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Chapter 11: 1914-18 Saddle Tank Types L and L50 Classes

11.1 Introduction

1 The L Class was designed to replace the E boats and was of the same saddle tank type but with higher surface and submerged speeds. E1 had been designed in 1910 and was in the 1910/11 programme, the modified E9 with two bow tubes was ordered in the 1911/12 Programme and orders continued until after the outbreak of war. By the beginning of 1916 the design was 6 years old. In this intervening period various types of double-hulled submarines to British and foreign designs had been built or were building. The war had been going on for a year and a half and many lessons to be incorporated in our submarines had already been learned, in particular the speeds obtained in service were below the designed speeds. Otherwise the E boats were very successful but it was time for the design to be modernised.

2. The L Class was therefore designed by the Admiralty based on the E Class, incorporating the lessons learned to that date and was ready for the first orders to be placed in February 1916. In that month an order was given to Vickers for two submarines to this new design. They were called E57 and E58, being practically elongated E Class. The improvements were so marked however that a new class was started and the two boats called L1 and L2. Six further vessels L3-8 were ordered in May 1916, L9 in August 1916 and twenty-five more L10-35 in December 1916. L13 was never ordered presumably for 'superstitious' reasons, L34 and L35 were later cancelled. The builders of the individual vessels are given in Appendix I.

In 1920 a number of the class were still building at Vickers. Of these L21, L22, L24 and L25 were completed and commissioned; L23 and L26 were towed to Chatham and Devonport Dockyards respectively with engines on board for completion. The hull of L27 was towed to Sheerness Dockyard for completion, the engines having been removed and re-erected at Barrow, tested and sent to Japan. L28 and L29 (minelayers) were cancelled and broken up on the slipway; L30 and L31 were also cancelled and broken up after commencement; the hull of L32 left Barrow in tow in 1920 to the Leith Salvage Co for breaking up.

11.2 Design

3. The class can be divided into three types:

  • L1-8 with 18-inch bow and beam tubes.
  • L9-33 (ex below) with 21-inch bow tubes and 18-inch beam tubes.
  • L14, L17, L24-27 fitted as minelayers with 21-inch bow tubes and no beam tubes.

4. A comparison of the main particulars as completed of L1-8 with the latest boats of the E Class is as follows:

L1-8 E9
Length overall 231ft 1in. 181ft 0in.
Beam maximum 23ft 5½in. 2 22ft 8 3/8in.
Displacement surface, tons 891 667
Displacement submerged, tons 1074 807
Bhp surface 2400 1600
Bhp submerged 1600 840
Speed surf ace. knots 17 14 (15.25 designed)
Speed submerged, knots 10.5 9 (10.25 designed)
Endurance surface, miles at knots * 1700 at 17 1630 at 14
Endurance submerged, miles at knots 14 at 10. 5 10 at 9
Torpedo tubes, bow Four 18-inch Two 18-inch
Torpedo tubes, beam Two 18-inch Two 18-inch
Torpedo tubes, stern   One 18-inch

* With 51 tons of oil fuel.

5. Changes were made from L1-8 in L9 onwards as follows:

  • Four 21-inch bow torpedo tubes were fitted in lieu of 18-inch tubes.
  • An additional main WT bulkhead was fitted at the after end of the bow torpedo tubes to separate the tube compartment from the torpedo room.
  • The length of the pressure hull was increased by 7ft 6 in. to accommodate the above.
  • There were a number of changes in internal tanks in individual ships.
  • In L18 onwards the fuel carried was increased to 78 tons and fitted retrospectively in the earlier boats.

In those submarines chosen to be minelayers the amidship torpedo tubes were removed together with their associated WRT, firing and adjusting tanks. In the first boats L14 and L17 sixteen mine tubes, eight each side, were fitted in the external tanks with sixteen mines. In the later boats, L24-27, the aftermost mine tubes were removed so that these vessels carried only fourteen mines. Otherwise the layout of the vessels remained unchanged.

6. An Arrangement of WT Compartments in L1-8 is shown in Fig 11.1, in L9 onwards in Fig 11.2. A General Arrangement of L1 is shown in Plate 33 and L33 in Plate 34.

