Unrestricted submarine warfare 2

The convoy system, belatedly introduced for merchantmen,
averted Britain's looming catastrophe

As recorded in the first part (See Post 16/2/17) the first three months of unrestricted U-boat warfare (UUW) had brought great success to the Germans and induced alarm and crisis activity in Britain. Holzendorff's prescription for the shutting down of Britain's war capability required six months of shipping tonnage at 600,000 tons a month, and the losses for February March and April had averaged more. Perhaps the German optimism was not so fanciful. They had taken the fateful (and ultimately ruinous) decision for UUW in early January. At that point they saw it as the only realistic way for Germany to win outright. They knew about the stretched and exhausted state of their own army, and the low morale and poor physical health of their own population. If they had appreciated fully the parlous state of the Entente powers they might have held off. The French Army was in a worse state than their own; and the Russian effort in the east would shortly cease to exist. The British Empire's resources were keeping the Entente going, but the country itself was on the verge of bankruptcy, and its debts to the USA could not be sustained unless the USA entered the war.  
But now Germany was irrevocably committed to UUW, described by Churchill as ".. one of the most heartshaking episodes of history. It was the greatest conflict ever decided at sea, and almost entirely a duel between Britain and Germany." 

Since their first short period of UUW in 1915, the German strength in U boats had increased greatly. Larger, faster and better armed boats were now capable of making 3-4 weeks patrols. The full complement was now over 150, of which around 70 would be at sea at any one time. Provided the Dover mine barrage could be bypassed (and it could with apparent ease) boats could take the short route from the German and Belgian bases to the vital southern approaches between Ireland and Brittany, and spend 2-3 weeks patrolling in wait for the inbound merchant men. Others took the longer route via the North Sea to patrol the northern approaches round Ireland to the coast of north west England and Scotland.
Once this system was in place, UUW had a rapid and dramatic impact. From a baseline of around 130,000 tons per month in 1916, there had already been a rise late in the year and in January 1917 to more than 250,000 tons sunk. For February the figure was 468,000; for March almost 500,000, and in April it rose to a staggering 869,000 tons. This proved to be the high water mark (sic) for UUW. It still stands as the highest figure ever*. This monthly average actually exceeded Holzendorff's target for strangling Britain in six months, and she was now running out of wheat as well as gold.
Belatedly, the Government and the Admiralty started to get a grip on this dangerous situation. Strenous afforts were made both to improve the protection of merchant shipping and to improve 'seek and destroy' tactics against U-boats. Both were woeful at the start of the year, partly because of the relatively low losses through most of 1916, and the competing pressures for resources to go elsewhere. Arming merchant men to ward off surface attacks from U-boats was the simplest and most obvious measure, but it was a victim of this prioritising - guns were needed everywhere, and only late in 1916 did capacity begin to catch up with demand. In the year 1916 of 600+ merchantmen experiencing surface attack, 76% of those armed escaped, whereas only 22% of those unarmed did so. Q boats (See Post 17/11/2015) were a clever and audacious riposte to surface attacks, although in all of 1915 and 1916 they sunk only 11 U-boats. Perhaps they deterred many more attacks, but by 1917 they were becoming redundant as the Germans were more wary, and were preferring torpedo attacks. Depth charges (still fairly rudimentary) as part of destroyer patrols comprised the other attacking option. There were insufficient destroyers to patrol all of home waters, but at least they were cheaper to build than submarines.
One of Lloyd George's early actions as Prime Minister was to create a new Board of Shipping, merging the offices of Admiralty Chief of Staff and First Sea Lord. Jellicoe was the first holder of this position. The aim was to free up time to focus on improving counter measures. Jellicoe drafted in younger officers to the befuddled Admiralty to deal with this new type of warfare. There were three main areas of focus: better mining; better technology (hydrophones and depth charges); and preparation of convoy plans. The last of these was to be the game changer.
Although convoys had been used for centuries, conventional wisdom held that they were inappropriate for protecting merchant shipping in WW1. This despite the fact that they were in use, successfully, for escorting troopships, and also carrying vital coal supplies across the channel to France**. It was argued that the varying (but slow) speeds and the sheer numbers of merchantmen involved made it too complex an undertaking (wrong). There were insufficient destroyers to protect large numbers of convoys (wrong); and that concentrating ships together would make them an even easier target for U-boats (the reverse proved to be true). Opposition to convoys came from every part of the establishment, the most senior being Jellicoe. Desperation at the position forced a re-think, and production of arguments - mostly from younger officers but also, to his credit, David Beattie, Grand Fleet Commander - to counter the blind logic. Lloyd George and his cabinet were more easily persuade, and prevailed over Jellicoe to insist upon a proper trial. The first experimental convoy left Gibraltar on May 10th 1917, and arrived completely intact. Otherwise overall losses for May remained dangerously high at 600,000 tons. The convoy system was rapidly extended to cover the inbound north and south Atlantic routes. So successful were they over the next three months that the Germans were forced to switch their attacks to un-escorted outbound ships, and gradually these too came under convoy protection (feasible because of the increasing American contributions to destroyer escorts). In October 1917, of 1500 inbound merchant ships only 24 were lost - an astonishing turnaround. Once again the conservative, slow thinking Admiralty had been demonstrably wrong. At Jutland, as Commander of the Grand Fleet, Jellicoe had been prey to this. This time he was the top man at home, but again he shouldered the blame, harshly.

Jellicoe as First Sea Lord.
His disagreement with Lloyd
George cost him his job.

The convoy system all but solved the protection problem and, with increased production from USA involvement, by late 1917 new tonnage far outweighed lost tonnage. The U boat teams were exhausted and many of their best and most audacious leaders had been lost. All possibility of starving Britain to surrender had gone. At the same time the re-invigorated approach to 'seek and destroy' was bearing fruit. Q boats were being replaced by flotillas of submarines that lay in wait for outbound groups of U-boats in the north sea and the channel. The effectiveness of mining tactics improved throughout 1917, particularly in the Channel with the appointment of the dashing Admiral Roger Keys to command of the Dover patrol (see Post 13/9/2015). The previous Dover barrage of 1916 had been ineffective in blocking the Belgian based U-boats from using the Channel route. In the November 1917 reshuffle when Wemyss replaced Jellicoe as First Sea Lord, Keyes replaced Admiral Bacon in the important Dover defensive role. Keyes immediately doubled patrols along the barrier, now laid with the new deep, horned mines, and organised night time illuminations to drive the U boats deeper toward them. This prompted great activity in the Channel, and ultimately closed it off as a route for U-boats.
By early1918, with improving depth charges and listening technology, the U-boats were becoming the hunted rather than the hunters. Britain had ridden out the blockade crisis and the risk of a precipitate defeat.

 * Even Hitler's modern U boat fleets in 1942 at their peak could not match it. 
** France's own coal fields being under German occupation