Because of their slenderness, some narrowboats seem very long. The maximum length is about , which matches the length of the longest locks on the system. Modern narrowboats tend to be shorter, to permit cruising anywhere on the connected network of British canals — including on canals built for wider, but shorter, boats. The shortest lock on the main network is Salterhebble Middle Lock on the Calder and Hebble Navigation, at about long. However, the C&H is a wide canal, so the lock is about wide. This makes the largest "go-anywhere-on-the-network" narrowboat slightly longer (about ) than the straight length of the lock, because it can (with a certain amount of "shoehorning") lie diagonally. Some locks on isolated waterways are as short as . Where it was possible to avoid going through locks, narrow boats were sometimes built a little larger. Wharf boats or more usually 'Amptons, operated on the Wolverhampton level of the Birmingham Canal Navigations and were up to 89 feet in length and 7 foot 10.5 inches wide.