Philip Watts was born at Deptford, Kent, on 30 May 1846, was the son of John Watts, of Havelock Park, Southsea, then chief assistant to John Fincham, shipwright at Portsmouth Dockyard, a famous builder of warships, and author of The History of Naval Architecture (1851), in the preparation of which John Watts assisted. His mother was Mary Ann Featherstone. Watts's father, grandfather, and great-grandfather were all master shipwrights, and his great-grandfather took part in the building of Nelson's flagship, Victory. Philip Watts was educated at the principal school at Portsmouth, and in 1860 was apprenticed as a shipwright in the royal dockyard, where he was taught mathematics and physical science in the dockyard school. He was selected to receive a ‘superior course’ in naval construction, and in 1866 was one of the three Admiralty students promoted to the Royal School of Naval Architecture and Marine Engineering at South Kensington. In April 1870 he left the Royal School with the title of fellow, and was appointed to the chief constructor's staff at the Admiralty (which in 1883 became the Royal Corps of Naval Constructors), where he made calculations with regard to the design of new ships and acted as Admiralty overseer on several ships then building by contract. In this capacity Watts made a practice of calculating on scientific principles the proper size of various parts of a ship's structure, and did much to break down the old ‘rule of thumb’ methods. On completing this work he spent over two years at Torquay in assisting William Froude, who was the first to develop an accurate theory of the behaviour of ships under way. This association was the beginning of a lifelong friendship with Froude and his son and assistant, Robert Edward Froude (1856–1924).
In 1872 Watts became a draughtsman on the constructor's staff. On completion of his service with Froude he was appointed assistant constructor at Pembroke Dockyard during the building of the battleship Shannon, launched in 1875, gaining a reputation as an ingenious designer of mechanical appliances and details of all sorts. On returning to the Admiralty he was entrusted with the organization and supervision of a ‘calculating section’ to serve the growing complexity of naval designs. He was also made responsible for the calculations relating to the torpedo-ram Polyphemus, laid down in 1878, a vessel of novel construction with a very small reserve of buoyancy, which made extreme accuracy necessary in calculating her range of stability and buoyancy. He was also concerned in the controversy over the battleship Inflexible, launched in 1876, and in connection with it conducted some steering experiments on the battleship Thunderer in Portland harbour, the results of which were afterwards issued for the instruction of the fleet. In connection with the Inflexible, Watts experimented with water chambers for the purpose of moderating the rolling of ships whose bulk was considerably higher than their centres of gravity. In 1883 he was promoted to the grade of constructor, and in November 1884 appointed to the staff of Chatham Dockyard; but in October 1885 he resigned from the Admiralty in order to succeed Sir William Henry White as naval designer and general manager to Armstrong & Co. at their warship yard at Elswick.
During his seventeen years at Elswick, Watts was placed in a position of great responsibility, both as a naval designer and as a captain of labour on a large scale; he brought the Elswick shipyard to the foremost position by designing and constructing foreign warships, and also by building several British warships designed by the Admiralty and secured for the Elswick yard in open competition. The ships designed for foreign powers were remarkable for their firepower and speed, which later were the outstanding features of Watts's additions to the British fighting fleet. He had signal success in obtaining high speed and heavy armament on abnormally small displacements. The cruisers he designed for Japan, Argentina, Brazil, Chile, Norway, Portugal, Romania, and Turkey established his reputation as a constructor and gave him experience for his later achievements. Nearly all the warship fighting done by the Japanese fleet in 1904 and 1905 was carried out by Watts's ships, the battleships Yashima and Hatsuse, and the cruisers Idzumo, Iwate, Asama, and Tokiwa. For his services he was awarded the Japanese order of the Rising Sun. Among Watts's Elswick ships may be mentioned especially the Italian cruiser Piemonte, built in 1888, then for her size the most heavily armed war vessel in the world, and the Japanese Yashima, launched in 1896, a battleship with the speed of a cruiser, and in many respects the forerunner of his battle cruisers of later times. From 1894 to 1910 Watts was lieutenant-colonel and honorary colonel of the 1st brigade of the Royal Garrison Artillery volunteers, and while at Elswick he equipped and sent out to the South African War the Elswick battery.
