Why cruiser support called for 28 knots

In this fleet model, cruising speed is about 18 knots and the battle line accelerates to roughly 21 knots on contact. The cruiser reconnaissance line operates at about 28 knots. Battlecruisers are expected to outrun that line, while fast battleships need to maintain contact so they can support it and reinforce the Grand Fleet's leading echelon against an enemy crossing its T. The tactical rule of thumb gives a three-knot advantage enough margin to control range. Six knots adds freedom to approach and withdraw, including running attacks. A seven-knot advantage provides extra allowance, explaining the 28-knot aim against a 21-knot battle line.