The Kriegsmarine deployed two types of “radar deceiver” (Funkmeßtäuscher) to protect U-boats from aircraft. Aphrodite (FuMT 1) was manufactured by AEG while Thetis IIc (FuMT 2) came from Telefunken. Both were directed against 120–180 cm waveband (166–250 mHz) and could be deployed by the submarines themselves or by the surface vessels supporting them.
This was a tethered balloon carrying metal foil radar reflector strips and was inflated and deployed from the surfaced boat’s deck. It was cleared for operational use on 15 June 1943, three days after crews had been assured that the hydrogen bottles used to inflate it would leak rather than explode if hit by gunfire. A comprehensive set of instructions for the system’s use was appended to the U-boat Command diary for June 1943. These included:
Although advice was given on how the decoys might be deployed in a variety of situations, the paper ended by saying: "Boats must try out suitable methods for themselves on the basis of these guiding principles and at their own discretion".
That August, another directive was issued to submariners:
The use of … Aphrodite is also most strongly recommended. It gives one boat the opportunity of simulating a whole group—for instance by letting several Aphrodite drift past the convoy at night and so make a gap in the escort before she attacks. Or if the boat wishes to attack from a certain quarter and the escort there is too strong, she can precede her attack by a group of scattered decoys so that the escort vessels will occupy themselves with them and thus make things easier for the boat.
What was thought to be Aphrodite’s first successful use was off the North African coast on 5 September when the U-boat observed the release point being illuminated 30 minutes later by shore searchlights. Three days later there came a report from the Atlantic:
By night flown over twice in quick succession by a locating flying boat. After each [pass] an Aphrodite released. After about three hours, dropping of parachute flares observed beyond horizon in direction in which Bolden released. Thereafter no further flight over U-boat.
Not two weeks after that incident boats were being told how the balloons had been approached by aircraft and chased by destroyers and that “skillful use is likely to result in great success, particularly in [the] case of a convoy”. On 21 November, U-969 used the device to escape detection by an aircraft. Nine days later, U-68 sank Fort de Vaux (5186 GRT) off Monrovia “after a successful Aphrodite deployment” drew off the freighter’s escorts. A directive to the »Borkum« wolf pack on 11 January 1944 melodramatically urged them to, “disseminate numerous Aphrodites over the area. When attacking the convoy, break up its escort by means of [them] … The eyes of the Fatherland are upon you.” On 11 February, U-413 was able to report “Aphrodite used successfully on several occasions against Naxos”. (German seamen used the name Naxos for both the warning receiver and the signals it was designed to detect). Crews were advised on 19 June 1944 that boats operating in coastal waters had successfully hidden in a “forest” of 20–60 Aphrodite while on the surface charging flat batteries. They remained unmolested despite cruising for 1–2 hours with a continuous strength-5 indication on Naxos.
A deciphered signal of 11 January 1943 described »Thetis« in some detail:
In order to interfere with the enemy location activity, type Thetis IIC decoy buoys (Täuschungs-Bojen) will be laid out in Biscay as far as approximately 12º West with effect from 11 January.
Description: a thin, upright wooden pole 5 metres long on a float, weighted at the bottom by a 5-metre long steel tube. On the wooden pole, thin metal dipoles visible only at very close range.
Method of operation: the buoy produces the same radar response as a U-boat.
On the morning of 30 January, two patrol boats, V401 and V402 were to sail from Royan “for Thetis experiments”. The intention of BdU earlier in the month had been for his boats to lay a line of 100–150 Thetis at 25-mile intervals across the Bay of Biscay from the 200 metre line out to 10º W: “The efficiency and life of these buoys will then be tested by aircraft”. On 18 March 1944 the Naval Staff was briefed that:
Radio-monitoring established repeated ASV locations on our own U-boats which were subsequently rescinded. Observation therefore permits the conclusion that the deployment of Thetis has proved itself.
An evaluation nine days later concluded:
Clearly … the enemy has had numerous false radar locations from Thetis laid in the Bay and this fact will have contributed to his change of tactics … [the] many reports of radar locations followed by a cancelled position are transmitted from areas where there are no U-boats but possibly Thetis buoys [provide] confirmation of [their] effectiveness …
As late as 23 June 1944, U-boat Command was noting that U-154 (sailed 20 June, lost 3 July) and U-802 (sailed 22 June, surrendered 11 May 1945) had dropped 10 Thetis apiece during their passage across Biscay from Lorient.
“Chimney Sweep” was a project to develop radar-absorbent coatings for U-boat conning towers and possibly for exposed hull areas also. Initial work was concerned with the 1.5 m waveband of ASV Mk. II and progressed only slowly. The adoption of the Schnorchel early in 1944 made the concealment of conning towers less of an issue and it was some time before it was appreciated that the Schnorchel itself was detectable by Allied centimetric radars. Work then switched to materials effective in the 3–9 cm band and these were fitted to a number of operational boats from late in 1944.