A Thames sailing barge is a type of commercial sailing boat once common on the River Thames in London. The flat-bottomed barges with a shallow draught and leeboards, were perfectly adapted to the Thames Estuary, with its shallow waters and narrow tributary rivers. The larger barges were seaworthy vessels, and were the largest sailing vessel to be handled by just two men. The average size was about 120 tons and they carried 4,200 square feet (390 m2) of canvas sail in six working sails. The mainsail was loose-footed and set up with a sprit, and was brailed to the mast when not needed. It is sheeted to a horse, as are the foresails so need no attention when going about. The topsail was usually first sail on and last sail off, being fixed to the topmast by hoops. In the upper reaches of the rivers and constricted harbours it reached into the clear air, and when approaching a berth casting off the halliard would drop it immediately killing the forward drive. The mizzen boom is sheeted down to the long shallow rudder, which helps the vessel through the wind when tacking. The masts are mounted in tabernacles so they can be lowered to pass under bridges, with the anchor windlass used to lower and raise the gear via triple blocks. This took considerable effort and to aid in the process 'hufflers' were often used who would come on board to help with raising the gear. The bowsprit where fitted could be 'topped' where space was limited.