In February 1902 Watts returned to the Admiralty as director of naval construction. At that time two of the King Edward VII class of eight battleships had been begun, and three more were to be laid down before the end of the year. In this class the main armament was much the same as that of the Collingwood class of battleships designed in 1880, though the secondary armament was heavier. Watts considered the class not powerful enough, though he was not able immediately to carry out his ideas to the full. In 1903 he produced designs for battleships of much greater gun power. The Admiralty, however, while approving what afterwards was named the Lord Nelson type, resolved to build first the three remaining vessels of the King Edward VII class, which were accordingly laid down in 1903, and were followed by the Lord Nelson and Agamemnon, of the new and more powerful type, in 1904 and 1905. The appointment on 21 October 1904 of Sir John Arbuthnot Fisher as first sea lord gave Watts the opportunity of realizing more completely his desire for powerful ships. As far back as 1881 he had accompanied the Inflexible in the Mediterranean when Fisher was in command, had had the opportunity of discussing with him matters of naval construction, and had brought away an outline design, evolved during the cruise, for a battleship with an ‘all big gun’ armament of four pairs of 16 inch 80 ton guns mounted in turrets. This design, however, had been rejected on account of the large displacement involved. Fisher, on becoming first lord, introduced a design for an ‘all big gun’ battleship having six pairs of 12 inch guns all mounted on the middle line, three pairs at each end of the ship in steps. A vessel so armed could fire six guns directly ahead and six directly astern, and all twelve guns on either broadside.
Fisher got a powerful committee of design appointed by Lord Selborne, including Prince Louis of Battenberg, naval officers such as John Jellicoe, Henry Bradwardine Jackson, Reginald Bacon, Charles Madden, Sir Albert John Durston, and Alfred Winsloe, and men of expert knowledge such as Lord Kelvin, Sir John Isaac Thornycroft, R. E. Froude, John Harvard Biles, Alexander Gracie, and Watts. Fisher's proposal of a twelve-gun battleship was generally approved, but on consideration it was found too large and costly. The dreadnought was as near an approach as was possible, taking into account dockyard capacity and naval estimates. The final result of the committee's aspirations, as interpreted by Watts, was the recommendation of the remarkable series of ship types of which the Dreadnought battleship and Indomitable battle cruiser were the chief. The principles underlying these new designs were a much more powerful armament on a given displacement, higher speed through the use of steam turbines and the water-tube boiler, unification of gun-calibre to secure gun control, greater manoeuvring power, the internal subdivision of the vessel into separate watertight compartments, each self-contained as regards access, drainage, and ventilation, and greater protection against guns and torpedoes in the arrangement of the armoured decks and the underwater protection of the sides. In the dreadnought battleship class the main armament was raised from the four 12 inch and four 9.2 inch guns of the King Edward VII class to ten 12 inch guns, mounted in pairs en barbette, three pairs on the centre line, and a pair on each broadside opposite each other amidships. The secondary armament disappeared and was replaced by quickfirers to deal with torpedo craft. But in later classes of dreadnoughts the secondary armament was restored, to meet the increased menace from air and torpedo attack.
The first Dreadnought was laid down at Plymouth on 2 October 1905, and began her sea trials on 3 October 1906. She had a speed of over 21.6 knots, and owing to her rectangular construction amidships was comparatively free from rolling. During his ten years of office Watts improved and developed the dreadnought type. He was the designer of the Bellerophon, St Vincent, Neptune, Orion, King George V, Iron Duke, and Queen Elizabeth classes, each containing several battleships, and each marking an increase in armament, displacement, and speed. He himself considered that the Orion class, laid down in 1909–10, practically realized Fisher's first design. In the Queen Elizabeth class, 1912–13, eight 15 inch guns were substituted for ten 13.5 inch guns. The main armament of the dreadnought type set the fashion to the whole world. ‘If Sir Philip's fame rested on no other basis than that of the armament of the Dreadnought, his name would be handed down as one of the world's great naval designers’ (Transactions of the Institution of Naval Architects, 68, 1926, 289). Lord Fisher wrote: ‘The Dreadnought could not have been born but for Sir Philip Watts’ (Fisher, 258).
