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One prolonged and two short blasts is 'D' in Morse signals. It is a signal to be used in restricted visibility by a vessel not under command[NUC], a vessel restricted in her ability to manoeuvre[RAM], a vessel constrained by her draught, a sailing vessel, a vessel engaged in fishing and a vessel engaged in towing or pushing another vessel.
lights must be on during periods of restricted visibility
one prolonged blast
one prolonged blast
One Prolonged Blast.
one prolonged blast
One prolonged blast
one prolonged blast
One Prolonged Blast.
one prolonged blast
One prolonged blast
Lights must be on during periods of restricted visibility. ref: EQ-2