When a vessel displays two prolonged blasts followed by two short blasts, what does this signal?

The correct interpretation of the signal consisting of two prolonged blasts followed by two short blasts indicates a vessel's intention to overtake another vessel on the port side. This signaling is crucial in maintaining navigational safety and understanding between vessels in close quarters or in congested waters.

According to the International Regulations for Preventing Collisions at Sea (COLREGs), a vessel intending to overtake another vessel must sound their horn in this specific sequence to clearly communicate their intentions. This approach helps to prevent confusion and potential collisions as vessels maneuver in tight spaces or along narrow channels.

The other options don’t align with this signal. For instance, a single sound would represent a different action, such as turning or indicating anchoring intentions. Following the proper signaling protocol is essential for safe navigation and effective communication on the water.