Just thinking,What would 60mm(a 20mm face plate riveted to a 40mm plate) of steel(with a tensile rating of either 350-400mpa or 450-500mpa) stand up to? Without seriously cracking, buckling, breaking or bending?
I'd assume quite a bit, considering your average unarmored ship's hull is about the same thickness of a dime, if I understand correctly.
Of course, materials engineering is just one component of how much of a beating a structure can take. We've discussed the advantages and disadvantages of "mercantile" vs "warship" construction and why many warships follow the former.