Britain’s Secret Weapon of the Seas

From World War I Torpedo Raider to Modern 3D-Printed RC Model

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Origins: A Bold Idea from Junior Officers

Few naval weapons have such an unlikely origin story as the Coastal Motor Boat (CMB). In the summer of 1915, three junior officers of the Harwich Striking Force — Lieutenants Hampden, Bremner, and Anson — hatched an audacious proposal: could a small, fast motor boat, armed with a torpedo, travel over enemy minefields and attack Imperial German Navy ships at anchor in their heavily defended bases?

The Admiralty gave tentative approval, and a Staff Requirement was issued for a new type of vessel to serve in the North Sea. The specifications were demanding. The boat had to be capable of at least 30 knots when fully loaded, carry enough fuel for a meaningful radius of action, and be armed with torpedoes, depth charges, or mines, supplemented by light machine guns. Several established shipbuilders were approached, but only one company believed the requirement could actually be met: John I. Thornycroft & Company.

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This was no surprise. Thornycroft had been at the cutting edge of fast naval craft for decades. As far back as 1875, the firm had delivered HMS Lightning to the Royal Navy — the world’s first purpose-built torpedo boat. By 1910, Thornycroft had produced Miranda IV, a 25-foot single-step hydroplane speedboat that achieved a remarkable 35 knots from just 120 horsepower, demonstrating the hull technology that would eventually become the CMB.

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The 40-Foot Prototype and Its Limitations

Based on the hydroplane design pioneered by Miranda IV, Thornycroft produced an initial 40-foot CMB that was accepted by the Admiralty for trials. Twelve boats were ordered in January 1916 under considerable secrecy — so secret, in fact, that trials were conducted at night to prevent the enemy from learning of their existence. All twelve were completed by August 1916, with further boats built to a total of 39.

These early CMBs were revolutionary in concept but constrained in capability. The weight limitations of the design meant a torpedo tube was out of the question; instead, the 18-inch torpedo was carried pointing forward in a rear-facing trough aft of the cockpit. When fired, a cordite charge drove a long steel ram which pushed the torpedo backwards out of the stern, entering the water tail-first. A trip-wire between the torpedo and the ram head would start the torpedo motor once pulled taut. The CMB would then turn hard over at full speed to get clear of the torpedo’s path. Remarkably, there is no record of any CMB ever being struck by its own torpedo — though on one occasion a premature trigger left a crew with an extremely tense 20 minutes near enemy territory while reloading.

The 40-footers gave distinguished service but were ultimately regarded as too small, their payload limited to a single torpedo. The stage was set for something bigger.

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The 55-Foot CMB: The Definitive Design

By 1917, Thornycroft produced what many consider the definitive version of the Coastal Motor Boat: an enlarged 55-foot variant that offered greater range, improved seakeeping, and a more versatile weapons load. This was the boat that would go on to define the CMB legend.

Specifications and Performance

The 55-foot CMB was a masterpiece of lightweight engineering. Built from a mahogany stepped-planing hull — the same hydroplane principle Thornycroft had been refining for years — the boats were designed to ride up onto their own bow wave and plane across the surface rather than pushing through it. The result was staggering speed for a combat vessel. Crews of between three and five men could push these boats to a breathtaking 40 knots, making them among the fastest naval craft in the world at the time.

Propulsion was provided by two Thornycroft 12-cylinder petrol engines developing approximately 450 brake horsepower in total, with some variants featuring different engine configurations. In addition, a small cruising engine of between 30 and 70 brake horsepower allowed the boat to loiter at a quiet seven or eight knots without consuming the powerful main engines — important for approaching targets undetected.

Armament: Torpedo, Guns, and Depth Charges

The 55-foot variant retained the distinctive aft torpedo trough of its 40-foot predecessor, housing a single 18-inch torpedo. Unlike the earlier type, it could also carry a mixed weapons load of one torpedo plus four depth charges, or could be configured for mine-laying instead of torpedo attack. Depth charges were released from individual cradles over the sides of the hull — a more versatile arrangement than a stern ramp.

