19.1 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. However, she shall not hail unless safety requires her to make a substantial course change to avoid the obstruction. Before tacking she shall give the hailed boat time to respond. The hailed boat shall respond by either

(a) tacking as soon as possible, in which case the hailing boat shall also tack as soon as possible, or

(b) immediately replying ‘You tack’, in which case the hailing boat shall tack as soon as possible and the hailed boat shall give room, and rules 10 and 13 do not apply.

19.2 Rule 19.1 does not apply at a starting mark surrounded by navigable water or at its anchor line from the time boats are approaching them to start until they have passed them or at a mark that the hailed boat can fetch. When rule 19.1 applies, rule 18 does not.