 The assaults on Zeebrugge and Ostend were eventually launched on 23 April , after twice being delayed by poor weather . The Ostend force arrived off the port shortly before midnight and made final preparations ; the monitors took up position offshore and the small craft moved forward to begin laying smoke . Covering the approach , the monitors opened fire on German shore defences , including the powerful " Tirpitz " battery , which carried 11 in ( 280 mm ) guns . As a long range artillery duel developed , the cruisers began their advance towards the harbour mouth , searching for the marker buoys which indicated the correct passage through the diverse <unk> which made navigation difficult along the Belgian coast . 
