Widebeam

A widebeam is a canal boat built in the style of a British narrowboat but with a beam of 2.16 metres (7 ft 1 in) or greater.

[citation needed] When English canals were first built to assist transport during the Industrial Revolution, locks were only 2.1 metres (7 ft) wide.

Inevitably, their greater beam makes widebeams more stable than narrowboats, but, with their bow welldeck vulnerable to flooding, they are still not true seagoing boats.

[16] At the junction of the Paddington Arm and the main Grand Union Canal, if the boat were to turn south through Brentford and navigate the Thames to Limehouse Basin, its maximum length is further restricted to 22.2 metres (72 ft 10 in).

If the boat were to turn north and travel to Rickmansworth, its maximum length must not exceed 21.75 metres (71 ft 4 in).

From Rickmansworth, a boat may travel north up the Grand Union, but at Leighton Buzzard there is a lock that is only 3.81 metres (12 ft 6 in) wide.

So any boat with a beam of 4.22 metres (13 ft 10 in) able to navigate canals and rivers in and around London will be unable able to proceed further north.