On 2/11/1914 the radio stations of the Kaiserliche Marine (Imperial Navy) at Zeebrugge and Oostende began operating. The first submarine U12 arrived at Zeebrugge on 9/11/1914, after the entrance of the harbour had been declared mine free. The withdrawing British and Belgians had failed to make the port installations unserviceable. The Germans quickly realised the importance of these ports near Britain and the sea routes to France.
The Unterseebootflotille Flandern was founded on 29/3/1915, and was to stay a thorn in the flesh of the Allies throughout the war. In September 1915 already 17 submarines were present, which increasingly attacked sea traffic in the Channel and on the east coast of England. The British reacted with the installation of the Dover Barrage, i.e. steel nets combined with sea mines placed at a certain depth. These were guarded by the trawlers and drifters of the Dover Patrol.
Korvettenkapitän (lieutenant commander) Bartenbach was appointed commander-in-chief of U-Flotille Flandern. On 1/10/1915 he became commander of the Flanders submarines that consisted of two flotillas. Like other war ships, like the small torpedo boats of “A” class, the submarines had their bases at Zeebrugge, Oostende and Brugge.
At the beginning of the war the submarines had directed their attacks towards the Royal Navy. After the British had installed their blockage of German sea routes, merchant ships became the prey of submarines since February 1915. The Germans declared the North Sea a military area. In April 1915 the Unterseebootsflottille Flandern came really into action, after submarine UB10 had arrived at Zeebrugge. The submarines of UB type were small vessels, suited to operate in the shallow waters of the North Sea. They had two 45cm torpedos on bord, and were assembled on the shipyards at Hoboken.
The Torpedo Boat Flotilla Flanders was founded on 21/5/1915. In August 1916 it consisted of the 1st Half Flotilla (A 4, A 16, A 9, A 15, A 10) and the 2nd Half Flotilla (A 2, A 5, A 8, A 14, A 11, A 13). Commander-in-chief was Kapitänleutnant (lieutenant) Assmann.
On 26/5/1915 a new type of submarine arrived at Zeebrugge. It was a mine layer of UC class that could drop 12 mines over the keel, but was not armed with torpedos. The mines were stored in six vertical shafts in the front part of the vessel. The UC mine layers came into action since June 1915, and were to disturb the sea routes to England throughout the war.
The inland port of Brugge was linked to the sea ports of Zeebrugge and Oostende by canals, and thus became a secure refuge for German war ships that were vulnerable by frequent artillery attacks from the sea. The Kaiserliche Werft Brugge (Imperial Shipyard Brugge) was equipped with floating docks and cranes, ammunition and supply depots and everything necessary for the repair of ships and maintenance of ship engines. As the British switched over from long range artillery attacks to air raids, the submarines were secured under concrete roofs.
Between March and July 1916 the Mariekorps could make use of larger torpedo boats of the AII class, and destroyers. They were to make use of their higher speed, perform attacks on British coastal sea traffic and sea traffic between Calais and Dover. Also the drifters that guarded the Dover Barrage were attacked.
the end of 1916 already 38 submarines and more than 25 torpedo boats and destroyers were to be found in the ports of Zeebrugge and Oostende. After it had for some time been forbidden to intrude the Dover Straits, sumarines began again to slip through the Dover mine fields. This saved time and fuel, and was done at night. The submarines travelled on the suface and submerged under the barrage nets when spotted.
In 1917 types of larger submarines came into use, i.e. the types UBII, UBIII, UCII and UCIII. Their operational radius was widened up to the western exit of the Channel, sea mines were even dropped off Boulogne. On the average 30 submarines passed through the Channel every month, scarecely hindered by the Dover Barrage. The British reacted by dropping even more mines off the Belgian coast. They tried to destroy the sea sluice of Zeebrugge with long range ship artillery, but the long distance artillery fire had no effect.
On 1/2/1917 the Germans declared unrestricted U-boat warfare. Merchant vessels were sunk, especially at night, without previous warning, and the losses of ship tonnage could not be made up for by building new ships. 63 submarines were active in British waters, and the German admirality thought that the war would be over in August 1917, if the submarines could cut off Britain from its supply routes. The submarines were exspected to bring the war into a decisive phase, and to win the war. But the Germans had overestimated their capacity, there were not enough submarines available to keep up an effectice blockade around Great Britain.
When, in the middle of 1917, the British decided to sail in convois guarded by an escort of war ships and sea planes, the Allied losses of ship tonnage decreased, while more and more submarines were lost. At that time one third of German submarines active in the North Sea was stationed at Zeebrugge and Oostende.
Furthermore three destroyer flotillas from Zeebrugge were active in the Channel, attacking submarine barriers and patrol boats of the Dover Patrol. Night attacks on ports like Ramsgate were no success. The destroyers were not so successful as the submarines, and when the destroyers G42 and G85 were sunk by HMS Swift ans HMS Broke on 21/4/1917, their activity decreased drastically.
During 1918 the passing through the Dover Barrage became a risky operation for the submarines. A new type of mine laid at a depth of 30 metres and scattered over a wider area, increased the losses of submarines. As well at daylight as at night the passage on the surface was very difficult. The barrage was guarded with searchlights, and more patrol boats stationed at the open passages.
In March 1918 the larger submarines received order to take the longer route round Scotland. Only the smaller boats of the Flanders Flotilla forced their passage through the Channel. UB103 was the last submarine of U-Flottille Flandern to try a break-through in the Channel. After having been damaged by a sea mine, the submarine of Kapitänleutnant Hundius was sunk by depth charges. The building of new submarines could not keep pace with the losses, and the Germans found no answer to the convoi system. Many experienced submarine commanders and crews had been killed.
After the start of the Allied final offensive on 28/9/1918, the submarines of the Flanders flotilla were, on 4/10/1918, given order to return to Germany. The last submarine left Zeebrugge on 5/10/1918. Only a few destroyed U-Boats remained. The evacuation of the Belgian coast began on 16/10/1918, not without first having destroyed the port utilities of Brugge, Zeebrugge and Oostende. At the end of the war U-Flotille Flandern could be counted responsible for one third of the losses of Allied ships.
