Prewar U-boats carried rescue buoys equipped with a light and telephone that could be released from inside a crippled submarine. Depth charges could dislodge them accidentally. A flashing light could then expose the boat's position when concealment was essential. The bulbs were removed before the war, followed shortly afterwards by the rest of the equipment.
Notable Point · 1:30:33-1:31:08
Rescue buoys could expose a damaged U-boat
Rescue equipment intended to identify a disabled submarine could reveal its position during a depth-charge attack.