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Chapter 4: Pre 1914 Saddle Tank Types D & E Classes

4.1 Introduction

1. The D Class was designed to eliminate as far as possible the limitations of the earlier classes. They were much larger than the C Class - the submerged displacement was nearly doubled - but the main change in design was the introduction of the saddle tank type of construction. The E boats were again considerably larger than the D boats but the change in design was mainly the result of the introduction of broadside torpedo tubes.

D1
D1

2. The programmes and total cost per boat up to November 1914 were:

Programme Cost
D1 1906/07 £79,910
D2 1908/09 £84,410
D3-8 1909/10 £89,100 ex gun
E1-6 1910/11 E1-8 £101,900 ex broadside tubes
E12-13
E7-11 1911/12 E9-11 £105,700 ex broadside tubes
E14-16
E12-16 1912/13  
E17-18 1913/14  


These costs were given by DNC in 1912 so that those for the E Class are either estimates or provisional figures.

All these boats were built by Vickers except D7 and D8 and E1, E2, E7, E8, E12 and E13 built by Chatham Dockyard. During this period Vickers also built AE1 and AE2 for the Royal Australian Navy and completed them at the beginning of 1914.

3. On 11 November 1914, a conference was held at the Admiralty with representatives of various shipbuilding firms to ascertain how many E boats each could build. As a result orders for thirty-eight boats were placed with Vickers (6), Beardmore (4), Fairfield (4), Brown (3), Scotts (1), Armstrong (2), Swan Hunter (3), Palmers (2), Cammell Laird (4), White (1), Thornycroft (2), and Yarrow (4). Later, on 20 November, two were placed with Denny. These orders covered E19-56.

Beardmore's were already building two boats of the E type for the Turkish Government under sub-contract from Vickers. They were taken over, slightly altered in design and formed two of the four boats placed with Beardmore, E25 and E26. Of the four allocated to Yarrow one was cancelled and two transferred one each to Scotts and Denny on 3 March 1915 because of more urgent work at Yarrow's. E28 was the boat cancelled at Yarrow's and an E boat of this number was never built.

4. Of the five boats ordered in the 1911/12 Programme E7 and E8 building at Chatham were completed as the previous boats E1-6. For E9-11 at Vickers some major modifications were made in the design which produced a more elaborate arrangement to build at slightly greater cost. It was the original intention that all boats from E9 onwards should be built to this modified arrangement. For some reason which is not apparent, since all the drawings etc must have been available at the time, E12 and E13 ordered from Chatham in the 1912/13 programme were not built to the full modified design of E9. In a way this was fortunate because when the large order was placed in November 1914 the boats ex the minelayers at all the firms other than Vickers were built to the E12 simplified design to assist rapid production.

For the many yards with no previous experience in building submarines, drawings, steel demands, etc, for E12 were available at Chatham. Orders for castings, main engines, main motors and switchboards, pumps, periscopes and the like, in fact most of the equipment and fittings required by these firms were ordered by the Admiralty.

E14 in 1914
E14 in 1914

5. The boats ordered before the war took 20-30 months to complete. E19 ordered in November 1914 created a record being built, equipped and handed over in 8 months. Several later boats completed in 8-10 months. They were actually fitted with engines being made at the time by Vickers ordered for the G Class in June 1914, which of course helped in this record. Completion was retarded in many cases by the introduction of equipment found necessary as the result of war experience.

6. E24, E34, E41, E45, E46 and E51 were fitted as minelayers and were without broadside torpedo tubes. This decision meant large structural modifications which caused delay in completion.

4.2 Design

7. The limitations of the C Class can be summarised as follows:

  • Single hull. The spindle hull whilst giving a good diving boat was not a shape which lent itself to surface speed and was inclined to be dangerous through small longitudinal stability on the surface.
  • The low reserve of buoyancy, not more than 10% of the surface displacement as designed.
  • Low speed both on the surface and submerged.
  • Limited endurance both on surface and submerged.
  • Single screw and petrol engine.
  • Single compartment vessels i.e. no internal main watertight bulkheads.
  • Bad habitability - no attention was paid to the accommodation for either officers or men.

8. The D Class design was approved by the Board in 1906 with the main improvements compared with the C Class as follows:

  • Increased safety in surface navigation and seaworthiness (obtained by increase in size and reserve of buoyancy).
  • Reduced risk of explosion and increased economy of working (heavy oil engine substituted for petrol engine).
  • Better manoeuvring power obtained by introducing twin screws.
  • Increase in speed and endurance.

The main object of the design was to get a submarine with high reserve of buoyancy and high longitudinal stability on the surface. The French and Italians at this time were considering the necessity of increasing the reserve of buoyancy. In France a series of trials was carried out between two vessels - the Emerande (7% reserve of buoyancy) and the Pluviose (40% reserve of buoyancy). The Pluviose was obviously double-hulled and probably a Laubeuf design. The trials were all in favour of the latter which is not surprising and the French adopted the double-hull form. The Italian designer Laurenti had the middle portion of his boats double-hulled with the ends single hulled. He got a very high reserve of buoyancy by having a watertight superstructure which was automatically flooded as the vessel dived. The Germans in their first submarine placed most of the double-hull on top of the pressure hull and at the ends and continued to do so. The USN did not adopt the double-hull until 1911.

9. The D Class was designed with saddle tanks which held the main ballast water. The reserve of buoyancy and the longitudinal GM greatly increased from that in the C Class. This, with increased size of boat, improved seaworthiness and safety in surface navigation.

The external ballast tanks in D1 carried 99 tons of water and this alone gave a reserve of buoyancy of just over 20%. However, five internal main ballast tanks were fitted under the batteries with a total capacity of 29 tons - practically the same as the total capacity in the C Class. On the completion of D1 it was found necessary to carry only 13 tons of water in these internal tanks and it is stated that 'the Kingstons could be kept closed and no pressure came on the battery flat.' In the true sense however they were internal main ballast tanks with the faults of those in the earlier classes. As things developed all the D Class and E boats had four or five of these tanks and one use to which they were put is explained in Paragraph 38.

10. With most of the main ballast water in the externals more space should have been available inboard to improve habitability. Reference (5) states that:

The most important advantage was their improved habitability as the radius of action of the C Class calculated from their speed, power and fuel was already greater than the power of the crew could endure as a matter of routine, the confinement and discomforts of existence at sea in these small ships in bad weather.

The endurance had been increased and it was essential to improve habitability to an even greater extent than would otherwise have been desirable. This was considered in conjunction with the reserve of buoyancy since Reference (5) also states

the reserve of buoyancy was 25%. Although habitability and safety could have been increased by greater reserve of buoyancy it was considered undesirable to incur the great complication and bulk of flooding and venting apparatus necessary to deal with the ballast water representing the (higher) reserve of buoyancy with the rapidity which is essential for the efficiency of any submarine boat.

This is undoubtedly a reference to the double-hull type submarine.

However with the introduction of a stern torpedo tube, greater power and an increased complement in a vessel with the same diameter of pressure hull as the C boats and only 20ft increase in length, it is doubtful whether there was much additional space available for accommodation.

4.3 Armament and Machinery

11. Although no experience had yet been obtained with the diesel engine in A13 (not completed until June 1908) heavy oil engines were adopted - a move which reduced the risk of explosion and increased economy of working. On service petrol engines had given a certain amount of trouble whilst other nations were experiencing even more trouble so the fitting of diesels was a risk worth taking. The introduction of twin screws improved manoeuvring.

