The beat

A sailing boat cannot sail directly head to wind. It therefore has to cross against the wind. A leg which is sailed head to wind is called a beat. America’s Cup yachts are able to sail very close to the wind – in an angle of 40 degrees facing the wind.

There are two "golden rules" to be observed on the beat. The first rule applies to the leading boat (usually the yacht which won the start) as well as to the trailing yacht. The rule of thumb is: If the wind shifts to the left, sail to the left – and vice versa. The boat following this rule generally enjoys the shorter path to the weather mark after the wind shift. In other words, the distance has been shortened.

The second rule of thumb only applies to the leading boat: Always stay between the breeze and the opponent. This results in two advantages: The rival has to battle against "bad air" caused by the sails of the leading yacht. Moreover, the leading yacht will catch a breeze before its opponent.

To turn the argument on its head, by using this tactic the leading yacht avoids the risk of the rival separating too much. Otherwise the rival could take advantage of a wind shift or gust, which the leading boat cannot access.

These tactics left aside, the right hand side of the course is favoured – given that wind speed and direction on the leg are constant. This is not least due to the fact that the boat which approaches the rounding mark from the right hand side sails on starboard tack and therefore enjoys the right of way.

Protect the Right Hand Side

Yellow is on the more favorable side of the course and wants to defend it. Yellow could easily cross Blue`s course. In doing so, however, Yellow has achieved exactly the opposite of what it wanted. Consequently, Yellow slows down. Thus, the right side of the course is not given to Blue "freely".

Blue, on the other hand, can try to dip Yellow's stern in order to become the lee boat and attain the right of way.

This is exactly what Yellow avoids by tacking herself leewards to Blue. Rule 11 and Rule 13 apply.

Now, Blue can follow Yellow or tack. There is a very good chance that option 1 won't happen because Blue has to sail in Yellow's "bad air" and won't be able to pass Yellow.

So, Blue has to turn away and can give it another try later on when both yachts tack again and sail right toward each other.

The Run

- Source: Oracle BMW Racing