Surcouf's low rangefinder limited practical gunnery range to about 12 kilometers, rising to roughly 16 kilometers when using the periscope. The same guns on a surface warship could reach about 26 kilometers. A British destroyer's 4.7-inch Mark IX or XII gun could reach about 15.5 kilometers, leaving only a narrow margin even under the submarine's more favorable estimate. Surcouf needed about three minutes and 35 seconds after surfacing before firing ahead, with additional delay to train the turret for a broadside. A roll exceeding eight degrees prevented turret training, night firing prevented observation of the fall of shot, and the ready magazines held only 28 rounds. Escorts stationed outside the convoy could already be within gun range when the submarine reached the merchant ships. They could then close on a large, unarmored target before its heavy guns produced the intended advantage.
Notable Point · 1:12:47-1:17:56
Why Surcouf's 8-inch guns offered little safety from escorts
Low sighting height, slow gun preparation, and turret limits erased much of Surcouf's theoretical range advantage over destroyer escorts.