During the Battle of the Atlantic, the Swordfish played a crucial role as a submarine-hunter, detecting and attacking U-boats that posed a threat to merchant ships traveling between North America and Britain.

According to Swordfish Pilot, Jack Thomas, Swordfish were sent to patrol various areas, based on intelligence received by the commodore of the convoy. Encrypted radio messages between U-boats and their command centres, intercepted and decoded by Bletchley Park, gave away the positions of the submarines. “The successful use of aircraft patrols was not only to sink the U‑boats,” Thomas adds, “but to deter them by keeping them submerged, where they were slower than the convoy—a case of preventative medicine.”

On sub patrols, each crewman watched a section of the ocean, says Swordfish pilot Lt-Cdr John Beresford, who served aboard both escort carriers and MACships. “The observer and rear gunner had binoculars,” he says. “Most U-boats cruised just below the surface in daylight, with the periscope leaving a small trail, the size of the trail depending on the sub’s speed. If the plane was reasonably low—say, 500 feet—you could see the periscope and the feathering trail that it left. And if the water was reasonably calm, you could see the hull just below the surface. If they were more than about 20 feet below the surface, or if the sea was very choppy, they couldn’t be seen.”

The U-boats had to come to the surface for speed and oxygen. They could travel twice as fast—about 10 knots—on the surface, but only about 5 knots submerged. Once they knew that they had been spotted, they could decide either to fight it out on the surface or dive to a safe depth.

Lt-Cdr John BeresfordSwordfish Pilot

Even if the Swordfish didn’t engage the enemy, their presence had the effect of keeping U-boats submerged, stopping them from manoeuvring into position to attack the convoys. 

The success of the Swordfish, combined with the bravery of the CAM pilots and the introduction of escort carriers and MAC ships, as well as the efforts of Bletchley Park, were critical factors in the eventual Allied victory. Without the shipborne air Anti-Submarine Warfare capability, which was first demonstrated by the Royal Naval Air Service in 1918, the Allies would not have been able to gain the decisive edge they needed to win the battle.