Like Aphrodite, Thetis was designed to counter radars in the 1.2–1.80 m waveband but to remain effective for longer. The IIC model, in production in December 1943, was designed to be more easily handled than Thetis I (which was produced only in small numbers) and to be deployable from U-boats while a further air-dropped version was under development The intention was to make both systems effective against the whole spectrum of Allied radars and capable of representing targets of various sizes. 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Radio-monitoring established repeated ASV locations on our own U-boats which were subsequently rescinded.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.
The Netzhemd (string vest) project sought to camouflage U-boats against radars operating on 1.2–1.80 m and 10 cm wavebands but the original target date of 1 November 1943 could not be met although practical trials did get underway. Early versions of Netzhemd were difficult to construct and allowed the boat to use only half its underwater performance. A signal from U-597 on 3 December seems to have been reporting some drawbacks of the system which may explain why it did not become standard equipment:
In sea force 3, net covering and nearly all lower frames broken. Wire netting makes a singing noise when proceeding submerged. Conversion hinders look-out and is very unsatisfactory from the point of view of wind resistance and spray.
A report of 7 January 1944 identified the three high-frequency research tasks rendered most pressing by the war situation one of which was “Black U-boat”, a project to reduce the boats’ radar reflectivity. This paper noted that thee had been comprehensive theoretical investigations and trials of Netzhemd, establishing that it cut detection ranges by 50%. Overcoming the remaining problems was of “extraordinary significance.” Compared with invisibility and actively combating radar-carrying aircraft, the other steps being taken were seen as half-measures
“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. In mid-June of that year the assessment had been that Schnorchel could be picked up on radar but not so precisely as to make an attack run possible. On a dark July night, U-763 suffered “well-placed” depth charging with its Schnorchel 3-4 metres above water but once it was lowered further attacks proved inaccurate. By 10 September, U-boat Command was warning that “Schnorchel carried high up can be fixed exactly and attacked with bombs like a boat on the surface.” 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.
Schornsteinfeger was still being worked on as late as 23 April 1945. A message from an Obltn. Weber on the Baltic island of Rügen to the Köthen Signals Experimental Regiment reported: “U-boat camouflage trials with Schornsteinfeger … Results not satisfactory. Interpretation [will follow] by courier on the 26th.” British Intelligence note that this was the first time they had heard this code name.