Final answer:
When a vessel is trying to overtake another vessel on the other vessel's port side, it would sound one short blast of the whistle.
Step-by-step explanation:
In a narrow channel, when a vessel is trying to overtake another vessel on the other vessel's port side, it would sound one short blast of the whistle. This signal indicates the intention to overtake on the port side. The vessel being overtaken should respond with one short blast to acknowledge the signal.