In WWI, How were the Germans able to maintain a submarine blockade of Britain?

Before the 1950s and 1960s submarines were not effective at hunting submarines, as such the British submarines were not especially expected to defeat German submarines. (uboats.net wwi sinking summaries, not exhaustive, U7 U40 U23 U51 UC10 U81 UC24 U99 UC63 UC65 UB16 U154 UB72 UB52 UB90 U78 16/202, 8% of losses)

In the Great War light vessels such as destroyers, Q-ships, patrol craft, armed trawlers, etc bore the brunt burden of submarine warfare. The battle line was not effective at this task, despite Dreadnaught having sunk a sub by ramming.

Submarines were primarily surface vessels which could occasionally submerge. They identified merchant vessels while surfaced, and engaged them primarily through surface gunfire. The British faced a number of problems in dealing with this threat. They lacked a way to protect their merchants. They lacked a way to locate submarines. They lacked effective weapons systems to destroy submarines. They had these lacks while responsible to keep a fleet in being to suppress the High Seas Fleet of Germany in case it sortied. They had these lacks while responsible to keep armed merchant cruisers on a distant blockade of Germany (and neutrals.)

Submarines were bleeding edge military technology, and there was a technical and military science lag in adjusting to them.

Merchant warfare is often considered unglamorous and receives inadequate staff attention. This compounded the problems caused by just how new submarine warfare was.

Why, you immediately ask, did the British not institute convoys immediately to protect their shipping? Convoys concentrate merchants for mutual defence. Convoys concentrate merchant shipping making submarines patrol longer to find targets. Convoys can be escorted. Convoys do all these things. They also drastically reduce the tons delivered per month as vessels must wait for a convoy to assemble, and must travel at the pace of the slowest vessel. The purpose of a merchant marine to a power at war is tons delivered per month. The purpose of submarine merchant warfare is to destroy potential tons delivered per month. As convoying would early in the war eliminate more tons per month due to slow transit than subs would sink, convoying would do the submarine’s job for them and starve the UK of war materiel. As long as submarines are sinking fewer ships than would be “lost” by delays in assembling and moving convoys, then the losses to submarines may be justified. Britain delayed convoying in part due to this sound if bloody economic reason.

This left ships blundering around the ocean attempting to locate subs, or more commonly attempting to not be located. On the rare occasion of locating a submarine, gunfire and ramming were the methods of destruction. These were not effective if a submarine submerged in a timely way, concealing itself from British eyes and weapons.

One response was to let the subs come to them. Q-ships and decoys were vessels designed to look like helpless and hapless merchants, but either heavily armed and hard to sink, or paired with a military vessel. Submarines were lured into surface gun battles with Q-ships, or attacked by the decoy’s protector.

One alternative to blundering was to catch submarines in transit. British submarines were stationed submerged outside harbours or at squeeze points to torpedo German submarines. British minelayers laid fields of mines to obstruct and potentially destroy German submarines. Mines accounted for far more submarines than torpedoing.

As German submarines became effective convoying was instituted. Depth charges were pioneered as a way to attack submerged submarines. ASDIC was invented to identify submerged submarines. More money went to light vessels. The problems were solved. Convoys meant that German submarines had difficulty finding and attacking targets. Convoys meant that targets were protected from surface action by mutual support. Convoys meant that submarines had to come to the escorts. Escorts had effective undersurface weapons. Escorts could have ways to identify and locate submerged submarines, once they had made themselves known. Importantly the Royal Navy had relearnt lessons of merchant warfare and trade protection, and could use these assets and techniques.

The Royal Navy failed to initially counter Germany’s submarines due to:

• a lack of institutional esteem for this arm of naval warfare
• competing higher priority responsibilities
• lack of doctrine
• technological shock
• lack of effective weapons systems to locate and attack submarines
• an economic need to not pursue the most effective methods, due to the high cost of these methods, until merchant losses became too great

Source: “WWI” uboat.net https://uboat.net/wwi/

How come that in WWI, the Germans were able to maintain a submarine blockade of Britain...

They weren't, at least not one which had a major impact on the war effort. Submarines and submarine tactics were in their infancy. They were slow, short ranged, largely blind, vulnerable on the surface, and they had to spend most of their time on the surface.

Rather than a blockade it was more like commerce raiding. The German objective after their resumption of unrestricted submarine warfare on Feb 1917 was 600,000 tons a month. They only reached that goal twice.

...when GB was clearly the dominant naval power?

They were on the surface. Just as submarines were in their infancy, anti-submarine tactics were also in their infancy. WWI was the first time submarines were used on a strategic scale. Because this was a new form of warfare it leveled the playing field between the British and the Germans.

All the technology used to counter the U-Boat peril in WWII were new or unavailable. Experimental hydrophones (passive sonar) became available to convoys in 1916. ASDIC (active sonar) was still in prototypes. By 1916 ships could carry a few depth charges. Anti-submarine air patrols were being experimented with, but with airships which had the range and endurance.

Ultimately the Germans lost half their submarines in WWI.

How come that the Royal Navy routed the German fleet at Jutland, but was unable to drive the U-boats out of home waters?

Anti-Submarine Warfare (ASW) is extremely different from surface warfare. Big gun ships are of little use against submarines, you need many smaller craft with specialized equipment and tactics. What having dominant surface navy and strong naval tradition gave Britain is a big head start in creating an effective ASW fleet.

Jutland wasn't a rout. The result is still hotly debated. I'd call it a "strategic victory" for the British as they kept their fleet intact, critical to Britain's survival, and convinced the High Seas Fleet to stay bottled up. It should have been a rout, but poor communications and poor training on the British's part, and some brilliant decisions by the Germans to survive the trap they walked into, turned it into a much closer thing than it should have been for the British.

Jutland shows that in WWI so many aspects of warfare were so new and so untested and so mixed up with tradition all at the same time that simply having more ships and a longer tradition was no guarantee of victory.

They ultimately did with a convoy system, though the British were quite resistant to implement it. A submarine can safely attack unarmed merchant ships (or can they?) but is no match for an armed escort vessel. Once convoys were in full swing in summer 1917 sinkings plummeted.

Did the British Empire have no subs of their own?

They did, but submarines in WWI (and WWII) were not effective at combating other submarines. Of the 217 German U-Boats lost only 16 were to other submarines. 14 to Q-Ships. About 20 to ramming. ~16 to depth charges. Mines were very deadly taking about 67 boats (6 to their own mines). A great many simply disappeared, U-Boats were not the most seaworthy vessels.

British submarines were largely E-class designed as warships, not commerce raiders. They largely operated in the North Sea as scouts and the Baltic Sea raiding German shipping and helping the Russians.