The CMB's transferred to the RAF from the Navy in the early 1920's were difficult to handle and did not have any form of reverse gear, which made mooring anywhere at any time a difficult proposition. However the RAF were happy with the fast speed of the CMB's and looked to the Admiralty, who at that time controlled all orders for fast craft, to purchase on behalf of the RAF some craft for similar employment in mind. The result was a craft, that was not purpose built for the RAF, as Thornycroft had other customers, bur arrived on the scene in 1927. The new design from Thornycroft was 63ft in length overall with a beam of 12ft and broke away from the traditional round bilge.
Most believe that HSL 100 from BPB was the first HSL in RAF service, however this is not correct as four of the 62ft Thornycroft craftwere built and taken on charge by the RAF, the first becoming HSL 150 in 1927, the last being HSL 162 in 1932. Speeds quoted unofficially for this type were about 27 knots, which is close to the Thornycroft recorded trials data for 150 as 26.5 knots on her trials. HSL 151 was recorded as attaining a speed of 26.57 knots, HSL 152 26.63 knots and HSL 162 a speed 25.315 knots during trials. HSL 162 recorded a fatest one way run of 27.907 knots. The craft were powered by a pair of 375 hp Thornycroft Y15 engines and "Power" reverse gear was fitted.
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