A rapid change in the weather data is the most likely reason for the decision for Leg 4 leader SHK Scallywag opting to go into Stealth Mode.

Previously, the David Witt skippered entry, which hails from Hong Kong, was set to be able to sail straight the Luzon Strait and then make any southing course adjustment required in the final hours before finishing.

However, over the space of two skeds, the recommended routing changed from just two of the four feeds used in Predictwind.com recommending an immediate gybe south, to a situation where all four feeds recommended the immediate gybe option. (On board the Volvo racers they have only organiser supplied weather data, and the feeds available through Predictwind, used by Sail-World are more extensive, giving a more balanced picture of what lies ahead and the options. The two feeds provided by the organisers are the green (GFS) and (Yellow ECMWF. As can be seen in the first scenario they tell quite a different picture from the other two.)

At that point, SHK Scallywag went into Stealth Mode where their position is concealed from the other boats and race fans.

It is a sure bet that she has gybed south, which means she will spend some of her lead which was put at around 80-90nm at the time she went into Stealth Mode.

However, her challenges don’t end there.

Libby Greenhalgh, navigator and former British Olympic Team Meteorologist, aboard Scallywag will have to make a call on whether to take a smaller bite south, recommended by two of the feeds and then still wind up close to the Taiwanese coast before heading down to the finish.

Or Scallywag can indulge in a more complex gybing sequence which will take her further south, at the risk of spending more of her lead, before striking out on a more direct route for the finish.

Complicating the decision is the string of islands, some large and some small extending across the mouth of the Luzon Strait which will have some impact on her course options.

A check on Google Earth shows that the chart does match the satellite picture, for those who remember the Vestas Wind incident on Leg 2 of the previous Volvo Ocean Race.

The course options for the second-placed Vestas 11th Hour Racing are similar but less extreme than those for the leg leader.

While Vestas 11th Hour is recommended to gybe south again, it is a lesser dip than Scallywag, but even following that modest dip south, Vestas then gets routed well to the north before heading for the finish line, and that scenario is likely to change.

Leg 4, Melbourne to Hong Kong, day 16, triple-headed (3 front sails in the air) tradewind sailing towards the Philippines on board Vestas 11th Hour. – photo © Amory Ross / Volvo Ocean Race

Scallywag does have two advantages outside the weather routing.

Firstly Libby Greenhalgh is a qualified meteorologist and will be well able to interpret the wider weather scenarios – other than what comes from the two official weather feeds allowed to be used by the competitors. Interpreting the wider weather scenarios also means sticking her head out of the hatch and reading the physical signs. She was also the British Olympic team meteorologist for the 2008 Olympics in Qingdao China – north but still on the China Sea.

Secondly, David Witt is now based in Hong Kong and has done some racing in the area – so in terms of local knowledge, Scallywag may have an advantage.

Long story short is that Scallywag is likely to place less weight on the routing data and recommendations and sail more by the seat of their pants.

Their hope, of course, is that the weather does not become too variable, in which case their lead and leg win which has looked so sure for several days, could come under threat as they run the gauntlet of the Luzon Strait.

by Richard Gladwell, Sail-World.com