RN Subs

Chapter 7: Double-Hull Overseas Types - Nautilus, Swordfish (1913), G & J Classes

7.1 Introduction

1. Nautilus was Vickers' reply to the requirements of the 1912 Submarine Committee for an 'Overseas' type submarine given in Chapter 5. The requirements laid down included a double-hull type of submarine with surface displacement about 1000 tons and Fleet speed, at that time, 20 knots. Vickers' design was double-hulled but had a surface displacement of 1270 tons and speed of 17 knots when the design was first submitted and to which building commenced.

There was general disappointment that only 17 knots could be obtained on an even larger displacement than suggested. The problem was the power plant and the power that could be developed on reasonable weight and size. Vickers probably had more experience with heavy oil engines than any other firm in this country but sea experience with submarine diesel engines was quite limited. A13 the first diesel boat had been in commission for only four years with a single engine of 500 hp. D1 with twin 600 hp diesels had been in service for three years and the remainder of the class about one year or less. The power required in Nautilus was 1850 hp per engine. Difficulties had been met with the early engines and it was not unreasonable that the much larger engines to be used in Nautilus were regarded with much misgiving.

The design problem was given to the Fiat Company, through Scotts, but that Company hesitated to guarantee the success of a heavy oil engine of the power required and recommended that we should await trials of similar engines then under construction at Turin. Major Laurenti produced a design with a steam plant, which on a smaller displacement than Nautilus guaranteed 18 knots. This design was developed by Scotts to become the Swordfish.

2. It must have been obvious that to achieve the requirements of the Submarine Commit tee at that particular time meant much development, change in principles and technique and delay. Nautilus was therefore ordered from Vickers on 23 April 1913 but progress was slow and she was not completed until October 1917. The delay in great part was due to the fact that the engines were experimental and the war prevented complete shop trials being carried out. Furthermore work was stopped on her to allow progress on types more urgently required. She was renamed N1 on 5 June 1917.

3. A report of a Committee under Commodore (S) in April 1914 on the Development of British Submarines states:

Nautilus is regarded by submarine officers as an exceedingly interesting experiment and on the results of her trials depend very largely the future of the large submarine in the British Navy. She is a bold experiment and it is hoped to learn a good deal.

Her main features are:

• Large surface buoyancy i.e. about 800 tons on 1270 tons.
• Large freeboard and upper deck giving, it is expected, good sea-keeping qualities.
• Complete double-hull on Laubeuf principle coupled with the watertight superstructure of the Laurenti boats.
• Good accommodation.
• Effective WT subdivision by strong bulkheads.
• Powerful armament.
• Heavy oil engines giving increased surface speed 17 knots, and a very large range of action 5 300 miles.'

All these features were thought to be true in 1914 but some changed before the vessel was completed.

7.2 Nautilus Design

4. Nautilus was a bold experiment with an increase in surface displacement as designed from the 660 tons of the E boats to 1270 tons, in hp of an engine from 800 to 1850 and from a saddle tank type of construction to a double-hull. She was a twin shaft vessel with two Vickers diesel engines each of 1850 hp, two main motors of 500 bhp and 352 Exide cells in two battery tanks. She was fitted with a gun forward of the bridge. The original design carried one bow, 21-inch torpedo tube, but experience early in the war showed the greater efficiency of attack with multiple bow tubes and two 18-inch tubes replaced the one 21-inch tube. In addition there were four beam 18-inch and two stern 18-inch tubes.

Exactly when the change in the bow torpedo tubes took place is not known, but it must have happened after building had commenced and probably after being launched in December 1914. Plate 22 shows the design General Arrangement with one 21-inch bow tube and Plate 24, an as built General Arrangement, with two 18-inch bow tubes. The layout is the same from aft to Frame 128 and for the two tubes the lengthened bow is obviously a compromise modification. The vessel was lengthened by about 15ft, the beam and depth of pressure hull and outer hull remained the same, as did the length of the double-hull. The distribution of main ballast water, controlled free flooding tanks and buoyancy tanks in the double-hull was changed with a considerable effect on surface displacement.

5. Nautilus was laid down in March 1913, although not actually ordered until the next month (a. provisional order appears to have been given in October 1912) and was not completed until the end of 1917. As late as 1918 the original design displacements of surface 1270 tons and submerged 1694 tons ex free flooding tanks were still being quoted and also a figure of 2014 tons for submerged displacement. Later on the figures changed to surface 1430-1440 tons and submerged 2025-2035 tons. During building the weight increased by about 160 tons and this was not all due to the additional bow tube and structure forward. The boat was obviously heavy aft. To overcome this necessitated a revision in principle in the externals from the Laurenti to the Laubeuf type. The controlled free flooding spaces were deleted and a number of buoyancy spaces fitted in the externals aft.

6. Plate 23 shows the original Construction Sections. No 3 longitudinals are watertight; the obvious Laurenti principle of baling flats as in the V Class. The space below was for main ballast water and that above for controlled free flooding water.

Fig 7.1 shows the Arrangement of WT Compartments as built. Forward of the conning tower a watertight longitudinal was fitted on the middle line at the top of and within the double-hull and the main tanks extended right round the hull. Aft of the conning tower the spaces above the 4th longitudinals Port and Stbd were made into buoyancy spaces and the main tanks extended round the hull below the 4th longitudinals. None of the double-hull was controlled free flooding. This arrangement was on the Laubeuf principle, as in the W Class.

