Why a U-boat screen could betray Operation Rheinübung

Would flooding the Denmark Strait with U-boats have helped Bismarck break into the Atlantic?

It might protect the immediate passage, but the communications required would expose the route and let a stronger British force wait beyond the submarines.

Traffic analysis can reveal useful intelligence without requiring every message to be decoded. Concentrating and directing many U-boats toward the Denmark Strait would expose the intended passage before Bismarck arrived. The Royal Navy could prepare Victorious and additional capital ships beyond the immediate submarine screen. Submerged boats could not keep pace with the German surface force, while surfacing exposed them to attack. The screen might clear the narrows but would sacrifice surprise and make the later interception stronger.