The total shp of the main engines was double that in the C Class and the bhp of the main motors nearly doubled, which meant an increase in speed both on the surface and submerged. The surface endurance was also increased considerably. Reference (5) stated that the economies effected by diesels and the increased fuel carried gave the D Class at 11.5 knots about 25% greater endurance than the C Class at 9 knots.

12. The two 18-inch bow tubes were one above the other which enabled a finer bow than in the C Class in which the tubes were side by side and this enabled the resistance to be somewhat reduced. A stern tube was fitted in a British submarine for the first time.

D4 carried a 12-Pounder gun on a mounting, which housed inside the superstructure. This was possibly the first submarine to be fitted with a gun.

4.4 D Class Design

13. D1 was laid down as a prototype in 1907 and completed in September 1909. The following vessels did not start to build until after that date, and alterations in detail to the D1 design were expected to give further improvement in submerged speed and endurance and in handling in the later boats. There were changes in the D1 design for D2 and again for D3-8 mainly in dimensions and displacement. The armament and machinery remained unchanged although improvements were made in bridges and periscopes and a novel method of flooding the external main ballast tanks introduced to centralise control.

D4 was the odd boat out in the D3-8 group. She was one of four boats D3-6 built at Vickers and was laid down, launched and completed in her correct sequence yet the arrangement of internal and external main ballast tanks was different. No flooding trunks were fitted in the externals as in the other boats and the internal main tanks reverted to the D2 arrangement. These changes were not connected in any way with fitting the 12-Pounder gun.

14. Even whilst the D Class were in the development stage circa 1908/9 there seems to have been a fear that the increased size of this class coupled with the increased submerged speed made end-on torpedo attack at short range difficult. DNC states that:

Provision of broadside tubes was considered in the D Class design and had some influence on the shape of the hull adopted but it would have been necessary to increase the size of the vessel and at the time financial considerations prevented acceptance of any more costly a submarine than the D Class design

The following extract from the report made some years later of a Committee under Commodore (S) in April 1914 gives the sense of that feeling:

With the D it was felt that the limit in length had been reached as regards end-on attack as it was becoming more and more difficult to manoeuvre clear after firing torpedoes at the short range necessary to ensure success. (Tactical diameter submerged three times length.)

Angle gyros were tried. (It is believed generally used in foreign boats). An accurate knowledge of range is however essential to ensure success and the results must necessarily compare unfavourably with the accurate short-range shooting which we practice and towards which our training is directed.

Because of these fears the necessity was seen for broadside tubes. In view of the lower value of bow discharge and the necessity of keeping the increase in displacement and cost over the D boats as low as possible only one bow tube was provided in the design of E1-6 but beam tubes were included. In fact opinion in the submarine service was against bow tubes and their complete omission was seriously considered.

It might be mentioned here that in 1910 three designs were produced in the Admiralty capable of carrying four beam tubes described as the 'French', 'Italian' and 'Improved E'. The first closely followed a Laubeuf type design, the second a Laurenti type design and the third was simply an enlarged E. DNC did not recommend the adoption of these larger boats involving new and untried principles and this was accepted.

4.5 E Class Design

15. The design of E1 was therefore based on the D Class but modified to include broad side 18-inch torpedo tubes. In all E1-8 had one bow, two broadside and one stern tubes and carried a total of eight torpedoes. Other main changes from D3-8 were:

  • Increase in length of 13½ft and in the moulded beam of pressure hull of 14½in. The increased beam was necessary to accommodate the broadside tubes although experiments made earlier had suggested that an additional 2½ft was necessary. In actual fact in the E Class the external structure in way of the tubes was built to pressure hull standards, the pressure hull was cut away and the beam tubes occupied the space from port to starboard saddle tank plating, in all nearly 23ft.
  • Increase in displacement of about 160 tons on the surface and 175 tons submerged.
  • Increase in surface shp and oil fuel to give increase in surface speed and endurance.
  • Increase in submerged bhp of main motors to allow full output of the battery on a one hour discharge rate and increase full speed submerged.
  • Two internal main watertight bulkheads were fitted so that for the first time the submarine had a certain amount of watertight subdivison.

16. In 1911-12 the following modifications were made in the design for E9:

  • The bow was lengthened 3ft to allow two bow torpedo tubes to be fitted side by side. This gave better protection to the bow caps and loading of the tubes than the arrangement in the D Class with one tube above the other. In E1-8 there was insufficient length for a torpedo between the after end of the tube and the watertight bulkhead at the after end of the torpedo room so that the tail of the torpedo had to project through the bulkhead door opening when ready for loading. This bulkhead was moved 2ft aft.
  • The engines were brought forward, giving 6ft 2in forward and 7ft 6in aft between the shafts instead of 5ft 6in in E1-8. The shaft lines were inclined slightly upwards 'which have some effect in reducing large bow hydroplane angles'. In general more room was given for the engines and motors.
  • The facilities for working torpedoes aft and amidships were improved. The after watertight bulkhead was moved forward 12ft 3in and the quarter battery of 56 cells formerly near the beam tubes moved aft between the shafts.
  • The conning tower was enlarged to form a steering position.
  • The number of internal main watertight bulkheads was increased to three.

It was possible to make the change from one to two bow tubes without increase in displacement due to the saving in weight from the original estimates for E1-8.

A detailed design incorporating these modifications was prepared and Vickers built E9-11 and 14-16 to that design. However, E12 and 13 built at Chatham appear to have been a compromise between E8 and E9. Before the vessels were laid down DNC stated that only one bow tube was fitted in E12 and E13 and the estimated cost mentioned in Paragraph 2 is based on that assumption. Very little information about E12 and E13 has been found but a change must have been made later and two bow torpedo tubes fitted but the quarter battery was not moved aft and the changes consequent on that move were saved. E27-56 ex the minelayers are said to have been built to the E12 arrangement.

17. In E17, laid down early in 1914, Vickers made another change in the design. The diameter of the pressure hull was decreased to 15ft 0-1/8in from 15ft 0½in for E16. There was also a decrease in the amount of external main ballast water due to a decrease in the length of the externals. As a result of these changes the submerged buoyancy decreased by 20 tons. The only reason that can be given for these small but relatively costly changes is that by the time E17 was ordered it was known that the earlier boats were carrying an excessive amount of permanent ballast.

These changes were incorporated in E17-26 all built by Vickers.

18. In E19-24 the bow was flared and carried forward 1ft 6in at the top of the superstructure. This improvement was undoubtedly fitted in all boats as built or retrospectively.

19. There were therefore five groups in the E Class with quite obvious differences:

  • E1-8 with one bow tube.
  • E12, 13, 27-56, excluding the minelayers, with two bow tubes and all the battery amidships.
  • E9-11, 14-16 with two bow tubes and part of battery moved aft.
  • E17-26, ex E24, as (c) with the submerged displacement reduced by 20 tons.
  • E24, 34, 41, 45, 46 and 51, which were built as minelayers. This meant that the broadside torpedo tubes were omitted and 20 mines carried in lieu, and involved considerable structural modification.

4.6 Additional Equipment

20. As a result of war experience many additional items of equipment were installed during building or retrospectively. Major items included:

  • An improved and much larger Wireless Telegraphy (W/T) installation than at first considered necessary. Large folding W/T masts were fitted.
  • Guns.
  • Hydroplane guards.
  • Sky searchers on periscopes.
  • Slop shoots.
  • Sounding machines.
  • Fessenden and 'jet' signalling gear.
  • WC's which could be blown at depth.