The total capacity of the double-hull remained the same when the change was made and a comparison of how the space was used is shown in the following table, excluding a few small tanks, which were similar in both arrangements:

The capacities quoted under (a) are based on known figures given under (b). The total for main ballast and controlled free flood under (a) of 687 tons is correct but the division between spaces has been estimated in the design stage

DNC gave the main ballast capacity as 424 tons, which is confirmed as reasonably correct by other references. But the submerged displacements being quoted at the time suggest that there was about 320 tons of controlled flooding space. This simply could not be carried in the double-hull and it must have been the intention to use some of the superstructure. One Vickers' reference gives main ballast 415 tons and watertight superstructure 314 tons. If these figures were correct the designed reserve of buoyancy would have been 58.5%.

7.3 Dimensions

7. As mentioned in Paragraph 5, it seems obvious that the change had to be made be cause of the added weight from the design figures and hence the need for more buoyancy. In addition the boat was undoubtedly heavy by the stern. There also seemed to be a shortage of space for auxiliary ballast and compensating water. These conclusions are reached because:

• 56 tons of buoyancy was added aft within the double-hull.
• The two aftermost main tanks were converted one into an auxiliary ballast tank and the other into a buoyancy space included in above.
• Four of the after internal oil fuel tanks were converted two for compensating water, one as a spare tank (could be used as an auxiliary ballast tank) and the other as a stowage for air bottles. The lost fuel was placed in tanks abreast the torpedo room amidships, which had been main ballast tanks.

8. The main particulars of Nautilus as modified were:

The estimated cost is given as £203 850 but because of the changes and time actually building this figure was undoubtedly exceeded.

Nautilus was a partial double-hull of the later Laubeuf type for 78% of the length of the boat with the external hull ship shaped and near vertical sided from mid-depth to the top of the external hull, which formed a wide deck structure. A narrow superstructure was fitted on this deck from the bows to aft of the bridge and was non-watertight except in the bows where it was controlled free flooded. The outer hull faired into the pressure hull at the forward and after ends. These were the only points of discontinuity. Bilge keels were fitted.

The pressure hull was not circular as in the Laubeuf designs but followed the Laurenti shape of hull.

9. The Loa was 258ft 4½in the Lbp 251ft 0in and the Lph 257ft, 0in. The latter figure is usually given as the Lbp but this is not correct.

The moulded beam of the inner hull amidships was 20ft 6in. All the pressure hull plating was worked inside the moulded beam, the frames being external and not joggled. The moulded depth of the inner hull was 18ft 0in which shows the out-of-circularity of the amidship sections, which applied generally to all sections.

The extreme beam over the outer hull was 26ft 0in and the extreme depth of hull 22ft 6½in. The ballast keel was 1ft deep, giving an overall depth of 23ft 6½in to underside of ballast keel.

The surface mean draught was of the order of 17ft 9½in. This gave a freeboard of about 5ft 9in to the main deck. 7ft 9in to the top of the gun deck superstructure and 11ft 9in to the bridge deck.

7.4 Displacement and Stability

10. Vickers gave the submerged displacement as 2026 tons. DNC was quoting 2035 tons and this probably included the bow controlled free flooding space. Vickers' figure is taken as built. The tank spaces, which affect displacement, are:

• Ten buoyancy spaces in the outer hull of 56.6 tons capacity. These will have been included in the submerged displacement of 2026 tons mentioned above. They do not have to be taken into account when calculating surface displacement and reserve of buoyancy.
• Eleven external main ballast tanks with a capacity of 584.7 tons.
• Two of the tanks in the above were fitted to carry emergency oil fuel. They were only 92½% filled with oil fuel and the other 7½% water. As main tanks they had a capacity of 121.4 tons.
• One controlled free flooding space of 13 tons in the bow superstructure.

Normally therefore the surface displacement was 1441 tons and the reserve of buoyancy 41.5%. When carrying emergency oil fuel the surface displacement was 1563 tons and the reserve of buoyancy 30.5%.

In the design stage DNC quoted the surface GM as 30in and the submerged BG as 15in. These were probably estimates by Vickers. Later on Vickers gave figures of GM 27in and BG 10.75in which are assumed to be inclined figures.

7.5 Speed and Endurance

11. The design figures were:

The surface endurance was based on using 98 tons of oil fuel without using emergency fuel. Later on Vickers quoted a maximum endurance of 4400 miles and also a submerged speed of 9 knots.

Since Nautilus did not complete until the end of 1917 and had little service, if any, as a submarine it is doubtful whether any reliable figures of speed and endurance in service were obtained. None have been seen. However with the changes that happened during the time building and additions such as hydroplane guards and the increase in displacement the design figures would not have been achieved.

7.6 Structure

12. The designed diving depth is given as 200 feet and compared with previous classes this seems reasonable.

Details of construction are shown in Plate 23. The pressure hull was worked in raised and sunken strakes of 20lb plating decreasing to 17lb at the ends. A reference has been seen that some 22½lb plating was worked in and this was probable in way of the amidships torpedo tubes where the main pressure hull was cut and extended to the outer shell plating. The outer hull of the double-hull portion was worked in raised and sunken strakes of 10lb plating generally with a 25lb keel strake and 20lb garboard strakes. In way of the amidships torpedo room the plating was made thicker with the structure to full diving depth strength.

The framing within the double-hull was a combination of plate frames and bracket frames spaced 21 in apart. The frame spacing forward and aft of the double-hull was 18in with frames forward of 11.53lb Zed bars and aft of web frames alternate with 11.53lb Zed bars.

13. Seven internal main watertight bulkheads were fitted so that the subdivision was reasonably good. The largest compartment was the engine room with a capacity of 286 tons and the next largest the officers' quarters and control room of 131 tons, both making no allowance for the equipment therein. With a reserve of buoyancy of nearly 600 tons it is reasonable to expect the vessel could survive the flooding of any two compartments on the surface.

