As in 1914, the most important success of German U-boats in January 1915 was against Allied warships. In the early hours of 1 January U24 sank the pre-dreadnought battleship HMS Formidable, the largest ship yet to be sunk by a submarine.
Formidable, along with the other 7 pre-dreadnoughts of the 5th Battle Squadron of Vice Admiral Sir Lewis Bayly’s Channel Fleet, left Sheerness at 10 am on 30 December 1914 in order to carry out gunnery practice. They were escorted to Folkestone by six destroyers, but from there were accompanied by only two light cruisers, HMS Diamond and Topaze. The destroyers on patrol in the Channel needed frequent maintenance because of weather damage. On the night of 28-29 December eight of the 24 based at Dover were under repair.
Kapitänleutnant Rudolf Schnieder’s U24 spotted the battleships at 9:50 am on 31 December. He was unable to get into a firing position, and had to abandon the attempt at 1:30 pm in order to re-charge his batteries.
At 7 pm Bayly ordered his squadron to change course in accordance with a standing order that ships sailing at less than 14 knots in areas where U-boats might be operating should change course just after dark in case they were being followed by a U-boat. The squadron was making only 10 knots.
At 10:30 pm U24 got underway, with her batteries re-charged. She spotted three large warships at 1:08 am on 1 January 1915. They were the lead ships of the 5th Battle Squadron. At 1:58 am U24 fired a torpedo at HMS Queen from 750 yards at an acute angle. It missed, but neither it nor the U-boat were spotted by any of the British ships.
The other five battleships then appeared. U24 crossed their wake and at 2:25 am fired two torpedoes at the last in the line, Formidable. One of them struck her abreast the forward funnel. She lost steam and developed a 10 degree list to starboard. Boats were launched, but more than 500 men were left on board. They brought tables and other wooden items up in order to make makeshift life rafts. A well lit liner then appeared, and Topaze, which was picking up survivors, signalled her to help. She acknowledged, but continued on her way.
By 3:10 am U24 had worked her way to Formidable’s port side. She fired another torpedo, which hit the battleship amidships. This corrected the list, but caused Formidable to settle by the bows. Captain Noel Loxley of Formidable then ordered Topaze to leave the sinking battleship because of the risk that the U-boat posed to her. The light cruiser spotted U24, but could not fire on her because of the positions of men in the water and Diamond. The U-boat then escaped.
The two light cruisers then attempted to rescue survivors, which was extremely difficult because of the wind and seas. Formidable sank at 4:39 am.
547 men of Formidable’s 780 strong crew were lost, including Captain Loxley. The survivors included 2 warrant officers and 71 men who were rescued from her sinking launch in a gale by the Brixham trawler Provident, which carried only four hands: Captain William Pillar,First Hand William Carter, Second Hand John Clarke and Apprentice Daniel Taylor, né Ferguson. All four were awarded the Sea Gallantry Medal.
The Board of the Admiralty admitted that the orders issued to Bayly regarding the movements of his squadrons could have been more precise. However, they ‘severely blamed him for faulty and careless conduct, which resulted in the disaster.’ He was ordered to exchange positions with Vice Admiral Sir Alexander Bethell, the commander of the Royal Naval College, Greenwich. Bayly was later given command of RN forces in Ireland, distinguishing himself in the battle with U-boats in the Atlantic.
 J. S. Corbett, H. Newbolt, Naval Operations, 5 vols. (London: HMSO, 1938). vol. ii, p. 57.
 Naval Staff Monograph (Historical) vol. xii, Home Waters part iii, November 1914 to the end of January 1915. pp. 147-48.
 Ibid., pp. 149-52.
 Ibid., p. 153.