Areal density (plating)

Areal density of plating is the mass per unit area of a plate or stiffened panel, used to roll up structural weight in a steel weight estimate. For bare plate it is the thickness times the steel density, about 7,850 kg/m^3, so a 12 mm plate weighs 94.2 kg/m^2. For a stiffened panel the stiffener mass is added back as an equivalent smeared thickness, giving an effective areal density that multiplies the panel area in the weight rollup. Naval architects apply this panel by panel across the structural model to build the structural weight group, which feeds the lightship and then the deadweight balance. Margins for weld metal and rolling tolerance are added.