Thursday, January 14, 2010

Pilot Chart

A friend of mine has invited me to cross the Atlantic on his forty foot sloop! At least, I think he has; he put the word out to a number of his friends, and I immediately threw my hat in the ring, so I hope to go along. The crossing is in June.

There are two legs, and I can do but one of them: Portland to the Azores; or the Azores to Gibraltar. Which one should I do, if given the choice? For me, the prevalence of good sailing outweighs all else, so of course I consulted the pilot chart for June North Atlantic.

In the early to mid nineteenth century, U.S. Navy officer Matthew Fontaine Maury distributed special logbooks to ship captains and asked them to record wind speed, direction, calm, ice pack, icebergs, fog and similar data. Maury created charts depicting this data in graphical form, and the pilot chart was born. The data continue to be collected, and one authority states that the April North Atlantic pilot chart depicts data drawn from more than four million observations. pilot chart is divided into rectangles of 5 degrees. Each rectangle contains a wind rose. The percentage of time the wind blows from a certain direction is indicated by the length of the arrows. The arrows fly with the wind, with the fletching up wind, as it were. When the wind is so often from one direction that the arrow would be quite long, the percentage may be given by number; in this example 40% of the time the winds are due north. The number of feathers indicate the average force of that wind by the Beaufort scale, so that in this example 40% of the time the wind is due north and the average north wind blows at force 7, or about thirty knots. The number in the circle indicates percent of calm, here 3%

Pilot charts also indicate the limit of pack ice, prevalence of bergs, percentage of fogs and gales, ship routes, ocean currents, typical tracks of rotating storms, and other information useful to anyone planning a voyage. God bless Matthew Maury!

Back to my dilemma: Portland to the Azores, or Azores to Gib? The June North Atlantic pilot chart (it's an 11 mb PDF file) shows that most of the time the winds are south to west about force four (about 15 knots). From the Azores to Gibraltar, on the other hand, winds are on the bow much of the time, so I'll ask to do the first leg.

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