You will perhaps already have noticed that, shortly before the Start, when the boats have lined up in the Start area, that the wind is significantly weaker there, even if you are in the front row and have clear wind.
The reason for this effect is the congestion which forms in front of the regatta field producing a not insignificant resistance to the wind.
In addition to this effect, the wind blows around this congestion and is significantly stronger at the edges, as shown by the narrower lines.
If we find ourselves on the left-hand side of the congested area, the wind flows to the right soon after the start and we can run higher. But take care, after the congestion zone the wind turns back to its original direction again.
This left wind shift should not be misinterpreted as an oscillation to the left, and mislead us into a tack to the right-hand side of the course.
If we find ourselves on the other side of the congestion, we have the exact opposite effect and feel a header and the leeward boats can sail closer to the wind than we can. If we have the freedom to tack, a quick tack and a short run to the right is called for. Sailing on port tack on the right side of the congestion we don’t have to sail against the wind shift and can also benefit from the stronger wind at the edge of the congestion.
The starting line is often relatively clear in the middle, and a group of boats forms on the right and left of it, each group forming their own independent congestion ‘cloud’.
In this case, the yellow boat can also benefit from the wind shift. In the middle, between these two congestion zones the wind will be at its strongest for a short time; a further advantage of this position.
However, don’t forget: the pre-requisite for being able to use these effects is a start in the first row. Under cover of other boats, you won’t even notice them.
Special thanks – Article by Peter Czajka – The Tactics of Sailboat Racing