The Collaborative International Dictionary of English v.0.48:

Steerageway \Steer"age*way`\ (-w[=a]`), n. (Naut.)
   A rate of motion through the water sufficient to render a
   vessel governable by the helm.
   [1913 Webster]

WordNet (r) 3.0 (2006):

steerageway
    n 1: (nautical) the minimum rate of motion needed for a vessel
         to be maneuvered