knuckle
A convex portion of a vessel's figure where a sudden change of shape occurs, as in a canal boat, where a nearly vertical side joins a nearly flat bottom.
knuckle: the hard corner where hull shapes meet sharply
A knuckle is a pronounced ridge or angle in a vessel's hull where two planes of plating meet at a sharp break rather than a smooth curve. In a canal boat, the most obvious example is where the nearly vertical side plating joins the nearly flat bottom plating, creating a distinct line that runs the length of the hull. This is not a gentle radius or a faired transition but a definite change in direction, typically occurring at 90 degrees or close to it.
Knuckles appear because they serve a practical purpose in certain hull designs. A canal boat with a knuckle maximizes usable internal space, since vertical sides and a flat bottom allow rectangular cargo holds with minimal wasted volume. The same principle applies to barges, lighters, and other working vessels designed for inland waterways and harbors where cargo-carrying capacity matters more than sea-keeping qualities. Ocean-going ships, by contrast, use rounded bilges and faired hull forms to reduce stress and improve hydrodynamic efficiency.
Structural and practical consequences
The knuckle creates a stress concentration at the plating joint. Designers and builders must reinforce this line with substantial framing, usually a strong stringer or bracket, to distribute loads and prevent cracking. The angle of the knuckle relative to the waterline also affects stability and trim; a knuckle at or near the waterline can cause hobby-horsing and increased wave-making resistance. In practice, working boats accept these drawbacks because they operate in confined waters where speed is not critical.
Knuckles also appear on other industrial equipment and structures wherever economy of space or material favors a sharp angle over a curve. The term carries over to large storage tanks, box-like vessels, and pressed metal components. In shipbuilding terminology, a knuckle must be carefully laid out during design and marked precisely on the loft floor, since the actual fabrication of the plating and its fastening depends on getting the geometry right at the point of intersection.