The ‘L’ class were originally designed as improved ‘Es’ but the changes were so great that they were re-classified as a new class. They were very successful. This photo of L 4 shows the original, low, gun position.

L 6 had her gun raised which became standard for the class.

The ‘L’ class began as ‘Improved Es’ – in fact, L 1 and L 2 were ordered as E 57 and 58. By 1916 the ‘E’ class design was 6 years old and there were many wartime lessons to be incorporated. Experiments with double hulls, steam etc, were abandoned and the well-proven saddle tank design was chosen. The main change in the first group of eight boats was increased surface speed using the 12-cylinder Vickers engine developed for the ‘J’ class. They achieved their design surface speed of 17kts on trial and the earlier boats reached 11kts submerged. Later boats were about ½kt slower due to the drag of a fixed bridge screen 5½ft high.

Harrison quotes some figures for speed in the 1930s which were about ½kt slower than on the original trials. This was almost certainly due to the increasing roughness of the hull as paint ripples and rust pits increased. When docked, they would be brushed and handscraped before re-painting but this would not produce a fair surface and the increasing roughness would certainly be enough to cause the loss of speed. Ten microns of roughness adds about 1 per cent to the power required for a given speed.

The first eight boats had four 18in bow tubes and one 18in on either beam. The gun armament of the earlier boats varied but from L 12 onwards a 4in gun was mounted at bridge level with its own access trunk. Earlier boats were modified similarly. The idea was to engage surfaced enemy submarines outside torpedo range with a gun well above water even in the low buoyancy condition.

Needless to say, these changes made the ‘Ls’ bigger than the ‘Es’. L 9 and later boats were further modified and larger still. The main change was in fitting four 2lin bow tubes in place of the 18in. An extra bulkhead was fitted between the tube space and the torpedo room. The beam 18in tubes were retained (these were removed between the wars in surviving boats). The beam tubes were omitted in those equipped as minelayers – L 14 and 17 with sixteen tubes and L 24–27 with fourteen tubes.

Even before the first ‘L’ class boat went to sea a further improved design was started. Six of the L 50 design were ordered in January/February 1917 and a further nineteen in April. Many were cancelled at the end of the war and only seven completed. This group had six 2lin bow tubes and none on the beam. They had a 4in gun either end of the bridge, each with its access trunk. The stern lines were modified to give better propeller immersion and it was hoped that they would also make 17kts. Early trials were very disappointing – cl2.4kts – but by refining appendage shape22 and fitting new propellers, L 71 reached 14kts.

All groups of the ‘L’ class had a diving depth of 250ft, quoted as 150ft from 1925. The test depth was 100ft.

Submarine Flotilla 1933 at Gosport, L52, L22, L20 & L6.

Some technical aspects

The success of a submarine design depends on the correct design of detail aspects to a much greater extent than in the case of a surface ship. In this section a few of these aspects will be considered in a little more detail.

Diving depth

In the earlier years of this period the modes of failure of a pressure hull, loaded externally, were not clearly understood and a confused nomenclature resulted. By the end of the war, there was a fairly good, subjective appreciation of the problem though only very simple calculations were possible.

In later years, three values of ‘Diving Depth’ were considered and, though they were not defined clearly in the early years, one can see a growing realisation of their significance.

COLLAPSE DEPTH: The design figure at which water pressure would cause the hull to fail assuming that all the plates had been rolled to the specified thickness and there were no manufacturing defects. Calculations were only possible on the strength of the plating between frames and though it was recognised that the frames could buckle, it was hoped that this was avoided by using heavy frames. Many early designs had numerous discontinuities or steps in the pressure hull which would have weakened it.

OPERATIONAL DEPTH: This was the maximum depth permitted in normal operation. It seems to have been introduced in 1925 when, for example, the quoted diving depth of the ‘Ls’ became 150ft instead of the earlier figure of 250ft. It allowed a margin of safety over the collapse depth for errors in the design calculation and for building defects and also for accidental depth excursions. In later years the operational depth was taken as about half the collapse depth. The operational depth would be reduced in older boats if surveys showed serious corrosion.

TEST DEPTH: In the period under discussion, the test dive was usually to about two-thirds of the operational depth.

There does not seem to have been any very clear definition of the point to which ‘Depth’ was measured. The gauge was roughly at eye level in the control room and this was the accepted base. At some date, this was formalised with depth measured to the axis of the boat, changed only with the nuclear programme to keel depth.