7. The L Class were of saddle tank type very similar to the E Class but about 50ft longer and 7 in greater in diameter of the pressure hull amidships. The sections were circular in the amidship portion but the stern was made chisel shape as in the H Class. They had the same advantages regarding the fitting of twin shafts and submerged control, but with the disadvantage that the propellers were higher than with the ordinary form of stern fitted in the E Class. This led to the propellers coming out of the water and the engines racing in a rough sea. The arrangement of rudder and after hydroplanes was also as in the H Class. The forward end lines were fined in and the lip end of the tubes kept well aft of the bows to give a good bow form.

11.3 Dimensions

8. In L1-8 the length from the stem to the after end of the hull was 221ft 9in and to the after end of the rudder 230ft 5in. The after side of the hydroplanes projected beyond the rudder by 8 in. The Loa was therefore 231ft 1in. The Lbp was about 225ft 6in and the Lph 200ft 9in.

The moulded diameter of the pressure hull was specified as 15ft 7½in. The plating was 20lb, except for a flat keel of 30lb, worked in raised and sunken strakes. The keel strake was a sunken strake and the top-plate a raised strake making the overall depth of pressure hull 15ft 9¼in. The maximum beam of the pressure hull would be 15ft 8½in with sunken strakes. Although 15ft 9in is some times given as in Fig 11.1, 15ft 8½in is considered to be correct as given in Fig 11.2. The ballast keel was 11½in. deep.

The maximum moulded beam of the externals was specified as 23ft 4½in. The external plating is taken as 10lbs and with raised strakes at the maximum beam makes the overall beam as 23ft 5½in. It will be noted that in Fig 11.2 a figure of 23ft 10½in is given, which means that, 2½in rubbing fenders were fitted.

9. In L9 onwards the length overall was increased by 7ft 6in. to accommodate the change in torpedo tubes from 18-inch to 21-inch. This increase was in the pressure hull. The rudder and after hydroplanes were rearranged although the areas remained the same. The Loa was 238ft 7in., the Lbp was approx 233ft 0in and the Lph 208ft 3in. The beam and depth remained the same as in L1-8

10. There is considerable confusion on draught which seems to arise from the amount of fuel carried - originally 51 tons and increased in L18 to 78 tons, the increase being fitted retrospectively in the earlier boats. Draught is given as either 13ft 3in or 13ft 6 n and both figures are coupled at various times with displacements ranging from 890 to 930 tons.

When carrying the full stowage of fuel of 78 tons the draught is taken as 13ft 6in. The freeboard to the superstructure deck would be 6ft 6in and to the bridge 12ft 9in. These are only approximate figures since the superstructure varied between boats. When not carrying fuel in external tanks the draught is taken as 13ft 3in. When in the 1930's a draught of 11ft 2in is quoted it represents the vessel at standard displacement.

11.4 Displacement and Stability

11. The original calculated submerged displacement for L1-8 was 1061 tons. When L2 was inclined the displacement had risen to 1065 tons. The submerged BG was 4.8in, which was unacceptable. The after end of the superstructure was made into a buoyancy space of 7.5 tons capacity, a buoyancy tank of 1 ton capacity was fitted in the superstructure near the engine room hatch and a tank for 1 ton of distilled water added just forward of the bridge. This increased the buoyancy by 9.5 tons. The vessel must have been heavy aft because in addition two auxiliary ballast tanks were converted into internal main ballast tanks, see Para 28. When L4 was inclined in February 1918 the submerged displacement was 1074 tons and the BG had risen to 7.4in.

L1-8 had tanks for 157 tons of external and 26 tons of internal main ballast water. The controlled free flooding space in the bows was of 7 tons capacity. Using all the main ballast water and a submerged displacement of 1074 tons, the surface displacement would be 891 tons and the reserve of buoyancy 21.3%. Sometimes 1080 tons is given for the submerged displacement but this includes the controlled tree flooding space.

