The Collaborative International Dictionary of English v.0.48:

Tunnage \Tun"nage\ (t[u^]n"n[asl]j; 48), n. [From Tun; cf.
   Tonnage.]
   See Tonnage.
   [1913 Webster]

WordNet (r) 3.0 (2006):

tunnage
    n 1: a tax imposed on ships that enter the US; based on the
         tonnage of the ship [syn: tonnage, tunnage, tonnage
         duty]