The Collaborative International Dictionary of English v.0.48:

Towboat \Tow"boat`\, n.
   1. A vessel constructed for being towed, as a canal boat.
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   2. A steamer used for towing other vessels; a tug.
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The Collaborative International Dictionary of English v.0.48:

Tug \Tug\, n.
   1. A pull with the utmost effort, as in the athletic contest
      called tug of war; a supreme effort.
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            At the tug he falls,
            Vast ruins come along, rent from the smoking walls.
                                                  --Dryden.
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   2. A sort of vehicle, used for conveying timber and heavy
      articles. [Prov. Eng.] --Halliwell.
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   3. (Naut.) A small, powerful steamboat used to tow vessels;
      -- called also steam tug, tugboat, and towboat.
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   4. A trace, or drawing strap, of a harness.
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   5. (Mining.) An iron hook of a hoisting tub, to which a
      tackle is affixed.
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   Tug iron, an iron hook or button to which a tug or trace
      may be attached, as on the shaft of a wagon.
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WordNet (r) 3.0 (2006):

towboat
    n 1: a powerful small boat designed to pull or push larger ships
         [syn: tugboat, tug, towboat, tower]