A formula used for calculating the stress in cylindrical boilers was:

This can be used for external loads provided the cylinder does not buckle and is truly circular. It was the only tool available to early designers and they made good use of it. Realising that the calculated values it gave were only approximate, they used it to calculate the stress in boats which (accidentally) had made an abnormally deep dive. This figure could be used, with caution, as the limiting value for new designs. This formula is surprisingly accurate for modern designs.

Harrison lists a few of the extreme depths recorded by early submarines.

Boat Depth (ft)

B1 95

E 40 318

G? 170

L2 300

L 2 was on patrol when she encountered three USN destroyers who took her for a U-boat. She dived to 90ft to avoid them but depth charges caused leaks and she sank to 300ft. She blew tanks and, on surfacing, was hit by a 3in shell at 1000yds which did not penetrate. ‘The three American destroyers apologised’.

Until the end of the First World War, the quoted diving depth seems to have been a calculated safe depth using the boiler formula with some factor of safety. Captains were generally ordered not to exceed half that depth. There do not seem to have been any cases of loss from structural failure, with the possible exception of K 5, though one cannot be absolutely sure since some boats disappeared during the war without trace. With all the uncertainty of structural design there must have been a touch of luck but the main reason was a wise degree of caution in sizing unknown components such as frames making the boats heavy but safe. There is little reliable information on diving times, but the early boats were slow by Second World War standards. The ‘Ls’ were said to reach periscope depth from full surface buoyancy in 1½ minutes which was probably better than earlier classes.

Hydroplanes

The Hollands had planes aft only, then called Submerged Diving Rudders, which moved through 60° from hard rise to hard dive. Initially they were worked by a compressed-air motor but this was unsatisfactory and hand operation was used. The ‘A’, ‘B’ and ‘C’ classes had a similar arrangement. In the ‘B’ and ‘C’ classes a balance weight was arranged so that if the control shaft broke, the planes would move into the horizontal position.

Control using planes aft is quite satisfactory at higher speeds but not at the low speeds which were all that these boats were capable of. To rise or dive the boat had to be put at a trim angle; they could not move up or down in a horizontal orientation. In 1905 approval was given to fit planes on the fore side of the conning tower of the later ‘As’ and the work was carried out after completion. A few ‘Bs’ and all ‘Cs’ were similarly equipped. In 1907 A 3 was fitted with bow planes for trial which appears to have been successful and it seems that most boats which had not already received conning tower planes were fitted with bow planes. All these planes were hand worked through rods and gearing – A 3’s gear took twenty-three turns of the handwheel in the control room to move from hard over to hard over. Bow planes are very vulnerable to damage in heavy seas and from impact with floating objects. Heavy guards were fitted but damage still occurred. The ‘D’ class had submerged bow planes, rather further aft than in the earlier boats and electric motors were provided to operate both bow and stern planes though hand operation was still possible.

Scott’s ‘S’ class had Italian-designed folding planes forward which were unreliable and gave them a bad reputation. On the other hand, the Scotts’ developed hydraulic operation of the planes in Swordfish was very successful and adopted in all later submarines including the last of the ‘E’ class. The drag of submerged planes and guards is very high and it was intended to fit housing bow planes in the ‘Gs’. The failure of the ‘S’ class planes caused this to be abandoned at the cost of 1–1½kts of speed on the surface.

Main engines

The first twelve ‘A’ class boats all had 16-cylinder Wolseley petrol engines but these were steadily developed from 350bhp in A 1 to 600bhp from A 5 onwards. The ‘Bs’ and the ‘Cs’ up to C 18 had the same design of engine but built by Vickers; from C 19 onwards the number of cylinders was reduced to twelve but still delivering the same 600bhp.

The first British submarine diesel for the ‘D’ class was a 6-cylinder Vickers engine. It was the only diesel design of the period; the ‘E’ class had the same cylinder design with 8 cylinders and the ‘J’ and ‘L’ classes had 12 cylinders. The basic design was refined but unchanged.

Submarine diesels

It was intended to try a variety of engines from different manufacturers (mainly German) in the ‘G ‘class, but the war prevented this.

Torpedo firing

Firing a torpedo from a submerged submarine is a complicated process. The torpedo is normally kept in a dry tube and when preparing to fire the tube must be flooded. This needs about half a ton of water for a 21in tube and must be taken from an inboard tank to preserve the trim. This is called the ‘Water Round Torpedo (WRT) Tank’. The torpedo is slightly heavier than water and when it is fired some water must be admitted from the sea to prevent the bow coming up. Before re-loading, the tube must be drained into an inboard tank.

