There can be no doubt that the Flanders U-boats, the thirty seven small UB and UC vessels based at Bruges and going to and from sea via Ostende and Zeebrugge were important factors in the strategic judgement that led to the Third Battle of Ypres. The fears of the Admiralty that the contribution these boats were making to the overall unrestricted submarine campaign plus the threat they and German torpedo boat forces posed to cross Channel communications were vital parts of the debate that led to Haig's offensive in Flanders. They were not the only reason, but they were significant, and rightly so.
The boats based in Flanders were small coastal UBs and UCs The largest UBs at this time were not quite 300 tons in submerged displacement and armed with two forward 19.7-in torpedo tubes. The UCs were 400 ton minelayers with eighteen mines, two external torpedo tubes forward and an internal torpedo tube aft. Together these boats sank almost 600, 000 tons of shipping between February and May 1917 almost half the tonnage sunk by the more numerous and larger High Sea Fleet U-boats. Their effectiveness was in part caused by their operational practices.
As Innes McCartney has shown in his ground breaking archaeological study of U-boat wrecks, the Flanders boats' tendency not to use radio and to hunt silently and stealthily in the narrow seas gave naval intelligence few,. if any, clues as to their presence High Sea Fleet U-boats could patrol further afield but their radio chatter (their major previous role had been reporting British fleet movements) meant they could be relatively easily avoided, especially when convoy was adopted.
There was thus good reason to wish that if the Flanders bases could be closed the U boat threat would diminish significantly. True, the boats could have withdrawn to Germany but their effectiveness would have significantly diminished given their limited range and their integration into High Sea Fleet radio practices. Although an amphibious landing was planned to support Haig's advance it was recognised that only a major advance by the BEF could seriously threaten the German forward naval bases. Jellicoe, the First Sea Lord, with his pathologically pessimistic assessments of the U boat threat was as much an author of Passchendaele as Haig. Against a united joint service view, Lloyd George, despite his doubts, had no alternative to give in.
The U-boats based at Bruges in 1917 were the UBIIs and and the UCIIs. These are not Flanders based examples but they show the types clearly. Note the external tubes on the UC.
Eric Grove (December 2017)
A longer version of this item will be published in a future edition of the Western Front Association magazine Stand To!