Source : Webster's Revised Unabridged Dictionary (1913)

Torpedo tube \Torpedo tube\ (Nav.)
   A tube fixed below or near the water line through which a
   torpedo is fired, usually by a small charge of gunpowder. On
   torpedo vessels the tubes are on deck and usually in
   broadside, on larger vessels usually submerged in broadside
   and fitted with a movable shield which is pushed out from the
   vessel's side to protect the torpedo until clear, but
   formerly sometimes in the bow. In submarine torpedo boats
   they are in the bow.

Source : WordNet®

torpedo tube
     n : a tube near the waterline of a vessel through which a
         torpedo is fired