The Thornycroft 67ft Mk1 craft were built with handed propellers - ie the Port screw turned left handed and the starboard screw turned right handed. One craft from the Mk1 production series, HSL 2505, was converted to have a pair of non-handed propellers fitted. Thus both propellers turned left handed, and gave an increase of speed of 1 knot over the Mk1 craft to a speed of 22 knots. The single unit so converted, probably as a trial craft for the subsequent production models, was designated 67ft HSL Mk1A. The further units with non handed propellers were designated 67ft HSL Mk1B.
The construction and layout of the Mk1A was identical to the Mk1 in all respects, including the power plant of twin Thornycroft RY/12 engines.
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