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pick your poison

The South China Sea is notorious for wave and current interactions that generate rough and challenging conditions. Strong monsoon winds and monsoon currents combine to make sailing especially difficult in this region.

The shortest route to Sanya would take the teams straight up the coast of Vietnam. But with the strongest current in this part of the South China Sea flowing south down the coast, it becomes a no-go zone for boats heading north. The coastal flow down the eastern edge of Vietnam can reach speeds of up to 3kts in a wide band reaching as far out as 60 miles offshore. The persistent current is further enhanced in being driven by the northerly monsoon winds.

With the fleet beating north, coupled with 2-3kts of current against them, the boats will have little choice but to head further offshore to get out of the stronger flow to maintain best velocity to the course. The vital aspect for the fleet is velocity made good to course (VMC). Current against the fleet does more than reduce the speed over ground, it compounds the effect on VMC already reduced in going upwind. Strong current on the nose is to be avoided.

It’s likely the teams will find some northward current further offshore, but with this comes the spectre of a punishing sea state. With wind against current, waves become shorter and steeper making for a rougher ride and some slamming conditions onboard. Finally, on approach to Sanya, a tidal current sweeps across the south-eastern edge of Hainan Island from east to west and across the entrance to Sanya itself.

Tidetech’s ocean current data for the teams 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.

Tidetech is a technical supplier to the Volvo Ocean Race providing teams with oceanographic data comprising ocean currents, sea surface temperatures and tidal data. Watch this video to learn more about the data being supplied to the Volvo Ocean Race by Tidetech