Despite the lessons learnt in WW1, at the very outset of WW2 the Sea Lords still considered submarines as 'not cricket' war vessels. After-all the senior naval officers weren't going to give-up their (flag) large and impressive battleships in favour of tiny submarines. The fact that U-18 and U-116 had been thwarted when attempting to enter Scapa Flow in WW1 led the Admiralty to reactivate it as a safe anchorage for British Home Fleet in WW2, albeit with additional blockships. These were positioned in the early weeks of the second world war. In mid-October 1939 U-47, with considerable luck, managed to thread its way through the blockships whilst surfaced and torpedo HMS Royal Oak. Winston Churchill then First Lord of the Admiralty declared (to the House of Commons) that the U-Boat attack had been a remarkable exploit of professional skill and daring... The rest of the rise of the submarine as a formidable war vessel is history, as they say! See www.uboat.net for more details.

Background
The Germans inspired by their at-sea successes continued development of their submarine technology throughout WW2, culminating in their streamlined, ocean-going type XXI Elektro-Boats:-

Photo: preserved type XXI submarine ex U-2540 riding high in the water

Notice the smoothed features, considerably different from the common conception of WW2 submarines. The XXI class of U-boat was designed for greater underwater operability and capable of higher submerged speed, to counter the increasing Allied anti-submarine capability by virtue of improvements in (surface ship) active sonar and depth-charge systems. As a counter to Allied radar, Snorkels (having a very small radar profile) were introduced to provide air for their diesel-engines whilst running submerged.

Three type XXI at the time of surrender

Here's a contemporary UK submarine:-

This photo is of an AMPHION Class diesel-electric submarine (in-service from 1945),  the only new class of UK submarine to be built during WW2 other than the X-craft. Unlike the German XXI none undertook wartime patrols; only two type XXI were active before the end of the war in Europe. Fortuitously, a (pretty radical) change of thinking occurred at Admiralty Board level, which resulted in development of UK submarines to counter that being undertaken by the Soviets who had acquired the lion's share of German submarine technology by the cessation of hostilities in Europe - see http://en.wikipedia.org/wiki/German_Type_XXI_submarine. Indeed, an Admiralty Policy Statement of 1948 declared that "The primary task of our submarine fleet is to hunt and kill other submarines" - for other, read Soviet.

Start of the Cold War
The Royal Navy, by all accounts, were not that impressed by the German type XXI having commissioned one (U-3017 as HMS N41) for test and evaluation purposes, which were concluded in 1949 and the submarine was scrapped. However, the concept of a streamlined hull form together with much increased capacity batteries, heavier-duty electric motors, and snort mast was carried forward as "the intermediate solution". In the same timescale, at the behest of FOSM, scientists and experimental staff at the Admiralty Research Laboratory ably assisted by the Admiralty Experimental Works and the Admiralty Engineering Laboratory, started to flesh-out improvements to in-service submarines. Quite a few RN submarines were modified including a few S-class of pre-war design, several of the last batch of twenty-one TRITON Class (in-service from 1942) and all 16 of the AMPHION class. (For further information visit the website of the Barrow-in-Furness Branch of the Submariners Association. The Premier UK resource on the internet for Submariners and anyone interested in the Royal Navy Submarine Service:- www.rnsubs.co.uk

Faster and increased underwater endurance
Although S-Class HMS/M Seraph was the first UK submarine to be modified and achieve much higher submerged speeds c1944, a slightly later converted S-Class HMS/M Scotsman was capable of short sprints of up to 17kt submerged and played a significant role in many improvements both in submarine design and ASW tactics to counter fast (submerged) submarines.

HMS/M SCOTSMAN in almost final form c1953 with prototype for future (bridge) fins fitted

The TRITON Class conversion programme commenced around 1948, followed three years later by the modernisation of the AMPHION Class. Two other major developments were also underway - see EXPLORER and PORPOISE classes, below.

Converted TRITON Class submarine featuring the final bow & forward topside 187 sonar dome,
which became distinctive features of UK diesel-electric submarines until the UPHOLDER Class

Modernised AMPHION Class submarine. Ultimately these had a bow and sonar dome as per the T-Class, above

The German submarine technology that really impressed the Royal Navy was the Hydrogen Peroxide Turbine powered U-boats; a Type XVIIB submarine U-1407, following salvage and refitting was commissioned into the Royal Navy in 1945 as HMS/M Meteorite for evaluation purposes. The evaluation raised considerable interest in this form of air-independent propulsion and hence the ability to stay submerged for extended periods with the on-board production of oxygen to breathe i.e. a true submersible. The Admiralty placed an order for two larger experimental submarines (the turbine-electric EXPLORER Class, based strongly on the Type-XVIIB craft) with the intention of building an operational class of twelve boats. Two boats were completed as designed: HMS/M Explorer in 1956 and HMS/M Excalibur in 1958. For a time these were the fastest submarines in the world with a top speed in excess of 25kt. Unfortunately Hydrogen Peroxide propulsion proved too troublesome and the two boats became known as Exploder and Excruciator. Neither boat survived much into the sixties, especially as the concept of nuclear powered submarines had become a reality with the USS Nautilus, as of Jan 1955.

