Eyes and Ears: Sonar and Radar

^{National Archives from Coast Guard}

^{Coast Guardsmen drop depth charges on German submarine located by sonar in 1943.}

In the early years of the submarine, it seemed that the only problems the undersea craft would have would be its own mechanical deficiencies. There was no way anyone on the surface could detect the presence of a submerged boat.

In the first part of World War I, the object of the British Navy was to catch German U boats on the surface. The main anti‑submarine weapons were the destroyer and the “Q ship,” a converted merchant ship, often carrying a cargo of lumber to inhibit sinking, with hidden deck guns. The former cruised the waters haunted by submarines and tried to catch one on the surface. Because the early subs had to spend most of their time on the surface, that task is not as hopeless as it sounds. The latter was a seagoing booby trap. To save on torpedoes and to comply with accepted standards of decency, subs in the early days of the war often approached freighters on the surface, told the crews to abandon ship and then sank them with gunfire. The Q ships aimed to attract these surfaced submarines and sink them with its guns. But after a few Q ship mis‑haps, submarine commanders just torpedoed all ships while submerged.

The first step toward the detection of a submerged U boat was the hydrophone. Hydrophones could pick up the sound of a submarine’s engines, but there were two big drawbacks. First, the hunter ship had to shut down its own engines so it could hear the subs. Second, one ship could not locate the sub by itself. Several ships working together were needed to get a rough approximation of the sub’s location. Once that was found, the navy ships would attack with depth charges.

The best anti‑submarine measure in the First World War was the convoy system, but not because convoys made it easier to locate or destroy U boats. It was because the convoy system bunched freighters up. Previously, the U boats waited for a freighter to come along. If its torpedoes missed, another ship would be along soon. A submerged submarine was about the slowest craft at sea. It couldn’t catch up with or even keep up with the slowest freighter. Convoys eliminated the steady stream of ships steaming across the Atlantic. U boats had to wait a long time between targets, and when a convoy appeared, it was guarded by naval ships. If the U boat were not positioned just right, it might miss all the ships in the convoy, and it was too slow to make up for poor positioning.

In the next war, the submarines were bigger, faster, and sturdier, but their enemies had something new, too. The British called it asdic; the Americans called it sonar. Basically, sonar sends beeping sounds into the water and listens for echoes caused by other objects in the water. An experienced sonar operator could distinguish between the echoes from a U boat or a whale. The convoy escorts also had better hydrophones, and whales don’t make engine and propeller noises.

Locating subs was also helped by another, more sophisticated method of detection: radar. Radar used radio waves instead of sound waves, but it, also, relied on echoes. The first important use of radar was in the Battle of Britain.

Britain had radar stations all along its shore. They were able to locate German planes long before they were in sight, allowing the British fighter command to concentrate its interceptors to meet the threat. The Germans had a primitive form of radar and had no idea that the British had any, much less the more advanced form they were actually using.

British scientists continually improved their radar devices. They made some small enough to be installed on ships. That further complicated life for subma‑riners. Radar could “see” in the dark, and it could “see” at much longer distances than human eyes. Submarines could no longer travel on the surface at night in safety, as they once did. It got worse. Radars became precise enough to “see” periscopes; they became small enough to install on airplanes. In daylight, unseen planes swooped out of the clouds and bombed surfaced submarines. At night, a surfaced submarine could be located by a plane and bombed before its crew even knew they were under attack. Airborne radar became operational in 1943, the year a German writer called “the year of the slaughter of the U boats.”

The miniaturization of radar went on. One result was the greatest advance in artillery in the 20th century: the proximity fuse (see Chapter 19). For the first time in history gunners could explode their shells, whether for antipersonnel ground fighting or for antiaircraft fire, at the optimum distance from the target, and do it without failure.

Radar and sonar were the first of a great array of detection devices that are at the heart of many of our modern “smart” weapons (see Chapter 50).