12. In L9 with increased length the displacement was specified as submerged 1086 tons and surface 920 tons. The make shift buoyancy tanks in L1-8 were dispensed with. With small additions, when L11 was inclined at Vickers in June 1918 the submerged displacement had risen to 1089 tons with a BG of 7.5in. The total main ballast water had been increased to 195 tons. The surface displacement was 894 tons and the reserve of buoyancy 22.6%. These figures apply to L9-17 as built excluding the minelayers L14 and L17.

13. In L18 onwards No 6 Pt and Stbd external main tanks were converted into oil fuel tanks. This reduced the amount of main ballast water by approximately 20 tons to 175 tons. The submerged displacement was as for L9. The submerged displacement was therefore 1089 tons, the surface displacement 914 tons and the reserve of buoyancy 19.8%. This applied to L18 onwards except for the minelayers. The external fuel was certainly fitted retrospectively in L9-13 and L15-16 and the displacement figures given for L18 would then apply to these vessels. This modification would appear to have been carried out also in L1-8 after completion.

14. In the minelayers each mine tube was responsible for reducing the submerged buoyancy by about 1.5 tons i.e. 24 tons in L14 and L17 with 16 mines and 21 tons in L24-27 with 14 mines. In all these vessels twelve of the mine tubes were in main ballast tanks so that main ballast water was decreased by 18 tons.

In L24-27 the submerged displacement became 1068 tons and the total main ballast fell to 157 tons. The surface displacement was 911 tons and the reserve of buoyancy 18.0%. In L14 and L17 both displacements were 3 tons less. In all cases the external oil fuel is being carried.

15. Summarising the above figures:

Displacement Reserve of buoyancy
Surface
Tons
Submerged
Tons
%
L1-8 (no external fuel) 891 1074 21.3
L1-8 (with external fuel) 911 1074 18.6
L9-18 (no external fuel) 894 1089 22.6
L9-18 (with external fuel) 914 1089 19.8
L18 onwards (with external fuel) 914 1089 19.8
L minelayers (with external fuel) 911 1068 18.0

The standard displacement for the class was 760 tons.

The above figures are based on the assumption that the full main ballast capacity could be used. The amount of main ballast water varied between the three types as built as shown in Paragraph 29. Records suggest that all the internal main ballast water could not be used on completion but ballast may have been adjusted to overcome this wholly or in part at first. However over the years it is to be expected that with improved and additional equipment the boats grew heavy. For stability reasons, this growth could not be compensated using permanent ballast. Some allowance also has to be made for water remaining in main tanks on the surface, which could increase surface displacement by at least 10 tons or more. About 1934 DNC gave the surface displacement (as inclined) for the boats then remaining as 930 tons with 150 tons of main ballast water being used and carrying 76 tons of oil fuel. With all these variables it is not surprising that the figures for displacement given by various authorities over the years vary considerably.

16. A further complication is recorded that L4, L23, L26 and L33 were fitted about 1930 to take fuel in Nos 5 Pt and Stbd main tanks to increase the total oil fuel carried to 98 tons. This would increase the surface displacement by about 23 tons. All the external fuel was carried aft to give more submergence to the propellers and prevent or reduce racing in a seaway.

17. After a number of the class had been built DNC was quoting figures for surface GM and submerged BG as 20in and 8.5in, in L1-8, and 20in and 7.5in for L9 onwards. Corresponding figures by Vickers were 18in and 8.25in for L1-8 and 18in and 7.6in for L9 onwards. When L11 was inclined in June 1918 the BG was 7.5in, in L4 in February 1918 the BG was 7.4in and in L17 a minelayer BG 7.6in and GM 15in. Reasonable figures to take for the class are, GM 18-20in and BG 7.5in with a GM in the minelayers of 15in.

18. The ballast keel in L1-4 as built with a 10-ton drop weight weighed 40.5 tons, which included 6.0 tons of lead ballast and 6 tons of steel billets. A figure of 40-43 tons seems general for the class. Sections of the keel were built of steel boxes for lead or other deadweight to adjust trim.