On top of the cockpit sat a twin Lewis gun mounting, which became a standard and characteristic feature of the type. This anti-aircraft capability had been hard-won: an earlier force of CMBs had been annihilated by German seaplanes, demonstrating in the most brutal fashion that these fast surface craft were highly vulnerable to air attack. The twin Lewis gun provided a measure of defence against aircraft and could also be turned on enemy surface craft during close-range actions.

A torpedo director — a small metal calculating device mounted on the front of the cockpit roof — allowed the crew to calculate deflection angles, accounting for the enemy ship’s course and speed before firing. This was sophisticated fire control equipment for such a small vessel, and it spoke to the seriousness of purpose that Thornycroft and the Royal Navy brought to the CMB programme.

The boats were also fitted with wireless communication equipment, allowing coordination with shore bases and other vessels — an important capability for craft operating on independent, long-range missions in enemy waters.

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Combat History: From the North Sea to the Baltic

The CMBs gave useful service operating from Dover and the East Coast ports during the First World War, with several notable engagements during the Zeebrugge and Ostend raids of 1918. CMB 5 torpedoed a German destroyer within the Mole at Zeebrugge; CMB 7 struck another destroyer alongside the Mole. These actions showed what the small, fast boats could do when well handled.

But the most spectacular chapter in CMB history came not during the war itself, but in the Baltic in 1919, in the murky conflict that followed the Russian Revolution.

The Kronstadt Raid: A Moment of Audacity

Operating from Finnish bases, a force of 55-foot CMBs was tasked with striking at the Bolshevik Baltic Fleet at anchor in the heavily defended naval base at Kronstadt. On the night of 17–18 August 1919, Lieutenant Augustus Agar — who had already won the Victoria Cross in a separate action for sinking the Russian cruiser Oleg while evading four escort destroyers — led a raid that entered legend.

The assault on Kronstadt was an act of extraordinary boldness. A large force of CMBs penetrated the harbour defences at high speed, using the darkness and their shallow draught to navigate through obstacles that would have stopped any conventional warship. The attack resulted in the sinking of two Bolshevik battleships — Petropavlosk and Andrei Pervosvanni — and a submarine depot ship in a single night’s action. It remains one of the most audacious small-boat operations in naval history, and it was achieved by the same Thornycroft 55-foot design that had been developed just two years earlier on the drawing boards at Hampton.

The surviving 40-foot CMB 4, in which Agar won his first Victoria Cross, is today preserved and on display in Boathouse 4 at Portsmouth Historic Dockyard, on loan from the Imperial War Museum Duxford — a tangible link to this remarkable period of naval innovation.

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Legacy and Later Development

The CMB concept did not end with the First World War. The design proved so successful that derived versions were developed into the Second World War. Later boats, built in the early 1940s, were described as “derived from the First World War Thornycroft 55ft Coastal Motor Boats” — essentially improved versions featuring a hard-chine bow and revised engine layout, powered by twin Thornycroft RY12 petrol engines capable of 40–42 knots.

These later CMBs served in clandestine roles, dropping agents in Greece and Crete in 1941 before being destroyed by German bombers when Crete fell. The design lineage that Thornycroft had begun in 1916 thus extended to the very heart of the Second World War’s secret operations.

The firm itself continued to be a major force in British naval construction, eventually becoming Vosper Thornycroft and building the fast patrol boats that carried on the tradition of high-speed naval craft long into the twentieth century.

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Bringing History to Life: The 1:16 Scale RC-Ready 3D Printable Model

For enthusiasts who want to own a piece of this extraordinary naval history, a highly detailed 3D printable model of the Thornycroft 55-foot CMB is now available on Cults3D, priced from US$46.73. Designed at 1:16 scale — a commanding size that does full justice to the original’s elegant lines — the model is specifically engineered for radio-controlled use, making it one of the most complete and practical CMB replicas ever offered to the hobbyist community.