21. A point of design interest arose in connection with rudders. The D Class were fitted with a normal balanced type rudder and in addition had an upper rudder on top of the pressure hull which operated in unison with the lower rudder. This same arrangement was fitted in the first E boats. Submerged turning trials were carried out in E4 with the following results:

  • At 13ft depth, 25° Stbd helm, 500 amps on main motors - time for circle 4 minutes 8 seconds.
  • At 13ft depth, 25° Pt helm, 600 amps on main motors - time for circle 4 minutes 3 seconds, tactical diameter 200 yards, heel inwards 0. 5°.
  • With top rudder removed at 13ft depth, 25° Pt helm, 500 amps on main motors, time for circle 4 minutes 17 seconds, tactical diameter 240 yards, heel inwards 0 1°.

These trials indicated that the upper rudder did not improve the manoeuvring power sufficiently to offset the disadvantages attending its use. The upper rudder was removed from the remainder of the class where fitted.

22. Drawings of these classes are shown in the following plates:

Details of the main machinery, armament and other equipment in these boats are given in Chapters 20-32.

23. Vickers, acting on instructions from the Admiralty, had designed the Holland to C Classes. The Admiralty prepared the design of D1 and this practice was followed for all later Classes except for the S Class, V Class and W Class, Nautilus and Swordfish (1914).

4.7 Form and Dimensions

24. The pressure hull in both the D Class and E Class was generally circular in section except that it was slightly flattened vertically aft and more so horizontally forward. The change aft was a move towards the 'chisel stern' found later in the H Class and L Class. The 'ovality' right forward in the E Class appears to have been very little different from the D Class in spite of the twin tubes in the latter being fitted one above the other whilst in the former they were side by side horizontally. Unlike the spindle hull types in which the centres of all sections were on a straight line - the axis of the boat - in the saddle tank types the centres of sections were raised going forward and aft from amidships. The effect was that the top of the pressure hull was much nearer the horizontal and the bottom of the hull more curved than in the spindle hulls.

25. The great advantage of the saddle tanks is shown by the effect on reserve of buoyancy. In the C Class it was 10% and rose to over 23% in D1 and 25% in D3-8. Even though the reserve of buoyancy fell in the E Class it remained above 20%. The longitudinal GM improved greatly rising from 27ft in the C Class to 163ft in D1. These two features together with increase in size greatly improved seaworthiness and safety in surface navigation. The E boats with extensive superstructure combined with a navigating bridge built over the conning tower were a big advance as sea boats and were easy to navigate even in the roughest weather.

26. D1 as a prototype for the class was completed at about the same time as D2 was laid down and therefore changes in the later boats were to be expected. D1 and D2 and again D3-8 can be taken as three types. Although D4 had a different lay-out of main tanks from the others it comes in the D3-8 group for dimensions. As seen in Appendix IIA the length overall, the moulded beam of the pressure hull and the extreme beam over the side tanks all varied between the types. The submerged displacement as designed rose progressively from 589 tons to 603 tons to 620 tons all due to changes in form, although some of the increase in buoyancy can be put down to the addition of ballast keels in D2-8. Within a few years of completion the submerged displacement of D1 had risen to 595 tons by additions e.g. ballast keel.

Similarly there were three changes in dimensions in the E boats. The changes between E8 and E9 are obvious and due to fitting two bow tubes in the latter. The reasons for the changes in E17-26 are not so obvious.

4.7.1 Length

27. As explained in Chapter 2, there was confusion between various authorities in the figures quoted for length. In fact in the drawings for D2, and this is taken to apply to the class, the forward bulkhead of the forward trimming tank is marked as the FP and the after bulkhead of the after trimming tank as the AP which was different from Admiralty practice. The actual figures for D1 are Loa 163ft, Up 158ft 6in and Lph 160ft. In D2 the forward and after ends were modified from D1 and the overall length decreased by 11in. The figures for D2 are Loa 162ft 1in, Up 158ft 2in and Lph 160ft. In D3-8 the length of the pressure hull was increased from D2 by 2ft 6in and the figures became Loa 164ft 7in, Lbp 160ft and Lph 162ft 6in. The rudder is angled forward and the after perpendicular is taken at the hull bearing. In all cases the Lph is taken from the forward side of the forward hull casting to the after side of the after hull casting.

28. For E1-8 Vickers give Loa 178ft 1in and Lbp 176ft 1in. DNC's corresponding figures were 178ft 6in and 176 ft. The Lbp quoted is undoubtedly that for the Lph. Actual figures are taken as Loa 178ft 1in, Up 173ft (the rudder in this class was again angle forward and the AP is taken at the hall bearing) and Lph 176ft 1in. In E9 the Loa was increased to 181ft to accommodate an extra bow tube. Bow shutters were fitted and the forward end became a free flooding space. The lengths were Loa 181ft, Lbp 176ft and Lph 171ft 4 in. These figures can be taken as applicable to E9-16, and E27-56 the later boats built by firms other than Vickers. E17-26 had an Loa of 181ft and it is taken that the Lbp and Lph were the same as in E9.

In some of the later boats the superstructure was extended forward of the stem a distance of about 1ft 6in. to give a ship-shape faired bow. This would increase the Loa to 182ft 6in. and Vickers quote this figure for E19-24. It is assumed that all the E Class were so fitted in time since some drawings of E l have been seen with a flared bow whilst others have had a straight bow.

4.7.2 Beam

29. The moulded diameter of D1 was 13ft 6in., the same as the C Class, and 20lb plating was used amidships. The maximum beam of the pressure hull was therefore 13ft 7in. In D2 the moulded diameter increased by 1in. to 13ft 7in. and in D3-8 by a further 3in to 13ft 10in. The plating was only 17lb at the maximum beam which therefore increased in D2 to 13ft 7⅞ in. and in D3-8 to 13ft 10⅞in.

The maximum beam over the saddle tank plating was 20ft 6in. in D1, 20ft 6½in. in D2 and 20ft 5in. in D3-6. This meant a progressive decrease in the width of the external tanks each side in the three groups. At the same time the length of the externals increased from approximately 90ft in D1 to 119ft in D2 and 116ft in D3-8. The effect on main ballast capacity in the externals was a variation of not more than 6 tons between the types.

30. In E1 the moulded diameter of the pressure hull was increased from D3-8 by 14½in to 15ft 0½in primarily, to accommodate the beam tubes. The maximum beam of the pressure hull was 15ft 1½in. These figures apply to all vessels of the class except E17-26 in which the moulded diameter of the pressure hull was 15ft 0⅛in and the maximum beam 15ft 1-1/8in.

The moulded beam of the externals was 22ft 6½in in all boats except E17-26 in which it was 22ft 6in. The maximum beam to outside of saddle tank plating in way of the torpedo tubes where a 19lb doubler was fitted was 22ft 8-3/8in. and 22ft 7-7/8in respectively. The length of the externals in E1-8 was about 127ft and this probably applied to all the class, except that in E 17-26 it was reduced to 110ft so that the capacity of the externals was decreased by about 10 tons from the remainder of the class.

In E1 as designed the beam of the pressure hull was 15ft and the breadth over saddle tanks 22ft 5in. Before building commenced Vickers were told to lay off the boats to figures of 15ft 1½in and 22ft 6½in respectively, the increase in section vanishing at about the ends of the saddle tanks to allow a margin of displacement over the estimated weights. As it turned out there was considerable saving in weight during building from the design weights and in consequence the early boats had a ballast keel of about 50 tons and so much ballast was undoubtedly an embarrassment. A figure of 15ft 0½in for the moulded diameter is the one quoted by the builders and is taken as the as built figure and is undoubtedly correct. If it had been 15ft 1½in. more permanent ballast would have been needed.

