Rule 20.1

A boat may hail for room to tack and avoid a boat on the same tack bu hailing 'room to tack'. However, she shall not hail unless

(a) she is approaching an obstruction and will soon need to make a substantial course change to avoid it safely, and

(b) she is sailing close-hauled or above.

In addition, she shall not hail if the obstruction, is a mark and a boat that is fetching it would be required to change course as a result of the hail.