German U-Boat

To reduce the sonar echo of U-boats, the Germans experimented with sound absorbing synthetic rubber which coated the outer hull of the boats. Radar absorbing materials were also used to coat the snorkel heads.

Alberich

Alberich consisted of synthetic rubber sheets of about 4mm in thickness which had sound absorbing properties. The material was Oppanol which was secured to the outer hull with adhesives, much like an outer skin. Although no conclusive tests were performed, but it was claimed that the echo reflection of a U-boat with Alberich was reduced by about 15 percent. In addition, it also acted as a sound dampener, containing the U-boat’s own engine noise.

Although the principle was a sound one, problems were encountered with the adhesive coat which was not strong enough to adhere the rubber sheet to the hull. This resulted in the sheets being partially washed off, which flapped in the wake of the ocean current, causing hydrodynamic resistance and noise. Further research into more reliable adhesives were conducted, but up to the end of the war, only a few U-boats had received this treatment. A further contribution was that treating the hull was a time consuming and laborious task.

The first U-boat to receive Alberich was U-11, a Type II coastal boat for trials on its sound absorbing properties. On April 1940, the first operational U-boat was treated, U-67 a Type IXC which was just being laid down. Thereafter, problems with the adhesive prevented further treatments until late 1944, when U-480, a Type VII was tested again using a new adhesive. The results were satisfactory enough and it was decided that all new Type XXIIIs and XXVIs would receive this coating, but ultimately only one Type XXIII, U-4709 had been completed with the coating.

Tarnmatte

This was a radar absorbing material used to coat snorkel heads to shield them from allied radar. It consisted of a compound of synthetic rubber and iron oxide and was claimed to have 90 percent effectiveness in reducing radar signature.