Seas under naval stress

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Naval warfare 1914-1916

 When WWI started, in August 1914, the German Navy had 28 U-boats. Their capacity was limited. By February next year, they had lost 7 U-boats, but had only sunk 10 vessels with a total tonnage of 20,000. This figure accounted for only 10% of all British losses during the first six months of war. Mines sank double as much over the same period, of the about 40 millions ship tonnage available to the Allies. From August 1914 until December 1916, the U-boats sank 2,200,000 tons. This represented the total number of 1,500 Allies’ vessels or about three vessels per day.  On the other hand, the loss of U-boats also increased, mainly due to a newly developed depth charge with 300 pounds TNT or amatol (in 1915), which became available and fully operable since 1916.

 Naval Warfare 1917-1918

 The situation became dramatic for Britain in early 1917. U-boats sank more ships than shipyards could deliver. In April 1917, the annual rate of the previous years was reached, with almost 860,000 tons. In 1917, U-boats alone sank 6,200,000 tons. This amounted to more than 3,000 ships.

 The total loss of the Allies’ shipping was of ca. 12 million tons: about 5,500 merchant ships, 10 battle ships, 18 cruisers, 20 destroyers, and 9 submarines. The total loss in naval units of the Allies and the Axis was of 650 ships (including 205 U-boats) with a tonnage of 1,200,000 tons. 

Depth Charges – What it meant to attack a U-boat?

 The onslaught by U-boats reached the pinnacle with almost one million tons sunk per month, like in April 1917. Although the British Navy was able to prevent hundreds of attacks, real or suspected, the result was not encouraging. Only a mere 11 U-boats were sunk in four months. New protection measures such as convoying, patrols and a new most promising weapon, depth charges, etc. were regarded as necessary.










While U-boats hunted and naval vessels during the early days of WWI, the scenario changed after 1916. They became the hunted ones and were depth-charged. Thousands of naval vessels steamed the seas around Britain day and night. In May 1918, the experience of the U-boat U-72 may illustrate the situation at sea. In early May, some 75 depth-charges were dropped on the boat by anti-submarine vessels and from an airship. Later, a destroyer arrived and attacked U-72 with another 20 charges. This caused a leak in a fuel tank leaving a trail of oil at the sea surface. 24 hours later, U-72 was again depth-charged, more than 20 times, by two naval vessels. A British submarine sank U-72 a few days later.

 Sea Mines

 Main minefields in the North Sea were on Britain’s East Coast, including the Strait of Dover, Helgoland Bight and the Northern Barrage. A rough figure for each of these areas is of 50,000 mines. The total number of mines in the North Sea was of 190,000 and the total number during the whole WWI was of 235,000 sea mines.

Minesweeping is an activity that stirs and shakes the sea on an unprecedented scale. The ‘stir impact’ on the seas could possibly be even many times higher than the mine laying and the impact of mines that ‘hit a target’ together. Between two sweepers in motion ‘hung’ a sweep wire, with a kite, to cut the mooring rope of the mines. Britain alone had more than 700 minesweepers in permanent operation and Germans also had a considerable number. Possibly 500 ships swept the North Sea day and night.

 Operating in a sensible area – Around the Shetlands

 Another example:  U-boats were a problem for the British. For this reason, between the 15th and 25th June 1917, four flotilla leaders, with about 50 destroyers and seventeen submarines, were sent to an area stretching from NW of Stornoway, round to the north of the Shetlands and eastwards into the North Sea. The idea was to force the U-boats to the surface and to attack them. On sixty-one occasions, U-boats were sighted and attacked twelve times.  In practice that must have meant that, in addition to the shelling operation of 75 naval vessels, many hundred depth charges had been dropped. No U-boat was sunk. This episode demonstrates that huge operations may have taken place in the sea, operations which did not go by without any impact on the sea area. However, these were not accounted for in relation to climate change.  

Barents Sea and Baltic Sea

 The matter is worth a detailed chapter but requires more pages. Although these seas were not the central stage of the operations, they saw immense naval activities and destruction. The intense encounters in the Barents Sea could have played an important role in the strong icing from the high North, in February 1915, and the harsh winter in North-Western Europe, 1916/17. Until early 1915, more than 450,000 tons of coal and 90,000 tons of weaponry had been shipped to the Russian port Archangel. Russian and German navies had laid thousand of sea mines and dozen of minesweepers were permanently in service. U-boats sank 25 ships, in late 1916, and 21 vessels from April until November 1917.

In the Eastern Baltic Sea, many dozen mine fields were laid with some ten thousand mines. Many naval activities took place every day over four years. British and Russian submarines operated successfully. The increasing sea icing during the war years, from 1914 until 1918, can be attributed to naval warfare in Baltic waters.


The sea profile were the Northern Barrage between Orkney Island and Norway had been laid



Northern Mine Barrage

 U-boats had been a serious threat to the Allies since 1916. They regarded it paramount to prevent U-boats from leaving the North Sea

and enter the Atlantic. To ‘close’ the northern outlet of the North Sea, a long barrage of about 150 sea miles (ca. 275 km), between the Orkney Islands and Norway, was required. Near the Norwegian coast, the water is 300 metres deep and near the coast of Orkney, of about 100 metres. Sea currents can reach 3-4 nautical miles/hour. That was a challenge and required the development of a new mine, the MK6.  The charge consisted of 300 pounds of grade B trinitrotoluol (TNT). The mine itself was supposed to have a destructive radius of 100 feet (ca. 30m) against submarines. Calculations showed that approximately 100,000 mines should effectively prevent U-boats from passing the line. Actually, only about 70,000 mines were laid until October 1918. On the other hand, 20,000 mines were disposed during the laying of the ‘Northern Barrage’. 

Mines were available by March 1918. Shortly after the mine lay had commenced, mines began to explode. As a report for the USA Government noted, between 3 and 4% of 3,385 laid mines blew up prematurely. In the middle section “A”, mines were supposed to be laid as it follows: 10 rows of mines at 80 feet submergence, 4 rows of mines at 160 feet submergence, and 4 rows of mines at 240 feet submergence.  The laying of mines ceased when the armistice from November 1918 was in sight. 

Mine sweeping started in the spring and ended in the autumn of 1919. From more than 73.000 mines

  • about 15% exploded prematurely soon after laying;
  • about 15% were disposed;
  • from the remaining ca. 50,000 mines
    • more than 30,000 mines were already ‘gone’ in the spring of 1919, either drifted away or exploded during winter storms;
    •  20,000 mines were swept in 1919.

Six months of sweeping operations comprised seven sweeping missions involving more than 70 vessels and 10 supply vessels.