the wings (clipped wings were fitted to most LF Spitfires), but by the engine, which had been optimised for low-altitude operation. All production Mk XVIs had clipped wings for low altitude work and were fitted with the rear fuselage fuel tanks with a combined capacity of 75 gal. Many XVIs featured cut-down rear fuselages with bubble canopies. On these aircraft the rear fuselage tank capacity was limited to 66 gal. Because of a slightly taller intercooler and rearranged accessories on the Packard Merlins a new, bulged upper cowling was introduced and also appeared on late production IXs. Armament for most Mk XVIs consisted of 2 × 20 mm Hispano II cannon – each with 120 rpg – and 2 × .50 calibre Browning machine guns – each with 250 rpg. 1 × 500 lb (227 kg) bomb could be carried underneath the centre rack, and 1 × 250 lb (114 kg) bomb could be slung under each wing. Some production aircraft had rear fuselage fuel tanks in addition to the main tank which allowed it to fly approximately as far as the Spitfire Mk VIII. Problems with the licence-built engines limited introduction to front-line squadrons for several months. A total of 1,054 Mk XVIs were built by Castle Bromwich. Spitfire Floatplanes (types 355 and 359) With the German invasion of Norway in April 1940 the RAF took an interest in the concept of using floatplane fighters in areas where airfields were not immediately available. To this end a Spitfire Mk I R6722 was taken in hand at the Woolston factory to be modified and mounted on Blackburn Roc floats. Tank tests were carried out at Farnborough, using a 1/7 scale model, it was found that the concept was basically sound, although the vertical tail surfaces would need to be enlarged to counterbalance the side area of the floats. The end of the Battle of Norway and the need for as many Spitfires as possible meant that R6772 was converted back to an ordinary fighter without being flown. With the entry of Japan into the war the concept was revived in early 1942. A Spitfire V W3760 was fitted with a pair of floats 25 ft 7 in (7.8 m) long, mounted on cantilever legs. This aircraft was powered by a Merlin 45 driving a four-bladed propeller of diameter (3.4 m). A Vokes filter was fitted to the carburettor air intake