HMS/M Excalibur - Peroxide turbine-electric submarine

Quieter
Somewhat belatedly (so some say, I say the delay was to the benefit of the class) HMS/M Porpoise the first of the PORPOISE Class of eight diesel-electric submarines was commissioned into the Royal Navy in 1956. The PORPOISE Class was a major step forward in quietness, generating 97% less noise than that radiated by the earlier submarines, and being so quiet proved a very capable platform for sonar. When submerged and operating quietly on electric propulsion they were virtually undetectable.

HMS/M Porpoise - PORPOISE Class diesel-electric submarine

The concept of peroxide turbine propulsion featured in the design of the OBERON Class (HMS/M Oberon launched 1959), the intention to build two submarines of the class as such was abandon in light of the experiences with the EXPLORER class boats. Consequently the whole class (13 submarines) was fitted with diesel-electric propulsion similar to the PORPOISE Class boats. A major feature of the OBERONs was the use of GRP for the superstructure, a feature that further reduced their radiated noise. Some of the OBERON Class features were retrofitted into the PORPOISE Class boats. These included the newly developed, by ARL, Passive sonar type 2007 that employed sonar type 186 hydrophone arrays, where fitted; the more fortunate (in my opinion) boats were fitted with flank arrays comprised of the ARL triple-hydrophone elements.

HMS/M Oberon - OBERON Class diesel-electric submarine

For the majority of the Cold War, the UK PORPOISE and OBERON classes were the quietest of all NATO submarines, and considerably quieter than the Soviet submarines. This gave the RN a significant tactical advantage when undertaking clandestine intelligence gathering missions and ASW barrier operations. Indeed the requirement for a suitable platform for these operations gave rise to the design and build of the UPHOLDER Class (see later) in the 1980s; at a time when the stealth of nuclear powered submarines was being improved enormously.

Even greater submerged speed and endurance
With the OBERONS becoming operational in the early sixties, thoughts were centered on nuclear-powered submarines for the Royal Navy. The first UK nuclear submarine HMS/M Dreadnought (S101) was commissioned in 1963 and at the same time the design was advancing for a much larger nuclear-powered submarine to carry very long range ballistic missiles. The aim was to complement the fleet of conventionally-powered submarines (SSKs) with nuclear-powered submarines of two types: hunter-killer (SSN) and a platform for the nation's nuclear deterrent (SSBN). The primary design goal for the SSNs was an underwater speed at least as high as surface combatants.

Concept of Operation for the UK Submarine Fleet 
The SSKs were to patrol designated areas in order to detect transiting Soviet submarines, with the SSNs to thwart attempts by the Soviet submarines to launch a land-attack with missiles or counter our patrolling SSBNs. RAF Maritime Patrol played a key support role in detection, tracking and trailing of Soviets submarines, using air-dropped Sonobuoys. The principal area of operations for the RN submarines was the North Altantic and Norwegian Sea as shown on the following map.

It can be seen that Soviet Northern Fleet submarines needed to pass through the gap between Greenland and Iceland, or the gap between Iceland and the UK, in order to launch missiles on either the UK and USA. Strategic deployments of RN (and US) SSBNs required them to pass through the gaps into the Norwegian Sea to be within missile strike range of Soviet land targets; the principal gap became that between Iceland and the UK with the forward-basing of USN SSBNs at Holy Loch in the Clyde estuary. Submarines of the Soviet Baltic Fleet could reach their operational areas by passage through the North Sea and around the north of Scotland, or via the English Channel.

Submarine-launched Missiles
The concept of a submarine carrying one or two missiles was embraced by the Soviets in the 1950s; this type of submarine was designated an SSG, or SSGN if nuclear powered, and featured an extended fin, or casing blister to accommodate the missiles. The US and UK, operating significantly less submarines, stayed with the SSBN as their primary platform for missiles until the late 1970s when they had developed a torpedo-tube launched missile for attacking surface and land targets. Initially RN submarines were equipped with the RN Sub-Harpoon missile, later the Tomahawk cruise missile.

Classes of Cold War UK Submarines
The following table summarises the evolution of UK submarines; from the outlines their relative sizes can be judged. I've included the ASTUTE class as the concept (essentially) originates from cold war days.

BACK