11.5 Speed and Endurance

18. The designed figures were:

(a) Speed surface 17 knots at 2400 bhp
(b) Speed submerged 10.5 knots at 1600 bhp
(c) Endurance surface 1700 miles at 17 knots with 51 tons oil fuel
(d) Endurance submerged 100 miles at 2 knots
14 miles at full speed

There is no doubt that a surface speed of more than 17 knots was expected in this class. L1 the first boat on trials in December 1917, obtained 17.2 knots. The design power of the engines was 2400 bhp at 380 rev/min; on test they developed 2600 bhp at 380-400 rev/min. Both powers are given at various times. Even higher powers could be achieved. In 1918 DNC quoted 17.2 knots for L1-8 and 17.6 knots for L18 onwards. In 1930 17.6 knots was given for the class. There is no doubt that the trial speed of 17 knots was achieved even when carrying the additional fuel in external tanks.

In 1930 the operational speed was 16.5 knots in a calm sea with reasonably clean bottom. The loss of speed with dirty bottom is a well-known fact and this operational speed of 16.5 knots shows a comparatively clean bottom. L3 on the measured mile achieved only 12.3 knots at full power eight months out of dock and L27 only 12.8 knots four months out of dock.

19. Although 10.5 knots was given as the design speed submerged more than 11 knots was actually anticipated in L1 only 10.8 knots was achieved on trials. This reduction was due to the fitting of a fixed bridge screen, 51ft high, which was not in the original design. 10.5 knots was the submerged speed specified for L9 onwards. This was consistently quoted thereafter and was undoubtedly achieved. A trial speed of 10.6 knots for 1 hour has been mentioned. In 1930 the operational submerged speed was 10 knots.

20. In addition to the design surface endurance of 1700 miles at 17 knots, 2500 to 3000 miles at 11 knots was mentioned. These figures were based on 51 tens of fuel. After 1918 DNC was giving 2850 miles at 17.2 knots for L1-8 probably based on the trials mentioned in Paragraph 18 and 2950 miles at 17.6 knots for L9 onwards. These figures must have been based on 78 tons of fuel. In 1930 DNC and RA (S) give 5000 miles at 8 knots for the class. Operational values in 1930 were 2380 miles at 16 knots and 3800 miles at 10 knots using 95% of 78 tons of fuel. These figures are reasonably consistent with the age of the class.

21. The design figures for submerged endurance were obtained. In addition 75 miles at 4 knots was quoted. Operational figures in 1930 were 13.5 miles at 9 knots, 65 miles at 5 knots, 112 miles at 2.5 knots and 100 miles at 1.5 knots (auxiliary drive).

11.6 Structure

22. It has been stated that the diving depth was 250 feet and since the E Class had been classed a few years earlier as 200 feet boats there is every reason to believe that the L Class could have been designed for 250 feet depth. In 1925 the officially used maximum diving depth was 150 feet and the depth to which tested 100 feet. This would be based on the age of the boats and wartime construction. Some of the class did reach 250-350 feet on service.

23. The following particulars are taken from an L Class specification and give a good idea of the details of construction used in saddle tank types at the time.

'The flat keel amidships was 30lb decreasing to 20lb at the ends of special steel with a UTS of 30-34 ton/in2. The remainder of the pressure hull was 20lb amidships decreasing to 17lb at the ends, all of special steel, except where furnacing was necessary. The strakes next to the keel had double chain riveted edge laps and butt straps. Other strakes had double zigzag riveted edge laps and butt straps. Flush 16in x 12in WT manholes were fitted in the shell plating under the battery spaces and wherever access was needed.

The vertical keel was of 15lb generally. Frames were of 6 in x 3 in x 3½in x 15lb Z bars spaced 21in apart. WT bulkheads were of 15lb plating and 20lb where the heavier tests were applied.

The external plating was of 7½lb (it is considered this would have been increased to 10lb at the maximum beam) with 12lb at the ends flanged to the main hull. Divisional bulkheads were of 10lb plating and external frames of 2½in x 2in x 31lb angle bars spaced 21in. apart. Flats under the batteries and engines were 18lb, elsewhere 15lb.

