The net to the left of "Nashorn"
The Soviet submarines had broken through the mine barrages in the Gulf of Finland too easily in 1942. To keep Soviet submarine force away from Baltic shipping stronger efforts were planned. The barrages would be larger and in addition a double submarine net would be laid from Porkkala to Naissaari, operation "Walross".
It was assembled and guarded mostly by Germans.
Not a single Soviet submarine managed to break through this obstacle and so shipping in Baltic Sea was relatively safe.
http://www.hut.fi/~jaromaa/Navygallery/Mines/mines.htm
The submarine net was 30 miles long, 60 to 90 metres high. It was made from 18 mm steel wire with 4 x 4 m mesh. The netlaying operation began on 28.3.1943 and it was finished on 15.5. The old Russian net on the Finnish coast was replaced 10.6. Because of the net all submarine hunting forces could be concentrated between Porkkala and Suursaari.
Soviet submarine operations
Baltic Red Fleet had four S-class, nine Shtsh-class and three M-class submarines operational. The Soviets were aware of the net and tried to destroy it with air attacks, but did not succeed. The first boat to sail was Shtsh 323, but it hit a mine between Leningrad and Kronstadt on 30.4. The boat was so heavily damaged that it had to be scrapped. The second boat was Shtsh 303 that sailed from Lavansaari on 11.5. In the night of 14.5. the boat tried three times to sail under the net but failed. On 15.5. the boat tried to surface after being two days submerged but was caught by German barges. The boat tried to hide by laying in the bottom at Vaindlo. However, one of the men panicked, blew the tanks and jumped from the surfaced boat. Again, Shtsh 303 managed to escape and after a week of rest returned to Lavansaari on 8.6.
While Shtsh 303 was escaping northwards the Finnish and German ships continued hunting. On 20. and 21.5. a submarine was detected around Vaindlo. On 22.5. Ruotsinsalmi and VMV 6 made an attack with depth charges that produced dead fishes and later some oil. The place was marked with a buoy and hunting continued. During the following two days the submarine tried to escape and hunters to maintain contact. On 24.5. the sister ship of Ruotsinsalmi, the Riilahti arrived and together the ships made a depth charge attack that produced oil, large air bubbles and debris. The destroyed boat was not Shtsh 303 as informed by the sailor that had deserted, but new Shtsh 408 that was on its first patrol. A few days later another boat, Shtsh 406 was most likely lost in the "Nashorn" mines.
The Shtsh-class boat had failed, and now it was time for the larger and more powerful S-class boats to try breaking to the Baltic Sea. The boats S 9 and S 12 sailed in the beginning of August. Neither of them reached Baltic Sea. Both of them were lost in the middle of August somewhere in or between "Seeigel" and "Nashorn" barrages. Of the five boats sent to Baltic Sea not a single one succeeded and only one returned. No ships were lost to submarines in 1943. The blocking of Gulf of Finland had been a 100% effective anti-submarine operation.
http://www.hut.fi/~jaromaa/Marine/cont43.htm