All main compartments were tested to only 15lb/in² except the two end compartments, which received 30lb/in² presumably to give a better test to the end bulkheads. This was a retrograde step since all portions of the pressure hull, even in the Holland Class had generally been tested to 35lb/in².

7.7 Tanks

14. The tanks inside the pressure hull were generally as in previous classes except that all the main ballast tanks were in the external, which was usual in double-hull boats. Other tanks were in the space between the hulls e.g. for oil fuel, auxiliary ballast and distilled water, but the structure of these tanks was built to pressure hull standards. No mention is made of a statical diving tank. A distilled water tank was built-in for the first time.

Tanks were tested to a mixture of D Class and E Class standards. Variations from E boat standard were:

• Although external oil fuel tanks were tested to 50lb/in², internal tanks were tested to 25lb/in² only. This pressure was applied also to the lubricating oil and drain tanks.
• Compensating tanks and spaces such as magazines, air bottle compartments and fresh water tanks to 25lb/in² instead of 50lb/in².
• Main ballast tanks to 15lb/in² instead of 25lb/in².

The tests applied were more to the standard in the V Class than the E Class and even lower for the main compartments tested to only 15lb/in².

7.7.1 Main Ballast Tanks

15. Eleven tanks with a total capacity of 584.7 tons were in the outer hull, each tank surrounding the pressure hull except in way of the buoyancy spaces. They were individually of large capacity by previous standards, the largest being of 95 tons. This one tank alone was of a capacity little less than the total of the six external tanks in the D Class and of the eight in the E Class. The Odin Class with 300 tons of main ballast water had fifteen tanks. The tanks in Nautilus were therefore very large and furthermore extended well above the waterline. They were fitted with Kingstons at about mid-height in the hull with tail pipes to the bottom of the tanks and the time to flood must have therefore been considerable.

Two of the main ballast tanks of 121.4 tons could be used as emergency oil fuel tanks for 97.6 tons of oil. They were only 92½% filled with fuel and the remaining 7½% with compensating water at sea pressure, the tanks being tested to 15lb/in² only.

The bow portion of the superstructure was a controlled free flooding space of 13 tons capacity.

7.7.2 Buoyancy Spaces

16. Fitted in the top of the externals between Nos 4 longitudinals Port and Stbd, ten in number of 56.6 tons capacity, these tanks have already been mentioned in Paragraph 6. They do not improve the reserve of buoyancy or seaworthiness and must have been a necessity of the design to obtain submerged buoyancy. The firing tanks shown in Plate 22 were converted into buoyancy spaces.

7.7.3 Oil Fuel Tanks

17. 96.3 tons of oil fuel was carried in seven internal tanks. Two of the external main ballast tanks were fitted as emergency oil fuel tanks to carry 97.6 tons of fuel. It is not known under what circumstances these emergency tanks were used, if ever, since the endurance using the internal fuel only was high by submarine standards. The mention of a very large range of 5300 miles by Commodore (S) in paragraph 3(g) is the figure using internal fuel only. The thought may have been to carry emergency fuel on long overseas surface passages.

7.8 Main Machinery

18. Consisted of a twelve cylinder vertical SA Vickers type diesel, developing 1850 bhp at 340 rev/min on each of the two shafts. They were of reversible type. The increase in power from the previous maximum of 800 bhp in the E Class was viewed with misgivings at the time. Full details of these engines are given in Appendix VIA.

The two single armature main motors each developed 500 bhp at 210 rev/min. The battery consisted of 352 Exide cells in two battery tanks. The working voltage was 340 volts in series and 170 volts in parallel.

7.9 Armament

19. As already explained, Nautilus was designed with one 21-inch torpedo bow tube. This was the policy at the time. The early E boats had the one bow tube, as also did the G Class design, which started early in 1914 and was changed later.

The Nautilus eventually had two 18-inch bow tubes which necessitated increasing the overall length of the boat by about 15 ft. in addition she had four amidship beam 18-inch tubes and two stem 18-inch tubes. She carried a spare torpedo for each tube making a total of 16 torpedoes, although Vickers state that only 14 torpedoes were carried as built. It may be that Vickers are quoting the original design figure in error, but if true, it is probable that no spares were carried aft to reduce weight aft for the reason mentioned in Paragraph 7.

One gun was fitted on the superstructure just forward of the bridge. Originally to be a 12-Pounder, it was changed to a 3-inch HA gun as built. It was raised and lowered on a vertical ram and stowed inside the superstructure.

20. Further details of the main machinery and armament and of other equipment are given in Chapters 20-32.

7.10 Subsequent Service

21. It is said that Nautilus was a failure. This may be true in that she had little service and experience as an operating submarine after completion in October 1917 and was used mainly as a Depot Ship for instructional purposes until being taken out of service in July 1919. However the step had been taken from small to large size submarines with a great increase in engine power and this must have provided considerable experience and confidence in building later classes.

7.11 Swordfish (1913) Design

22. As already mentioned, there was general disappointment that in the Nautilus design a speed of only 17 knots could be expected instead of the 20 knots asked for in the 1912 Submarine Committee' s requirements for an Overseas type submarine. Furthermore the design was even then, of nearly 30% greater displacement than had been envisaged. There were also misgivings about the heavy oil engines of 1850 hp being proposed which were much larger than ever before used, especially in view of the difficulties being experienced with the much smaller engines then in service.

