In addition to the prospect of gale force winds and huge waves on the approach to Sanya, strong ocean currents are set to further complicate the teams’ tactical options.
"Strong wind against current produces a punishing sea state with shorter steeper waves making for dangerous slamming conditions"
The South China Sea has a notorious reputation for strong monsoon winds and currents which, when combined, can make sailing especially difficult in the region.
The shortest route to Sanya takes the boats close to the coast of Vietnam, but according to race technical partner, Tidetech, suppliers of the detailed data used by the fleet’s navigators to plot ocean currents, the strongest flow runs south down the coast, potentially making this area a no-go zone for boats.
Up to three knots flows down the eastern edge of Vietnam in a wide band reaching out as far as 60 nautical miles miles offshore. Over the final few days of this leg the effects of the current could be further enhanced by strong northerly winds.
The crews will be keen to avoid sailing directly into the current and near the Vietnam coast there will be two to three knots against them, potentially keeping the fleet further offshore to avoid the worst of the flow.
There the boats could find some helpful northward current, but the downside is that strong wind against current produces a punishing sea state with shorter steeper waves making for dangerous slamming conditions for the boats.
On the final approach to the Leg 3 finish, a tidal current sweeps across the south-eastern edge of Hainan Island from east to west and across the entrance to Sanya itself.
Tidetech is a technical supplier to the Volvo Ocean Race providing teams with oceanographic data comprising ocean currents, sea surface temperatures and tidal data.
The data is obtained from satellite measurements of sea surface height, which oceanographers use to construct a global map of ocean surface heights.
The strength and direction of the current can be calculated from this information, similar to the way a weather map of high and low pressure systems allows meteorologists to estimate wind.
Find out more about the data Tidetech makes available to the Volvo Ocean Race teams in this video.