{"id":19678096,"date":"2026-09-03T14:05:09","date_gmt":"2026-09-03T12:05:09","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/blackburn-b-20-retractable-hull-flying-boat\/"},"modified":"2026-09-03T20:53:35","modified_gmt":"2026-09-03T18:53:35","slug":"blackburn-b-20-retractable-hull-flying-boat","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/fr\/blackburn-b-20-retractable-hull-flying-boat\/","title":{"rendered":"Blackburn B-20: The Flying Boat With a Retractable Hull"},"content":{"rendered":"<style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\n<!-- mfsh:top -->\n\n<style>\n.mfsh-trigger{display:inline-flex;align-items:center;gap:10px;height:44px;padding:0 18px;margin:0 0 26px;border:2px solid #0F1720;background:#fff;color:#0F1720;font-family:inherit;font-size:14px;font-weight:700;line-height:1;letter-spacing:0;text-transform:none;cursor:pointer;box-shadow:none;transition:background .15s,color 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To get off the water you want a deep hull, to keep the propellers clear of spray, and a wing sitting at a high angle so it will unstick. To fly efficiently you want neither of those things.<\/p>\r\n\r\n<p>Blackburn&rsquo;s answer, in 1940, was an aircraft that changed shape. The entire bottom half of the hull dropped away on hydraulic jacks for take-off and landing, then folded back up flush for cruise. They called it the nutcracker.<\/p>\r\n\r\n<p>It flew for twelve days.<\/p>\r\n\r\n\r\n<div style=\"background:#f5f5f5;padding:20px 22px;margin:24px 0;font-size:15px;line-height:1.8\"><p style=\"margin:0 0 10px;font-weight:600;font-size:17px\">Quick Facts<\/p><p style=\"margin:4px 0\"><strong>Designer:<\/strong> Major John Douglas Rennie, chief seaplane designer at Blackburn; patented 1935<\/p><p style=\"margin:4px 0\"><strong>Specification:<\/strong> Air Ministry R.1\/36, which the Saro Lerwick won<\/p><p style=\"margin:4px 0\"><strong>Serial:<\/strong> V8914, one aircraft, built at Dumbarton on the Clyde<\/p><p style=\"margin:4px 0\"><strong>Engines:<\/strong> two Rolls-Royce Vulture II X-24s, around 1,700 to 1,800 hp each<\/p><p style=\"margin:4px 0\"><strong>Hull travel:<\/strong> the pontoon dropped about 5 ft 8 in on four hydraulic jacks<\/p><p style=\"margin:4px 0\"><strong>First flight:<\/strong> 26 or 27 March 1940, Flt Lt Henry Bailey<\/p><p style=\"margin:4px 0\"><strong>Lost:<\/strong> 7 April 1940 off Bute &mdash; three killed, two survived<\/p><p style=\"margin:4px 0\"><strong>Kept secret until:<\/strong> June 1945<\/p><\/div>\r\n\r\n\r\n<h2 style=\"padding-top:22px\">How the nutcracker worked<\/h2>\r\n\r\n<p>The lower hull was not a fairing. It was a sealed pontoon divided into five watertight compartments, and it carried all of the aircraft&rsquo;s fuel. Four hydraulic jacks lowered it, while hinged triangular frames ahead of each jack pushed it forward as it went down, so that dropping the hull also set the correct nose-up attitude for the water.<\/p>\r\n\r\n<p>The wingtip floats did the same trick in miniature, retracting outward until they formed the wingtips themselves.<\/p>\r\n\r\n<p>None of this was easy in 1936. British Lockheed built the hydraulics, running at around 2,500 psi at a time when 1,400 psi was considered normal practice, with a gravity system to drop the hull if the pressure failed. The engineer who wrote it up after the war was frank about how it had looked at the start.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><em>&ldquo;When Maj. Rennie, chief seaplane designer of Blackburn Aircraft, first approached Lockheeds in 1936, the problem seemed almost insuperable with the hydraulic equipment which was at that time available.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>H. P. Henry<\/strong> &mdash; Writing in Flight, \"A Teaser in Hydraulics\", 18 April 1946<\/div><\/div>\r\n<p>There was an unexpected bonus. With the hull extended, the crew could stand on the pontoon, which made mooring and boarding far easier than on a conventional boat.