Coastal Motor Boat
| Class overview | |
|---|---|
| Name: | 40 foot CMB |
| Builders: | Thornycroft, Tom Bunn, Taylor & Bates, J W Brooke, Frank Maynard, Salter Bros, Wills & Packham |
| Operators: |
 Royal Navy
|
| Completed: | 39 +2 not taken into service as CMB |
| Cancelled: | 16 |
| Preserved: | 1 (CMB 4) |
| General characteristics | |
| Length: | 45 ft (14 m) o/a |
| Propulsion: | Single screw, various choices of petrol engine |
| Complement: | 2-3 |
| Armament: | Single 18" torpedo, 2-4 Lewis guns, depth charges or mines |
| Notes: | Mahogany plank on frame construction, single-step planing round-form hull |
| Class overview | |
|---|---|
| Name: | 55 foot CMB |
| Operators: | Royal Navy |
| Completed: | 88 |
| General characteristics | |
| Displacement: | 11 tons |
| Length: | 60 ft (18 m) o/a |
| Beam: | 11 ft (3.4 m) |
| Draught: | 3 ft (0.91 m) |
| Propulsion: |
|
| Speed: | 34–42 kn (63–78 km/h) |
| Crew: | 3-5 |
| Armament: | 2 18" torpedoes or 1 18" torpedo plus 4 depth charges, 4 Lewis guns |
| History | |
|---|---|
 United Kingdom
|
|
| Name: | MTB 331 |
| Operator: | Royal Navy |
| Builder: | Thornycroft |
| Launched: | 1941 |
| Class overview | |
| Name: | 55 foot CMBT (1941 class) |
| Operators: | Royal Navy |
| Completed: | 14 |
| Preserved: | MTB 331 |
| General characteristics | |
| Displacement: | 17 tons |
| Length: | 60 ft (18 m) |
| Beam: | 11.5 ft (3.5 m) |
| Draught: | 4 ft (1.2 m) |
| Propulsion: | Twin screws & twin 650 hp (480 kW) Thornycroft RY12 petrol engines |
| Speed: | 40 knots (74 km/h) |
| Armament: | Twin 18" torpedoes, depth charges or mines |
| Notes: | Mahogany plank on frame construction, single-step planing round-form hull |
During the First World War, following a suggestion from three junior officers of the Harwich destroyer force that small motor boats carrying a torpedo might be capable of travelling over the protective minefields and attacking ships of the Imperial German Navy at anchor in their bases, the Admiralty gave tentative approval to the idea and, in the summer of 1915, produced a Staff Requirement requesting designs for a Coastal Motor Boat for service in the North Sea.
These boats were expected to have a high speed, making use of the lightweight and powerful petrol engines then available. The speed of the boat when fully loaded was to be at least 30 knots (56 km/h) and sufficient fuel was to be carried to give a considerable radius of action.
They were to be armed in a variety of ways, with torpedoes, depth charges or for laying mines. Secondary armament would have been provided by light machine guns, such as the Lewis gun. The weight of a fully loaded boat, complete with 18-inch (450 mm) torpedo, was to not exceed the weight of the 30-foot (9.1 m) long motor boat then carried in the davits of a light cruiser, i.e. 4.5 tons.
The CMBs were designed by Thornycroft, who had experience in small fast boats. Engines were not proper maritime internal combustion engines (as these were in short supply) but adapted aircraft engines from firms such as Sunbeam and Napier.
In 1910, Thornycroft had designed and built a 25 ft (7.6 m) speedboat called Miranda IV. She was a single-step hydroplane powered by a 120 hp (89 kW) Thornycroft petrol engine and could reach 35 knots (65 km/h).
A 40 ft (12 m) boat based on Miranda IV was accepted by the Admiralty for trials. A number of these boats were built and had a distinguished service history, but in hindsight they were considered to be too small to be ideal, particularly in how their payload was limited to a single 18-inch torpedo.
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Wikipedia