Source : Webster's Revised Unabridged Dictionary (1913)

Towboat \Tow"boat`\, n.
   1. A vessel constructed for being towed, as a canal boat.

   2. A steamer used for towing other vessels; a tug.

Source : WordNet®

towboat
     n : a powerful small boat designed to pull or push larger ships
         [syn: {tugboat}, {tug}, {tower}]