Origin of the Dover Patrol
At the outbreak of war the main British naval force in home waters was the Grand Fleet, based in Scotland. There were, however, four different forces based in the south east of England: the 1st and 3rd Destroyer Flotillas under Commodore (T) and the 1st Submarine Flotilla under Commodore (S) at Harwich; Cruiser Force C at the Nore; the 6th Destroyer and 2nd and 3rd Submarine Flotillas under the Admiral of Patrols at Dover; and the Channel Fleet of 20 pre-dreadnought battleships. These forces were not under a single command.
The 6th Destroyer Flotilla of two light cruisers and 20 destroyers was responsible for the defence of the Dover Straits and was named the Dover Patrol. The loss to U-boats of the cruiser HMS Pathfinder off Berwickshire on 5 September 1914 and of three armoured cruisers of Force C in the southern North Sea on 22 September showed the importance of the Patrol Flotillas in anti-submarine warfare. At the same time, the work of the Admiral of Patrols was becoming more complex, including minelaying, prevention of enemy minelaying and organising the transport of British troops to France and Belgium and the evacuation of Belgian refugees. On 7 October it was decided that the Dover Patrol should become a separate command. Rear-Admiral Horace Hood took it over six days later.
Hood was replaced by Vice Admiral Reginald Bacon in April 1915. Bacon later wrote a two volume history of The Dover Patrol, which is now out of copyright and available to read at Naval-History.net or to download at The Internet Archive.
Bacon listed the achievements of the Dover Patrol: drifters, crewed by pre-war fishermen, maintained anti-submarine nets, which stretched for 45 miles in 1917; trawlers, also crewed by fishermen swept for mines across 250 miles per day; 120,000 merchant ships passed through the straits with light losses; 5,600,000 troops crossed the Channel without loss; the enemy held coast was bombarded from sea 28 times from ranges of up to 15 miles versus a maximum of 12 miles on ranges in peacetime; examination of merchant ships; and laying of minefields.
The first anti-submarine drifters arrived at Dover in January 1915 and there were over 130 of them there by June. They dragged an average of 1,000 yards of nets with a mesh pattern of 10 foot squares to a depth of 120 feet. The straits were a maximum of 21 miles wide and 180 feet deep, with an average depth of 108 feet. This meant that 36 drifters could in theory block the passage, but in practice tides and current made the task of such small craft difficult even in good weather. They were at best armed with a 6 pounder gun and sometimes with just a machine gun, requiring them to be protected by destroyers and armed auxiliary steamers. German records show that U8, scuttled on 4 March 1915 after being caught in the nets, was the only U-boat lost to the Dover barrage in 1915 or 1916. It did, however, force the larger U-boats that were based in Germany to take the longer route round Scotland to the Atlantic, with the smaller UB coastal submarines and UC minelayers based in Flanders using the shorter route through the Dover Straits.
The Allies used mines to combat U-boats but U-boats also laid mines. The 15 boats of the UC I class carried 12 mines but had no deck gun or torpedo tubes. The 64 UC II class boats carried 18 mines and had two bow and one stern torpedo tubes. They initially had no deck gun although some were fitted with a 105mm gun in 1918. The 16 UC III class boats that were commissioned had 14 mines, the same torpedo armament as the UC IIs and either an 88mm or a 105mm gun.
Most German mines laid in British waters were laid by U-boats. In the second half of 1916 an average of about six merchant ships were sunk per month in British waters. This increased to 10 in the first half of 1917 but fell back to four in the second half of that year. On average 178 mines were swept in each month of 1916, rising to 355 in 1917. Even the English Channel was too big an area to sweep completely and only about 10 per cent of the waters around Dover could be swept regularly.
As an example of the size of the Dover Patrol, in October 1916 it comprised:
The Auxiliary Patrol of 2 yachts, 78 trawlers (56 fitted as minesweepers), 10 paddle minesweepers, 130 net drifters, 24 motor launches and 5 motor boats.
The 5th Submarine Flotilla of the light cruiser HMS Arrogant and 10 submarines.
The 6th Destroyer Flotilla of the light cruiser HMS Attentive, 33 destroyers of 400-1,000 tons, 12 monitors with 7.5 to 15 inch guns, 12 gun or patrol boats, 5 paddle minesweepers and a seaplane carrier.
In late October the light cruiser HMS Carysfort and the 8 L class destroyers were detached from Harwich.
