Signals to Attract Attention.
If necessary to attract the attention of another vessel, any vessel may make light or sound signals that cannot be mistaken for any signal authorized elsewhere in these Rules, or may direct the beam of her searchlight in the direction of the danger, in such a way as not to embarrass any vessel Any light to attract the attention of another vessel shall be such that it cannot be mistaken for any aid to navigation. For the purpose of this Rule the use of high intensity intermittent or revolving lights, such as strobe lights, shall be avoided.
Light or sound signals, which could be mistaken for signals authorised elsewhere in the Rules, must not be used to attract the attention of another vessel. In particular, signals, which could be confused with those authorised under Rule 37 and Annex IV, are not to be used, unless the vessel is in distress. A very long blast on the whistle could, for instance, be taken to be “a continuous sounding with any fog signalling apparatus” (Annex IV paragraph 1 (b)).