The spritsail barge developed on the Thames from lightering barges. Originally there were no deep-sea docks in London and trade into the city had to be transferred from sea-going vessels into lighters to get up into the city centre. Because the Thames is a tidal river all the way through London, these barges could be unpowered other than a single steering "sweep" and  could drift in and outwith the tide. But adding a sail, and enough crew to control it, could be a trade advantage.

          By the early 1800's barges designed specifically for sail were emerging. Hull shapes evolved away from the simple box, and the sail rig evolved to provide some weatherliness but also simplicity of handling, because crews. and their costs, were kept to a minimum. In the days before the railway and the motor car, a mainstay of the trade was hay from the fields around the estuary to feed the horses, and the inevitable result of the hay back to the fields to grow more. That was never going to pay really well.
          The spritsail was the result. The single long diagonal spar would set a huge area of mainsail, which would be furled by brailing lines, gathering the sail up against the mast instead of dropping it. Brailing could be done easily by one man instead of the two or three that would be needed to haul up a gaff spar and the weight of canvas attached on a similar size of conventional rig. Above the main could be set a large main topsail, especially useful in crowded anchorages and
light airs above the lower level turbulence. The topsail was also furled by brailing, having a very small, short gaff. Ahead of the mast there was a substantial foresail, set on a wooden or iron 'horse' across the deck so it was self-tacking. This was set on a heavyweight stay to the top of the mainmast, and at the bottom of that stay was a 6 sheave tackle that, in conjunction with the foredeck windlass, could be used to lower the mainmast to the deck and raise it again to “shoot” under bridges over the rivers. A bowsprit added two more foresails, one set to the mainmast head, and another to the head of the topmast. With the main topsail and the jib topsail down, the topmast could be lowered parallel to the mainmast and the barge, then referred to as "stumpy-rigged” was ready for bridges. In port, the bowsprit would be raised to the vertical to save on dock space. 
          The last sail was the mizzen, typically a very small sail set on a sprit with the mast aft of the helm, and the sail sheet to the aft end of the rudder. This provided very little in the way of power, being used mainly to assist in steering. Because the hull was flat bottomed, and the barge would regularly be beached at high tide to be loaded by cart at low tide. the rudder didn't extend beneath the flat bottom. It could be very ineffective, and the little mizzen set to move with the rudder could help to blow the stern around. (As it happens, the pictures to go with this text all show a larger, gaff mizzen.)
          The other big distinctive feature on the Thames barge was the leeboards These were a Chinese invention, brought to Europe by the Dutch. The fin on the lee side of the barge would be lowered. extending three or more feet below the flat bottom, and it would provide resistance to sideways slip of the shallow hull, and therefore a respectable performance across the wind. Winches to raise and lower the leeboards were put just ahead of the helm so they could be operated by the helmsman while the other hand was attending the foot of the mainmast when tacking. Barges up to 80 feet in length were crewed with only two men and a dog.
          So by 1900, there were some two thousand of these barges trading around the Thames estuary and the east coast of England. They were astonishingly versatile, operable by a tiny crew, loading and unloading almost any cargo, whether at a dock, alongside other craft, or sitting flat on the mud at the side of a field. They could operate in very shallow waters. their crews loved to boast that if a gull wasn’t walking, they could keep sailing. Their masters were a breed apart, knowing the east coast waters like no-one before or since, handling their unique craft and their balky customers equally well.

          By the mid-1920's over two centuries of development had influenced the design of these barges, and in the preceding 50 years had been accelerated by the institution of annual sailing barge matches, the oldest sailing races in the world after the America's Cup. They could, and did. handle the conditions in the English Channel and the North Sea, voyaging from southern Scotland to Plymouth, from the Channel Islands to Hamburg. 
          One of these barges, Will Everard, was destined to have one of the most interesting and varied histories of all, one that still continues. Will Everard reached Southampton with a cargo of cement on the Sunday morning, 1st. September,1939 in time to hear the declaration of war broadcast by the BBC. From that day until peace came back in May 1945 the Will Everard made 147 coasting voyages, and carried a total of 38,345 tons of commodities such as grain, sugar beet, oil cake, fertilisers, sugar, coal, and cement. She used none of the nation's precious oil fuel. She roamed the east and south coast of England from London north to Yarmouth and south to Southampton.

Nautical lore – Thames spritsail barges - Part 1.