HMS Thetis had suffered one of the worst peacetime submarine accidents ever. She had sunk during sea trials in 1939 with the loss of 99 lives. The salvaged ship had been renamed HMS Thunderbolt.
HMS Thunderbolt made her first successful attack when she spotted the Italian submarine Tarantini in the Bay of Biscay. The Naval Intelligence summary gives the full details:
H.M.S. THUNDERBOLT SINKS ITALIAN U-BOAT
H.M.S. Thunderbolt patrolling the Bay of Biscay on 15th December, attacked a U-Boat which was in close company with three trawlers.
At 0835 when diving 12 miles south-west of the mouth of the Gironde, an object resembling the conning tower of a U-Boat was sighted. Thunderbolt altered course to close, and later observed two trawlers on the same bearing. Smoke appeared to be coming from the vessel originally sighted. Assuming that these were the three armed trawlers which had been seen on previous occasions, Thunderbolt reset course for patrol position.
At 0909, however, when the range of the enemy vessels had decreased considerably, a U-Boat, in company with three trawlers was clearly visible 5,000 yards away, bearing Red 110°. Course was altered directly towards the target and all tubes were brought to the ready.
Whilst approaching the firing course, another sight was taken, which showed that the target was now stern on.
Thunderbolt continued to close at half speed grouped-down.
When 130 degrees on the U-Boat’s starboard quarter, periscope range estimated to be 4,000 yards, the disposition of the trawlers was thought to be reminiscent of the start of an A/S exercise.
Thunderbolt therefore allowed the enemy a low nominal speed of 6 knots, and altered a few degrees to reach the firing course. Commencing at 0920, six torpedoes were fired at 12 second intervals, an alteration of three degrees to port being made after the third torpedo.
After what seemed an interminable delay, a tall column of water was seen to rise into the air, followed by an explosion four minutes nine seconds after firing the first torpedo.
Part of the U-Boat, either bow or stern was seen to protrude out of the water, and subsequently no trace of her could be seen.
Five minutes nine seconds after the first, fifteen further explosions were heard, and Thunderbolt could not determine whether these were all depth charges, or if some were torpedoes striking the sea bed, the depth of water being 18 fathoms.
The enemy trawlers could not have known from which direction the torpedoes came, as all the depth charges sounded fairly distant.
The Italians’ very early admission that one of their U-Boats had not returned from the Atlantic is explained by the fact that their patrol craft witnessed the sinking, and our public announcement of the incident has produced no comment.