What does one prolonged blast plus one short blast signal to a river lock attendant?

You want to go through the lock

Explanation:

When an individual or sailor is nearing the Lock, s/he can delay movement for at least about 130 meters away from the lock, such that the signal can enter the lock. Thus, an individual will then notify or alert the lock attendant that s/he wish to pass through the lock. To achieve this, the sailor will then sound one prolonged blast followed by one short blast of ones boat's sound-producing device.

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