How 21 knots constrained US standard battleships

The American standard battleships combined heavy armor and numerous guns in a coherent battle line. A 21-knot design speed was defensible when chosen in 1911, but ships ordered in 1916 and later retained it as machinery and rival designs improved. Britain's plan was moving toward a battle line of roughly 23 knots. A fleet tied to 21-knot capital ships also constrained the movement of its cruisers and destroyers. Treating 21 knots as a minimum could have preserved a common baseline while allowing later ships to exploit more powerful machinery.