4.7.3 Depth

31. Both classes had circular sections amidships so that the moulded beam and depth were the same. D1 had 20lb plating throughout so the overall depth to the underside of the flat keel (USK) was 13ft 7in. She had no ballast keel when built but assuming that one was fitted retrospectively as in D2, which was 5in deep, the overall depth would become 14ft.

In D2-8 the keel was 30lb and top plate of the hull 17lb so that approximately 1⅛ in has to be added to the moulded beam to give an overall depth to USK. For depth to the underside of ballast keel a further 5in has to be added in D2 and 11in. in D3-8. The relevant moulded depth, overall depth to USK and maximum depth to underside of ballast keel figures are therefore for D2 - 13ft 7in, 13ft 8⅛in and 14ft 1⅛in and for D3-8 - 13ft 10in, 13ft 11⅛in and 14ft 10⅛in.

32. The E Class had a 30lb keel plate and a 19lb top plate. The ballast keel was 11in, deep. The moulded depth was 15ft 0½in. In all boats of the class, except for E17-26, and the overall depth to USK 15ft 1¾in and to underside of ballast keel 16ft 0¾in. These figures will be slightly less for E17-26.

4.7.4 Draught

33. The as built draught of D1 is given as 10ft 0½in at 464 tons displacement. The surface displacement was however 483 tons and DNC (1912) gives draught at this displacement as 10ft 5in. Since the Tons per Inch (TPI) was of the order of 4. 5 tons these draughts are in reasonable agreement. The draughts do not include a ballast keel.

For D2 corresponding figures are 10ft 6¾in at 477 tons as built and 10ft 9½in at 489 tons (DNC 1912). The TPI at the latter draught was 4.58, again reasonable agreement. The draught has increased from D1, primarily because of the 5in. ballast keel now fitted.

In D3-8 figures are given as 11ft 3¾in as built and 11ft 5in (DNC 1912) both at 494 tons displacement. The TPI has fallen to 4.4 and the ballast keel increased to 11in deep. The draught of 11ft 5in is the more reasonable figure.

34. Although variation in figures are given for the E Class as for the D Class a reasonable value taken for the draught of class is 12ft 6 ½in. The TPI was 5.34. There were slight variations between groups of the class due to variations in submerged buoyancy and the amount of main ballast water carried. Typical figures given by Vickers are for E3-6 as 12ft 6¼ in at 656 tons displacement, E9-11 as 12ft 4½in at 658 tons and E17-18 as 12ft 9 in at 665 tons.

4.7.5 Freeboard

35. The following are approximate figures for freeboard to (a) the bridge deck and (b) the superstructure:

(a) (b)
D1 10ft 6ft
D2 9ft 6in. 6ft 3in.
D3-8 12ft 6ft 3in.
E1-8 12ft 6ft
E9-11 12ft 6ft 3in.

4.7.6 Ballast Keel

36. D1 was not fitted during building although C19 onwards, which were completed after D1, did have one. There can be no doubt that D1 was fitted retrospectively, possibly as in D2.

In D2 the ballast keel was 45ft long centred amidships with a depth of 5in. In D3-6 it was 61ft long about amidships and made 11in deep by 24 in wide. In the E Class the depth was kept at 11in. and the width increased to 2ft 8in and the length increased to 82ft in E1 and 97ft in E17-24. In all cases the depth of keel remained constant throughout its length and so followed the curvature of the hull. The effect was to increase the submerged buoyancy by about 3.5 tons in the D Class and 7 tons in E17-24, and to increase the draught by the depth of the ballast keel. Bilge keels were not fitted.

4.8 Displacement and Stability

37. With the introduction of saddle tanks the troubles regarding freeboard and longitudinal stability in the spindle hull type boats were overcome or at least greatly reduced. The reserve of buoyancy reached a peak of over 25% in D3-8 which was not equalled again until the Swordfish Class in the 1930's, except in the double-hulled boats. The longitudinal GM was enormously improved and the problem of trim on the surface greatly eased. Freeboard was much more and reached a peak in D3-8 of 12ft to the bridge deck and 6ft 3in. to the superstructure, which was as good for this size of boat as has been achieved since.

38. All the earlier classes had restrictions imposed on the use of main ballast water and with relatively small total capacities the restrictions had a marked adverse effect on surf ace displacement and reserve of buoyancy. In the D and E Classes a much larger quantity of main ballast water was in external tanks and there was never a restriction on the use of these tanks. They also had some internal main ballast tanks on which in some cases a restriction was imposed. The effect on surface displacement and reserve of buoyancy was relatively small. This restriction happened in many of the later classes and seems to have been accepted as a way of compensating for added weight from the design figures during building and in preference to decreasing permanent ballast, if carried, with an adverse effect on stability.

The D1 design was approved only four years after A1 was laid down. It was an entirely different and more complicated type than the previous classes. To calculate the weight of the boat in the design stage and make a reasonably correct compromise between weight, buoyancy and main ballast water must have been a difficult task and some insurance against an original underestimate of the weight and/or additions made during building was desirable. The internal main ballast water was the eventual insurance.

Some detailed weights for D1 give the total weight as 482 tons and adds a 'margin of weight or internal ballast of 16.5 tons'. These would appear to be 'weighed weights' during building. This means that at the time only 16.5 tons of the internal main ballast water of 29 tons could be used. This fell later to 13 tons. No mention is made in the weights of permanent ballast and as far as can be ascertained none was ever carried. As built D1, D2 and E17-26 were the only boats of these classes which had to forfeit internal ballast water.

39. D1 had 99 tons of main ballast water in the externals and 29 tons in internal tanks a total of 128 tons. Vickers gave the submerged buoyancy as 589 tons and the surface displacement as 464 tons by emptying the main ballast tanks of 125 tons of water. These are design figures.

In 1912 DNC gave the submerged buoyancy as 595 tons probably due to changes after completion e.g. ballast keel added. Assuming all main ballast tanks empty the surf ace displacement would be 467 tons and the reserve of buoyancy 27.5%. In practice it was found that only 13 tons of internal water ballast was needed which makes the surface displacement 483 tons and the reserve of buoyancy 23.2%. This is taken as the normal service condition.

It must have been thought that too much space had been allocated to internal main ballast in D1 because one tank less was fitted in D2 and the capacity fell to 23 tons. The moulded diameter of the pressure hull had been increased so that the submerged buoyancy increased to 603 tons. The capacity of the external main ballast tanks was 103 tons. Therefore as designed the surface displacement was 477 tons and the reserve of buoyancy 26.5%. In 1912 DNC was stating that only 11 tons of internal main ballast was being used. The service figures are therefore taken as submerged displacement 603 tons, surface displacement 489 tons and reserve of buoyancy 23.4%.

In D3-8 another 3in. was put in the diameter of the pressure hull and the submerged buoyancy rose to 620 tons. The external main ballast fell to 97 tons and the internal main ballast increased to over 27 tons so that the total main ballast capacity was 125 tons. The surface displacement became 495 tons and the reserve of buoyancy 25.3%. It is interesting to note that the full tank capacity was now used when diving.

40. The design surface GM is given as 15in for the class. In 1912 DNC gave figures, probably inclining experiment results, for D1 as 15.2in, D2 as 15.7in and D3-8 as 18in.

