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ANTONY PRESTON, JOHN BATCHELOR
The Dutch had originally built two types of submarine, those with "K" numbers for the East Indies, and those with "O" numbers for home waters, but in 1937 the two series were combined under "O" for "Onterzeeboot". The O21 class of five boats were laid down in 1937/38 for general service at home and in the Far East. They were conventional in all ways but one: they were the first to incorporate an "air mast" for charging batteries while running at periscope depth, a vital step in extending the submerged endurance of submarines.
British X1. X1 was the largest British submarine ever built until the advent of nuclear propulsion, and was in many ways superior to other cruiser-submarines. Her radius of action was 12,400 miles on the surface, and she could remain submerged for over two days, thanks to her large battery capacity. In addition, her two twin gun-mountings were carried high above the waterline to free them from spray interference, but despite all these advantages her unreliable machinery prevented her from being a success
Alas, like other prophets the Dutch were without honour in their own country, and with their Allies for that matter; when O21-24 arrived in England in May 1940 the first thing the Royal Navy did was remove the air masts, and only in 1943 did the Germans realise the worth of the gadget they had found in a Dutch shipyard. Looking around in desperation for an antidote to the danger of recharging batteries on the surface at night, when aircraft and escorts were using radar, they re-examined the air mast and perfected it as the "schnorchel".
French Surcouf. Not only the largest submarine in the world, the Surcouf was also the only one to have the maximum calibre of guns allowed under the Washington Disarmament Treaty. Her 8-in guns were carried in a twin power-operated turret, and could each fire three 260-lb shells per minute to a range of 30,000 yards. In addition, she carried a seaplane in a cylindrical hangar abaft the conning tower. She operated under the Free French flag in the Second World War, but was accidentally rammed by an American freighter in 1942
After their spectacular experiment with the Surcouf the French Navy reverted to more conventional submarines. In March 1920 the Chairman of the Naval Estimates Committee in Parliament had suggested quite seriously that a fleet of 250 to 300 submarines would answer all needs hitherto met by cruisers and battleships. Fortunately this extreme argument was met by a reasoned rebuttal, for it was clear to naval officers that, despite its potency, the submarine had only recently suffered a catastrophic defeat. Furthermore it was correctly argued that on a ton-for-ton basis the complexity of a submarine made it just as expensive as a surface ship to build and maintain, and also reduced its effective life. Although submarines were to be built in large numbers, they were nonetheless part of a balanced fleet. Two types were built after 1922, 1st Class boats of some 1,000 tons, and 2nd Class boats of 600 tons, the larger being intended for overseas patrol duties and the smaller ones for defensive patrol duties in home waters.