Air Ministry Specification R.1/36 (to meet Operational Requirement 32) was issued in March 1936 to several companies that had experience in building flying boats. The specification was for a medium-range flying boat for
anti-submarine,
convoy escort and
reconnaissance duties to replace the Royal Air Force's biplane flying boats such as the
Saro London and
Supermarine Stranraer. The specification called for a cruise speed of and a weight of no more than . Designs were tendered by Saunders-Roe (S.36),
Supermarine (Type 314),
Blackburn Aircraft (b. 20) and
Shorts. The
Blackburn B.20 was a radical design which offered much better performance by reducing the drag associated with a flying boat hull, and so a prototype was ordered to test the concept. Of the other designs, the Supermarine was the first choice with Saro and Shorts tied in second place. The Supermarine was ordered "off the drawing board" (without requiring prototypes to be produced and flown first). Supermarine's commitment to the Spitfire meant that work was not expected to start for two years and so the Ministry looked to the other designs. Saunders-Roe had redesigned the S.36 in the meantime, replacing the low hull and
gull wing with a deeper hull and a high wing, and the Supermarine order was transferred to the S.36. The contract was issued in June 1937 to buy 21, which was given the service name Lerwick (after the town of
Lerwick). The aircraft was a compact twin-engined, high-winged
monoplane of all-metal construction, with a conventional flying boat hull, a planing bottom and two stabilising floats carried under the wings on long struts. It was powered by two
Bristol Hercules radial engines and initially had twin fins and
rudders. For defence, the Lerwick was equipped with three powered
gun turrets. The nose turret had a single 0.303 inch
Vickers K gun; the other two had 0.303
Browning machine guns, two guns in the
Nash & Thompson FN.8 turret in the dorsal position and four in the Nash & Thompson FN4.A turret in the tail. Offensive armament was a total of of bombs or depth charges; four or eight bombs, or four depth charges, carried in two streamlined nacelles behind the engines, similar to the
Martin PBM Mariner. The first three aircraft were used as prototypes, with the first being launched on 31 October 1938, after numerous delays during design and construction. The Lerwick was immediately found to be unstable in the air, on the water and not suited to "hands off" flying, a major problem in an aircraft designed for long-range patrols. Numerous adjustments, including the addition of a greatly enlarged single fin and an increase in the wing
angle of incidence, failed to remedy its undesirable characteristics, which included a vicious
stall and unsatisfactory rates of roll and
yaw. In service, several aircraft were lost because of wing floats breaking off, suggesting this was a structural weakness. Persistent problems with the hydraulics resulted in bomb doors sometimes dropping open during flight. On one engine the Lerwick could not maintain altitude or maintain a constant heading, as the controls could not counter the torque of one engine on maximum power. An engine failure would inevitably see the aircraft flying in slowly descending circles. On one occasion, the loss of an engine forced a Lerwick to make an emergency landing in the
Caledonian Canal. The aircraft was then towed to
Oban at the end of a string of coal barges. ==Operational history==