The design submerged BG was 7.25in. for D1, 8in. for D2 and 9 in. for D3-8. Again DNC gave figures in 1912 taken to be from inclining experiments as 7.3in. for D1, 6.3in. for D2 and 9 in. to 9.5in. for D3-8. The reason for the low result in D2 is difficult to understand and is taken as an error for 8.3in.

4.9 E Class Displacement and Stability

41. The E Class was a large one and there were a number of changes over the four years they were building. Details are known of the boats built at Vickers but little information is available of E12 built at Chatham which is unfortunate since it was the type used for building by all yards other than Vickers during the war. However, the displacement range of the known types is small and E12 undoubtedly comes within that range. As far as can be ascertained all the E boats used the full capacity of both internal and external main ballast tanks to dive when first built except for E17-26.

In E1-8 the submerged displacement was 796 tons. The calculated hull was 784 tons less appendages. They carried 110 tons of external and 31 tons of internal main ballast, a total of 141 tons. With all main ballast used the surface displacement was 655 tons and the reserve of buoyancy 21.5%. The service displacement would have been a few tons heavier due to water remaining in the tanks after blowing.

In E9-16 the submerged displacement rose. Vickers quote 800 tons and their calculated hull less appendages was 796 tons. In 1912 DNC quoted 807 tons and this is the more likely figure. The total main ballast was 140 tons so that the surface displacement was 667 tons and the reserve of buoyancy 21%. It can be assumed that these figures approximately apply to E27-56 ex minelayers.

In E17-26 the submerged buoyancy fell and 780 tons is normally quoted. Vickers hull calculation gives 776 tons less appendages. Since the large firing tanks are now inboard these figures are compatible. With 131 tons of main ballast the surface displacement was 649 tons and the reserve of buoyancy 20.2%. Vickers records state that only 14 tons of internal ballast was used and give the surface displacement as 664 tons which gives a reserve of buoyancy of about 17.5% and are taken as the correct figures.

The modifications in E24, E34, E41, E45, E46 and E51 to build as minelayers were considerable and it is thought that both submerged and surface displacements would have changed. However, Vickers for E24 quotes the submerged displacement as 765 tons, a reduction of 15 tons from E17, with no change in surface displacement.

42. The stability in this class was extremely good. Before E1 had been completed DNC gave estimates for GM and BG of 20in. and 10in. respectively for E1-8 and 20in. and 8in. for E9-16. Vickers quoted figures in considerable detail of GM starting at 17.5in. in E9 decreasing to 16in. in E17, and BG of 11.5in. in E1 decreasing to about 9.5in. in E9 and then to 8.5in. in E17; they also give 6.5in. in E24 a minelayer. The main point about these figures is that they are decreasing with the different types which is to be expected.

The Vickers' figures must be as built values and they would be expected to decrease with years of service. However some later inclining experiment results for E1-8 have been seen in which the GM was of the order of 23in, and the BG around 13in. After the war DNC was still quoting the surface GM as 23in with the submerged BG as 12in for the class. It would be reasonable to take these figures for E1-8 with corresponding figures for E9-16 as 23in and 10in and for E17-26 as 21 in and 9 in.

4.10 D Class Speed and Endurance

43. The estimated figures of speed and endurance for D3-8 were:

  • Surface speed 14.5 knots at 1200 bhp at 500 tons displacement.
  • Surface endurance 2500 miles at 10 knots.
  • Submerged speed 9 knots at 580 bhp.
  • Submerged endurance 70 miles at 5 knots.

44. D1 completed building 18 months before D2 and there was a further gap of 6 months between D2 and D3. Many changes were taking place some of which would affect speed and endurance especially between D1 and D2. The major items were:

  • Changes in form between D1, D2 and D3 which were likely to improve speed.
  • In D1 both the forward and after hydroplanes were some distance from the ends of the boat, were well drowned and fitted with plate guards in line with the hydroplanes. In D2 onwards the planes were moved much nearer the ends and were raised; a much larger guard was fitted aft but the forward one deleted because it was seemingly impossible to fit one.
  • The bridges became progressively larger and in D2 onwards large periscope brackets appeared. These had an adverse effect on submerged speed.
  • The oil fuel carried increased in D2-8 by over 11% from that in D1.
  • The battery power and duration of discharge at full speed was greater in D2-8 than inn D1.

45. In 1912 DNC quoted the following figures:45. In 1912 DNC quoted the following figures:

(a) Surface: Speed Endurance
D1 14 knots 3075 miles at 8 knots
D2 14.5 knots 3300 miles at 8 knots
D3-8 14.5 knots 2150 miles at half power
(b) Submerged:
D1 9.8 knots 50 miles at 5.5 knots
D2 10.0 knots 60 miles at 5.5 knots
D3-8 9.0 knots 60 miles at 5.5 knots
(c) Surface:
D1 14.5 knots 2000 miles at 11. 3 knots
D2 14.5-15 knots 2000 miles at 11.5 knots
D3-8 14.5-15 knots 2000 miles at 11.5 knots
All boats   1400 miles at full seagoing speed
CB 1815 of August 1914 gives:
(d) Submerged:
D1   9 knots for 1 hour
7 knots for 4 hours
D2 9 knots  
D3-8 10 knots  


46. Although Vickers' claim only 14 knots for the D Class and DNC gave this figure in the 1930's, it is considered to be pessimistic for the later boats whilst the CB 1815 figures are perhaps optimistic. For the service maximum surface speed, 14 knots is taken as reasonable for D1 and 14.5 knots for D2-8.

47. Based on Vickers consumption figures given in Chapter 25, the figures for surface endurance using all the fuel on board would be at full speed about 1300 miles for D1 and about 1500 miles for D2 onwards. At half power about 1830 miles in D1 and 2100 miles in D 2 onwards. Half power would be just over 11 knots in D1 and about 11.5 knots in D2-8. These figures compare reasonably well with those given in Para 45 allowing for 95% of fuel being used.

Service endurance figures taken as reasonable are for D1 at full speed of 14 knots about 1230 miles and at 11.2 knots about 1750 miles and for D2-8 at a full speed of 14.5 knots approximately 1400 miles and at 11.5 knots approximately 2000 miles.

48. It is extremely doubtful whether any of the class achieved a submerged full speed of more than 9 knots although the later boats might have done short bursts of 10 knots. The estimates for speed in Para 43 and those by DNC in Para 45 were given without any real evidence of the effect the changes mentioned in Para 44 would have on performance. It is relevant that the figure of 9 knots for D3-8 given by DNC in 1912 as against 10 knots for D2 was one given by DNC to the First Lord in January 1913. It can well be assumed that there was a worry about the effect on speed of the growth of topside fittings undoubtedly being pressed into the boats. Vickers claimed only 8.7 knots as the full speed submerged for the class.

It is therefore considered that 9 knots was the maximum speed possible perhaps slightly more in D2-8. In the latter boats the motors could be overloaded to give 300 rev/min as against the normal maximum of 265 rev/min and at these higher revolutions bursts of speed nearing 10 knots might be achieved.

49. The submerged endurance figure is taken as 9 knots for 1 hour perhaps slightly more in D2-8. Vickers quote 50 miles at 5 knots for D1 and 65 miles at 5 knots for D2-8.

