The Zeebrugge Raid: Block-ships during WWI

“Ahead of her, as she drove through the water, rolled the smoke-screen, her cloak of invisibility, wrapped about her by the small craft. The north-east wind moved the volume of it shoreward ahead of the ships; beyond it, the distant town and its defenders were unsuspicious; and it was not till Vindictive was close upon the Mole that the wind lulled and came away again from the south-west, sweeping back the smoke-screen and laying her bare to the eyes that looked seaward. There was a moment immediately afterward when it seemed to those in the ships as if the dim coast and the hidden harbour exploded into light. A star shell soared aloft, then a score of star shells; the wavering beams of the searchlights swung around and settled to a glare; the wildfire of gun flashes leaped against the sky; strings of luminous green beads shot aloft, hung and sank; and the darkness of the night was supplanted by the nightmare daylight of battle fires.” [1] 

Image: [H.M.S. Intrepid], A.S. English Fonds, Collections CCGW/CCGG, 2016.3.1.1-138.
On April 23, 1918, the British Royal Navy deliberately sunk three of their ships in an attempt to block the entrance of a strategic Belgian port. Proposed by Sir John Jellicoe, the Zeebrugge Raid was launched in response to a growing German presence in the English Channel. This presence was largely made possible because of German access to the Bruges shipping canal.
There had been two previous attempts at the attack, capable of a success if only the weather permitted. The night of the 22nd, however, offered nearly perfect conditions, including a prepared smoke-screen to reduce visibility for the Germans. The function of the Vindictive was to attack the Mole which guarded the Zeebrugge Canal, “land bluejackets and marines upon it, destroy what stores, guns, and Germans she could find” and create a diversion while the H.M.S. Thetis, Intrepid, and Iphigenia ran in and sank themselves in their appointed place. [2] Each packed with concrete and mines, their strategic location was to limit German access to and from the port.
When the wind unexpectedly changed and lifted the smokescreen, however, the Vindictive and the three block-ships came under heavy German fire. Losing many of their men, the Vindictive was no longer able to provide crucial support for the three block-ships. Without the support and under continuous fire, the Thetis, Intrepid, and Iphigenia were unable to sink at their pre-assigned locations, as planned. While they were still successfully sunk at the mouth of the canal, the Intrepid being the most successful, it would only remained blocked for a few days before German vessels were able to move freely again.
The Zeebrugge Raid was nonetheless reported as a victory by the British Navy. At a cost of more than 500 casualties, however, the raid did not in reality hinder German movements for more than a few days.
References
[1] Howard, Keble. The Zeebrugge Affair. New York: George H. Doran Company, 1918, 45.
[2] Ibid, 44.