Hardly less remarkable was the creation of the battle cruiser. At the outbreak of the First World War in 1914 all the effective battle cruisers in the Royal Navy were of Watts's design. The battle cruisers Indomitable, Inflexible, and Invincible were launched in 1907. They were armed with eight 12 inch guns, and had a speed of 24 knots. On account of their heavy armament and substantial armour protection they might have been classed as battleships. They were followed by the Indefatigable class, launched in 1909, which included besides the Indefatigable the Australian cruisers Australia and New Zealand. In 1910 were launched the Lion, Princess Royal, and Queen Mary, of greater size and a speed of 28 knots, and in 1912 the Tiger, with a speed of 30 knots. It was the speedy arrival and intervention of the Inflexible and Invincible—wholly unexpected by the enemy—which proved the decisive factor in the action with Admiral von Spee off the Falkland Islands on 8 December 1914. Considerable improvements were also made by Watts in the designs of light cruisers and destroyers.
All these changes in design were fundamental and not merely developments of pre-existing types. As they were made in time of peace it was not easy to explain their desirability to the public and they met with considerable opposition. In parliament in July 1906 Mr Balfour criticized the designing and building of dreadnoughts, and Sir William White, Watts's predecessor as constructor, suggested in the Nineteenth Century in April 1908 that Britain had started an unnecessary naval armament race. There were strong reasons, however, for thinking that the improvements in naval construction were making the race inevitable. It has been asserted that Germany designed more powerful ships as early as 1904. If an advance was certain, it was important to have the initiative. Subsequent events showed this. Watts's work designed in time of peace stood the vital test of war. Naval developments in Britain caused the continental powers to change their whole outlook on naval affairs. The Kiel Canal had to be widened and harbours reconstructed, and when war broke out in 1914 the German navy, with a greatly inferior firepower, was not in a position to meet the demands made on it by the military chiefs in Berlin. The fact that Britain from the beginning was greatly superior at sea was of primary importance. At the battle of Jutland (31 May 1916) twenty-nine of the thirty-four British battleships and battle cruisers engaged were of Watts's design.
In August 1912 Watts resigned his post as director of naval construction, but his services were retained in an advisory capacity until January 1916, when he returned to Elswick and became a director of Armstrong, Whitworth & Co. He was created KCB in 1905, and in 1900 was elected a fellow of the Royal Society, of which he was afterwards a member of council and a vice-president. He received the honorary degree of LLD from Glasgow University and that of ScD from Trinity College, Dublin. He became a member of the Institution of Naval Architects in 1873, and contributed to its Transactions, a member of the Institution of Civil Engineers in 1901, and of the Institution of Mechanical Engineers in 1902. He wrote on ‘Ships’ and ‘Shipbuilding’ for Encyclopaedia Britannica. On the formation of the Society of Nautical Research in 1910 he was made a vice-president. In June 1921, at the annual meeting of that society, he called attention to the dangerous state of Nelson's Victory in Portsmouth harbour, and as a result of his efforts and those of Sir F. C. Doveton Sturdee the famous ship was refitted and preserved. In restoring her to her Trafalgar conditions Watts was assisted by old plans of the ship which had come down to him from his great-grandfather. He contributed an article on this subject in 1923 to the Transactions of the Institution of Naval Architects. Like all great organizers, he attached a high value to education. He assisted in the creation of the professorial chairs and the schools of naval architecture in the universities of Glasgow, Durham, and Liverpool. Watts married in 1875 Elise Isabelle, daughter of Chevalier Gustave Simoneau de St Omer, of Brussels. His wife and his two daughters survived him. He died of pneumonia at his residence, 4 Hans Crescent, Chelsea, London, on 15 March 1926 and was buried in Brompton cemetery.