4.11 E Class Speed and Endurance

50. The design figures for the E Class were:

  • Surface speed 15.25 knots at 1600 bhp at 660 tons displacement.
  • Surface endurance 3000 miles at 10 knots.
  • Submerged speed 10.25 knots at 840 bhp.
  • Submerged endurance 99 miles at 3 knots.
DNC quoted figures for E1-16 in 1912 as follows:
Surface speed - 15 knots
Submerged speed - 10.25 knots for 1 hour 30 minutes
Surface endurance - 2600 miles at half power
Submerged endurance - 99 miles at 3 knots
These were estimates because the first E boat was not completed until April 1913.
Vickers gave figures for E17-26 as follows:
Full speed surface - 15 knots
Full speed submerged - 9.5 knots
Surface endurance - 2250 miles at full speed
- 3800 miles at 10 knots
Submerged endurance - 15 miles at full speed
- 65 miles at 5 knots


51. Although a large class building over a number of years the characteristics between the various groups did not alter appreciably. The changes in form, dimensions and displacement were not of sufficient nature to make any marked difference in speed and endurance. The power both surface and submerged remained constant and the only change in oil fuel carried was an increase of about 7% from E1-8 in the later boats.

However as a result of war experience large guards with supports were fitted around the hydroplanes and these appendages are stated to have decreased the speed by 1.25 knots, presumably based on the design figure of 15.25 knots. This would bring the surface speed to 14 knots and the submerged speed to perhaps 9 knots. There would also be losses in endurance.

52. Some actual trial results in E1-8 are available. The surface displacement was 655 tons and the vessel usually trimmed by the stern about 0.75° when stopped. The maximum power was 1600 bhp at 380 rev/min.

  • E1. February 1913 on Maplins measured mile. Mean speed 15.14 knots at 375.5 rev/min. The displacement was 586 tons (i.e. 69 tons light) and the draughts Fwd 10ft 6in, Aft 12ft 7in equivalent to a trim of 0.7° by stern. At full displacement and maximum revolutions the above figures would represent a speed of under 15 knots.
  • E4. March 1913 on Stokes Bay mile. Mean speed of 15.41 knots at 376.5 rev/ min; draughts Fwd 11ft 10in, Aft 12ft 7in - trim 0.25° by stern. This represents a displacement of about 647 tons. The speed is higher than could be expected.
  • E8. On Maplins measured mile. Mean speed of 14.98 knots at 380 rev/min and 1600 bhp (estimated) and 14.49 knots at 357 rev/min and 1430 bhp; displacement 614 tons at draughts Fwd 10ft 10in, Aft 12ft 11in representing a trim of 0.7° by stern. The displacement is light by 40 tons and the true full speed would be about 14.8 knots.

All these trials were pre-war runs and represent a speed of about 15 knots. When the hydroplane guards mentioned in Paragraph 51 were fitted the maximum speed would fall to about 14 knots. However, no authority has quoted anything less than 15 knots even after the war. Yet there undoubtedly was a loss of speed and trials in E 51, which must have taken place in 1917 or afterwards, gave a speed of 14.31 knots at 375 rev/min at draughts Fwd 12ft 2in, Aft 13ft 3in. Although this corresponds to a displacement of about 675 tons, 20 tons heavier than the E1-8 trials mentioned above and she was a minelayer, it does represent a considerable loss in full speed. It is considered that not more than 14 knots is a reasonable speed for the class after all modifications were made.

53. The surface endurance figures quoted vary. After the war DNC quoted 2000 miles at 15 knots. Vickers had previously quoted 2250 miles and later on 1680 miles at full speed.

An undoubtedly trial result gives an endurance of 3400 miles at an economical speed of 10 knots and fuel consumption 33 gallons per hour. Vickers gave the fuel consumption as the boats left Barrow as 0.513lb per bhp hour at full speed and 0.6lb at cruising speed (assumed 10 knots). Using these fuel consumption figures will give endurance figures of approximately 1630 miles and 3200 miles respectively, based on using 40 tons of fuel. Allowing for true capacities and using only 95% of the fuel the figures taken as representative, are:

(a) E1-8 1500 miles at full power and 3000 miles at 10 knots.

(b) E9 onwards 1630 miles at full power and 3200 miles at 10 knots.

54. The design full speed submerged was 10.25 knots at 840 bhp for 1 hour 30 minutes. E1 carried out some submerged speed runs early in 1913 and obtained a mean of two runs of 9.64 knots at 294.7 rev/min and approximately 830 bhp. For some time DNC quoted 10 knots and then later on 9 knots. This would agree with the introduction of the hydroplane guards. Vickers quoted 9.5 knots. The service speed after all the war time modifications were made is taken as 9 knots. However at the 30-minute battery discharge rate the motors could develop 1060 bhp and short bursts of 10 knots could undoubtedly be achieved.

55. E1-16 had Mercury batteries with a duration of the battery discharge at full speed of 1 hour 10 minutes. From E17 onwards Exide batteries were fitted with a higher output, in fact Vickers give the submerged endurance at 5 knots as 65 miles in E1-16 and 70 miles in E17 onwards. These figures are accepted as reasonable and the endurance at full speed is taken as one hour which is perhaps slightly pessimistic.

56. Figures of speed and endurance considered as reasonable for these classes are given in Appendix IIIA. They depend on so many factors that they must be given some latitude, especially those for the submerged condition.

4.12 Structure

57. Construction Sections of D3-8 are shown in Plate 12 and for the E Class in Plate 15. In D1 and D2 the scantlings vary slightly from those in D3-8.

4.12.1 Pressure Hull

58. In all vessels the pressure hull plating was worked flush with external straps. Amidships in D2-8 the keel strake was 30lb, the garboard strakes 20lb and the remainder 17 lb. In D1 it is probable that 20lb plating was used throughout this was the practice at the time D1 was designed when ballast keels necessitating a thicker keel strake were not fitted. This assumption agrees with the overall dimensions for depth given by the builders. The thickness of plating undoubtedly decreased towards the ends of the vessels. Special mild steel plating was used for the first time in D3-8.

In the E Class, with an increase in diameter of just over 14in, the keel strake was 30lb and the remaining plating 19lb amidships. This plating decreased progressively towards the ends to 17lb and 15lb.

4.12.2 Externals

59. In the D Class 10lb plating was used generally for the externals and fitted in raised and sunken strakes with a raised strake at the maximum breadth. A 10lb non-watertight flat was fitted at mid height in the externals.

In way of the broadside torpedo tubes in the E Class the pressure hull was cut away and the externals built to pressure hull strength. The plating amidships was 19lb reducing to 10lb at the ends, with a 19lb doubler fitted in way of the broadside tubes.

4.12.3 Frames

60. In D1 and 2 the pressure hull frames were of 31in x 3in x 7.8lb angle bars spaced 18in apart as in the C Class. In D3-8 the angles were replaced by 3in x 3in x 2½in x 8.5lb Zed bars still spaced 18in apart.

In the E Class frame spacing was increased to 21in with heavier frames amidships of 5in x 3in x 3in x 14.17lb Zed bars decreasing to 4in x 3in x 21in x 11. 53lb Zed bars at the ends.

Frames to external tank plating were generally 2½in x 2in x 3.75lb angle bars whatever the plating thickness.

4.12.4 Bulkheads

61. In the D Class the collision bulkhead fitted in the C Class was sited about 16ft from the forward end of the pressure hull. No other internal main watertight bulkheads were fitted so in truth the D Class were one compartment boats.

62. In E1-8 this forward bulkhead was omitted. Two main watertight bulkheads were fitted, one at the after end of the forward torpedo compartment and the other at the fore end of the engine room. They were therefore three compartment boats. As stated at the time 'the boat was thus made safer against damage to the pressure hull and against internal explosion of battery gas or fuel'. They also strengthened the hull against external pressure.'

