The leaders of the Vendée Globe have been in the South Seas for just a week, and the Roaring Forties have already claimed numerous victims. This is certainly not an easy area to sail, and it is far from over. As we will see, the leading group faces another violent storm in the next few days.
But let's look back first - and at a number of failures that is worrying in several respects. Three top skippers have been hit in the past few days. Two of them - SamanthDavies on "Initiatives Cœur" and Sébastien Simon on "ArkéPaprec" threw collisions with unidentified floating objects out of the running. Kevin Escoffier even lost his boat when he shot down a steep wave at breakneck speed as if in a dive and the hull of his "PRB" literally shattered.
It may seem like a coincidence that all incidents occurred around the same time and in the same sea area. But there is some evidence that it had to do with the specifics of the region in which they sailed: almost exactly south of the Cape of Good Hope.
Figure 1: The Agulhas Current can reach up to 5 knots on the East African coast before breaking into separate eddies in the Southern Ocean. The positions of "Initiatives Cœur" and "ArkéPaprec" show that their collisions were exactly on the borderline of the current vortices. "PRB" broke nearby, where strong currents aggravate the sea
There is a powerful ocean current, the infamous Agulhas Current, which originates from the Indian Ocean and descends south along the east coast of Africa.
Where it separates from the continent, at about 40 degrees south latitude, it meets the eastward flowing currents of the South Sea caused by the wind. In this zone, small eddies of warm water split off from the Agulhas Current.
There are two things to consider here: First, the Agulhas Current carries flotsam that it gathers along the East African coast. The objects are carried in the surface water up to the mentioned convergence zone at 40 degrees south latitude - this is exactly where Sam Davies and Seb Simon sailed when they had their collisions.
Second, the south-westerly setting Agulhas Current acts against the prevailing wind and wave directions, moving eastwards. These opposing forces lead to a very irregular, stepped sea state, and they can also increase the wave height.
The region is notorious for monster lakes - called "freak waves" in English, which are at least twice as high as the mean wave height. We are waiting to learn more about what exactly happened to Kevin Escoffier's "PRB"; it is quite likely that conditions like these contributed to its sudden disaster. When looking at the weather for the next few days, there is an interesting point regarding the strategy - and that is how slowly the top-ranked skippers sail
Figure 2: The infrared satellite image from Sunday 0815 UTC. One can clearly see the cold front, which stretches from northwest to southeast and moves in the direction of the fleet
In fact, the conditions seem too tough for foiling. It is becoming increasingly clear that many sailors deliberately slow down their pace in order to keep the boats going. We have rarely seen averages above 20 knots. Both the accelerations and the decelerations in the waves are much greater - the speed often oscillates between 10 and over 30 knots. Some of the conventional hydrofoil boats that accelerate less extremely have therefore managed to make up many miles on the front runners, such as Benjamin Dutreux and Damien Seguin. The skippers of the foiling Imocas must take this into account when determining their strategies for the fifth week of the Vendée Globe. At the moment they are only sailing at around 80 percent of their optimal speed, which has an impact on their position and future route selection - in particular on the question of whether or not they manage to stick to a weather system
At the moment a severe depression is approaching the fleet. It pulls in from the south in a north-easterly direction over the top field. It is already a very large and powerful system, and in the next few days we will see it strengthen along the associated cold front. A secondary low that crosses the course of the fleet is also developing along this cold front. An explosive situation. Warm air directed south by a high in the north is the reason for this. A very active cold front with strong winds and storms develops where it meets the polar air of the deep
Figure 3: Monday, December 7th at 0900 UTC. Boris Herrmann's "Seaexplorer" (dark gray diamond) on the back of the front after he has necked on the port bow. Charlie Dalin and Thomas Ruyant (yellow and blue diamonds) are still sailing in the north-west wind off the front on the starboard bow
Over the next 36 hours, the skippers have to estimate and decide how long they can or want to sail in front of the cold front before they are overtaken by it. The north-westerly wind direction that prevails in front of the front enables a direct east route - and thus the chance to break away. However, this strategy also requires caution - because an intensifying cold front can lead to extremely difficult conditions in the Southern Ocean. A more cautious course might end up being quicker, but in any case less risky.
Boats like "Apivia" and "LinkedOut" can theoretically stay in front of the front until Wednesday, while Imocas a little further back like Boris Herrmann's "Seaexplorer" will probably stay astern tomorrow morning. It will be a really difficult balance to strike - especially since only a third of the lap around the world has been made and there are still about 50 days of this race to go.
Figure 4: Friday, December 11, 1500 UTC. The interplay of two highs creates a zone of light wind that slows the leaders and allows the boats behind to catch up a few miles
Once this front is pulled through, the wind will abruptly turn southeast and decrease to a reasonable 20-25 knots. Such wind speeds are actually much better for sailing faster so getting dropped may not be the worst part of it. However, the wind angle does not fit optimally - it becomes a little too low for the direct course. When the low moves east, the high in the northwest will determine the weather. The wind turns slowly from west to southwest.
Next Saturday the leaders will likely be level with Western Australia. A difficult transition between the high in the west and a high near Tasmania may develop here. In between there is a zone with weak wind. This can be a chance for the following skippers to catch up a few miles as the fleet shrinks. Boris Herrmann will see current route calculations at the end of next week only 250 nautical miles behind Charlie Dalin. But first of all, you have to master a few days with difficult conditions in the southern ocean.