20  Room to Tack at an Obstruction
20.1 Hailing and Responding
When approaching an obstruction, a boat sailing close-hauled or above may hail for room to tack and avoid another boat on the same tack. After a boat hails,
(a) she shall give the hailed boat time to respond;
(b) the hailed boat shall respond either by tacking as soon as possible, or by immediately replying ‘You tack’ and then giving the hailing boat room to tack and avoid her; and
(c) when the hailed boat responds, the hailing boat shall tack as soon as possible.

20.2  Exoneration
When a boat is taking room to which she is entitled under rule 20.1(b), she shall be exonerated if she breaks a rule of Section A or rule 15 or 16.

20.3 When Not to Hail
A boat shall not hail unless safety requires her to make a substantial course change to avoid the obstruction. Also, she shall not hail if the obstruction is a mark that the hailed boat is fetching.