law: Indian Ports Act, 1908
  51. Master to hoist number of vessel 
  (1) The master of every inward or outward bound vessel, on
  
  arriving within signal distance of any signal-station established within the
  
  limits of the river Hooghly, or within the limits of any part of a river or
  
  channel leading to a port subject to this Act, shall, on the requisition of the
  
  pilot in charge of the vessel, signify the name of the vessel by hoisting the
  
  number by which she is known, or by adopting such other means to this end as
  
  may be practicable and usual, and shall keen the signal flying until it is
  
  answered from the signal-station.
  (2) If the master of a vessel arriving as aforesaid offends
  
  against sub-section (1), he shall be punishable for every such offence with
  
  fine which may extend to one thousand rupees.