How much lift did Gouge's patented flaps add to the Sunderland on landing?

The Sunderland's flaps relied on unusual devices, patented by Gouge, which slid backward along curved tracks: as they moved back they also lowered, increasing wing area and generating 30% more lift for landing.

Those wings were thick, and on them sat the four Bristol Pegasus XXII radial engines mounted in nacelles, along with six drum-type fuel tanks with a combined capacity of 9,200 litres (2,025 imperial gallons, 2,430 US gallons). With supplementary tanks fitted, the Sunderland's total fuel capacity reached 11,602 litres (2,550 imperial gallons, 3,037 US gallons).

In November 1933 the British Air Ministry issued Specification R.2/33, calling for a new-generation, long-range, general-purpose flying boat. Short developed the S.25 Sunderland from its well-known S.23 to meet that general reconnaissance requirement. Its chief designer, Arthur Gouge, had initially envisaged a 37 mm COW gun in the nose alongside the single Lewis gun fitted in the tail. The prototype K4774 made its first flight on 16 October 1937. Production of the Short Singapore ended in June 1937, and its replacement by the Sunderland was well advanced by the outbreak of the Second World War.