Beam

The maximum width of a vessel, measured across the hull at the stated reference level.

Beam is a principal dimension, but published figures may refer to moulded beam, extreme beam or another defined measurement and can include or exclude projections. The widest point is not necessarily at the vessel’s longitudinal midpoint.

Beam influences space, resistance and stability together with hull form, displacement and loading; a wider vessel is not automatically safer or slower. The word also means a transverse structural member, and “on the beam” describes a direction approximately at right angles to the centreline.