The fleet-submarine concept assumed that boats could accompany the battle fleet, submerge near the point of contact, and ambush enemy ships. Pre-nuclear submarines normally made long passages on the surface, while the unusually long-ranged or fast examples were exceptions. First World War planning still revolved around an expected decisive fleet action of the kind Jutland was supposed to provide, which made submarine participation especially attractive. Yet an 18-knot submarine could not match a 21-knot battle line or 28-knot battlecruisers, and its roughly 10-knot submerged speed widened the gap. Effective use therefore depended on intelligence: the submarine had to reach the anticipated battle area first, take up an ambush position, and hope the enemy passed within range.
Chapter · 6:31-9:02
Why a fleet submarine still could not keep up
A fast pre-war submarine still lagged behind battle lines and battlecruisers, making intelligence and pre-positioning central to its tactical use.