The model can be found at our Cults3d page: https://cults3d.com/en/creations/thornycroft-55ft-coastal-motor-boat-1-16-scale-rc-ready-modular-builder-fr

A Modular Approach to a Complex Subject

One of the most impressive aspects of this model is its modular construction philosophy. The hull is divided into five numbered sections — each split into a top and bottom half — allowing the builder to print the model on standard consumer printers without needing a large-format machine. The sections interlock cleanly and can be assembled into a complete, watertight hull ready for electronics installation.

Each hull section also features dedicated RC access hatches — front, middle, and rear — through which motors, servos, receivers, and batteries can be installed and later accessed for maintenance. This practical detail shows a designer who has thought through the RC conversion seriously, not as an afterthought.

Extraordinary Detail Throughout

The component list is remarkable in its ambition and completeness. The model includes:

Hull and Deck: Five-section modular hull in top and bottom halves, with deck hatches complete with porthole glass inserts (eight in total, four per side), front hatch cover with air intake, guard rails in two-piece left and right configurations at the bow, and accurately reproduced hull strakes and rivets in configurable versions.

Captain’s Bridge and Conning Tower: The bridge is available in multiple variants — an open frame type, a version with full windows, and an option with a roof. Individual window panes for the bridge front and sides are provided as separate clear-print parts, and a torpedo director cradle is included on the conning tower roof. A functional hatch for the bridge frame version is also included.

Weapons and Equipment: Twin Lewis gun mountings with individual gun halves, drum magazines (two types), crosshair sights, and gun stands are all present in meticulous detail. The torpedo trough assembly includes the torpedo itself, release tray braces, holding springs, barrier components, and hook plates — capturing the complex mechanism that made the CMB’s distinctive firing system work. Four depth charge cylinders are included, complete with holders, lids, and stopper wedges.

Topside Fittings: Anchors (left and right), bollards (bow and deck variants), a flag post, a working flag, ladders, chainlinks, air intakes in both standard and funnel types (each in two-piece and single-piece versions), large fuel or water tanks in two-piece halves, high-pressure tank legs, and various other deck fittings bring the model to life with a level of authenticity that stands comparison with professional display models.

Display Stand: One of the most thoughtful inclusions is a complete display stand modelled on the launching trolley used for the real boats. The stand features a wheeled transport car with front, middle, rear, and extension pieces, rolling wheels with axle options for both direct installation and ball bearing upgrades, wheel stoppers, I-beam rails, hull support cradles for front, middle, and rear positions, and a ground base. When not running on water, the finished model can sit in its display stand just as the real CMBs rested on their shore-side trolleys — a beautiful contextual touch.

Files and Format

The download package provides files in both 3MF and STL formats in a ZIPped archive, with each component individually named and logically organised for easy print management. The comprehensive part count — running to well over one hundred individual STL files — reflects the depth of research and modelling effort that has gone into recreating every external detail of this historic vessel.

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Why the Thornycroft 55-Foot CMB Matters

The Coastal Motor Boat was more than just a fast torpedo boat. It represented a fundamental rethinking of naval warfare — the idea that speed, surprise, and audacity could substitute for armour and firepower. In this sense, the CMB was a direct ancestor of all modern fast attack craft, special operations vessels, and offshore patrol boats.

Thornycroft’s contribution to this story cannot be overstated. The firm’s willingness to push the boundaries of hull design, engine technology, and weapons integration — at a time when most shipbuilders considered the specification impossible — produced a vessel that changed the course of naval history. The raids on Kronstadt and the Baltic operations of 1919 demonstrated what small boats, well handled by brave crews, could achieve against far larger and more powerful opponents.

For the scale modeller or RC enthusiast, this 1:16 Cults3D model offers a rare opportunity to engage with that history in a hands-on way. Whether displayed on its period-accurate launching trolley or running at speed on a lake or harbour, a finished build of this model will stand as both a tribute to Thornycroft’s extraordinary engineering legacy and a showcase of what modern 3D printing can achieve in the hands of a skilled designer.

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The Thornycroft 55ft Coastal Motor Boat 1:16 Scale RC-Ready model is available on Cults3D. The model files include over 100 individually named STL components covering the full hull, all deck fittings, weapons systems, and a complete display stand. <<< Click to get the file pack.