<\/p>\r\n\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img data-opt-id=218702015  fetchpriority=\"high\" decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/ml5psubhxdln.i.optimole.com\/cb:0e0_.b970\/w:auto\/h:auto\/q:mauto\/ig:avif\/https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/saro-lerwick-flying-boat-takeoff.jpg\" alt=\"A Saro Lerwick flying boat taking off from Loch Ryan\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The Saro Lerwick, which won the R.1\/36 contract the B-20 was designed against. It was not a success. The Air Ministry funded a single B-20 prototype anyway. Photo: public domain<\/figcaption><\/figure>\r\n<h2 style=\"padding-top:22px\">Twelve days<\/h2>\r\n\r\n<p>V8914 first flew from the Clyde at the end of March 1940 in the hands of Blackburn&rsquo;s chief test pilot, Flt Lt Henry Bailey. On 7 April, on a high-speed run over the Sound of Bute with five men aboard, severe vibration set in. Slowing down did not help. Bailey ordered everyone out.<\/p>\r\n\r\n<p>Ivan Waller, a Rolls-Royce flight engineer in the co-pilot&rsquo;s seat, had his parachute deploy early and snag on the wireless mast, leaving him trapped half out of the hatch until he tore himself free. Fred Weeks got out of the other roof hatch and hit the rudder on the way past, an injury he carried for the rest of his life. Both were picked up from the water about a quarter of an hour later.<\/p>\r\n\r\n<p>Bailey stayed at the controls until it was too late and jumped from too low. He drowned. The two Blackburn riggers aboard, Duncan Robertson and Samuel McMillan, were never found. Crew of a nearby ship reported seeing a large flat object fall out of the cloud &mdash; almost certainly the aileron, which had fluttered and broken away.<\/p>\r\n\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img data-opt-id=964145211  fetchpriority=\"high\" decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/ml5psubhxdln.i.optimole.com\/cb:0e0_.b970\/w:auto\/h:auto\/q:mauto\/ig:avif\/https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/avro-manchester-rolls-royce-vulture-engine.jpg\" alt=\"An Avro Manchester running up a Rolls-Royce Vulture engine\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">A Rolls-Royce Vulture being run up on an Avro Manchester. The same troubled X-24 engine powered the B-20, and helped sink the Manchester too. Photo: Imperial War Museum, public domain<\/figcaption><\/figure>\r\n<h2 style=\"padding-top:22px\">The idea outlived the aircraft, briefly<\/h2>\r\n\r\n<p>The Air Ministry concluded that the crash had nothing to do with the retracting hull, and that the principle was proven. Blackburn were asked for a bigger version, the B-40, to replace the Sunderland; two were ordered in September 1941 and cancelled four months later, the gain in range judged too small. A fighter floatplane derivative, the B-44, got as far as a mock-up before it too was dropped.<\/p>\r\n\r\n<p>By then the whole question was becoming academic. Long-range land-based aircraft were making the big flying boat obsolete, and no amount of clever hydraulics was going to change that. The engineer who described the system in 1946 said as much.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><em>&ldquo;Successful experimental flying was cut short when the boat was lost due to a flying accident and, as this type of retractable bottom is only seen to its best advantage in small and medium-size flying boats, it is unlikely that this highly ingenious arrangement will be developed any further.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>H. P. Henry<\/strong> &mdash; Flight, 18 April 1946<\/div><\/div>\r\n<p>He was right. Nothing else has ever used it.<\/p>\r\n\r\n<p>The existence of the B-20 stayed classified until June 1945. The wreck lies off Bute and is a designated war grave. In 1998 a trawler snagged it and Royal Navy divers recovered one of its Vulture engines, which is now at the Dumfries and Galloway Aviation Museum &mdash; effectively the only piece of the aircraft anyone can go and look at.