Terminology
Purists tend to write the term with a space (narrow boat) when referring to an original (working) boat or a replica, omitting the space when referring to modern canal boats of around 7' beam and used for leisure including residence. The single word 'narrowboat' has been adopted by authorities such as the Canal and River Trust and the magazine 'Waterways World' to refer to all boats built in the style and tradition of the narrow canal locks.
Although some narrow boats were built to a design based on river barges and many conform to the strict definition of the term, it is incorrect to refer to a narrowboat (or narrow boat) as a barge. In the context of the British inland waterways, a barge is usually a much wider, cargo-carrying boat or a modern boat modelled on one, certainly more than 7 feet (2.1 m) wide.
It is also incorrect (or at least incongruous) to refer to a narrowboat as a longboat, although this name was sometimes used in the Midlands in working-boat days.
Usage has not quite settled down as regards (a) boats based on narrowboat design, but too wide for narrow canals; or (b) boats the same width as narrowboats but based on other types of boat.
Read more about this topic: Narrowboat