The four Coastal Motor Boats (CMBs) were loaned to the RAF from the Royal Navy for the purpose of assisting with the Schneider Trophy Races. Created in 1912, the Schneider competition was primarily intended to promote the development of flying boats. Ultimately the contest became a circuit race over a fixed distance, where speed was of the essence. The RAF set up the High Speed Flight in 1926 to challenge for the Schneider Trophy, in which role it was unsurpassed. The Schneider Trophy races kept the RAF in the public eye at a time when it was most needed. During the practice sessions for the 1927 race, the 56ft Seaplane lighters (see before) and the various ferry and servicing boats were indispensable. From the rescue point of view, the 35ft Brooke type seaplane tenders, the 50ft Kelvins and the Brooke Seacars were inadequate. As the racing speeds had doubled in the preceding 10 years, the RAF required fast ambulance launches.
As an interim measure, the RAF borrowed three CMBs. The craft were 56ft long, with a pencil-slim beam and a stepped planing hull. They were originally designed by Thornycroft as fast attack craft, capable of 35 knots in calm weather, and carried 2-18inch torpedoes fired through the stern. When stripped of their weapons in RAF service they were capable of 38 knots and a maximum of 40 knots. The CMBs had several undesirable features however. Their Thornycroft Y12 petrol engines had a knack of catching fire on starting, a feature that subsequently wrote of one of the three boats, CMB 115DE. They were also very difficult to handle in confined waters as they were not fitted with any form of astern gearbox. When slowing down from high speeds, a drogue had to be streamed to create drag. CMB 115DE caught fire and was destroyed on the 31st May 1929. She was replaced by the fourth CMB in RAF service, namely 84E.
CMB 81C was completed on the 02.04.19 having been ordered from Thornycroft as Yard No. 1571 but sub-contracted to Camper & Nicholson for completion as their Yard No. 244. The CMB was allocated to Dover, but was selected for retention at Portsmouth for the Reserve Flotilla on the 11.02.26. The CMB had previously been re-engined in 1925 with Thornycroft Y12 engines when she was redesignated CMB 81E. The craft was sent (loaned) to the RAF for a short period in 1927, but was returned to the RN in 1930 after the Schneider Trophy races. Subsequently she was converted into a Target âKâ and ended up being brought to produce at Invergordon in 1940.
CMB 83C was completed by Thornycroft at their Hampton yard on 19.12.19 and went to Dover. The CMB was re-engined in 1925 (as for CMB 81C above) and re-designated CMB 83E. On the 11.12.26 she was lent to the Air Ministry for the RAF experimental establishment at Felixstowe. Subsequently she was returned to Sheerness for repairs on the 14.09.28 which completed on the 24.06.29. The RAF declared that they no longer needed the craft on the 20.09.29 and the CMB was returned to the RN for conversion to a target in 1930. The engine was removed in that year and the craft was sunk and abandoned on the 06.10.33.
CMB 84C was completed on the 29.12.20 with twin Sunbeam 450 bhp engines and was likewise allocated to Dover. She was retained in Reserve at Portsmouth and similarly re-engined and transferred to the E class by 1926. After the loss of 115DE in 1929, the CMB 84E was supplied as a replacement on the 29.06.29. The RAF no longer required her services as of the 20.09.29 and the CMB was returned to RN use. As Target âOâ she was brought to produce in 1940.
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