 U-boat commanders who survived such attacks reported a particular fear of this weapon system since aircraft could not be seen at night, and the noise of an approaching aircraft was inaudible above the din of the sub's engines.Subsequently, the common practice of surfacing at night to recharge batteries and refresh air was mostly abandoned as it was safer to perform these tasks during daylight hours when enemy planes could be spotted.A drop in Allied shipping losses from 600,000 to 200,000 tons per month was attributed to this device.