St. Lucia this year, as it did last years, due to the constrictions on cruising during a pandemic. We did spend about a half a year in St. Lucia and half in Antigua. Sailing was very limited, but the cruising lifestyle carried on albeit in a lower key. Social distancing, masks, on again-off again curfew’s and beaches, restaurant and business closures were the rule but didn’t stop all social activities. We managed to take 5 covid tests each to facilitate our limited movements and expect at least 2 more to fly to and from NYC in the next week. Vaccines are to be included, so at the end of the day, nasal swabs, masks, shots in the arms have become routine.

But enough about that. As I wrote above, this season’s voyage was a bit short of actual sailing. Two 30 hours sails: St. Lucia to Antigua and Antigua to St. Lucia was just about it. We did sail to Barbuda from Antigua and did sail around, literally, Antigua and dropped anchor in a number of harbors. But it was good to stretch our and Kalunamoo’s sailing legs and leave Antigua. And nothing broke! Although Maureen does have a sore back that may have been aggravated from the natural “motion of the ocean” and its effects on Kalunamoo.

Video of sailing “between islands” when the winds were up to about 20 knots.

The sail down from Antigua is on the western side of three intermediate islands: Guadeloupe, Dominica and Martinique. This means that the actual sailing is a relay of ocean sailing between islands with strong trade winds interspaced with calm or variable winds on the lee side of the islands. Every four or five hours conditions change drastically. The arrival and departure on the ends of the islands also has changing wind and sea conditions that add to the mix. This makes the non-stop passage rather interesting. A video of the interisland part is here. . The conditions on this passage were almost ideal for us. Wind was slightly north of east in the 16-20 knot range with some gusts to 22. This made for a beam reach and allowed us to carry full sail. In the lee of the islands, westerly wind was not unusual, and motor sailing and sail changes were the way to go. But many ask, what do you do for 30 straight hours?

Passing Montserrant

This is also asked of our longer passages, up to 13 days from the U.S. to the Caribbean (we are not a fast boat!). The answer is, of course, sail. That in-itself, despite an auto pilot, will keep one busy. This last passage was a busy “sail” as alluded to above. But one thing I thought about was the Sargasso seaweed that is ubiquitous here. A few years ago, it was at its most prevalent but even now is substantial. One effect of this weed is to keep our trolling for fish to a very minimum. Trolling a fishing lure takes constant attention to remove the weed off the lure. Too much work! The origin, life cycle, and movements of the Sargasso weed are still not settled science, but I was struck by the brown seaweed streaks that litter the ocean. Some people look at clouds. I have looked at streaks. Do they look like rows and flows of angel’s hair? (Didn’t Joni Mitchel sing about this?). In any case, it is one thing that can be done sailing. 

The brown seaweed is a visible mar of the iridescent blue of the Caribbean water. When winds blow, and they do 99% of the time and from one direction only, the weed forms streaks many hundreds of feet long flowing in the direction of the wind. It’s a good indication of true wind direction. The weed itself grows in mats and float on the surface of the ocean. The mats can become quite large, we have seen them around 3000 square feet or more in the open ocean. They extend only an inch or so above the water and less than a foot below the surface. So the question is, how do these streaks form from mats?

It seems like a simple answer. The wind breaks up the mats and push the small pieces down wind. But why do they form streaks, like a line of soldiers off to war. Are they connected with invisible thread? No. My first thought was that the edges of the mat must break free first and would then normally float down wind one after the other. It certainly looks like that is what happens. Then when we last flew into Antigua I saw from the plane mats of weed in the ocean. It was clear that the streaks were a trail left from the mats movement and not the other way around. But in either case, that doesn’t explain why mats and the trail would travel at different speeds. If the wind breaks up the mats, all the pieces should travel together. Maybe the waves do it. But the streaks conform to the wind direction and are perpendicular to the wave train.

It turns out that Irving Langmuir, saw the seaweed lined up in the Sargasso Sea in 1927 and asked the same question. He happened to be an American physicist and formulated why. The result is now known as the Langmuir Circulation. Fluid dynamics is not my specialty, but it is interesting to learn the solution to what appears to be a simple problem. In short, the weed is pushed into these streaks by rotating tubes of water. The axis of the tubes lie together horizontally for lengths of considerable distance. The diameter of the rotating tubes can be tens of feet from the surface. Adjacent tubes rotate in opposite directions so that anything floating on the surface is pushed to the side where it meets the adjacent tube. The other tube pushes anything floating in the opposite direction. The end result is a line of floatables between the tubes, hence the streaks. This works the same for anything floating, including bubbles or garbage.

How the tubes are formed are really complicated and not fully understood but is powered by wind, wind driven waves and currents. These tubes not only result in streaks but in vertical mixing of surface water. Oddly, the tubes are generally parallel to wind flow and develop quickly. They can also intersect other tubes.

So the streaks are not really blown by the wind but by the invisible circulating water made visible by the weed. Thanks Irving. I’ll never look at streaks the same way again. And yet, there is some beauty in the abstract forms that defy interpretation of fluid dynamics.

We will spend the Hurricane Season in St. Lucia keeping an eye out for threatening weather. Hope is high for a more “normal” cruising season next November but, like the Langmuir Circulations, we may be pushed and ordered by forces we know little of or simple assumptions that we don’t know at all.

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