If you are unsure about the other vessel's actions and the risk of collision is increasing, what signaling action is appropriate?

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When there’s doubt about the other vessel’s actions and the risk of collision is growing, you must clearly warn the other vessel and prompt a response. Five short, rapid blasts on the whistle is the internationally recognized danger signal. It communicates, in a clear audible way, that you are unsure of the other vessel’s intentions and that there is a real collision risk, so the other vessel should pay attention and take appropriate action.

Other signaling options don’t convey that urgent warning. A single long blast has a different, distinct meaning and isn’t used to indicate imminent danger. A few short blasts or other sequences are used for maneuvers or acknowledgments, not to warn about potential collision. Not signaling at all leaves the situation ambiguous and increases the chance of a misunderstanding.