Official rule text as published in the U.S. Coast Guard Navigation Rules
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. [International: 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.]
Color note: black text applies generally; blue is International Waters; green is Inland Waters.
MyCOLREGS explanation
Summary
If you need to get another vessel’s attention, you may use light or sound signals, as long as they cannot be mistaken for a signal this rule book already defines elsewhere — flashing your running lights or sounding a whistle pattern that means something else just creates confusion. You can also point a searchlight toward danger without shining it so as to embarrass (blind or confuse) another vessel. The International text specifically cautions against high-intensity strobe-type lights for this purpose, since they too are easily mistaken for something else.
Real World Example
A well-meant warning that accidentally mimics an existing signal - flashing a running light instead of a plain spotlight, or blowing a pattern close to a maneuvering signal - can be read as that maneuver actually happening, sending the other vessel the wrong way instead of putting her on alert.
Practice Questions
Q1. You want to get the attention of a vessel that seems unaware of a hazard ahead of her. Which of the following is consistent with Rule 36?
A. Use the pilot vessel's four-short-blast identity signal.
B. Sound the five-short-blast danger signal even though there is no risk of collision with your own vessel.
C. Fire a red flare, since any attention-getting method is acceptable.
D. Direct your searchlight beam toward the danger, without shining it so as to embarrass another vessel.
Q2. Rule 36 says any attention-getting light or sound signal must meet what condition?
A. It must be a strobe light for maximum visibility.
B. It must be sounded on the whistle only, never by light.
C. It must match one of the maneuvering signals in Rule 34.
D. It must not be capable of being mistaken for a signal authorized elsewhere in the Rules.
Practice Question Answers
Q1.
D. Direct your searchlight beam toward the danger, without shining it so as to embarrass another vessel. Rule 36 specifically permits directing a searchlight beam toward the danger as an attention-getting signal, so long as it is not done in a way that embarrasses (blinds or confuses) another vessel. A borrows a signal reserved for a different purpose entirely. B misuses the five-short-blast danger signal for a situation where your own vessel isn't at risk. C overstates Rule 36 - it doesn't sanction just any attention-getting method, only ones that won't be confused with an authorized signal.
Q2.
D. It must not be capable of being mistaken for a signal authorized elsewhere in the Rules. Rule 36 requires that any signal used to attract attention cannot be mistaken for a signal already given a specific meaning elsewhere in the Rules, to avoid creating confusion instead of clarity. A invents a specific device requirement. B wrongly limits attention signals to sound only, when light is also allowed. C assumes it must match an existing maneuvering signal, when the actual condition is simply that it must not be mistaken for one.