<\/p>\r\n\r\n<p>Because so little survives, there are almost no photographs of it. This is the best account of the design:<\/p>\r\n\r\n\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/x2WKXcdLt8E\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n<p>And a look at the B-20 alongside the B-40 and B-44 that were meant to follow it:<\/p>\r\n\r\n\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/L-hTWm9h9qM\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n\n<style>.mfq{margin:34px 0 8px}.mfq h2{font:26px\/1.3 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117;padding-top:22px;margin:0 0 6px}.mfq .qa details{border-top:1px solid #5C91FF;margin:0;padding:0}.mfq .qa details:last-of-type{border-bottom:1px solid #e2e7ee}.mfq .qa summary{cursor:pointer;list-style:none;display:flex;justify-content:space-between;align-items:center;gap:16px;padding:22px 26px;font:18px\/1.7 \"Gilroy semiBold\",Helvetica,Arial,sans-serif;color:#0d1117}.mfq .qa summary::-webkit-details-marker{display:none}.mfq .qa summary::after{content:\"+\";font:24px Helvetica,sans-serif;color:#3568e0;flex:0 0 auto}.mfq .qa details[open] summary::after{content:\"\\2013\"}.mfq .qa details:hover summary{color:#3568e0}.mfq .qa .a{font:16px\/1.7 \"Gilroy regular\",Helvetica,Arial,sans-serif;color:#454e5e;padding:0 26px 24px}.mfq .qa .a a{color:#3568e0}@media(max-width:680px){.mfq .qa summary{padding:18px 14px;font-size:17px}.mfq .qa .a{padding:0 14px 20px}}<\/style>\n\n\n<section class=\"mfq\"><h2>Frequently Asked Questions<\/h2><div class=\"qa\"><details open><summary>What was the Blackburn B-20?<\/summary><div class=\"a\">An experimental British flying boat of 1940 with a hull that changed shape. The entire lower half dropped away on hydraulic jacks for take-off and landing, then folded back flush for cruise. Blackburn nicknamed it the nutcracker.<\/div><\/details><details><summary>Why would you make a hull retractable?<\/summary><div class=\"a\">To resolve the flying boat&rsquo;s oldest compromise. Getting off the water needs a deep hull to keep the propellers clear of spray and a wing at a high angle of attack; flying efficiently needs neither. A hull that extends for water work and retracts for flight gives you both.<\/div><\/details><details><summary>How did the mechanism work?<\/summary><div class=\"a\">The lower hull was a sealed pontoon of five watertight compartments carrying all the fuel. Four hydraulic jacks lowered it about 5 ft 8 in, while hinged triangular frames pushed it forward as it dropped, setting the correct nose-up attitude for the water. The wingtip floats retracted outward to form the wingtips themselves.<\/div><\/details><details><summary>How long did the B-20 fly?<\/summary><div class=\"a\">Twelve days. It first flew at the end of March 1940 and was lost on 7 April 1940 over the Sound of Bute.<\/div><\/details><details><summary>What caused the crash?<\/summary><div class=\"a\">Aileron flutter, with the aileron then breaking away, leaving the aircraft uncontrollable. Crew of a nearby ship reported seeing a large flat object fall out of the cloud. Three of the five men aboard died, including chief test pilot Flt Lt Henry Bailey, who stayed at the controls too long and jumped from too low.<\/div><\/details><details><summary>Did the retractable hull idea go anywhere?<\/summary><div class=\"a\">The Air Ministry concluded the crash was unrelated to the hull and the principle was proven, and ordered a larger B-40 and a fighter floatplane B-44. Both were cancelled. Nothing else has ever used the concept.<\/div><\/details><details><summary>Is anything left of the B-20?<\/summary><div class=\"a\">Very little. The wreck lies off Bute and is a designated war grave. In 1998 a trawler snagged it and Royal Navy divers recovered one of its Rolls-Royce Vulture engines, now at the Dumfries and Galloway Aviation Museum. Its existence stayed classified until June 1945, which is why almost no photographs of it exist.<\/div><\/details><\/div><\/section>\n\n\n\n<script type=\"application\/ld+json\">{\"@context\":\"https:\/\/schema.org\",\"@type\":\"FAQPage\",\"mainEntity\":[{\"@type\":\"Question\",\"name\":\"What was the Blackburn B-20?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"An experimental British flying boat of 1940 with a hull that changed shape. The entire lower half dropped away on hydraulic jacks for take-off and landing, then folded back flush for cruise. Blackburn nicknamed it the nutcracker.\"}},{\"@type\":\"Question\",\"name\":\"Why would you make a hull retractable?