The Collaborative International Dictionary of English v.0.48:

Primage \Pri"mage\ (?; 48), n. [F.] (Com.)
   A charge in addition to the freight; originally, a gratuity
   to the captain for his particular care of the goods
   (sometimes called hat money), but now belonging to the
   owners or freighters of the vessel, unless by special
   agreement the whole or part is assigned to the captain.
   --Homans.
   [1913 Webster]

Bouvier's Law Dictionary, Revised 6th Ed (1856):

HAT MONEY, mar. law. The name of a small duty paid to the captain and
mariners of a ship, usually called primage. (q.v.)