First of these factors is the South Atlantic High. This is a high pressure that develops over the South Atlantic and changes from day-to-day in its size, shape and position. It normally starts to develop pretty much offshore of Rio de Janeiro, fairly small and weak. Then it grows in size and strength, gradually stretching out diagonally across the ocean toward Cape Town. Then it stretches out further, bending around below the Cape of Good Hope and generating the gales for which that area is so well known. Then the piece that goes south of Africa generally breaks off, during which there can be very intense and damaging gales in that area. The piece that breaks off moves off to the east into the Indian Ocean and the piece left in the Atlantic weakens considerably and retracts to become the start of the next cycle.
|Surface pressure map today, with the high stretched out diagonally between Rio and Cape Town, showing areas of light and stronger winds and directions. The lower right end of the high looks set to start ridging around the southern tip of the continent. Click on the images for a larger view. Image courtesy of stormsurfing.com|
Another factor is the type of boat that we are sailing. Modern shallow-bodied light displacement boats with fractional rigs go very well in light breezes and heavy traditional boats don't. That means that a modern boat can sometimes cut the corners and sail in light breezes that would stop an older and heavier boat. You can risk trying to sail the modern boat through the high but don't try to do it with a traditional boat, which must sail around the outside of the high to stay in as much wind as it can find.
The potential speed of the boat also affects where it should be at any time. The lead boats can sail through an area of decent wind but by the time that the slower boats get there the high will have a totally different character and that area may be totally windless.
This has been a problem on various editions of this race, when all boats have started on the same day and a changing high has split the fleet dramatically into two separate fleets separated by the high, leaving the slower boats dead in the water. In the 2000 race we on "Black Cat" made it through before the high blocked the slower boats behind us. In that case the high was much further north than normal and we beat/fetched all the way across the Atlantic by sailing south of the high. Then it stretched itself out behind us and blocked the unlucky boats behind us.
This time the organisers made two starts, 6 days apart. All boats rating lower that IRC 1.0 were in the first start, all others in the second start. "Black Cat", with a rating of 1.054, is one of the smallest and slowest of the 2nd start boats. Now the boats are approaching the middle of the Atlantic, the two fleets are near to each other and will soon start to mix. The net result is that the two fleets are now in a similar part of the ocean and having to take decisions based on the same atmospheric conditions.
The tracking map is interesting to study today. The first start boats are mostly grouped to the north, sailing around the outside of the high. The only exception is "Pinto Russell Marie Galante", the light mauve track leading the cluster of three at lower left. The other two are 2nd start boats that will rapidly overtake her today, currently sailing at double her speed.
|Overall view of the South Atlantic on the tracking map. The leaders are approaching half-way across. Image courtesy of xtra-track.com|
"Black Cat" has been sailing around the outside of the fleet, seen as the purple track, in close company with the Stadt 65 "INSV Mhadei", the green track. Skipper Dave Immelman chose this tactic of sailing more miles in stronger winds, anticipating that the high may block boats taking a more direct course.
|Zoomed-in view of the tracking map. Image courtesy of xtra-track.com|
This analysis is my own, from watching the tracker until now. I should have another report from "Black Cat" in another day or two, then will post the skipper's take on the voyage.