23. Although an order for Nautilus was eventually placed in April 1913, the design problem was given through Scotts of Greenock to the Fiat Company and M Laurenti. That Company was wary about using heavy oil engines and hesitated to guarantee the success of such engines of the power required. At the same time Laurenti prepared a design with geared steam turbines having a speed of 18 knots on a surface displacement of 856 tons. This Investigation must have taken place mid-1912 and some notes prepared for DNC in September 1912 include a design named 'Fiat 140 bis' of 856 tons surface displacement and 18 knots speed, which was undoubtedly the one by Laurenti.

This design was criticised by some on the grounds that the radius of action would be reduced and the time for getting under way increased. Regarding the radius of action, Laurenti offered 3500 miles at 10 knots; at the time Vickers were giving 5300 at 11 knots for Nautilus. This criticism was countered by a statement to the effect that:

The high reserve of buoyancy permits of oil fuel being carried in the externals and a few minutes delay in getting underway is not considered a serious disadvantage'. It is interesting that emergency fuel tanks of about 100 tons were included in Nautilus later on, but if the fuel was carried it reduced the reserve of buoyancy from 41.5% to 30.5%. Arguments in favour of steam were 'certainty of running, improved manoeuvring power, less strain on personnel and economy in upkeep

The optimistic remarks in quotes above were in a report by Commodore (S), and were undoubtedly made at the time (1912) in an endeavour to obtain an objective.

24. Laurenti's design was developed by Scotts presumably with advice from the Fiat Company and the Admiralty and became the Swordfish. From the information available it would appear that the main dimensions were kept but the surface displacement increased and the surface endurance fell. Guns were added.

25. Swordfish was ordered from Scotts on the 18 August 1913. Like Nautilus she experienced delays in building and was not completed until July 1916. She was the first steam propelled submarine built for the Royal Navy. A figure for cost of £180 000 was given in 1912 and of £175 000 in 1914. Both are undoubtedly low because of the time taken to build.

7.12 Dimensions

26. Detailed information about the vessel is difficult to find mainly because the records held by Messrs Scotts were destroyed by enemy action in 1941. A Schematic Profile supplied by Scotts is given in Fig 7.2. A General Arrangement drawing is shown in Plate 25.

The main particulars of Swordfish were:

27. Being a Laurenti design it was based on the Laurenti principle of double-hull, which extended for 75% of the overall length of the boat. Watertight flats (baling flats) were fitted between the inner and outer hulls about one-third of the depth of the hull from the top. The portion of the double-hull above the baling flats was controlled free flooding and the portion below used for main ballast, oil fuel and compensating tanks. The outer hull was of ship shape form and the top formed a wide superstructure deck, which was extended to the bow and the stern.

Remarkable features of this design are the many discontinuities in the pressure hull and the shape of the sections as shown in Fig 7.2 and Plate 25.

In way of the beam tubes the pressure hull was extended beyond the outer shell plating and fairing plates fitted to fair the excrescence's into the outer hull, as was done later in the G Class. The keel was cut up and back from the stem to the caps of the bow torpedo tubes. By bringing the bow caps some way aft the bow form was made quite fine. The stern was of wide ducktail form.

28. Other points of interest are:

• Dished main watertight bulkheads. It is stated that these bulkheads were fitted with double doors so arranged that in the event of one compartment being flooded the pressure acted on the door to keep it against the bulkhead. Seven main bulkheads were fitted so subdivision was good. Scotts state that being dished, the bulkheads required no stiffeners. E1-8 were the first British submarines to be fitted with watertight internal main bulkheads. They had two and E9 onwards three such bulkheads. The S Class and Swordfish building at the same time with ten and seven bulkheads respectively emphasize the practice adopted in foreign navies at the time regarding watertight subdivision.
• Dished divisional bulkheads in the externals.
• Built-in passageways at the side of the forward and after battery rooms and the boiler room.
• The conning tower, with a built-in recess to take the lower conning tower hatch when open.
• The shape of the torpedo embarkation hatch. This is as was first fitted in the S Class.
• Divers air connections on the fore side of conning tower - there were three such connections.
• The watertight housings for the guns.

29. The Loa was 231ft 3½in, the Lbp approximately 218ft 6in and the Lph approximately 210ft.

The maximum beam of the vessel was 22ft 11in in way of the amidships torpedo tubes. The plating projected beyond the fair line of the outer hull and was in fact a part of the pressure hull. The maximum beam of the actual outer hull was about 21ft 1in. Fairing plates were fitted. The beam of the pressure hull varied in each main compartment. Approximate figures were 14ft in the control room. 16ft in the boiler room and slightly less in the engine room. Both the beam and depth were kept constant within compartments for most of the main amidship compartments.

The maximum depth of the outer hull was approximately 18ft 6in and depth of ballast keel about 1ft 9in. The depth of the pressure hull varied between main compartments. In the control room it was about 9ft 6in at the middle line with a headroom of 7ft above the floor. In the next compartment, the boiler room, the depth was increased to something like 15 ft.

In the Swordfish the base line was taken as the horizontal through the bottom of the outer hull amidships. The design LWL is 12ft 6in above the base line and the baling flats at the outer hull amidships are about 12ft 9in rising to about 13ft 9in inboard at the pressure hull.

For the 'Fiat 140' design, the draught is given as 14ft 1in with a displacement of 856 tons and a freeboard to the upper deck of 6ft 1½in. The maximum depth of hull was 18ft 6in and the draught is therefore to the underside of a 21in ballast keel. This depth of keel agrees approximately with that shown in Fig 7.2. Scotts give the draught as 14ft 11in above the baseline on a displacement of 932 tons. If this was correct the baling flats would be drowned by over 2ft and the freeboard to the upper deck would be only 3ft 7in. It is considered that the 14ft 11in draught given by Scotts is to the underside of ballast keel. Using a reasonable TPI of 6-7 tons the difference in displacement of 76 tons between the two displacements mentioned above represents about 10in in draught and gives good agreement.

