Why Vanguard and Iowa trade speed advantages

Could Vanguard be faster than Iowa in the North Sea while Iowa was faster in the Caribbean?

Yes; Iowa's design favors calmer water, while Vanguard's seakeeping lets her sustain speed better in rough seas.

Iowa's hull and bow reflect requirements that included passage through the Panama Canal and high speed in calmer water. Vanguard's bow was shaped for rougher conditions, allowing her to maintain speed better as the sea state worsened. The later comparison describes Iowa's bow as extending forward without providing positive buoyancy of its own, although the precise cantilever point is recalled uncertainly. This arrangement helps explain the tendency of the bow to drive into waves. The faster ship in calm water can therefore lose its advantage in heavy seas.