From E9 onwards 'two more bulkheads were fitted, one between the motor room and the engine room and another forward to ensure greater safety against damage by mines'. The former was actually at the after end of the motor space. This was the original intention. One only was fitted at the after end of the motor space in E9-11 and 14-26 but apparently not in the other boats.

The E Class were the first British boats to be fitted with internal watertight bulkheads. Although foreign powers had adopted watertight bulkheads we had not considered them necessary. Now, with increase in size, adoption was considered imperative. Even with the number of bulkheads fitted the boat was not sufficiently sub-divided with the reserve of buoyancy available to be able to keep the boat afloat if the battery compartment or machinery compartment was flooded. But the risk was taken that compartments would not be breached close to the top and that some air would be trapped sufficient to keep the vessel afloat with the remaining compartments intact.'

The internal bulkheads did strengthen the pressure hull, but they were not taken into account in the calculations for strength of hull. As in previous vessels the E Class 'were designed to have a reasonable factor of safety at 100 feet beyond which it was considered a submarine would never descend voluntarily.' Events during the war proved this to be wrong and there was a desire to go deeper.

The designers at the time stated that with the strengthening of the hull from earlier designs they 'now considered the boats capable of 200 feet diving depth.' A proviso was added later that it was impossible to be quite sure on this matter without actual trials. Actually, the E Class were eventually classified as 200 feet diving depth boats.

4.13 Internal Structure

63. The general pattern of decks and tanks was very similar to that in the earlier classes but the structure was generally stronger. More use was made of the vertical keel by extending it to flat level and in the D Class was watertight except in internal main ballast tanks. The result was a large increase in the number of tanks, most of them quite small. In the E Class the tanks were not subdivided since the vertical keel was non-watertight which must have meant a considerable saving in fittings and cost.

Although internal main ballast tanks were sited under and at the sides of the batteries in the D Class and the plating of the battery tanks remained at 15lb as previously the stiffening was improved. In the E Class in which tanks were kept below the battery tank floor level the watertight flat plating was increased to 17lb and the side plating of the battery tanks decreased to 12½ lb.

4.13.1 Tests

64. From the C Class the only change in test pressures of the tanks was that although in D1 the oil fuel tanks were tested to 25lb/in2 as in the C Class, from D2 onwards the test was raised to 50lb/in2.

In addition new types of tanks had appeared such as compensating and adjusting tanks, fresh water tanks, magazines and Forbes log tanks, all tested to 50lb/in2. Also cable lockers to 100lb/in2. The battery tanks although still covered with wooden boards were tested to 2lb/in2 in the E Class.

The list is therefore:

  • All main hull compartments and conning tower - 35lb/in2.
  • Cable lockers - 100lb/in2.
  • Firing tanks - 95lb/in2.
  • Battery tanks - 2lb/in2.
  • All other internal tanks ~ 50lb/in2.
  • External tanks - 25lb/in2.

4.13.2 Superstructure

65. Superstructure was more extensive than in the earlier classes and a good walking platform was provided extending from forward very nearly to the after end, except in E9 onwards when it was cut short some way from the stern in an effort to save weight.

The plating was generally 10lb with 2in x 11in x 2lb stiffeners. An extensive structure faired in the conning tower forming a bridge that grew as the classes developed. Portable guard stanchions and wires were provided for the first time.

The forward superstructure was cluttered with anchors, capstan, cable holders, cable and wire stowages. Firing tanks were sited in the structure. The anchor and cable arrangements had reached their peak in weight and complication.

In E19 onwards the bow was extended forward 1ft 6in and built as a ship shape, flared structure to cut down spray from the bow wave. This flared bow was fitted in other vessels of the class retrospectively.

The superstructures and bridges were now as good as ever likely to be expected and must have had an adverse effect on diving time. The size of the superstructure in E1 may be judged by the fact that It had a capacity of 140 tons about the same as the main tank capacity.

4.14 Hatches

66. In the D Class torpedo hatches were fitted forward and aft of the type in the C Class and of course the upper and lower conning tower hatches which were of the balanced type and opened by hand. In addition an escape chamber called the 'after conning tower' was fitted over the motor room without a hatch at the pressure hull and a watertight scuttle in the superstructure deck at the top of the tower. This was primarily an escape hatch although undoubtedly used as an engine room access in harbour.

67. The after conning tower was deleted in the E Class and the after torpedo hatch moved to the forward end of the engine room and used for loading torpedoes to both the amidships and after tubes. Both torpedo hatches were mechanically opened through gearing from handwheels operated at flat level. The hull hatches were connected to the superstructure hatches over so that they opened together primarily for escape purposes.

4.15 Ballast Keels

68. Dimensions of the various ballast keels fitted are given in Paragraph 36. The keels were connected to the flat keel by, 5in x 5in x 14.46lb angle bars. A portion of the ballast keel amidships was fitted as a 10 ton drop weight, or drop keel as it was known later, which could be released from inboard.

The ballast keels were made of wood and cast iron blocks according to ballast and trim requirements but it is unlikely that 'ballast boxes' in which to stow portable ballast were included at this early stage. In fact the keels up to D2 were too shallow to allow such boxes to be used effectively.

A statement by Vickers on E3-6 as built gives the ballast keel as 52 tons including a drop weight of 10 tons. The amount of ballast that had to be carried in E1-8 was an embarrassment. This agrees with a statement made by DNC that due to saving in weight from the design figures in E1-8 twin torpedo tubes could be carried forward in E9 onwards on the same displacement. In E9 onwards the weight of the ballast keels dropped by 15-20 tons.

4.16 Minelayers

69. Six E boats were fitted with mine tubes in the saddle tanks. The broadside torpedo tubes were removed. The tubes were open at the top and bottom to the sea and the mines 20inber, stowed in them were released by mechanism operated from inboard. This necessitated considerable structural change.

4.17 Tanks

70. The types of tanks fitted in the D Class and E Class followed generally those in the C Class except for the differences mentioned hereafter. For tests see Para 64.

Presumably to reduce free surface effect middle line watertight bulkheads were fitted in all auxiliary ballast, compensating, oil fuel and lubricating oil tanks in the D Class. The result was a large increase in the number of tanks and in the piping services to the tanks. This practice was a change from the earlier classes and was undoubtedly overdone; it was discontinued in the E Class. As a result however and with the externals there were many sided tanks. In D1, D2 and D4 all the sided tanks were numbered with odd numbers on the starboard side and even numbers on the port side. In D3 and 5-8 external tanks were numbered the same way and this practice continued to the K Class when they were numbered No 1' Port and No 1 Stbd and so on, i.e. the present day practice. The internal tanks in D3 and 5-8 were numbered No 1 Port and No 1 Stbd and so on, but the E Class reverted to the D1 practice, i.e. odd numbers starboard and even numbers port which continued in subsequent classes.

In the early D boats the internal main ballast tanks were numbered A, B, C, D and E. From D3 onwards this was changed to A, B, X and Y presumably to give some indication of forward and after tanks, although in some boats all these tanks were forward of amidships. This however is a principle that continued and was later used for other types of tanks.

Due to the large number of boats built in the E Class including the War Emergency Programme of November 1914, the number of changes that took place during the programmes, the number of builders and the fact that Chatham built to a compromise design for E12 and E13 based on E8 and E9, there were many changes in the arrangements of tanks throughout the class. Other than for the boats built at Vickers which are shown in Appendix IVA, details are not known. Even in the groups shown in the Appendix there were slight changes between E9-11 and E14-16. The former had nine external main tanks of 113 tons capacity and the latter eight tanks of 111 tons capacity. Although cable lockers and Forbes logs were not fitted during building until E17 they would have been fitted retrospectively in earlier boats of the class.

