A lock sets the boat’s usable width
A canal lock is a chamber between gates that raises or lowers boats between stretches of water at different heights. Its inside width is a hard limit: a boat must clear the walls and gates not only when it enters, but also as the water level changes. Narrowboats are therefore built with a little clearance rather than to match the lock exactly, leaving room for careful steering and small variations in the masonry.
That clearance matters in a confined chamber. The boat has to be positioned between the gates before they close; then water flows in or out until the boat is level with the canal ahead. A hull wedged against a wall can make an already slow maneuver more difficult, so a fit that works on paper is not enough.
Width trades capacity for access
The slim hull is a compromise. A wider boat could carry more in a single trip, but it would be excluded from locks built for narrower craft—and from the connected canals beyond them. Narrow dimensions let a boat use a larger network of restricted waterways, while limiting how much it can carry and how much space it offers aboard.
The lock thus shapes more than a boat’s outline. It determines which routes remain open to it. A few inches of clearance can be the difference between joining a long chain of canals and stopping at the entrance to a lock.
