A Mersey flat is a two masted, doubled-ended barge with rounded bilges , carvel build and fully decked. Common from the 1730's to 1890s.
As the name suggest these Flats originbated on the River Mersey but where also used on Rivers Irwell and Weaver.
The length of a flat was from 62 to 70 feet long, with a 6-feet draught and a beam of 14 feet 9 inches to 17 feet. They could carry up to 80 tons of cargo.
See also
Other Types of Sailing Craft