71. Details of tank capacities are given in Appendix IVA and typical arrangements of tanks are shown in:

4.17.1 Main Ballast Tanks

72. In D1 twelve tanks were fitted In the saddle tanks, six each side, with a total capacity of 99 tons. In addition there were five internal tanks all under the battery tanks with a total capacity of 29 tons. They were all fitted with Kingstons hand operated and vents also hand operated. When first completed only 13 tons of the internal ballast could be used.

D2 had only six external main tanks, three each side, with a total capacity of 99.3 tons that is practically the same capacity as the twelve tanks In D1. The principle used in flooding the tanks was completely changed, each tank being flooded from a flooding trunk near amidships each side. This principle is explained in detail in Chapter 23. The capacity of the flooding trunks was 4.23 tons so that the total amount of external main ballast water was 103.5 tons.

Four internal main ballast tanks with a total capacity of 23 tons were fitted as against the five tanks in D1. This was a reduction of nearly 6 tons. There is no doubt that the actual amount of Internal main ballast used in D1 was less than design expectation and this was undoubtedly the reason for reducing the capacity. Even then only 11 tons of internal ballast was needed in D2 on completion.

In D3-8 except D4 the arrangement of tanks was as in D2. The capacity of external decreased by about 6 tons to 97 tons because the diameter of the pressure hull increased by 3in and the maximum beam to the outside of the saddle tanks decreased by 1½in. To make up for this loss the capacity of the internal ballast was increased to 27 tons. In fact on completion these boats required the internal ballast tanks full when diving since the buoyancy had increased. D4 was the odd boat of this group. Flooding trunks were not fitted in the externals. She had six external ballast tanks of 97 tons capacity and four internal ballast tanks of 28.4 tons.

Although tested to 50lb/in2 it appears to have been the practice to close the Kingstons of the internal tanks as soon as they were flooded and never to open them when excessive pressure was likely to come on the battery tanks. It is probable that they were blown only after the boats had surfaced on the external tanks.

73. The E boats followed the general pattern of main ballast tanks as in the D Class except that the flooding trunks were deleted and each tank was flooded direct from the sea through its own Kingston. Details varied between the groups within the class. E1-8 had eight external and five internal main tanks with a total capacity of 141 tons; E9-11 nine external and four internal of total capacity 141 tons; E14-16 eight external and four internal of total capacity 140 tons; whilst E17-26 were as E14-16 but the capacity was reduced to 131 tons.

4.17.2 Auxiliary Ballast Tanks & Compensating Tanks

74. Oil fuel was not yet self-compensating so that oil fuel compensating tanks were required in the D Class although as already mentioned they were not named as such D1 had eight compensating tanks and four auxiliary ballast tanks with a total capacity of 26 tons. The same pattern followed in other boats of the class although in D3-8 one compensating tank is particularly specified as 'Compensating Tank for Anchor Weight' and had a capacity of 0.4 tons.

75. In the E Class oil fuel was self-compensated. Middle line bulkheads in tanks were no longer fitted as a general policy. E1-8 had four compensating tanks with a total capacity of the same order as the eight tanks in the D Class. However, two adjusting tanks were fitted amidships in the externals (although really part of the pressure hull) which were undoubtedly used in conjunction with the broadside torpedo tubes. Only two auxiliary ballast tanks were fitted amidships with a total capacity of 5.25 tons as against 8.25 tons in the D Class. The arrangements in the later boats were very similar, except that the number of compensating tanks was reduced to two or three with reduced capacity.

4.17.3 Oil Fuel Tanks

76. The 29 tons approximately of fuel in the D Class was only just over double that in the C Class yet it was carried in fourteen tanks as against two in the latter. The majority of these tanks were less than two tons in capacity. This must have been most costly in weight and space for servicing piping, etc. This complicated arrangement seems to have been unnecessary since the bulk of the fuel was in two blocks. Water jackets round the tanks were now of course unnecessary with diesel oil.

77. The E Class carried just over 40 tons of oil fuel in much larger tanks with no middle line bulkheads, and the fuel was now self-compensating. E1-8 had six tanks only but this number increased to eight in later boats of the class and they were then named 'oil fuel or compensating tanks'.

4.17.4 WRT & Firing Tanks

78. The pattern in the C Class was followed, that is one WRT and one firing tank for each torpedo tube. The size of the WRT tanks increased; from 0.44 tons per tube in the C Class it grew to an average of 0.69 tons in D1, fell to 0.44 tons again in D3 and rose to about 0.9 tons in the E Class. Firing tanks continued to be fitted in the superstructure but at some time between E8 and E17 they were moved inboard to near the torpedo tubes. A firing tank for the after tube was not fitted after E18. Built-in firing tanks do not appear to have been fitted in any of the classes that followed.

4.17.5 Lubricating Oil Tanks

79. The tank capacity for engine lubricating and cylinder oil greatly Increased from 1.34 tons in the C Class to near 6 tons in D2, in six tanks which seems to be an excessive number of small tanks. In D3-8 two of these tanks were used as 'lubricating oil drain tanks' for the first time and the practice of having these drain tanks continued in the following classes.

4.17.6 Fresh Water Tanks

80. A built-in tank for fresh water was first fitted right aft In D1 with a capacity of 2.64 tons. In D2 it was moved forward and the capacity decreased to 1.73 tons. It fell further to 1.57 tons in D3-8.

E1 had only one tank with a capacity of 2.31 tons (520 gallons) but after E8 the capacity was increased to over 4 tons (900 gallons) in two tanks one each forward and aft, although a statement has been seen that these vessels were limited to carrying 550 gallons. The amount that could be carried In the D Class was certainly limited. In fact at the time fresh water was in a low category. In March 1912 Capt.(S) of HMS Bonadventure suggested that the amount of fresh water in the D Class should be increased. It was stated in reply that 'the amount of freshwater that can be carried depends on the condition of the boat as regards weight when completed'.

4.17.8 Other Tanks

81. Other tanks Introduced for the first time were:

  • Anchor weight-compensating tank in D3-8, called drop-weight-compensating tank in E1-8. This was a small tank under the forward torpedo tubes 0.4 tons capacity in the former and 1.95 tons in the latter. From E9 onwards the dropweight was deleted and a special tank not required.
  • A magazine was fitted for the first time in E1, in the forward torpedo compartment. Although D4 carried a gun it was for experimental purposes and no special magazine appears to have been built in that vessel.
  • Cable lockers were fitted during building for the first time in E17. They were square trunks built into the tanks below the flat port and starboard sides at the after end of the torpedo room with chain pipes to the pressure hull over. They were therefore open to the sea when dived and were tested to 100lb/in2.
  • Forbes log tank. First fitted in E17.

4.17.9 Sounding Tubes

82. About 1911 it was considered desirable to have some method of gauging the amount of water in compensating tanks, internal main tanks and fresh water tanks. Dip rods were suggested and fitted in D4 and D5. They were found most useful and the practice adopted generally.

Fig 4.1
Fig 4.1
Fig 4.2
Fig 4.2
Fig 4.3
Fig 4.3
Fig 4.4
Fig 4.4
Fig 4.5
Fig 4.5
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Chapter 3: The Spindle Hull Types - Holland, A, B and C ClassesChapter 5 - Double-Hull Submarines