There are essentially two phases to German commerce raiding in WWI. The first was carried out by cruisers and auxiliaries that were based in the German colonies. However, these were hunted down relatively soon after the declaration of war. In 1915-17, we see the second phase, consisting mainly of converted merchant ships which tried to run the British blockade.

The main raiders in the first phase of the war were the cruisers that had been based in German colonies before the war. In addition to von Spee's squadron based at Tsingtao, the cruiser Königsberg was based in German East Africa, while the Karlsruhe was in the Caribbean. They were joined by a number of passenger liners which were given guns; Kronprinz Wilhelm was armed by Karlsruhe, Prinz Eitel Friedrich by von Spee at Tsingtao and Cap Trafalgar by the gunboat Eber off Brazil. The liner Kaiser Wilhelm der Grosse was the only raider of this phase to be in European waters at the start of the war. Armed in German waters, she slipped out before the British established an effective blockade.

Of these, the cruisers Emden and Karlsruhe had the greatest effect. Emden was detached from von Spee's squadron before it crossed the Pacific on the route to the Falklands. She operated in the eastern Indian Ocean, capturing or sinking 23 merchants, a French destroyer and a Russian cruiser, before being sunk by HMAS Sydney. Karlsruhe operated against British shipping in the South Atlantic, capturing 16 ships, but would be sunk by an internal magazine explosion in November 1914. Königsberg captured one merchant, and sank the British cruiser Pegasus, but would be forced into the delta of the Rufiji river by engine issues. She was quickly bottled up by the British, and sunk by British monitors in July 1915.

For the most part, the liners were more able to remain at sea. Cap Trafalgar was caught and sunk by the British armed merchant cruiser Carmania off the Brazilian island of Trinidade on the 14th September 1914 (ironically while Cap Trafalgar was disguised as Carmania). Kaiser Wilhelm der Grosse captured two merchants, but was herself sunk by HMS Highflyer. Kronprinz Wilhelm managed to capture 15 ships. However, British patrols caught her tender (carrying coal and supplies). Low on fuel, and suffering from engine problems, she was forced to seek internment in the USA. A similar fate would befall the Prinz Eitel Friedrich. She sank eleven ships in the Pacific, but would seek internment in March 1915.

Following this, the German Navy was forced to change tack. With no ships left outside Germany, any raiders had to run the British blockade. Aware that this would catch most cruisers or liners, they began to give merchant steamers hidden armaments, in the hope they could sneak past the British patrols. These ships would also have more efficient engines than liners, and hence a longer range. The first such ship was the Meteor, which made two sorties, primarily as a minelayer. On the second, a British patrol caught her, and her crew scuttled her. In December 1915, the most successful of these raiders, the Moewe made her first sortie. She laid two minefields, one off the Orkneys, sinking the battleship King Edward VII, and the other off the Gironde. She continued into the Atlantic, before returning to Germany in March, having captured 57,776 tons of shipping. Moewe's second sortie came in November 1916, slipping out of the North Sea disguised as a Swedish ship. She made another patrol of the Atlantic, narrowly avoiding a British cruiser, and being damaged by the armed merchantman Otaki. She sank 24 ships, totalling 119,600 tons, on this cruise, but would not make another sortie. Also leaving Germany in November 1916, the Wolf slipped out of Kiel, carrying 465 mines and (in an innovation for auxiliary cruisers) a seaplane. She laid a number of minefields off key ports in the Indian Ocean and western Pacific, and raided Allied shipping in the region. She sank 14 ships, while her minefields sank another 13. The last successful raider was the Seeadler, leaving Germany on the 21st December 1916. Somewhat unusually for a raider, she was a sailing ship. On Christmas Day, she was stopped by a British AMC, the Patria, but managed to escape serious inspection. Seeadler patrolled the Pacific, capturing 28,000 tons of shipping, before she was wrecked on the island of Mopihaa on the 31st July 1917.

Not every raider managed to escape Germany. In February 1916, a raider (confusingly also called Wolf) ran aground and sank while leaving Kiel. Later that month, on the 29th, the Greif was encountered by the British AMC Alcantara, as the RN had been pre-warned by radio decrypts and a submarine sighting. However, Greif was able to close. Fire was opened at a range of 800 yards, with the German ship initially doing heavy damage. However, the AMC Andes and the light cruiser Comus were close to the battle, and reinforced Alcantara. Grief was soon sunk, but Alcantara would join her, having been hit by a torpedo fired by Grief. The last raider, the Leopard, was stopped by the boarding vessel Dundee on the 16th March 1916. While Dundee was heavily outgunned (she was armed with two 4in guns, compared to five 5.9in and four 3.4in on the German ship), clever positioning on her captain's part allowed her to do heavy damage to Leopard. Reinforced by the armoured cruiser Achilles, the British soon sank Leopard.

This description of the experiences of German raiders should show some of the reasons why the Germans made comparably few attempts to use surface raiders. Without a network of bases at which ships could rearm, refuel and refit, they lacked the ability to make effective long patrols. The scale of Allied patrolling made prolonged action hard to achieve. Similarly, the British blockade meant that attempting to reach the main shipping routes was difficult. Finally, the Germans had comparatively few ships that could operate in this way - cruisers were more useful with the fleet, while freighters were useful for trading with the Scandinavian neutrals, or as blockade runners. Submarines had none of these issues. For much of the war, they could evade British patrols in the North Sea, which had little effective way to find a submerged submarine. Subs could sit in shipping lanes for prolonged periods without fearing Allied retaliation, unlike a surface raider. Submarines also gave little warning to a merchant target. As a result, the main way in which the Germans fought the commerce war was with submarines.

Sources:

Castles of Steel: Britain, Germany and the Winning of The Great War at Sea, Robert K. Massie, Pimlico, 2005

Fighting the Great War at Sea: Strategy, Tactics and Technology, Norman Friedman, Seaforth, 2014

German Commerce Raiders 1914-18, Ryan K. Noppen, Osprey, 2015

Blockade: Cruiser Warfare and the Starvation of Germany in WW1, Steve R Dunn, Seaforth, 2016

History of the Great War - Naval Operations, Julian S Corbett and Henry Newbolt, Longmans, 1920

History of the Great War - The Merchant Navy, Archibald Hurd, John Murray, 1921

Review of German Cruiser Warfare 1914-1918, Admiralty, O.U. 6337 (40), 1940