German U-Boat

As allied radar technology advanced, the Germans deployed radar decoys to fool the allies into thinking that a U-boat was present. Radar decoys were deployed either by balloons or floating buoys and were designed to reflect radar waves which could look like a surfaced U-boat. Depending on the type used, they could either be launched from the standard torpedo tube, or had to be assembled by hand on deck. It was popularly used with many U-boats carrying up to 30 decoys, but was eventually phased out with the advent of the snorkel.

The allies knew all about the decoys, but nevertheless, it caused confusion among the ASW forces. With more than a few occasions, U-boats had successfully used radar decoys to shake off their pursuers, but the allies had quickly developed counter measures.

Aphrodite

Deployed in September 1943, the Aphrodite was a three feet diameter sized balloon which floated above the surface and was anchored to the sea surface by a floating raft. The line was 50 meters long, three strips of aluminum foils were attached to the line to act as radar reflectors. The device had to be assembled on the upper deck, where the balloon was inflated with a hydrogen filled can. It had a lifespan of three to six hours after deployment.

Thetis

The Thetis entered service in February 1944 and was designed to confuse Allied warship radars. It consisted of a floating buoy, metal sheets and steel tubes which resembled somewhat like a small sail. It took an inordinate amount of space and time to deploy and the original Thetis had to be assembled on the upper deck. Later versions could be launched from the standard torpedo tube. As it was capable of being afloat for months at a time, the strategic intention was for each U-boat to release these decoys as they crossed the Bay of Biscay - where it was hoped that it would cause confusion to allied ASW forces.

However, by the time the first Thetis was launched, the allied had introduced a new airborne radar, the ASV Mark III which did not pick up the signature of the decoys. The Germans realized that the entire effort was a complete waste of time.