and force Britain to terms. The situation had become so bad, and militarily embarrassing, that the RNAS was ordered to concentrate solely on ASW. The turning point in the Atlantic came when escorted convoy systems came in, and merchant ships ceased sailing independently. Added to the system was aircraft, which although incapable of sinking a submarine, could render valuable psychological damage to U-boat crews and provide reconnaissance support which eliminated the U-boat threat. Despite the preference for flying boats and seaplanes, non-rigid blimps were also developed for anti-submarine patrol, the airships being built in a number of types, such as the SS class, the SST class, the SSP class, and the SSZ class. For flying boats, having started initially with Curtiss H boats bought from the United States, a modified version with a new, more seaworthy hull, was designed by RNAS Commander John Porte at the Seaplane Experimental Station at Felixstowe, and these aircraft became known as the Felixstowe F.1, Felixstowe F.2, and Felixstowe F.3, culminating in the Felixstowe F.5 of 1918. These, along with the blimps, were to make a valuable contribution to the protection of convoys. Unrestricted submarine warfare made aircraft more important than ever in the war against the U-boats. By 1917 some 39 seaplane and 26 aircraft stations in Britain were all assigned to ASW operations. By December 1918 Britain's land-based coastal air forces was to have consisted of 353 flying boats and seaplanes and 920 land-based aircraft. The actual figures obtained were 305 and 382 respectively, but the clear intended expansion was an indication of air power's importance in trade and maritime defence. The figures and statistics for the coastal air force's impact on the U-boat fleet are disputed. Air Ministry figures claimed aircraft sighted 361 U-boats, attacked 236 and sank 10. It is unlikely that these figures are correct owing to the lack of capable offensive weapons. But 'scarecrow' tactics did result in 96 sightings and 46 attacks. U-boat crews, ignorant of aircraft's capabilities, were often forced to dive to escape observation and the attention of escorting destroyer screens which aircraft could direct for the kill. By 1918 only six convoys under escort by aircraft were attacked. One unnamed U-boat captain stated that "aircraft are our worst enemy". By the end of the war, British maritime air power was the most advanced in the world. Totalling a maximum of 3,000 aircraft (in all services),