The superstructure was of 5lb plating. In L1 a 3-inch gun was fitted on a heightened superstructure some way forward of the bridge; this changed to a 4-inch gun at the bridge deck level. A gun access trunk was fitted for the first time in which a man could stand with the lower hatch closed. There is one statement that 'perhaps the only disadvantage was the large silhouette'.

24. L1-8 had five internal main watertight bulkheads and subdivision was probably not as good as in some previous classes. In particular the fore end was a large compartment which 'in the event of damage by mine might have had disastrous results'. In L9 onwards an additional bulkhead was fitted between the rear end of the tubes and the torpedo stowage compartment. WT doors fitted in the WT bulkheads were 4ft 6in high x 2ft 3in wide.

25. The ballast keel, 11½in. deep amidships, was made horizontal at the bottom and could be used as a docking keel. Sections of the keel were steel boxes for the stowage of lead or other deadweight to adjust trim.

11.7 Tanks

26. The pressures applied in tank tests go back to the practice used before the H Class changes; internal tanks in general to 50lb/in2, main hull compartments to 35lb/in2 and oil tanks tested individually when sided to 10lb/in2 although each group received 50lb/in2.

11.7.1 Main Ballast Tanks

27. Except in way of the beam tubes where the pressure hull extended to the saddle tank plating the saddle tanks were completely occupied by main ballast tanks test to 15lb/in2. In L1-8 the total capacity of the twelve tanks was 156.7 tons. In L9 onwards the capacity dropped very slightly to 155.9 tons because No 1 main tank was decreased in length by 3ft 6 in. In the minelayers L14 & L17 the mine tubes were in the externals and the total capacity of the main tanks fell to 137.8 tons.

In L18 onwards No 6 Pt and Stbd tanks were used for oil fuel. With a capacity of 20.4 tons of water they would carry about 17.9 tons of fuel. The external main tank capacity dropped to 135.5 tons. This modification was made retrospectively in the earlier boats.

28. L1-8 originally had two internal main ballast tanks near amidships with a total capacity of 16.7 tons. At some time before completion two auxiliary tanks, one right forward of 3 tons and the other amidships of 6.8 tons were converted into internal main tanks. These modifications were probably made in conjunction with adding buoyancy spaces aft as a result of the first inclining in L2 mentioned in Paragraph 11. These tanks, lettered A, B, X and Y, were all fwd of amidships and had a total capacity of 26.5 tons. In L9 onwards No 1 auxiliary ballast tank right forward in the earlier boats was also converted into an internal main ballast tank giving a total capacity of 39.5 tons in all boats from L9 onwards.

The bow superstructure was a controlled free flooding space of just over 7 tons capacity.

29. The various amounts of main ballast tank capacity in tons available as first built were:

External Internal Total
L1-8 156.7 26.5 183.2
L9-13, 15-16 155.9 39.5 195.4
L14, 17 (minelayers) 137.8 39.5 177.3
L18-23, 28-33 135.5 39.5 175.0
L24-27 (minelayers) 117.4 39.5 156.9

There were undoubtedly slight differences between individual boats in the amount of main ballast water available but this table does show the trend from at first an increase and then a considerable decrease in the capacity of the main ballast tanks. The decreases were due to the introduction of oil fuel in the externals and mine tubes in the minelayers.

30. The external main tanks were flooded through holes in the bottom except for the two tanks carrying emergency oil fuel which had hand operated Kingstons. The policy regarding the fitting of Kingstons in main tanks is discussed in Chapter 23.

A vent pipe led from the highest point of each external tank into the pressure hull and thence outboard again into the superstructure. Valves were fitted where the pipes led into and out of the pressure hull. For the internal main ballast tanks a pipe from each tank led to a group chest having a large branch connected to a main vent valve which could be either power or hand operated.

11.7.2 Oil Fuel Tanks

31. As designed, all the oil fuel was carried inboard in three groups with a total of fourteen tanks. L1-8 started with a forward group of three tanks (14.3 tons), an amidship group of five tanks (14.3 tons) and an after group of six tanks (22.2 tons). This gives a total of 50.8 tons of oil fuel. In, or adjacent to, these fuel groups were three spare tanks, one in the midships group (capacity 4.14 tons water, 3.62 tons fuel) and two adjacent to the after group (capacity 9.86 tons water, 8.65 tons fuel). There is little doubt that originally these spare tanks were meant to carry oil fuel. In later vessels the spare tank of the amidships group was used as a provision room and this was probably adopted retrospectively in the earlier boats.

