Although it may have had a different name, the Coastal Motor Boat (CMB) was the true antecedent of the MTB. Its creation in 1915 and 1916, when the Royal Navy desperately sought a new weapon at sea, was the first episode in the history of Coastal Forces.
John Isaac Thornycroft was born in 1843 and began working at a shipyard before he even went to university. Armed with a diploma in engineering he opened his first boat works in Chiswick in 1866 and, as new boats and launches were produced, he quickly became a renowned boat builder. In 1904 the firm moved to new premises at Woolston, Southampton and in 1908 they acquired the former Immisch Company works on the small island of Platts Eyot — on the Thames at Hampton — and renamed it the Hampton Launch Works.1
Miranda IV was launched from Platts Eyot in 1910. Abandoning multi-step hulls in favour of a single step, the boat incorporated graceful curving lines throughout its 26ft length. Below water the hydroplane avoided flat hull sections to prevent the boat slamming into the sea at high speed, whilst above water, the upper hull dropped away sharply to the rear of the boat, creating an early form of whale-back design. At the back the stern narrowed to a thin wedge, allowing water to flow past smoothly and without the drag of a wide flat stern. The engine was tailored to the boat and, although not of high power, it could push the hull over the water with ease.2
Miranda IV was perhaps the world’s first true motor racing power boat, a pinnacle of the sporting drive that revolutionised the hydroplane. But, only four years after it was launched, Britain was at war. The next evolutionary jump would come from an offensive requirement.
As the First World War headed into stalemate on land, both Britain and Germany looked to the sea in the hope of delivering a knockout blow. Both nations had built up significant fleets of battleships in the preceding decade and the opposing admirals both thought that this was where victory would lie. Each tried to orchestrate a battle in which they may have the advantage and several skirmishes in the North Sea resulted, although none delivered the crushing victory that both sides wanted.
In early 1915, keen to assist the Royal Navy, Thornycroft proposed plans for a planing, stepped hull boat equipped with a torpedo. Despite the evolution in size of the early torpedo boats following the launch of HMS Lightning, Thornycroft had already experimented with a small, singular function torpedo boat — a 40ft vessel built for the Russian government in 1904, powered by an internal combustion engine but based on a conventional hull design. Fitted with Thornycroft’s patented launching gear, a torpedo could be slung over the side of the vessel and fired into the water. Other nations had also started to marry the combustion engine with small torpedo boats, although few had yet got beyond experimental models.3
Meanwhile, three young officers of the Royal Navy had developed ideas of their own. The Royal Navy had established several destroyer and cruiser flotillas at ports along Britain’s east coast and, by 1915, the crews of the Harwich Force were growing frustrated. German warships only occasionally carried out operations in the North Sea and when they did, they were able to make quick strikes and steam for home again before British ships could reach the area. Lieutenants Hampden, Bremner and Anson thought that a game changer was needed and believed that small, fast, torpedo craft would be the solution. Shallow draught vessels capable of lightning strikes in enemy held waters would be able to clear the numerous unguarded sand banks and minefields along the European coast and more easily navigate enemy harbours.4
The only problem was that no such craft existed. Supported by their commander, Commodore Tyrwhitt, the officers approached the Admiralty in July and, after tentative approval, submitted a more detailed proposal in September.4 The craft would need a powerful engine capable of 30 knots and sufficient fuel for a sustained voyage. At the same time, it could not weigh more than 4¼ tonnes (including an 18in torpedo) so that it could be carried by a cruiser.5
The Admiralty produced a Staff Requirement and made inquiries with numerous boat builders, but few considered such a vessel was practicable and only Thornycroft responded. Thinking that his earlier ideas had been ignored (indeed there’s no certainty that Hampden, Bremner and Anson knew anything about his earlier proposal) Thornycroft was keen to develop such a craft, although it was not easy. The weight of fuel was perhaps the biggest barrier, although the launching of the torpedo at high speed was no small problem itself.6
The three officers worked with Thornycroft on the design of the boat and the complications caused by the torpedo. The sheer weight of a standard tube launcher meant that it could not possibly be used and therefore launching a torpedo forwards, over the bow, was impossible. Thornycroft’s original proposal had used his patented side launching gear, but this required the boat to significantly reduce speed to fire the weapon, negating the whole point of this new boat.6 Eventually the solution was suggested by the young officers and perfected by Thornycroft: the projectile was launched tail first, over the boat’s stern. This made it possible to launch the torpedo whilst the boat was travelling at high speed — indeed speed was somewhat essential. After launching, the boat would need to turn out of the way as soon as it could before the torpedo began to accelerate. Of course, the boat would need to turn away from the target as well, so this was an effective arrangement. Trials in Portsmouth, using one of Thornycroft’s own launches, showed that the unlikely design worked.4
Incredibly, by January 1916, the designs for Thornycroft’s Vedette Skimmer were ready to present to the Admiralty. Using Miranda IV‘s hull form as a basis for the design, he proposed a 40ft boat, powered by one of the company’s V12 engines and equipped with a single 18in torpedo. The boat only required a crew of two — there wasn’t really room for anyone else.3
The Admiralty was impressed and immediately placed an order for twelve boats. They were presumably delighted to discover that Thornycroft had, in fact, already started building one — Boat No. 781 (yard number 874)1 was launched from the Hampton Launch Works on 6 April 1916. The remaining eleven boats followed over the next few months and were classified as Coastal Motor Boats (CMBs) to disguise their true identity. In covert trials on the Thames, CMB 1 reached speeds of 33½ knots with a full load.6 The first true torpedo boat was ready.
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- Kenneth Barnaby, 1964. 100 Years of Specialized Shipbuilding and Engineering. Hutchinson.
- R. W. Crowly, 1910. The Thornycroft Racers. Motor Boat, July 25. pp.39-41
- Anon, 1920, A Short History of the Revival of the Small Torpedo Boat (C.M.Bs.) during the Great War and Subsequently in the Kronstadt, Archangel and Caspian Sea Expeditions of 1919. Thoryncroft. pp.3-7
- Norman Friedman, 2014. Fighting the Great War at Sea: Strategy, Tactics and Technology. Naval Institute Press. pp.180-181
- David Brown, 1999. The Grand Fleet. Warship Design & Development, 1906-1922. Chatham Publishing. p.150
- Keiran Phelan & Martin Brice, 1977. Fast Attack Craft. McDonald & Janes Ltd. pp.62-63
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