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It's hard sometimes. We've all read about those passages, weeks of the wind aft of the beam and never touching a sheet. No doubt they happen, and when they do they are also doubtlessly noteworthy. I'm still waiting for one of those myself. In fairness, I guess I should admit that the wind can be consistent when it is strong and my destination is directly upwind. Over the last few years I have been distributing weather observation forms to cruising yachts, and this article is a sort of summary of findings. It might be a verse to that old song from Porgy and Bess, "Taint Necessarily So".
The way that I summarize the logs which follow is that their wind observations are first pulled out and listed. Then each observation is compared with the next one. Usually, in my logs, these are at about eight hour intervals. If the observation is the same, say Beaufort force 3, in both occasions, then that is an instance of equal wind. If four succeeding observations are force three, that is one instance, with a duration of four observations. The same procedure applies to wind direction. If both wind speed and direction are the same, that is a separate tabulation. The idea is that if wind speed is the same, then you probably don't need to change sails. If wind direction is the same, you may need a sail change, but don't need to alter course. If both are the same, you are having the "classic" tradewind passage, at least for as long as it lasts.
Looking at my Marquesas passage in 1985, with recorded observations about three times per day, the wind blew from the same direction on two or more successive observations twenty times in twenty-three days, sixty out of eighty-five observations. This apparent uniformity disguises the fact that wind direc tion varied 360 degrees over that period. The largest variation in direction watch to watch was about 22 1/2 degrees, and within one watch about forty-five degrees. Another way of saying that is that changes, like frontal passages, tend to be abrupt. On eight occasions, for about one-fourth of observations, wind force factors were equal from one observation to the next. The highest wind was force 8, with three observations over force seven and eight under force 3. On the trip to Hawaii, eighteen days, the wind direction was equal watch to watch twelve times, about two-thirds of the observations. Ninety-five percent of the observations were wind directions in the in the SE to NE quadrant. On seven occasions, amounting to about one-third of the observations, wind speed was equal watch to watch. About 20% of the observations were under force 3, and 3% over force 5. Wind direction and speed were constant, though only 17% of the time on a watch to watch basis.
This compares with a winter passage back to San Francisco from Hawaii in which fourteen occasions in twenty-five days, about one half of the observations, had wind direction consistent between two or more observations. Variation on one watch was 20 or more degrees about twenty percent of the observations. The wind speed was equal between observations on seven occasions, comprising about 30% of the observations. The greatest wind speed variation between observations was three Beaufort force factors, but it also varied that much during one observation period twenty percent of the time, and two of the occasions of equal wind speed were force eight.
Looking at a 1983 log, an El Nino year, a Galapagos-Marquesas passage with daily observations showed five consecutive days of SSE winds followed by ten days of SE. In the last three days the wind fluctuated between ESE and SE. Wind strength was more variable, being constant Beaufort factors on four occa sions, totalling thirteen days. The highest daily observation was force 5, and the lowest, force 2. Gusts were not noted. Is this the ideal tradewind passage, or are the variations smoothed out by stretching out the time scale?
In a 1993 Mexico-Marquesas passage, there were four occasions in which the wind direction was con stant, observation to observation, and those accounted for 13 out of 22 observations sampled on a twice daily basis. Wind speed was constant within five knots on five occasions, about 55% of the total.
The highest wind speed observation was 20 knots, and SE was the most common direction. Gusts and squalls were not noted. Both wind speed and direction were constant on three occasions, 36% of the total.
From Panama to the Marquesas in 1993, in a sample of 32 twice daily observations, wind direction was constant on 6 occasions for a total of 21 observations, (66%). Wind speed was constant within 5 knots on 7 occasions; 53% of the time. Both wind speed and direction were the same on only 2 consecutive occasions, 19% of the observations.
These can be compared with a coastal California passage in sumer 1993, with observations every four hours. In a total of 18 observations wind direction was constant on three occasions, 50% of the total. The most common direction was NW, 33%. Wind speed was within five knots variation on 3 occa sions, 16 observations, with the greatest wind being 14 knots and the least being less than five. Wind direction and speed were constant on two occasions, 37% of observations. Dana in Two Years Before the Mast did characterize the northwesterlies of California as a form of tradewind.
By comparison, Heart of Gold, a Schumacher 50, in passage between Mopelia and Rarotonga in July, 1993 showed no constant wind direction over a 72 hour period, it backing or veering constantly between south to west with excursions through north. Wind speed was constant on three occasions, 8 out of nineteen observations. This summary was taken from the graph, attached. It is interesting to note the fine scale variation of wind speed. It varies continually, and the curve is drawn using four minute aver ages.
Looking at the complete passage records in the tropics, watch to watch, wind speed stayed the same within one Beaufort force factor or five knots 35% of the time; direction, 68% of the time; and both, 18% of the time. I suppose that there are rare occasions when you could make a good passage without touching a sheet. Normally the passage would be pretty slow because you would be wandering off course a lot of the time, and to make up for the heavy stuff, you would be reefed even when it was light.I knew some people who did a 33 day passage from Moorea to Hawaii that way.
All of this has to be put into the perspective of the Atlas buoy chain. An article in TOGA Notes, the periodical of the Tropical Ocean-Global Atmosphere project, gives average wind data for 1987-8 for the eastern tropical Pacific as approximately 11 knots, the bottom of Beaufort force 4, with a standard deviation of about 3 knots. In other words, two-thirds of the buoy observations are within the range of 5 to 17 knots, force 2 to force 5. These data are consistent with the yacht observations. For that matter, the October Pilot Chart of the South Pacific shows a wind rose at two degrees north and 132 degrees west which indicates no calms, and wind from the southeast 59% of the time, and from the east 34% of the time. For the purpose of navigation by sail, the variation is more interesting and important than the average.
In the 1995 Tahiti Cup race, Yukon Jack had very little wind variation, and a 20 minute passage of the doldrums. A day or so later and a bit farther east, Petard had a 24 hour doldrum drift, and wide variation of wind with squalls in the south Pacific Convergence Zone.
I have the feeling that wind variation may be similar to wave height variation. Out of any group of waves there is a statistical possibility of a freak wave. I'll bet that the same holds true for wind. Wind is never exactly steady, and out of any series of gusts and lulls, there is probably some chance of an ex treme gust. With what I have at the moment, though, there isn't enough to figure the probabilities. So things rest with Bougainville's observation from the 18th Century that Pacific trades are less regular than Atlantic.