Following the German attack of 17-18 March 1917 on the anti-submarine net barrage in the Dover Straits, the Admiralty told Vice Admiral Sir Reginald Bacon, C.-in-C. Dover, that his forces should be more concentrated on nights when enemy attack appeared to be likely. The night of 20 April appeared to be such a night, as it was there was a new moon the next day and high water was at 2230.
The British had two flotilla leaders (large destroyers) on the western part of the barrage and four destroyers to the east. There was a light cruiser, a flotilla leader and three destroyers and a monitor protecting shipping in the Downs, off the east coast of Kent and six destroyers at Dover.
The Germans had strengthened their destroyer force at Zeebrugge by transferring Korvettenkapitän Kahle’s 3rd Flotilla from the High Seas Fleet. The operation on 20-21 April was to be controlled by Kahle. He would not be at sea but instead would command from Bruges, where the Germans collected and deciphered intercepted British orders. According to the British Official History, two groups would again attack the net barrage, but this time there would be no raid on the Downs.
The force on the Dover side of the straits consisted of six destroyers from the 3rd Flotilla (822-960 tons, three 10.5 cm (4.1 inch) guns, six 50 cm (19.7 inch) torpedo tubes, 33.5-34 knots). That on the Calais side comprised four from the Zeebrugge Flotilla and two from the 3rd (852-990 tons, three 105 cm guns, six 50 cm torpedo tubes, 32-34 knots).
However, Mark Karau’s history of the German naval forces in Flanders during the First World War says, based on German archives, that the Calais force included only three destroyers, with the other three operating in the Downs.
A bombardment of Calais started at 2310 and one of Dover at 2330. Little damage was done to either town but the armed trawler Sabreur was damaged at Dover. British batteries returned fire but without hitting the enemy. However, the bombardment caused Bacon to keep his Dover destroyers in port until 2355, by when the Germans had gone.
At 0038 on 21 April, the Dover raiders sighted the flotilla leaders HMS Broke (1,610 tons, six 4 inch and one 1.5 pounder (20mm) guns, two .303 inch MGs, four 21 inch torpedo tubes, 31 knots) and Swift (2,170 tons, one 6 inch and two 4 inch guns, two 18 inch torpedo tubes, 35 knots). Broke was named after Sir Philip Broke, a successful frigate captain from the War of 1812.
Firing commenced at 0044. Swift, which was ahead of Broke, turned towards the German line with the intention of ramming one of the smaller German ships. She missed and was hit by several shells, but one of her torpedoes hit SMS G85. Swift then pursued the Germans as far as the barrage but then had to give up because a shell hole had caused flooding in one of her forward compartments.
Broke also fired torpedoes and attempted to ram a German destroyer. She missed first time but struck SMS G42 with her second attempt. She took several minutes to extricate herself, during when she came under fire from two other German destroyers. She tried to pursue the Germans but was too badly damaged to keep up with them. She therefore returned to G42, which was sinking by the stern but fired on Broke, which replied until the German guns fell silent about 0120. Broke’s engines then broke down and she began to drift towards G42. Some other British destroyers then arrived and took Broke under tow.
G42 and G85 both eventually sank. The British picked up about 100 survivors from crews of 87 and 85 respectively. British losses were 40 killed or wounded on Broke and one killed and four wounded on Swift. The British casualties are listed on Naval-History.net.
The two British captains, Commander Ambrose Peck of Swift and Commander Edward Evans of Broke, were awarded the Distinguished Service Order and promoted to Captain. Evans became a national hero as ‘Evans of the Broke‘ and reached the rank of Admiral, serving in the 1940 Norwegian Campaign. He had previously been second in command of Captain Robert Scott’s ill fated 1911-13 Antarctic Expedition.
The losses from this operation caused a change in German strategy. Future attacks would be aimed at the Netherlands to UK convoys rather than the Channel patrols and barrage.
 Naval Staff Monograph (Historical) 1922 vol. vi, The Dover Patrol i. p. 97.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. vol. iv, , pp. 373-74..
 Ibid. vol. iv, p. 372,
 Ibid. vol. iv, p. 378; R. Gray, Conway’s All the World’s Fighting Ships, 1906-1921, pp. 168-69.
 M. D. Karau, The Naval Flank of the Western Front : The German Marinekorps Flandern, 1914-1918, p. 124.
 Naval Staff vol. vi. p. 98; Corbett, Newbolt, Naval, p. 375; Karau, Naval, p. 125.
 Gray, Conway’s 1906-1921, pp. 73, 77-78; Karau, Naval, p. 125.Karau gives German times that were an hour ahead of British time. All times quoted have been adjusted to British time.
 Naval Staff vol. vi. p. 99; Karau, Naval, p. 125.
 Corbett, Newbolt, Naval. vol. iv, pp. 377-78.
 Ibid. vol. iv. p. 378; Gray, Conway’s 1906-1921, pp. 168-69.
 Karau, Naval, p. 126.