There is some confusion about the amount of oil fuel carried. Vickers consistently gave 51 tons and gave endurance figures comparable with this capacity. DNC on the other hand gave 76 tons.

From L18 onwards as built No 6 Port and Stbd external main tanks were used for oil fuel. This would increase capacity by 17.9 tons making a total of 69.7 tons. No reason is seen why the two spare tanks in the after fuel group should not be used for oil fuel and they undoubtedly were to bring the total fuel carried to about 78 tons. CB 1815 (1930) quotes the L Class as carrying 78 tons of oil fuel. There is little doubt that the boats prior to L18 were fitted retrospectively to carry this additional oil.

11.7.3 Other Tanks

32. Points of interest are:

  • (a) Firing tanks were adopted for the beam tubes, a practice that had been abandoned in all classes since the E boats in 1914. They were in the externals and tested to 95lb/in2.
  • (b) In L1-8 a specially built distilled water tank tested to 75lb/in2 was fitted in the superstructure. This continued until about L18 when two tanks were built in the external tanks with increased capacity.
  • (c) More consideration is given to stores. In L1 one provision room was provided but from about L18 onwards a second store appears.
  • (d) There is no doubt that in the design L1 had ten auxiliary ballast tanks (approx 45 tons) but during building this was cut back to seven tanks (33 tons) and that the other three tanks were used as internal main ballast water and fresh water tanks. In L9 the number of auxiliary water tanks was reduced to six and the capacity reduced still further to 24.5 tons another tank having been made into an internal main ballast tank. All vessels had two adjusting tanks of total capacity of 6.3 tons.

11.8 Machinery

33. The twin screws were driven by two twelve-cylinder diesels of the J Class type with a total of 2400 bhp at 380 rev/min. Some authorities quote 2600 bhp which was the bench test power of these engines at 380-400 rev/min as against 2400 bhp which was the design figure. Details of this engine are given in Appendix VIA. A specification for what Vickers call the 'L11 type' states that the engines could develop 1500 bhp each at 450 rev/min. In each case the shipbuilders made the main engines except for L5, by Crossley, L7 by Bellis and Morcom, L8 and L16 by Thornycroft, L9 by Westinghouse and L15 by Beardmore.

34. The main motors were of the open shunt wound double armament type developing 400 bhp each, a total of 1600 bhp at 300 rev/min for 11 hours (armatures in parallel), with speed range down to 106 rev/min (armatures in series).

An auxiliary drive for low speeds submerged consisted of a 20 hp motor which could give a speed of 1.75 knots driving the starboard shaft only through a worm drive.

The battery consisted of 336 cells in three sections of 112 cells each stowed in 3 tanks. These tanks were fitted with teak covers and tested to 3lb/in2 after fitting.

11.9 Armament

35. L1-8 had four 18-inch bow torpedo tubes and two 18-inch beam tubes and carried a total of ten torpedoes. No spares were carried for the beam tubes.

In L9 onwards the four bow tubes were changed from 18-inch to 21-inch. The two 18-inch beam tubes were retained except in the minelayers. Again spare torpedoes were carried only for the 21-inch bow tubes. In the minelayers L14, L17 and L24-27, the beam tubes were removed and sixteen mines carried in tubes in the externals in L14 and L17 and fourteen mines in L24-27.

36. One gun was mounted on the superstructure forward of the bridge and the type of gun varied considerably between individual boats originally. Of the known types L1 started off with a 3-inch HA disappearing type some way forward of the bridge without a gun access trunk. From about L12 onwards the gun deck was raised to the level of the bridge deck immediately forward of the bridge canopy and a gun access trunk was fitted. This was done for the purpose of quickly attacking an enemy submarine out of torpedo range but within gun range with the boat floating at low buoyancy. See Chapter 28 Para 12.