The draught is therefore taken as 14ft 11in to the underside of ballast keel with a displacement of 932 tons. The corresponding freeboards are 5ft 4in to the superstructure deck and about 10ft 6in to the bridge.

7.13 Displacement and Stability

30. Various figures have been quoted for displacement and the following are representative:

At (a) the main tank capacity given by Scotts has been used since this is a figure obtained from quoted tank capacities. In all cases the figure for the free flooding spaces has been calculated from the other columns except that in the Fiat 140 design it was actually quoted.

Of the above figures those by Scotts are the most likely to represent correctly the vessel as built. The value of 173 tons for main tank capacity is as given in Fig 7.2. The surf ace displacement of 932 tons is given by Scotts in association with 'a draught when loaded of 14ft 11in' which is accepted as a measured draught after completion. One of Laurenti's design standards appears to have been that the main ballast carried should not be less than 20% of the surface displacement. On a surface displacement of 856 tons and a main tank capacity of 173 tons it was 20.2% and these were probably the correct figures in the Fiat 140 design. Because of the increase in weight during the development of the design and building the Scotts' figures give 18.5%.

With a surface displacement of 932 tons and a main tank capacity of 173 tons the submerged displacement was 1105 tons.

It is difficult to assess the actual capacity of the controlled free flooding spaces. The capacity of the double-hull below the baling flats was of the order of 300 tons including main ballast, oil fuel and compensating tanks. The capacity of the double-hull above the baling flats, which was controlled free flooding, must have been considerably less than the capacity below the flats. The Scotts' figure of 1475 tons for the maximum displacement when submerged must be the total volume of the vessel to the outside of all plating. The figure of 246 tons of free flooding space mentioned in the table is considered the most reasonable although on the high side. Since the reserve of buoyancy only is affected 246 tons has been taken as the amount of controlled free flooding water, which gives a reserve of buoyancy of about 45%. This was undoubtedly the maximum possible - it could be less.

31. The only figure for stability of Swordfish that has been seen is one given in 1914 and therefore an estimate of a submerged BG of 12in with 10% tolerance. Laurenti in his design gave surface GM 20in, submerged BG 10in and a metacentric height of 8in whilst diving. This last figure is interesting in that it demonstrates the loss of stability likely to occur at some low buoyancy condition when diving or surfacing, a fact that caused trouble in some RN submarines years later.

7.14 Speed and Endurance

32. Particulars of the Fiat 140 design were:

All authorities thereafter quoted the same figures for maximum speeds for Swordfish. The only figures for endurance given were 3000 miles at a cruising speed of 8.5 knots and 60 miles at 6 knots submerged. With the increase in displacement etc these speeds and endurances would not have been achieved in Swordfish on completion.

7.15 Tanks

7.15.1 Main Ballast Tanks

33. There were eight main ballast tanks, six in the double-hull and one forward and one aft, with a total capacity of 172.8 tons. See Fig 7.2.

The whole of the double-hull above the baling flats was used for controlled free flooding spaces with telemotor operated flooding valves and vent valves. The forward 35 ft of the superstructure and 14ft of the bows were ordinary free flooding which from the point of view of seaworthiness is rather surprising. The capacity of the controlled free flooding spaces is taken as 246 tons.

7.15.2 Oil Fuel Tanks

34. Swordfish carried 4186 cu ft or approximately 102 tons (sg 0.88) of oil fuel. It is stated to have been carried in forty-four tanks arranged in ten groups, five each side of the boat, in the double-hull. The fuel was self -compensated. Each fuel tank was fitted with a gauge glass to show the height of the water level. Fuel was passed to an observation tank in the boiler room and all the compensating water valves and oil fuel suction valves were also situated in the boiler room. Further details are given in Chapter 25.

7.15.3 Other Tanks

35. Those shown are one main compensating -tank amidships of 43 tons, forward and after trimming tanks of 13 tons total, and WRT and torpedo compensating tanks. No mention is made in Fig 7.2 of a feed water tank although presumably fitted.

7.16 Main Machinery

36. The Swordfish had twin screws each driven by one set of geared Parsons impulse reaction turbines of a total shaft horse-power of 4000 at a speed of 3500 rev/min. The propeller revolutions were 530 rev/min. A figure of 3250 bhp has been quoted and used in connection with the speed of 18 knots. An astern turbine was incorporated with each of the LP turbines. A Yarrow type boiler of 4551 square feet total surface designed for a working pressure of 250lb/in² supplied the steam through a superheater, which raised the temperature by 100°F.

37. The problems of the funnel and conditions in the machinery spaces when diving had to be tackled in this country for the first time and the following extracts are from a statement prepared by Scotts:

As the Swordfish was the first submarine to be steam propelled, many new problems had to be faced; for example, in connection with the housing of the funnel and the closing up of the ship for diving. Many designs were made embodying ingenious ideas, and from these an arrangement was adopted employing electric gear for the funnel and valve at funnel base, and. hydraulic gear for the top cover. The whole operation of closing down took about one minute and a quarter, which was considered good at that time.

The same problem arose later in the K Class in which vastly improved arrangements were fitted as explained in Chapter 8.

The Swordfish being the first British steam driven submarine there were no available data for guidance as to the temperatures that would be experienced due to the closing down of the machinery spaces of such a vessel. Arrangements therefore were made by Scotts for taking very complete records of the temperatures in and around the engine and boiler rooms under such conditions, and for this purpose there was installed in the vessel a thermograph and thermometers of maximum and minimum type. After a preliminary trial of the machinery had been made in the fitting-out basin, the engine and boiler rooms were closed down with steam on the boiler; the boat however, was not submerged on this occasion. The temperatures were taken at half-hour intervals in all the required positions, the engine room being entered periodically for the purpose.

