The U-boat became the German navy’s main weapon in both the World Wars of the twentieth century, but Germany was initially slow to adopt the submarine. Holger Herwig notes that Admiral Alfred von Tirpitz, the architect of the German navy, favoured battleships and had little time for either cruiser warfare or submarines. The first U-boat was not completed until late 1906. Rudolf Diesel was German, but the first U-boats with diesel engines were not introduced until 1910. French submarines had used diesel engines from 1904. The switch to the diesel engine meant that the U-boat was considered a potentially decisive weapon for the first time, but against the enemy battle fleet, not merchant shipping.
In 1914 Germany possessed only 24 operational boats. Another four were used for training and 16 were under construction. Only 10 of the operational boats had diesel engines; the others used Körting heavy fuel oil engines that produced a great deal of smoke and sparks. This made them very visible on the surface and required ventilation pipes; stowing these slowed diving.
On 6 August 1914 the German navy sent 10 U-boats into the North Sea, but U9 had to return home early because of engine problems. Two days later the dreadnought HMS Monarch, carrying out gunnery practice near Fair Isle, between Shetland and Orkney, was narrowly missed by a torpedo fired by a U-boat.
Paul Halpern comments that Roger Keyes, then Captain (S), the commander of the RN’s submarines, later wrote ‘that the fact that the Monarch was performing such a task within 500 miles of Helgoland was an example of the navy’s general ignorance of submarine powers and limitations.’ The RN had proved in 1910 that its submarines could operate more than 500 miles away from their bases, but apparently did not consider the possibility that the Germans could do the same.
About dawn on 9 August the light cruiser HMS Birmingham, part of a screen 30 miles ahead of the Grand Fleet, observed through the mist a submarine stationary on the surface. The sounds of hammering coming from the submarine, which was U15, suggested that her crew were trying to repair an engine fault.
Birmingham closed the range and opened fire. U15 moved forward slowly, but Birmingham rammed her and cut her in two. U15 sank with all hands. Robert Massie says that 23 men went down with her, although Conway’s All the World’s Fighting Ships 1906-1921 gives her normal crew as 29.
The need for ships meant that only temporary repairs could be carried out to Birmingham at first, so she retained two scars along her bows for some months. U15’s sister boat U13 also failed to return home from the cruise, probably after hitting a mine.
This mission had cost the Germans two U-boats in return for minor, self-inflicted damage to a light cruiser and had shown that the early U-boats were mechanically unreliable. R. H. Gibson and Maurice Prendergast state in the history of the German Submarine War 1914-1918 that:
Not even the hardiest optimist could pretend that the submarine had vindicated its war value by this expedition. Yet, had the Germans known, the effect of the cruise had caused uneasiness to their opponents…it was confidently predicted by some – but not by naval officers – that all other U-boats would soon follow U15 to the bottom.
As soon as the requirement to cover the crossing of the British Expeditionary Force to France was over, Admiral Sir John Jellicoe withdrew his Grand Fleet north west of Orkney. Future sweeps into the North Sea were carried out as quickly as possible.
Jellicoe was concerned that Scapa Flow was poorly defended, so ordered the preparation of a secondary base at Loch Ewe on the west coast of Scotland. Halpern notes that ‘Jellicoe actually felt safer at sea than he did in Scapa Flow.’
This German cruise featured the first attack by a submarine launched torpedo on a moving warship. The Turtle, which made several unsuccessful attacks on British ships in the American War of Independence, and the CSS Hunley, which sank the USS Housatonic in the American Civil War, attacked by attaching explosive devices to the hulls of stationary ships. U15 was the first submarine to be sunk by an enemy ship in the war.
 H. H. Herwig, ‘Luxury’ Fleet: The Imperial German Navy, 1888-1918 (London: Allen & Unwin, 1980), p. 88.
 P. G. Halpern, A Naval History of World War I (London: UCL Press, 1994), p. 29.
 R. Gray, Conway’s All the World’s Fighting Ships, 1906-1921 (London: Conway Maritime Press, 1985), p. 175; R. K. Massie, Castles of Steel: Britain, Germany, and the Winning of the Great War at Sea (London: Jonathan Cape, 2004), p. 79.
 R. H. Gibson, M. Prendergast, The German Submarine War, 1914-1918. (London: Constable, 1931), pp. 3-4.
 Halpern, Naval, p. 29.