Considering the vessel as a compound beam (the hull girder), the shearing force at any section is the algebraic sum of the vertical forces (loads) to one side of the section, i.e. the net load (weight - buoyancy) acting on that part. The bending moment at any section is the algebraic sum of the moments of the loads to one side, i.e. the integral of the shearing force. The shearing force is the rate of change of the bending moment, and the bending moment is the integral of the shearing force. The point of maximum bending moment occurs where the shearing force is zero (where the shearing force changes sign), which is usually at or near midships for a ship in still water, and at the point where the net load changes sign. The maximum bending moment is the largest hogging or sagging moment, and the hull girder must be designed to withstand it.

The hatch coaming is the vertical structure around the hatch opening that raises the hatch above the deck to prevent water entering and to provide strength. Sketch: the hatch opening is bounded by a vertical coaming plate (about 600-900 mm high for a bulk carrier) welded to the deck, with a top flange (or a horizontal stiffener) and vertical stiffeners (brackets) connecting the coaming to the deck. The coaming is made of thick plate and is stiffened to resist the loads of the hatch cover and the cargo, and to provide the longitudinal strength of the deck (the coaming acts as a longitudinal girder). The hatch cover sits on the coaming with a gasket and is secured by cleats. The coaming corners are rounded and reinforced to avoid stress concentrations. The coaming provides the watertight seal for the hatch and contributes to the longitudinal strength of the hull girder.