Data collected in this manner proved that the temperatures did not rise very much after closing down, and were not higher than existed in parts of boiler rooms of torpedo boat destroyers and light cruisers. Further trials at sea with the vessel submerged (which meant that the inner hull was water cooled), proved conclusively that the temperatures met with were little if any higher than with the oil engines which had been invariably fitted in submarines up to this time. It had been anticipated that there would be considerable rise in temperatures in proportion to the time the vessel was closed down, but the records did not bear this out, and while the temperatures increased up to a point they were always within reasonable limits.

In addition to the question of temperatures, another important problem which came up for consideration referred to the quality of air in the boiler room due to closing down and diving with steam up, and advantage was taken of the temperature trials to investigate this matter. It has already been stated that for these trials the boat was floating in the basin with the engine and boiler rooms closed down and steam up. It was necessary to know if it would be safe for a man to enter the boiler room at any time after closing down; it being feared that there was a possibility of harmful vapours arising from drops of oil fuel reaching the hot plates. To determine the point, samples of the air were drawn at intervals throughout the trials from the boiler room by the aid of specially designed connections. These samples were then chemically analysed and the only vapour found was of 6 parts of carbonic acid in 10,000 parts of air. So it was found that even under the worst conditions that could exist, that is, with the boiler room closed down and steam up without the vessel being submerged, the air in the boiler room remained safe for breathing purposes, and no liability of explosion existed when relighting the burners.'

38. Two main motors giving 1400 bhp total, were sited in the engine room. A total of 128 cells were fitted in two battery rooms with 64 cells in each, one forward of the broadside tubes and the other aft of the engine room. These battery rooms had built in passageways at the side to allow communication through the boat without entering the battery compartments.

7.17 Armament

39. Two 21-inch torpedo tubes were one above the other in the lower half of the hull forward. The bow caps were electrically operated. Two spare torpedoes were carried. Swordfish was the first British submarine to have 21-inch torpedoes but not the first in service.

In addition four beam 18-inch tubes were amidships with four spare torpedoes very similar in arrangement to that in the Nautilus.

Two 3-inch guns were fitted, one well forward and the other aft. They were of the disappearing type to present no obstructions when submerged and the gun houses were watertight in order to preserve the guns. Designs of the watertight cover and housing arrangements were got out in conjunction with Messrs Armstrong Whitworth by which the guns were raised by the action of a hydro-pneumatic cylinder and lowered by releasing the air pressure.

7.18 Central Control

40. Scotts claim to have made in Swordfish great strides forward in the central control of the operation of the submarine. Their claims are as follows:

'A new and far-reaching departure from established practice was made when we designed a telemotor system by which valves at a distance could be controlled from a central position. In the Swordfish this system was applied successfully to the superstructure vent and flooding valves.'

'The advantages of central control in connection with submarines was appreciated from the outset and in the Swordfish much time and attention was devoted to this matter. in this vessel the main vent valves were placed in the control room and the main vents were joined to a master vent valve so that, Kingstons being open (the normal condition), all tanks which required to be flooded for diving could be filled by opening this one valve.'

'All the HP air bottle group connections also were brought to a master valve box in the control room, and from this the various connections were tapped off for blowing and other purposes. The blow valves for all the main ballast and compensating tanks were placed in the control room and these tanks could also in emergency be blown from the forward or the after end of the boat. The telemotor valves already referred to were likewise operated from the control room. All the remaining steering and diving gear and other important fittings were in the Swordfish operated from the control room.'

7.19 Safety Arrangements

41. These have been mentioned briefly for the S Class. The whole picture for the S Boats and Swordfish is given below in Messrs Scott's words:

'In the Laurenti type submarines various safety appliances were introduced which had not hitherto been fitted in British submarines. It will be of interest to enumerate some of these devices:

Each boat was fitted with safety buoys, which were capable of being released from the interior of the boat, and, by means of attached lines, arrangements could then be made for raising the vessel.

A telephone buoy was also provided. This buoy was arranged to be released from the control room and carried in addition to the telephone transmitter and receiver, a food pipe through which liquid food could be passed from the surface. In the case of the Swordfish the telephone buoy was further improved by the fitting of an electric lamp to indicate its position in the dark.

The question of safety was taken into consideration when the blowing arrangements were being designed. In these boats the main ballast tanks were arranged to be blown when desired by one valve fitted in the control room. Two similar valves were fitted, one at forward end and one at after end of the boat to enable all the main ballast to be blown from either end should the control room have to be vacated, thereby enabling the boat to reach the surface.

Another safety arrangement provided an emergency fresh air pipe extending through all the habitable spaces. This pipe was always charged with high pressure air which could be released at will into any compartment, thus helping to freshen the air. By these arrangements, incidentally, a damaged compartment which had become flooded might be blown out, should the damage be anywhere except on the top of the hull; the air releasing valve being operated from either side of the bulkheads. Each of the boats was fitted, also, with three divers air connections so designed and arranged that they were independent of any action on the part of the crew.'

Further details of the main machinery and armament and of other equipment are given in Chapters 20-32.

