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):

PRIMAGE, merc. law. A duty payable to the master and mariner of a ship or
vessel; to the master for the use of his cables and ropes to discharge the
goods of the merchant; to the mariners for lading and unlading in any port
or haven. Merch. Dict. h.t.; Abb. on Ship. 270.
     2. This payment appears to be of very ancient date, and to be variously
regulated in different voyages and trades. It is sometimes called the
master's hat money. 3 Chit. Com. Law, 431.