What was the purpose of the shipbuilding program mandated by Second Vinson Act ?
It was an expanded " South Dakota " , carrying either twelve 16 - inch / 45 caliber Mark 6 guns or nine 18 - inch ( 460 mm ) guns — the latter armament being dropped after the 31 March agreement — with more armor and a power plant large enough to drive the larger ship through the water at the same speed as the " South Dakota " s , .