U-boats can coordinate with a raiding operation, but they cannot act as a dependable moving screen for 31-knot ships. A submarine that stays submerged is too slow to reposition with the surface battle. One that runs fast on the surface becomes visible and vulnerable to Royal Navy ships and aircraft. The failed efforts to bring submarines into useful positions around Bismarck illustrate the timing problem. Unless the British force happens to cross a pre-positioned patrol line, the raiders should not expect U-boats to break a pursuit.
Notable Point · 2:47:17-2:48:04
U-boats cannot keep pace with the raider battle
Submarines cannot reliably rescue a fast surface group because they are slow submerged and conspicuous when they sprint on the surface.