1:600 HM submarine Trident, group 1 "T" class. This was the original design, with a bulbous nose housing two external torpedoes, and two more external torpedoes firing forward from amidships. The bulbous nose created a large wake, and this hampered vision through the periscope. Most of the surviving boats later had the finer group 2/3 nose, and the amidships tubes angled backwards. Some even had a stern tube added. © Andrew Arthur

1:600 HM submarine Thrasher, group 2 "T" class. The main change was the fining of the nose, to create less wake, exposing the external nose torpedoes. Later, surviving boats had their amidships tubes angled backwards, and some had an external torpedo added aft and even a 20mm cannon at the back of the conning tower. © Andrew Arthur

1:600 HM submarine Tudor, group 3 "T" class. An all-round improved group 1/2, with a fined nose, rear-firing amidships torpedo and an aft torpedo. A 20mm cannon was fitted at the after end of the conning tower. © Andrew Arthur

This class was designed to replace the "O", "P" and "R" classes in the colonial role, but in a smaller hull to suit them more to home & Mediterannean operations, and a 42 day patrol was specified.
The London Naval Treaty allowed 16,500 tons of submarines, so to secure enough units, a maximum displacement of 1,100 tons was specified.
400 tons smaller than the subs they replaced, they were never-the-less better armed, longer ranging, faster and deeper diving if slightly slower on the surface than their predecessors. They were the first class to have reversible diesel engines, so it was no longer necessary to use the electric motors for manouvering. Torbay and Tridenthad one ballast tanks converted to a fuel bunker, for an aditional 46 tons of fuel.
In the original group 1 ships, all torpedoes were grouped firing forward, but this was found not to be the best arrangement, and in group 2 and 3, and group 1 ships after refit, the amidships tubes were moved aft and angled backwards, and in the group 3, and group 2 under refit, a stern tube was added, and the casing extended right aft. In the group 1, the bulbous fairing covering the forward external tubes caused a large wake at periscope depth, impairing vision forward. In later groups, and some refitted ships, the casing was fined, exposing the tubes, and giving a knife edged appearance.
In 1944, Truant was fitted with a snorkel, but this could not be implemented class wide until post war. This was not a real problem, British submarines never really faced stiff competition from an advanced enemy escort force, and had the relative luxury of being able to surface at night 90% of the time to re-plenish air and re-charge batteries. Also, unlike many other countries' submarines, a lot of attention and space was paid to air scrubbers, coolers and re-circulators to keep a submarine's air bearable for many long hours.
Late war, Tactician, Tally-Ho!, Tantalus, Tantivy and Taurus had a rivited ballast tank welded shut and changed into a feul bunker for a further 86 tons of diesel, extending range to 11,000 nautical miles and displacement to 1442 tons on the surface.
The classes were first partly, then wholly welded, and had the longest endurance of all WW2 RN subs, Tantalus setting a record of 56 days from home, 40 on station. They were most sucessful, and useful, and only replaced when the equivalent "A" class was introduced.