The culmination of Keyes and Lynes' work in this office was the Zeebrugge Raid of April 1918 and the subsidiary raids at Ostend in April and May. The plan in these operations was to sink obsolete nineteenth century cruisers in the canals linking Zeebrugge and Ostend with the vital naval base at Bruges, thus bottling up the German force in the base and preventing its use by the U-boat fleet during their war on Allied commerce ("Handelskrieg"). The operations suffered heavy casualties amongst the attacking sailors and marines but did succeed in sinking the blockships and partially obstructing the canal, although the full effect of these attacks had been subject to debate ever since.