The torpedo was blown out of the tube by compressed air. In this era the pressure was 250lbs/in2 which was too high, producing a big air bubble which could be seen from the target vessel and the shock swung the depth-keeping pendulum back so that the torpedo ran deep for a considerable distance. The torpedo was not a very accurate weapon, particularly against fast-moving, manoeuvring targets. Compton-Hall quotes figures (from N Lambert) showing German submarines scored 12 per cent hits against British warships but 52 per cent against merchant ships. British submarines averaged some 15 per cent hits, mainly against warships. A crude fire control device was developed in the form of a slide rule called ISWAS (Where it IS based on where it WAS – still used as a backup even after the Second World War).

Radio communications

Even the Hollands had a radio receiver but transmitters were not fitted in submarines until 1912 when it was approved to fit Type 10 (3kW) to ‘Ds’, ‘Es’ and some ‘Cs’. This was a Poulson arc set with a theoretical transmission range of 250–300 miles and could receive from shore stations at up to 600 miles. It was not very reliable and required a mast or masts to be raised. Later boats had valve sets which were more reliable and had greater range. By the end of the war some boats had the SA set which could receive with the boat at shallow submergence – bridge bulwark level with the sea. The Fessenden sound oscillator permitted communication between submerged submarines at up to 30–40 miles. Pigeons were carried in the early boats and were reliable and could fly at 30mph – if not over-fed. Compton-Hall quotes a message sent from Terschelling at 0400hrs which reached the Admiralty 12 hours later.

Miscellaneous

A submarine contains a remarkable variety of technologies, many of which have no other application and far too numerous and complicated for more than a mention in this brief account. There were problems with magnetic compasses even in surface ships and these were much more difficult in submarines. The compass was outside on the bridge and had to be surrounded by a heavy brass structure. A small, upside-down periscope enabled the helmsman to see it – with difficulty. Gyro compasses were introduced in Swordfish and in the ‘E’ class. These early Sperry units were unreliable and the wise officer of the watch compared them frequently with the slightly less unreliable, though inaccurate, magnetic compass.

Permanent bridge screens (as opposed to canvas dodgers) were fitted from 1917. Whilst greatly improving life for bridge personnel, they were heavy and added considerably to submerged resistance, reducing speed by about ½kt.

Though the RN were probably the first to fit periscopes, they were soon overtaken by superior units from other countries. Keyes’ team purchased a number of French and German periscopes in 1911 and, though they seem not to have been used, the British manufacturer (Sir Howard Grubb) was inspired to greater efforts.

Other topics which can only be listed but all of which presented their own problems included air bottles, compressors, LP blowers, batteries and their ventilation. A crude escape chamber was fitted in some of the ‘C’ class in 1908 and by 1911 a breathing helmet was issued.

How good were they?

Of course there were problems with these submarines; almost every aspect of their technology was novel as were their tactics. Every other navy had problems but only the USN and the German navy were suitable for comparison and the USN had no direct war experience. The best comparison with German submarines is in a paper to the INA by Arthur Johns in 1920. Johns began with a factual description of the main types of German submarine. He emphasised the rising cost per ton which rose from 4000 marks per ton in 1914 to 9000 in 1918 (it is not clear how much of this is due to inflation). Johns says this is about double the figure for British boats but exchange rates in wartime are almost impossible to evaluate. However, the building time of 800-ton submarines increased from 24 months to 30 months. The big cruisers took about double the time to build of a standard U-boat.

All U-boats were double-hull style over most of the surface. However, the top section was usually free flooding and the bottom omitted so that the difference from the British saddle tank was not great. It was noted that the captain controlled the boat from the conning tower, not the control room in the main hull as in the RN. This gave greater submergence for the same length of periscope at the expense of less team contact, a dilemma never resolved.

Johns points out that far from possessing the exceptional speed rumoured for U-boats they were actually rather slow for their power, probably due to large and poorly-aligned appendages. Stability was marginal and some classes required girdling. Captured boats were tried after the war and were thought to be good sea boats, dry and manoeuvring well, but British officers thought their own boats handled better under water.