11.10 Accommodation

37. There is no doubt that the accommodation was greatly improved in this class compared with the E Class. An interesting change concerning the siting of accommodation occurred as shown on some of the original drawings. In L1-8 compartments from forward read torpedo room, officers' quarters, crews' quarters, and control room. The officers' quarters seem excessively large. In L9 with the change in the bow tubes and the introduction of an additional bulkhead to separate the tube space and the torpedo room, 5ft was taken out of the officers' quarters and put into the torpedo room but still the officers' quarters appear to be excessive. By L18 the torpedo room is called the forward mess deck and the former officers' quarters have become the after mess deck. The officers' quarters are sited in a smaller space between the after mess deck and the control room. This was undoubtedly an effort to improve accommodation for the crew although it did, at the same time, bring the wardroom next to the control room with advantage.

11.11 L50 Class - Design

38. The design of the L50 type of patrol submarine was based on the L Class modified to give an improved bow torpedo armament and an increased gun armament. It was developed without any sea experience in the L Class, in fact the orders for L50-55 were placed in January/February 1917 some four months or so before L1 the first of the L Class, was launched.

The changes in armament in the L50 Class were:

  • Six bow 21-inch torpedo tubes as against four bow 21-inch tubes and two beam 18-inch tubes in the L Class.
  • Two 4-inch guns sited at bridge level one just before the bridge and one just aft as against the one 4-inch gun in the L Class.

The main engines, motors and batteries were the same in both classes. The speeds were expected to be about the same with the normal stowage of oil fuel which was all internal in L50, but to increase the surface endurance the total fuel could be increased by using external tanks and in this condition the surface speed would be slightly reduced.


Fig 11.1

Fig 11.2

Fig 11.3

Fig 11.4

39. Six boats L50-55 were ordered in January/February 1917 and a further nineteen L56-74 in April 1917. Only seven of the class were completed and none before the end of the war, L52, L53, L54, L55, L56, L69 and L71. Four of these boats started to build at private yards and were eventually completed at HM Dockyards, the last in January 1925. Details are given in Appendix I. The remaining boats were cancelled.

40. They were of the saddle tank type with dimensions very similar to the L Class, the beam and depth being the same but slightly shorter in overall length. The stern however was of ducktail shape and above water as in the J Class and K Class instead of the chisel shaped and below water stern In the L Class. Shaft brackets supported the shafts and gave a much deeper immersion of the propellers and less racing in bad weather. This arrangement also gave a finer run down aft with increased headroom. In addition, more space was available for accommodation with the omission of broadside tubes. A General Arrangement is shown in Plate 35.

11.12 Dimensions

41. The Loa was 235ft 0in, the Lbp 230ft 6in and the Lph 206ft 6in. The beam of the pressure hull and over externals and the depth of the pressure hull as designed are given to be the same as in the L Class with the same circular pressure hull. Therefore the maximum beam of the pressure hull was 15ft 8½in and the maximum depth 15ft 9¼in. The maximum beam over externals was 23ft 5½in. The ballast keel was 11½in deep. The draught was 13ft 1½in at a surface displacement of 960 tons.

11.13 Displacement and Stability

42. Figures given consistently for displacement are submerged 1150 tons and surface 960 tons with the normal stowage of 78 tons of oil fuel. After completion DNC amplified these figures with a reserve of buoyancy of 19.8%.

All the main ballast water was in external tanks. In addition there were three buoyancy tanks in the bow superstructure. From the figure for reserve of buoyancy the capacity of these buoyancy tanks has been included in both the submerged displacement and the amount of main ballast water used. About this time the practice began of fitting HP blows to bow controlled free flooding spaces which then came into the same category as main ballast tanks.

The standard displacement for the class was 845 tons. This is made up by subtracting 78 tons of oil fuel, 6.5 tons of lubricating oil, 19.5 tons of compensating and trimming water and 11 tons of fresh and distilled water - a total of 115 tons of liquid from the surface displacement.

43. When carrying 39 tons of oil fuel in Nos 6 and 7 Port and Stbd external main tanks the total main ballast water fell by 45 tons to 145 tons. The submerged displacement remained at 1150 tons and surface displacement became 1005 tons and the reserve of buoyancy 14.4%.