7.20 Subsequent Service

42. Swordfish had a very short life as a submarine. She was commissioned on 28 April 1916 as a tender to Dolphin and renamed S1. The first S1 of the S Class had been sold to the Italian Government in July 1915. Building at Scotts completed on 21 July 1916. She was taken in hand at Portsmouth on 27 June 1917 for conversion to a Patrol Boat and reverted to her old name of Swordfish on 5 August 1917 for service as a surface Patrol Boat. Commissioned as such on 10 August 1917 as a tender to Victory she completed tier conversion on 24 January 1918 and was paid off for sale on 30 October 1918. She was sold in July 1922.

7.21 G Class

43. A conference was held in the First Lord's Room at the Admiralty on 9 December 1913, at which the German submarine programme was discussed. It was found that the Germans had devoted nearly the whole of the money available for submarine building to overseas type vessels of double-hull type of approximately E boat displacement. It was decided that the Admiralty should prepare a design of an overseas patrol boat of about E boat surface displacement of partial double-hull construction with single 21-inch torpedo tubes forward and aft and two 18-inch tubes amidships. It was named the G Class.

The G Class is rightly included in this chapter as an Overseas double-hull type but in the design no attempt was made to obtain the speed of 20 knots asked for in the Nautilus, Swordfish and J Class so that they are not comparable.

44. In June 1914 five G Class submarines G1-5 were ordered from Chatham Dockyard. They were to be fitted with Vickers engines of the E Class type. Tenders were also called for from outside firms and the type of engine was left to the builder to propose, the purpose being to encourage the building of types other than the Vickers engine which had been fitted in all previouS Boats. As a result of the tenders Armstrong Whitworth obtained an order in July 1914 for two submarines, G6-7, one to be fitted with Nuremberg engines (MAN) and the other Sultzer engines. Because of the difficulty of obtaining Sultzer engines when the war started and the impossibility of a MAN design, Vickers type engines were subsequently fitted in these two boats. Scotts were given one boat, G14, to be fitted with a Fiat engine and Samuel White one boat to be fitted with their type of MAN engine. The latter boat was subsequently cancelled.

On 24 November 1914 Vickers were given an order for six boats, G8-13. It was decided that the building of these vessels was not to interfere with the building of the additional E boats (six in number) ordered earlier that month. Before the war started Vickers had obtained an order for the engines for four of the G boats to be built at Chatham. They were the same engines as fitted in the E Class and good progress had been made on them, so they were fitted in the lead ships of the Vickers E Class ordered in November 1914. This accounts to some extent for the short time in which E19-22 were built, that is 8, 9, 11 and 12 months respectively after order.

7.22 Design

45. The original design contained a single 21-inch tube forward but experience early in the war showed the greater efficiency of attack with multiple bow tubes. Two 18-inch tubes replaced the one 21-inch tube. The bow form had to be changed to do this. The armament of torpedo tubes was therefore changed to two 18-inch bow, two 18-inch beam amidships and one 21-inch aft. This was the beginning of the 21-inch torpedo in RN submarines at sea, although Swordfish with two 21-inch bow tubes had been ordered a year earlier.

46. In general the characteristics of the G Class resembled the E Class except that it was a partial double-hull type. The overall length and the beam of the pressure hull were slightly more and the internal watertight subdivision was improved. The main engines and main motors were generally the same and although fewer cells were carried they were of greater capacity.

The main particulars were:

*The speeds given in the above table are design figures. Due to changes and in particular the fitting of hydroplanes guards the surface speed was reduced by about 1.25 knots and the submerged speed by probably as much as 1 knot in both classes.

An estimated cost of £125 000 was given in 1914. A General Arrangement drawing is shown in Plate 26.

47. They were twin-shafted vessels with two eight-cylinder diesels giving 800 bhp each and two single armature motors of 420 bhp each. They carried 200 cells in two battery tanks. One 3-inch QF gun was fitted just forward of the bridge and one 2-Pounder at the after end of the bridge.

As designed it was intended to fit housing type hydroplanes forward and aft to reduce resistance but the serious troubles experienced in the S Class with this type of hydroplane caused the drowned type to be fitted eventually with a consequent loss of surface speed.

48. Of partial double-hull construction on the Fiat-Laurenti principle, the double-hull formed the middle 55% of the length of the boat. Horizontal WT flats were worked in the externals, port and starboard sides, about 3ft 6in below the datum and formed the top of the main ballast tanks. The portion above the WT flats was divided into three controlled free flooding spaces. The top of the outer hull was extended forward to the bows and although part of the space was controlled free flooding it was mainly open to the sea. The, outer hull was also extended 2ft for about 10 ft in a non-WT structure until it faired in with the pressure hull. All this provided a wide deck over 4 ft above the waterline and an excellent recreation space. A narrow superstructure was fitted on the middle line above this deck to take the conning tower, guns, exhaust tank, etc.

The outer hull was of ship shape form with the mid-depth of sections keeping more or less horizontal to the bows but rising at the stern. The sections at the bow and the stern were elliptical about a vertical axis.

49. A feature of this design is the number of discontinuities in the pressure hull. Within the double-hull the upper half of the pressure hull appears to be fair throughout its length except for the necessary change in way of the beam tubes but the overall depth is kept constant within various sections changing from one section to the next with definite discontinuities. This was a compromise between the V2-4 arrangement with the pressure hull completely fair in beam and depth and the Swordfish with discontinuities in both beam and depth. The V2-4 design was undoubtedly the best from the point of view of strength of hull. However the discontinuities my have been accepted because of the strength of the framing within the double-hull and as the most economical in weight and space to keep within an approved displacement.

7.23 Dimensions

50. The Loa was 187ft 1in. Vickers and DNC gave the Lbp as 185ft, but this is not correct by the standard adopted. The Lbp was about 178ft 0in and the Lph 174ft 9in.