Since Johns had designed most of the British boats, one may be suspicious of his impartiality but his views were not disputed by RN operators or by overseas designers. On the contrary, every speaker in the discussion paid tribute to Johns. Constructor Commander E S Lands USN, already an experienced submarine designer and destined to become a leading designer between the wars said:

Boat for boat I consider the L 50 class of the British design to be the equal if not the superior of the U-boat. If the engines of the two were traded, the British boat would completely outclass the German boat. The British boats are better designs so far as the design of submarines is concerned … For fleet purposes the British ‘K’ class are superior to the UAs …’

Other speakers expanded on these points. The DNC, d’Eyncourt, said that the German engines delivered 300hp per cylinder whilst British engines had only 100hp. Rear-Admiral Dent, head of the submarine service, paid the users’ tribute to Johns and his designs. He said that ‘during the war we built the largest submarine, the fastest submarine on the surface, the fastest submarine submerged, the submarine with the heaviest gun armament and the submarine with the heaviest torpedo armament.’ The only great advantage possessed by the U-boats was plenty of targets.

Displacement: Groups I and II: 890 tons (surfaced), 1080 tons (submerged), Group III: 960 tons (surfaced), 1150 tons (submerged), Group IV: 897 tons (surfaced), 1195 tons (submerged), Group V: 996 tons (surfaced), 1322 tons (submerged)

Dimensions: Group I: 231910 x 23960 x 1 3930, Group II: 238970 x 23960 x 13930, Group III: 235900 x 23960 1 3 . 20, Group I V: 250900 x 23960 x 13930, Group V: 250900 x 24930 x 12940

Machinery: 2 diesel engines, 2 electric motors, 2 shafts. 2400 bhp/1600 shp = 17/10.5 knots

Range: 3800 (Group IV: 7000, Group V: 5500) nm at 10 knots surfaced, 80 nm at 4 knots submerged

Armament: Group I: 6 x 180 torpedo tubes (4 bow, 2 beam), total 10 torpedoes, 1 x 40 gun, (final 4 Japanese boats omitted beam tubes), Group II: 4 x 210 torpedo tubes (bow), 2 x 180 torpedo tubes (beam), total 10 torpedoes, 1 x 40 g u n , Group III: 6 x 210 torpedo tubes (bow), total 12 torpedoes, 2 x 40 guns, Group IV: 4 x 210 torpedo tubes (bow), total 8 torpedoes, 1 x 40 gun, Group V: 6 x 210 torpedo tubes (bow), total 10 torpedoes, 1 x 30 AA gun, 1 x 7.62mm machine gun; minelayers: 4 x 210 torpedo tubes (bow), total 4 torpedoes, 16 x mine tubes and mines

Complement : Group I: 35, Group II: 38, Group III: 44, Group IV: 48, Group V: 60

Notes: This design was developed as a re- placement for the successful E -class. It reverted to the single-hull type with saddle ballast tanks that had proven itself with the earlier boats. Later series made the transition to 21-inch torpedo tubes. L-13 was not used in a superstitious reaction to the disastrous career of the K-13.

The L-10 was sunk by German warships north of Terschelling on 3 October 1918; the L-55 was sunk by Soviet warships off Kronstadt on 4 June 1919 (and was later recovered by the Soviets, commissioned in October 1931 as the Bezbozhnik, damaged and laid up in May 1941, and scrapped about 1953); the L-9 sank in a typhoon at Hong Kong on 18 January 1923; the L-24 was accidentally rammed and sunk by the battleship Resolution on 10 January 1924. The other boats, after serving actively into the 1930s, were sold for scrap between 1930 and 1936, apart from the L-23, the L-26, and the L-27, which were used for training during World War II and were not scrapped until 1946. The Japanese boats were redesignated the RO-51 through the RO-63 in 1924. The RO-55 was stricken in 1939. The RO-62 collided with the RO-66 off Wake Island and sank it on 17 December 1941; the RO-60 wrecked at Kwajalein on 29 December; the U. S. destroyer Reid sank the RO-61 off Atka Island on 31 August 1942; U. S. aircraft sank the RO-65 in Kiska Harbor on 4 November. The other Group IV boats served as training vessels from 1941 and were joined by the remaining Group V boats from late 1942. The RO-64 was mined in Hiroshima Bay on 12 April 1945, and the other boats were scrapped in 1946. The Hrabri was seized by the Italians in April 1941 but was broken up later that year. The Nebojs a escaped to Alexandria in April 1941 and operated with British forces. After World War II, the Nebosjare- turned to the Yugoslav Navy and was renamed the Tara. It was stricken in 1954.