It was developed during World War II as a highly efficient engine for airborne lifeboats, providing a long range from little fuel.
A search and rescue aircraft would drop the lifeboat by parachute to the crew, who could then attempt to make their own way back to a friendly shore.
[citation needed] Several designs of airborne lifeboat were developed, using both sails and pairs of small Britannia outboard motors, mounted inboard as Z drives.
These were adequate for crews crossing the North Sea from operations in Europe, but a longer-ranged design would be required for the expected air war against Japan.
Unusually for a small two-stroke, there was no reliance on the Kadenacy effect, although the transfer ports were angle-drilled to encourage swirl.
At the rear of the engine, outboard of the crankshaft coupling chain, a rectangular enclosure kept the electrical accessories dry.
[6] This was due to the unexpectedly rapid end to the war in the Far East, and the lack of urgency that created within the Air Ministry.
Although the basic engine was ready on time, much development after this was spent on ancillaries such as electric starting and a reverse gear.
[7] This engine was unsuccessful owing to problems with the complex crankcase casting warping after machining, preventing a reliable seal.