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"To resolve the flying boat\u2019s oldest compromise. Getting off the water needs a deep hull to keep the propellers clear of spray and a wing at a high angle of attack; flying efficiently needs neither. A hull that extends for water work and retracts for flight gives you both.\"}},{\"@type\":\"Question\",\"name\":\"How did the mechanism work?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"The lower hull was a sealed pontoon of five watertight compartments carrying all the fuel. Four hydraulic jacks lowered it about 5 ft 8 in, while hinged triangular frames pushed it forward as it dropped, setting the correct nose-up attitude for the water. The wingtip floats retracted outward to form the wingtips themselves.\"}},{\"@type\":\"Question\",\"name\":\"How long did the B-20 fly?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Twelve days. It first flew at the end of March 1940 and was lost on 7 April 1940 over the Sound of Bute.\"}},{\"@type\":\"Question\",\"name\":\"What caused the crash?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Aileron flutter, with the aileron then breaking away, leaving the aircraft uncontrollable. Crew of a nearby ship reported seeing a large flat object fall out of the cloud. Three of the five men aboard died, including chief test pilot Flt Lt Henry Bailey, who stayed at the controls too long and jumped from too low.\"}},{\"@type\":\"Question\",\"name\":\"Did the retractable hull idea go anywhere?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"The Air Ministry concluded the crash was unrelated to the hull and the principle was proven, and ordered a larger B-40 and a fighter floatplane B-44. Both were cancelled. Nothing else has ever used the concept.\"}},{\"@type\":\"Question\",\"name\":\"Is anything left of the B-20?\",\"acceptedAnswer\":{\"@type\":\"Answer\",\"text\":\"Very little. The wreck lies off Bute and is a designated war grave. In 1998 a trawler snagged it and Royal Navy divers recovered one of its Rolls-Royce Vulture engines, now at the Dumfries and Galloway Aviation Museum. Its existence stayed classified until June 1945, which is why almost no photographs of it exist.\"}}]}<\/script><p><em>Sources: H. P. Henry, \"A Teaser in Hydraulics\", Flight, 18 April 1946; Old Machine Press; Dinger&rsquo;s Aviation Pages (citing Derek N. James, Aircraft Illustrated, February 1974); Helensburgh Heritage.<\/em><\/p>\n<!-- mfsh:bottom -->\n\n<button type=\"button\" class=\"mfsh-trigger mfsh-bottom\" data-mfsh-placement=\"bottom\" aria-haspopup=\"dialog\"><svg><use href=\"#mfsh-i-share\"\/><\/svg>Share this story<\/button>\n\n<!-- \/mfsh:bottom -->\n\r\n\r\n\r\n<div style=\"background:#f0f4ff;border-left:4px solid #5C91FF;padding:16px 20px;margin:32px 0 8px\"><p style=\"margin:0 0 8px;font-weight:600;color:#333\">Related Posts<\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/saunders-roe-princess-flying-boat-too-late\/\">Britain&rsquo;s Giant Flying Boat, Born Too Late<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/avro-manchester-failed-bomber-vulture-engine-lancaster\/\">Avro Manchester: The Failed Bomber That Became the Lancaster<\/a><\/p><p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/saunders-roe-sra1-jet-flying-boat-fighter\/\">Saunders-Roe SR.A\/1: The Jet Flying-Boat Fighter That Needed No Runway<\/a><\/p><\/div>\r\n","protected":false},"excerpt":{"rendered":"<p>Blackburn solved the flying boat\u2019s oldest compromise by making the bottom of the hull drop away on hydraulic jacks. The only one built flew for twelve days.<\/p>","protected":false},"author":27,"featured_media":19677601,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[666,664],"tags":[],"class_list":["post-19678096","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-history-and-legends","category-military-aviation"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Blackburn B-20: The Flying Boat That Split in Half | MiGFlug<\/title>\n<meta name=\"description\" content=\"Blackburn solved the flying boat\u2019s oldest compromise with a hull that dropped away on hydraulic jacks. 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