On 30th June 1942 Lieutenant R. E. Schreder was commanding a Martin PBM Mariner on an Anti Submarine patrol over the north Atlantic. His plane was part of Patrol Squadron Seventy Four, based on Bermuda. In the long hours of patrol the surface of the sea was constantly monitored by the radar – positive signals that might be U-boats had to be investigated for visual confirmation and action initiated immediately. The post action report summarises the simple sequence of events:
This attack was delivered after radar contact made while the plane was flying above scattered cumulus clouds. Visibility was unlimited and the pilot found himself in ideal position for attack when the U-boat was sighted visually. Surprise of the U-boat was apparently complete.
The hypothesis advanced in [Lieutenant R. E. Schreder's report],that the second bomb stuck in the deck and was carried to set depth where explosion occurred is reasonable and is strengthened by the possibility that the explosion of the first bomb lifted the stern and caused submergence at a greater angle than usual which would account for part of the stern being visible at the time of the second explosion.
The only other possibility appears to be that the bomb rolled off the deck, sank to set depth and exploded directly under and very close to the U-boat. In either event, it is believed that damage must have been fatal and this attack is, therefore, regarded as having resulted in a sure kill.
Original reports can be seen at U-Boat Archive
The British submarine HMS Thrasher had had a narrow escape when bombs had stuck in the outer casing – on that occasion they had been bombs rather than depth charges which had not exploded when they reached depth.