A destroyer screen needed enough speed to move from one side of a battle fleet to the other while the larger ships continued at speed. A speed advantage of at least seven knots over the battle fleet was desirable as battlecruisers and battleships grew faster. Destroyers waiting on the side away from the enemy could avoid obscuring their own fleet's visual gunnery with smoke and reduce exposure to secondary batteries and short-falling shells. They still needed to pass through the battle line quickly if a torpedo threat appeared on the engaged side, without forcing the larger ships to slow down. Exercises explored different ways for destroyers to peel off and cross the fleet's course. The requirement was for speed usable in rough weather as well as a high trial figure.
Notable Point · 42:03-47:29
Why fleet screens needed a speed margin
The demand for high speed came from the tactical problem of moving destroyers around a battle line without blocking fire or losing contact with a torpedo threat.