The Germans made several attacks by destroyers on the Dover Straits. Click on the links for more details of the larger attacks.
The first of these, on 26-27 October 1916, resulted in the sinking of the old destroyer HMS Flirt, called a 30 knotter after her designed speed, the transport Queen, six drifters, a trawler and serious damage to the destroyers HMS Nubian and Amazon. The Germans suffered no losses but missed opportunities to do more damage. The German success was helped by their previous inactivity, which made the British complacent. This action showed that the barrage had limited effect, as 14 British destroyers had crossed it without being damaged.
The Dover Patrol was reinforced by destroyers from the Humber and Harwich. Destroyers had then to be sent from the Grand Fleet to the Humber. This meant that the Grand Fleet might have had to leave part of the 4th Battle Squadron behind when it put to sea because of a lack of destroyers. The Germans were unable to base a large number of destroyers at Zeebrugge because of the risk of air attack, meaning that they face a lengthy canal journey from Bruges. This meant that the British normally detected their operations early, The Germans usually reinforced their Flanders Flotillas with extra destroyers from the High Seas Fleet before raids. 
The second, on 23 November 1916, was ineffectual. Six German destroyers approached the Downs and fired at the drifters, damaging one without causing any casualties, before turning away before the British destroyers in the area could engage them. They made no attempt to enter the Downs, where over 100 merchantmen were moored. The Germans claimed to have bombarded Ramsgate, but no shells landed on land.
On 25-26 February 1917 the Germans sent destroyers to attack the traffic rout from England to the Hook of Holland, the Downs and the barrage. The only effects of this raid were that the destroyer HMS Laverock was struck by a torpedo that did not explode and that a bombardment the Thanet coast slightly damaged some houses.
A German raid on 17-18 March resulted in the sinking of the destroyer HMS Paragon and the merchant ship SS Greypoint and the damaging of the destroyer HMS Llewellyn.
The next attack came on 20-21 April 1917. It was the first to end in a major German defeat. The destroyers SMS G42 and G85 were sunk by the British flotilla leaders HMS Broke and Swift.
This was a loss of almost 10 per cent of the destroyers based in Flanders and could not be replaced. The Germans therefore changed their strategy. Future attacks would be aimed at the Netherlands to UK convoys rather than the Channel patrols and barrage. Raids on shipping at the mouth of the Thames on 26 and 30 April encountered no shipping, although Margate was bombarded on 26 April. 
On 10 May a planned attack on Netherlands to UK convoys led to a battle between German destroyers and three British light cruisers and four destroyers., Neither side suffered any losses, but the British achieved their objective of protecting the convoy.
A week later the Germans attacked a convoy in fog, sinking the merchantman SS Ciro. The British destroyer HMS Setter also sank after collided with HMS Sylph.
A further raid on 23 May was unsuccessful. Three days later a raiding force encountered two monitors and two French torpedo boats, but a fifteen minute gun battle caused no losses to either side.
The German surface forces in Flanders remained on the defensive for the remainder of 1917, fearing that the British might try an amphibious attack as part of their Passchendaele offensive. The British planned such an operation, but the land offensive did not go well enough for it to be carried out. The main tasks of the Flanders Flotillas in the rest of 1917 were minesweeping and coastal patrols. The British carried out a number of coastal bombardments, which were normally accompanied by major air battles, as both sides attempted to drive off the enemy’s observation aircraft. There were some naval encounters, but none resulted in losses to either side. By the end of 1917 too many vessels had been transferred away from Flanders, mainly to take part in Operation Albion, an amphibious assault in the Baltic Sea, for them to carry out offensive operations.
Keyes takes Command
By late 1917 the Admiralty was concerned that up to 30 U-boats a month were evading the barrage. Rear Admiral Roger Keyes in December 1917, the Admiralty’s Director of Plans, proposed illuminating the mine and net barrage with searchlights at night in order to force U-boats to dive into the minefield. Bacon argued that this would reveal the barrage and make it vulnerable to attack. On 18 December he was ordered to institute an illuminated patrol. The next night UB56 was forced to dive and was destroyed by mines. Admiral Sir John Jellicoe, the First Sea Lord, was a supporter of Bacon, but he was dismissed and replaced by Admiral Sir Rosslyn Wemyss in late December. Soon afterwards, Bacon was replaced by Keyes.
