Warning about Using Deckman or Expedition

For those using routing software like Expedition or Deckman, consider the following: Computer routes suggest courses that are too close to highs when sailing around them downwind, but it isn’t due to any bugs or defects in the polars, software, or grib files. Of course there is a numerical incentive to go close to the high because it is a shorter course, and the routing algorithms assume that the wind is perfectly steady and the helmspeople are perfect, so the computer model takes you right to the edge. In the real world, however, there are light spots, the kite collapses occasionally, and helmspeople lose concentration, and in every event, you have to sail higher briefly to regain speed. And of course whenever you sail higher the wind gets lighter, which causes you to have to sail higher still, and the boat spins out up into the high It’s like asking a computer to tell you the best path to walk around the Grand Canyon.

The computer will correctly tell you to walk exactly on the edge. Of course we know that that would be too perilous because just one misstep would cause us to fall in, so instead we walk along a few feet away. Similarly, when rounding a High downwind, use the computer route to assess the closest edge of any sensible course, but instead sail somewhat farther away. The tricky part is evaluating the “somewhat”. If the High is very well formed, stable, moving North, or growing, and if you have terrific helmspeople, you can come pretty close to the route (30 mi). If the high is wobbly, dicey, unsupported by an upper level ridge, weakening, or moving South, then it is dangerous and you need to give it a wider margin. It nearly never makes sense to sail N of the computer route. Also, keep a very sharp eye out for cutoff lows. If a cutoff appears you can either pass N of it or if you have to pass S of it then try to pass well S. The winds are quite light S of a cutoff.

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