44. In 1918 DNC gave the stability as surface GM 20in and submerged BG 12in. These figures were repeated after 1930. The ballast keel as built was 40 tons.

11.14 Speed and Endurance

45. Design figures were:

(a) Speed surface 17.5 knots
(b) Speed submerged 10.5 knots
(c) Endurance surface 3000 miles at 17.5 knots
(d Endurance submerged 80 miles at 3 knots

Based on the L Class with the same transverse section and what was hoped to be an improved form and with better immersion of the propellers there was cause to be optimistic that the design speeds would be obtained but this was not to be.

  • Reference (a) above. The speed was a disappointment but modifications to appendages made an Improvement. L71 before modification obtained only 12.4 knots. By streamlining the hydroplanes, modifying the ballast keel and making other small appendage modifications over 14.0 knots was achieved with new propellers. By modifying certain other underwater fittings, which was not done in the RN submarines because of expense, Armstrong obtained 15.6 knots in the same type they built for Yugoslavia.
  • The design speed was at 960 tons displacement carrying only 78 tons of oil fuel. CB 1815 (1930) gave the operational speed for the class as 12.5 knots carrying 120 tons of fuel, and with moderately clean bottom. It is reasonable to assume that 14 knots was unlikely to be exceeded in the best possible conditions when carrying the maximum stowage of fuel.
  • Reference (b) above. 10.5 knots was certainly not achieved and could not have been expected due to the growth in bridges and topside fittings generally whilst building. CB 1815 (1930) gave 8 knots and no other figure has been seen to expect anything higher.
  • Reference (c) above. The design figure was based on 78 tons of oil fuel. Some years after completion accepted figures were 3780 miles at 12 knots and about 5000 miles at economical speed (8 knots) with 120 tons of oil fuel.
  • Reference (d) above. The only reasonable service submerged endurance seen is 38 miles at 5 knots.

11.15 Structure

46. The design diving depth was as for the L Class. Figures of 250 feet and 200 feet have been seen. The former is the logical figure based on the 200 feet of the E Class. Some boats of the class did reach 250 feet to 300 feet. In 1925 the diving depth was set at 150 feet and test depth 100 feet. This would have been based on the length of time to build with part wartime construction.

The weight of the 'hull and fittings group' was 473 tons in the L50's compared with 455 tons in the L Class near enough to assume that the hull scantlings were the same and as described in Paragraph 23.

11.16 Tanks

11.16.1 Main Ballast Tanks

47. All the main ballast water was carried in fourteen tanks externally in the saddle tanks and in three buoyancy tanks in the bow superstructure with a total capacity of about 190 tons. There were no internal tanks. In service Nos 6 and 7 Pt and Stbd external tanks were used for oil fuel with a reduction of about 45 tons in main ballast water.

This class experienced troubles when surfacing mainly due to cross venting of the LP blows to the external tanks. By making the blows separate the surfacing was greatly improved but was never as good as desired.

11.16.2 Oil Fuel Tanks

48. The normal stowage of 78 tons of oil fuel was in internal tanks. This stowage was used in the design for the legend displacement, speed and endurance. At the same time it was stated that carrying additional oil fuel in two of the after external tanks, bringing the total fuel to 100 tons, could increase the radius of action. In actual fact Nos 6 and 7 Pt and Stbd main tanks, in the externals, were converted to carry fuel bringing the total to 117-120 tons and this became the normal fuel stowage.

11.17 Main Machinery

49. The main engines, main motors and batteries were a repeat of the L Class. Armstrong Whitworth made the main engines for L52 and L53, L54 and L69 by Bellis and Morcom, L56 by Williams and Robinson and L71 by Thornycroft.

11.18 Armament

50. The torpedo armament consisted of six bow 21-inch tubes and a total of twelve torpedoes. Two 4-inch guns were sited at bridge deck level one at the fore end and the other at the after end of the bridge.


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Chapter 10: 1914-18 Single Hull Types H and R ClassesChapter 12: Submarine Policy 1919 - 1930