Keyes strengthened the minefields and employed a patrol of a monitor with a 12 or 15 inch gun, four 30 knot destroyers, patrol boats, 14 trawlers, 60 drifters, four motor launches and two paddle minesweepers. At night the minefield was illuminated by flares from the trawlers and the destroyers’ searchlights.
The larger U-boats stopped using the Straits in February and the smaller boats based in Flanders became less active from April. They laid 404 minefields in 1917 but only 64 in 1918. A 1922 Admiralty document claims seven U-boats sunk in the Dover Straits area in the first four months of 1918 and six in the rest of the year, 12 of them being UB or UC type boats.
Uboat.net lists five boats lost to mines and one to depth charges in the Dover area in the first four months of the year, with the seventh described as missing. It gives four lost to mines in the Dover area and one off Flanders in the rest of the year, with the final boat having been rammed by the steamer Queen Alexandra off Cherbourg.
The Germans bombarded Yarmouth on the evening of 14 January 1918. They did not encounter any British warships and the only German ships damaged was the torpedo boat SMS V67, which struck a mine and had to be towed back to port. No ships on either side were sunk in minor actions on 23 January and 5 February.
The last and seventh German raid on the Dover Barrage came on 14-15 February 1918. It was the most successful, sinking seven drifters and a trawler and severely damaging five drifters, a trawler and a paddle minesweeper without loss. Six of the raids had been successful, but they were at least a month and as much as nine months apart, with the result that the losses from one attack had always been replaced by the time of the next one. The Dover Straits Barrage therefore continued to keep U-boats out of the busy shipping lane of the English Channel, and to force them to sail round the British Isles on their way to the Atlantic, reducing their time on station. It is unclear why the Germans stopped attacking the Dover Barrage, especially when their last effort was so successful.
The Dover Patrol took part in the attacks on Ostend and Zeebrugge on 23 April 1918, for which eight VCs were awarded, and the follow-up raid on Ostend on 9 May 1918, for which another three VCs were awarded. These raids were intended to close the canals that connected the ports of Zeebrugge and Ostend to the German naval base at Bruges. The first raid closed the Zeebrugge canal to larger destroyers until 14 May, but smaller torpedo boats and U-boats were able to use it and both raids failed to block the Ostend canal.
In 1918 the British launched a series of aerial bombing raids on the German naval bases in Flanders. From 17 February to 1 April five destroyers and torpedo boats and a U-boats were damaged by bombing. The Germans were forced to reinforce their fighter defences, but the raids became heavier from 10 May. Between then and 2 June 12 destroyers and torpedo boats and two U-boats were damaged by bombing. On the night of 28 May the Zeebrugge canal lock gate was hit by a bomb, putting it out of action for a week. On 9 June it was damaged by a coastal bombardment, closing the canal for the rest of the month to all shipping.
The English Channel, a vital communications link for British troops in France and Flanders, remained open to Allied shipping throughout the war. By April 1918 it was largely closed to U-boats.
 Naval Staff Monograph (Historical) 1922 vol. vi, The Dover Patrol i, pp. 5-7.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols (London: HMSO, 1938). vol. i, p. 16
 Naval Staff vol. vi, pp. 8-9.
 R. H. S. Bacon, The Dover Patrol, 1915-1917, 2 vols (London,: 1919). vol. i, pp. xii-xiv.
 L. Sondhaus, German Submarine Warfare in World War I: The Onset of Total War at Sea (Boulder MD: Rowman & Littlefield, 2017). Kindle Edition, locations 1902-50.
 M. Faulkner, A. D. Lambert, The Great War at Sea: A Naval Atlas 1914-1919 (Barnsley: Seaforth, 2015).
 Ibid., p. 105.
 Corbett, Newbolt, Naval. vol iv, pp. 66-67; P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), pp. 346-47.
 Naval Staff vol. vi, p. 88.
 Ibid., pp. 88-91.
 M. D. Karau, The Naval Flank of the Western Front: The German Marinekorps Flandern, 1914-1918 (Barnsley: Seaforth, 2003), p. 126.
 Naval Staff Monograph (Historical) 1939 vol. xix, Home Waters part ix, May to July 1917, pp. 10-12.
 Karau, Naval, p. 126.
 Ibid., pp. 161-65.
 Corbett, Newbolt, Naval. vol. v, p. 309.
 Naval Staff vol. vi, p. 136.
 Karau, Naval, pp. 174-75.
 Corbett, Newbolt, Naval. vol. v. p. 217; Karau, Naval, p. 179.
 Karau, Naval, pp. 207-10.