As found in the E Class the minimum beam required in way of the broadside tubes was 22ft 8in. Presumably because of the ship shape form and to save weight, the maximum beam of the outer hull in the G Class was 19ft 1in. In consequence the tube space extended beyond the outer hull and was faired into it by a form of 'blister'. This form of construction was being adopted at the time in Swordfish. The maximum beam over these blisters was 22ft 8in. This was also of course the beam of the expanded pressure hull in way of the tubes but the beam of the true pressure hull at this point was 15ft 3½in moulded.

The maximum overall depth amidships was 16ft 7in. The amidship depth of the pressure hull was 12ft 0in moulded. The ballast keel was 1ft deep so that the overall depth to the underside of ballast keel was 17ft 7in.

The mean surface draught to the underside of the ballast keel was 13ft 4in which gave a freeboard to the top of the outer hull deck of 4ft 3in and to the bridge deck of about 12ft. The baling flats in the externals were about 8in above the normal surface waterline.

7.24 Displacement and Stability

51. Figures for submerged displacement varying between 837 and 1026 tons are quoted. There does however seem to be general agreement that the surface displacement was near 700 tons. The differences in submerged displacement are due to the fact that various amounts of free flooding space have been included.

The most reasonable figures are by Vickers of submerged displacement 837 tons, surface displacement 703 tons and the reserve of buoyancy 36.6%. These figures are coupled with 122.5 tons of main ballast water in external tanks, 11.43 tons in internal tanks, i.e. a total of 134 tons of main ballast water. With controlled free flooding spaces of 123.7 tons the overall displacement of the outer hull would be 961 tons. These figures represent the boats as first built and before the modification that follows had been made.

52. A statement has been seen to the effect that the boats did well on service but as a trial 'one boat had the bows raised and a greater amount of surface buoyancy added forward. This improved sea-going qualities so much that all the class will be modified.' This modification would appear to be justified since the first 10ft of the boat from the stem to the foremost bulkhead was non-watertight, the foremost 32ft of the superstructure was open to the sea and the pressure hull forward was only about 1ft above the waterline.

A displacement calculation made by Vickers states that the displacement of the outer hull was 970.6 tons and that the free flooding portion 'above the scoops, was 120.6 tons. Vickers undoubtedly made this calculation in connection with the above modification. The displacement of the outer hull had increased by 10 tons. The controlled free flooding portion had remained unchanged (see below) and therefore a 10 ton watertight buoyancy chamber had been built into the bow superstructure. The displacement became submerged 847 tons and surface 713 tons and the reserve of buoyancy 36.1%.

It will be noted that the portion of free flooding tanks 'above the scoops' is mentioned which shows that 3.0 tons of water would remain in these tanks when on the surface. A certain quantity of water will always remain in main tanks when blown and in controlled free flooding tanks when surfaced but this has so far been ignored In assessing surface displacement and reserve of buoyancy in all previous classes. The difference is of course marginal. The effect on the figures given in Paragraph 51 would be to increase the surface displacement by 3 tons and decrease the reserve of buoyancy by about 12%.

53. No inclining experiments have been sighted but figures for surface GM and submerged BG of 24in and 8.5in respectively were given by Vickers either designed or as built and of 26in and 9in by DNC in 1918 which should be inclined figures.

7.25 Speed and Endurance

54. The designed surface speed was 15.5 knots. Due to the change in the form of the bow to take two 18-inch torpedo tubes instead of one 21-inch tube and the fitting of drowned hydroplanes and their heavy guards the actual measured mile speed was only 14 knots. Apparently only one boat carried out an official speed trial on a measured mile on completion.

Although the submerged speed was quoted early in 1914 as 9.5 knots, thereafter it was given as 10 knots by most authorities and was undoubtedly the design speed. After the war 9.5 knots was generally used although at one time CB 1815 again gave 10 knots whilst at the same time giving the surface full speed at its true value of 14 knots. It is accepted therefore that 9.5 knots was achieved probably with overload on the motors. With the introduction of hydroplane guards etc after the design had been completed it is to be expected that a loss in submerged speed occurred of the order of 1 knot at normal full speed.

55. The design surface endurance was 2600 miles at 12.5 knots and submerged endurance 99 miles at 3 knots. By 1918, at full speed, a surface endurance of 1900 miles was shown.

Vickers gave the estimated fuel consumption figures as their boats left Barrow as 0.5lb per bhp hour at full speed and 0.6lb per bhp hour at cruising speed. Using 95% of 44 tons of oil fuel the endurance at full speed would be of the order of 1800 miles at 15.5 knots or 1650 miles if only 14 knots was achieved at full power. It seems reasonable to expect no more than about 1650 miles at full speed in service although CB 1815 gave 1890 miles at seagoing full speed. At 10 knots and using a consumption figure of 0.6lb per bhp hour the endurance is of the order of 3160 miles. DNC quoted 3800 miles at economical speed, which was probably lower than 10 knots. Reasonable figures for submerged endurance appear to be of the order of 10 miles at 9 knots and 95 miles at 3 knots.

7.26 Structure

56. Although stated to have a diving depth of 200 feet being a pre-1914 war design it probably followed the E Class pattern of being designed for 100 or 150 feet and classified 200 feet after the war started. The pressure hull was of 20, 18 and 16lb plating very similar to the E Class on a similar diameter of hull and the same frame spacing although the hull was not circular as it was in the E Class. The outer hull was of 14. 12, 10 and 8lb plating except in way of those tanks tested to 50lb/in², which would be built of thicker plating.

The framing was on the same lines as in the previous double-hull boats with plate frames amidships. Internal frames fitted outside the double-hull were 15lb channel bars probably decreasing towards the end

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