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.

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.

An old submarine, loaded with explosives, was also run under a bridge connecting the mole, or breakwater, at Zeebrugge with the shore, and there blown up, so as to prevent interruption of the raiders while they were doing their work alongside the mole.