Monday, January 09, 2006

Gates and Great Circles

One of the simplest rules of thumb for yacht navigators on ocean races is that the shortest distance is often the quickest. The other is of course to follow the winds. For leg 2 of the Volvo Ocean Race, both thumbs would be pricking with the same message - head down into the furious fifties.

The figure below shows the direct great circle route from Cape Town to Melbourne - and its clear to sail the shortest distance you have to get down to about 58S.
Then of course theres getting the strongest wind, and wind in the right direction. Again as the NOAA forecast below shows, a good latitude is below 50S.

But the fleet has stayed up at around 41 - 43S, where as can be seen in the graphic above the weather is the sort of mix of variability and complexity that gives navigators nightmares of sitting in a wind hole while the whole crew insists they don't blame him really.

The answer is the existence of a set of ice gates and way points as shown in the top figure. The two ice gates are both at 42S and the fleet must also round Eclipse Island forcing them much further north than their routing programs would otherwise have liked. But with ice spotted up at 47S thats probably just as well.

Meanwhile Mike and the boys on Black Betty have a stomping 326 mile lead over the Pirates who are working hard to fight off the ABN juniors just 7 miles astern.

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