Neither Zeebrugge, Ostend, nor Bruges could be rendered untenable to the enemy with the guns available during 1917, although Ostend in particular, and Zeebrugge to a lesser extent, could be, and were frequently, brought under fire when certain conditions prevailed, and some temporary damage caused. Indeed, the fire against Ostend was so effective that the harbour fell into disuse as a base towards the end of 1917. We were arranging also in 1917 for mounting naval guns on shore that would bring Bruges under fire, after the enemy had been driven from Ostend by the contemplated operation which is mentioned later. When forced to abandon this operation, in consequence of the military advance being held up by the weather, these guns were mounted in monitors. In the matter of blocking the entrance to the ports of Zeebrugge and Ostend, the fact had to be recognized that effective _permanent_ blocking operations against destroyers and submarines were not practicable, mainly because of the great rise and fall above low water at ordinary spring tides, which is 14 feet at Ostend and 13 feet at Zeebrugge for about half the days in each month. Low water at Ostend also lasts for one hour. Therefore, even if block-ships were sunk in the most favourable position the operation of making a passage by cutting away the upper works of the block-ships was not a difficult matter, and the Germans are a painstaking people. This passage could be used for some time on each side of high water by vessels like destroyers drawing less than 14 feet, or submarines drawing, say, 14 feet. The block would, therefore, be of a temporary and not a permanent nature, although it would undoubtedly be a source of considerable inconvenience. At the same time it was realized that, although permanent blocking was not practicable, a temporary block would be of use, and that _the moral effect alone of such an operation would be of great value_. These considerations, together with the abandonment of the proposed landing on the Belgian coast, owing to unfavourable military conditions, led to the decision late in 1917 to undertake blocking operations concurrently with an attack on the vessels alongside the Mole at Zeebrugge.