The hull girder is an abstraction of a vessels structure used to quickly estimate longitudinal stress issues. The hull girder is an equivalent box girder formed by considering the vessels continous longitudinal strength members present in the midship section. Decks and horizontal shell plating are considered similar to flanges, and and longitudinal bulkheads and sideshell are considered as webs. Sloping or curved members are resolved into their equivalent horizontal and vertical components. Applying hogging or sagging moments allows for the estimation of stress, primarily on the weather deck (a common failure point for ships).
Working from the midship section, all structure which is discontinous over the length of the vessel is disregarded. The moments of inertia of the remaining structure is calculated, as well as the midships sections neutral axis. Using the parrallel axis theorem, the moments of inertia of the constituent components is summed and considered at the neutral axis. Applying a bending moment, the stress is considered at the most distant fibres: As many vessels are designed to have a low centre of gravity, have double bottoms, and heavier lower plating, the furthest fibre is typically the weather deck.