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Boating Weather for the Long Haul

A cruising couple downloads and analyzes GRIB files to find the best route across the South Pacific.
Boating Safety

October 9, 36°44.7’S, 90°49.5’W

>> Synopsis: As the low-pressure system (center pressure 1017 millibars) pulls away from us, we find ourselves in an increasing southeasterly flow from the leading edge of a high-pressure system (center pressure 1035 millibars) advancing from the south. (See 10/10 0600 (24 hours).) A low-pressure system (center pressure 1013 millibars) is forecast to come up out of the Southern ocean moving from southwest to northeast 48 hours later, creating a crush zone with a high-pressure system approaching from the west (center pressure 1032 millibars). (See 10/11 1800 (60 hours).) this low didn’t appear in any of the previous GRIBs , including the one from the day before. (Compare 10/11 1800 (84 hours) and 10/12 0600 (96 hours) for the same period from the GRIB of the day before.) After this low passes, a strong westerly flow fills in beneath the high-pressure system (center pressure 1030 millibars) that should carry us all the way to Canal de Chacao. (See 10/14 0600 (120 hours).)

>> Analysis: the low advancing out of the Southern ocean creates an ugly pattern. In the southeast winds before the low, we don’t have the option of sailing the great-circle course. We’ll be able to sail on starboard tack a bit north of east or on port tack a bit west of south. Sailing east will put us in the leading edge of the low with moderate south or southeast winds. It may also pin us against the coast with a strong southerly flow if another low follows the same track. Sailing south will put us onto the back of the low-pressure system with gale-force southwest winds, which will be just forward of the beam on the great-circle course to Canal de Chacao. In the last few days, the GRIBs have been showing a consistent southerly flow off the Chilean coast, where a current runs to the north. Beating down to Canal de Chacao against wind and current will be very difficult, so we want to avoid that possibility.

>> Strategy: Given the prevalence of southerly winds along the coast, we can’t afford to give up any southing. If this pattern holds and the low looks as if it really is going to develop in 24 hours, we’ll tack to the south even at the expense of losing some easting. this will get us south and give us greater flexibility to handle whatever might come next, but at the expense of making few miles toward our destination.

>> Actual: the southeast flow continues, and the GRIB on october 10 shows that we won’t be able to avoid the low. We tack to the south. In the next 24 hours, we sail 130 nautical miles and reach 38 degrees 30 minutes S, but only make good 10 miles toward our destination. By the early hours of October 12, we’re under staysail alone in southwest winds averaging 35 knots with gusts to 45 on a course just south of east with the wind just aft of the beam. Our strategy pays off when a second low follows the track of the first when we have only 350 nautical miles to go. This time, we were far enough south that we’re able to keep the winds on the stern throughout the gale. We arrive at Canal de Chacao just as the wind from this second gale begins to ease, and we drop anchor on October 16, just after midnight, concluding a 24-day passage.

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The U.S. Coast Guard is asking all boat owners and operators to help reduce fatalities, injuries, property damage, and associated healthcare costs related to recreational boating accidents by taking personal responsibility for their own safety and the safety of their passengers. Essential steps include: wearing a life jacket at all times and requiring passengers to do the same; never boating under the influence (BUI); successfully completing a boating safety course; and getting a Vessel Safety Check (VSC) annually from local U.S. Coast Guard Auxiliary, United States Power Squadrons(r), or your state boating agency's Vessel Examiners. The U.S. Coast Guard reminds all boaters to "Boat Responsibly!" For more tips on boating safety, visit

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