From about 1921, Oswald Short had been thinking about the construction of seaplane floats and flying boat hulls made from metal, specifically duralumin, rather than the traditional wood.

He assembled a team, including C. P. T. Liscomb who had extensive experience with that alloy to look into the hydrodynamics and corrosion characteristics of such hulls, and by 1924 was looking out for an opportunity to apply their results.

[1] A contemporary source[2] claimed it to be "the first light seaplane to be built [in the United Kingdom] and possibly in the world" and the first British all-metal flying boat.

The hull was a duralumin monocoque structure with a concave V-shaped planing bottom incorporating two steps, the main one near the centre of gravity.

Originally the Cockle had a shallow triangular fin and rudder,[3][4] but this was later extended upwards to a curved and slightly pointed profile which more than doubled the area, to cope better with single-engine flying.

[5] Deterred from using 32 hp (24 kW) Bristol Cherub flat-twin engines owing to vibration problems, the Cockle began with a pair of V-twin Blackburne Tomtits.

Before the aircraft was complete the bare hull (always Short's main concern) was floated for a day in April 1924 and found to be satisfactorily watertight.