Build quality and class variation

Warships are not assembly-line products. Even nominal sister ships can leave a yard with bulkheads a few inches from their intended positions or different steel fitted. On older ships, builders sometimes compensated for a misplaced bulkhead with thicker steel instead of moving it. A later engine variant can also be slightly heavier or lighter than the unit originally specified. A hundred-ton difference in a 10,000-ton design is only one percent, yet many modest substitutions can accumulate into tens or hundreds of tons across a large vessel. Trials establish how the ship that was actually built compares with its design and with the rest of its class.