If the ship receiving the distress alert is unable to do so (e. g. because it is disabled with an engine breakdown) or, in the special circumstances of the case, considers it unreasonable or unnecessary to do so.
(The master of a loaded gas carrier might consider the obligation to assist a tanker on fire unreasonable. The master of a ship in a busy shipping area might consider the obligation to assist a ship 100 miles away unnecessary, where he knew that many other ships would be closer to the distress position.)
