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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

At the end of September 2007, my husband, Evans Starzinger, and I were in the Îles Gambier, at the southeastern edge of French Polynesia, aboard our 47-foot aluminum Van de Stadt Samoa, Hawk. We were about to embark upon a 3,400-nautical-mile passage to Canal de Chacao, the narrow channel that leads to Puerto Montt, at the north end of the channels in Chile.

The passage would take us across three of the global wind systems: the southeast trades, the variables, and the westerlies of the Roaring 40s. During what turned out to be a 24-day (and 3,800-nautical-miles-sailed) passage, we relied upon the weather information derived from Gridded Binary Files to help us pick our way through the complex weather systems we encountered. By sharing the exact information we had at our disposal, we hope to help others understand these valuable forecasting tools and how we make use of them to benefit from or avoid weather systems.

The Îles Gambier are located in the belt of southeast trade winds that extends to around 30 degrees S. To avoid beating dead into the trade winds for several hundred miles, the traditional sailing route between French Polynesia and the Chilean coast calls for vessels to sail as close to due south as they can manage through the southeast trades and the variables until reaching the westerlies of the Roaring 40s. Here, voyagers turn east and run to Chile.

To avoid being becalmed in the Southern Hemisphere summer (December through February), vessels may need to drop below 45 degrees S to remain in the westerly flow of winds under the semi-permanent high-pressure system centered around 30 degrees S, in the vicinity of Easter Island. In the early Southern Hemisphere spring, when we were about to embark, the South Pacific high would be expected to be north of its summer position and not yet fully established. Low-pressure systems with strong westerly flows would be tracking as far north as 30 degrees S, which we hoped would allow us to sail more directly to our destination.

When departing on a long passage, we start with the assumption that we will follow the conventional sailing directions, but two weeks before our departure we start watching how systems are actually tracking and whether we should modify our route. Because we were coming to the end of the trade-wind season, low-pressure systems had begun tracking across the Gambiers from the north and west, out of the subtropics, pushing easterly winds ahead of them. Since the South Pacific high wasn’t yet established, both lows and highs from the Roaring 40s regularly rolled northward into the variables; these, too, brought easterly winds well above 35 S. Taking the traditional route with these weather patterns would mean many days of beating into easterlies and the likelihood of several easterly gales or even storms. By mid-September and our departure, it was becoming increasingly apparent that we’d have to modify the traditional route to avoid a significant percentage of strong headwinds.

During this passage, we’d download GRIBs once a day. When determining where we’d be on each successive time period in the GRIBs, we assumed an average of 150 nautical miles per day.

Here’s a look at four 120-hour GRIB files we downloaded during our trip, how we interpreted them, and how the actual weather systems played out.

For much of this passage, we felt like a pinball ricocheting between the various low-pressure and high-pressure systems, but by using the GRIBs, we never experienced sustained winds of more than 40 knots or headwinds of more than 20 knots true. Few passages are this complicated, and those that are rarely work out so well. But in this case, the GRIBs made for a much easier passage than we would’ve had if we’d followed the conventional sailing route.

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