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Home - Dits & Bits - BR3043 - Part One - The Individual Classes - Chapter 8

Chapter 8: Fleet Type K and K26 Classes

8.1 Introduction

1. By the spring of 1915, in the effort to obtain a Fleet speed of 20 knots, the Nautilus and the Swordfish were already building and the J Class had just been ordered. The speed likely to be achieved in these vessels was 17, 18 and 19.5 knots respectively. It was then that the demand was again made for submarines to accompany the Fleet; the speed had risen to 24 knots. Reference (11) states 'the K Class were asked for by the Grand Fleet and were not the product of the Submarine Branch'.

8.2 Design

2. In the spring of 1913 when Nautilus was ordered and discussion was going on about the Swordfish design, DNC investigated the design of a steam propelled submarine and ran a model at the Admiralty Experiment Works, Haslar. The design was broadly 33 8ft long by 29ft beam by 11ft draught with a surface displacement of 1700 tons. Geared turbines of 10 000 shp would give a speed of 24 knots. It included four 21-inch bow and four 21-inch broadside torpedo tubes and was of partial double-hull type of construction.

At a conference at the Admiralty held by the Third Sea Lord, it was decided not to proceed further with the DNC design on the grounds of:

  • The great increase in size over existing boats, 1700 tons as against 660 tons of the E boats. It appeared desirable to proceed by easy stages in increasing size and to await the results in Nautilus, 1270 tons, laid down in March 1913 before proceeding to anything larger. It was not of course known at the time of the delay in completing Nautilus and that the displacement would rise to 1440 tons.
  • The use of steam propulsion. Although the French used steam because their heavy oil engines were unsatisfactory, it was only of relatively small power. It was proposed to await the results in Swordfish's geared turbines before proceeding to this much greater power. The power in Swordfish was 4000 shp. Swordfish was ordered in August 1913.
  • The problems of control in diving and when submerged.

These were very good reasons for proceeding with caution and advancing in reasonable steps through Nautilus and Swordfish before taking such a big step both in size of boat and type and power of machinery. Furthermore the double-hull construction was a type about which the RN had no practical experience.

3. In the spring of 1915 this design by DNC was in the background. To obtain the power required of 10000 shp obviously meant steam, since using diesels would have required eight engines of the twelve cylinder type of 1200 shp which were to go into the, J Class. But steam propulsion had only recently been turned down for the J Class.

4. On 15 April 1915, Vickers forwarded a design for a large steam driven submarine. They were building two sets of engines for the J Class and undoubtedly realised the limitations on the surface speed that could be obtained with diesel engines. With the usual Vickers' foresight they tried to anticipate the next step. Their design was 280ft long by 28ft beam by 23ft depth with 2000 tons surface displacement. 14000 shp with a surface speed of 23 knots, 1500 bhp submerged. 284 cells of special type, four 18-inch bow tubes and four 18-inch broadside tubes, one 4-inch and one 12-Pounder gun. This design was circulated and it was then approved to proceed with DNC's 1913 design since it was of smaller displacement than the Vickers' design and offered 24 knots as against 23 knots thus a complete reversal of previous policy.

5. The Admiralty design was developed and Vickers were given the outline particulars, form and general arrangements on 4 May 1915 and were requested to proceed with the detailed drawings. Two submarines K3 and K4 were ordered from the firm in June 1915. In August 1915 K1, K2 and K5 were ordered from Portsmouth Dockyard, K6 and K7 from Devonport Dockyard, K8, K9 and K10 from Vickers, K11 and K12 from Armstrong Whitworth, and K13 and K14 from Fairfield. In February 1916 further orders were placed with K15 at Scotts, K16 at Beardmore and K17 at Vickers.

K18 to K21 were ordered but were subsequently built as the four M Class submarines. K13, which sank on trials in the Gareloch, was, after salvage, completed as K22.

6. Some radical changes were made in the final design from that mentioned in Paragraph 2:

  • One 800-bhp oil driven engine driving a 700-hp dynamo was added.
  • 18-inch torpedo tubes were fitted in lieu of 21-inch tubes. In addition the design included twin 18-inch deck tubes to be used on the surface at night.
  • Two 4-inch guns and one 3-inch HA gun included.

The net result of the changes was that the final design surface displacement came out at about 1850 - 1880 tons. This grew during building to 1980 tons.

8.3 Dimensions

This class may be called the first Fleet Submarines. As a matter of interest one of the early drawings prepared by Vickers is headed 'HM Flotilla Leaders'.

7. The main particulars of the class were:

Length overall 339ft
Maximum beam 26ft 6¾in.
Maximum depth 20ft 11¼in.
Displacement surface 1980 tons
Displacement submerged 2566 tons
Speed surface 2 1 knots
Speed submerged 8+ knots
Bhp surface 10 500
Bhp submerged 1440
Torpedo tubes, bow Four 18-inch
Torpedo tubes, beam Four 18-inch
Torpedo tubes, deck Two 18-inch

A figure of £340 000 for estimated cost has been seen.

A General Arrangement is shown in Plate 28 . An arrangement of WT Compartments is shown in Fig 8.1 and Design Construction Sections In Fig 8.2.


Fig 8.1

Fig 8.2

8. Although called double-hull boats they were not really so, or even partial double-hull, in the sense that has been used for earlier classes i.e. with an outer hull completely surrounding an inner hull. In section they were rather like an early Laubeuf type in which the external hull was mainly abreast the upper half of the pressure hull and the outer hull plating faired into the pressure hull some way below the waterline. Fig 8.2 shows this type of construction. It was the original intention to fit baling flats as shown with controlled free flooding spaces above the flats. This was changed. The baling flats were made non-watertight and the whole of the external hull was used for main ballast water in port and starboard-sided tanks with a divisional bulkhead on the middle line at the top. The change is explained in Paragraph 21. The top of the outer hull formed a good reasonably flat recreation deck but the sides of this outer hull had considerable curvature and were unlike the ship shape form of the double-hull boats already discussed. At the forward end of the externals the top plating was extended to the bows with considerable sheer, the enclosed structure being controlled free flooding. Aft the external plating was extended through controlled free flooding superstructure until it faired in with the pressure hull. As was stated 'the ends of the boat, which might be damaged without endangering the immediate safety of the boat, were only single-hulled at the sides'. In the Laubeuf boats the ends were enclosed in external plating.

The pressure hull was of elliptical form quite fair except for discontinuities at the extreme forward and after ends of the external hull. In general the shape was good to withstand its designed diving depth of 200 feet.

A narrow superstructure was built from the conning tower aft to enclose the funnels, etc. A totally enclosed bridge deckhouse was built over the conning tower as additional protection for the bridge personnel'.

9. The main hull was dictated to a large extent by the main machinery, which occupied 35% of the overall length. The openings for the two funnels and boiler room air intakes presented a big problem. The funnels hinged downwards into the superstructure and two watertight doors were used to close each of the funnel openings. The upper doors hinged over and closed the openings on top of the superstructure funnel uptakes and were clipped on the inside. The lower doors on the pressure hull hinged in similar fashion but had no clips. The lower doors were hemispherical in shape and were made self-adjusting to allow for change of shape clue to temperature. Water circulation for cooling the door seatings was also fitted. The hinging of the funnels and closing of the doors was done simultaneously by an electric motor sited in the turbine room. In K15, K16 and K17 the operation was done by a hydraulic semi-rotary engine, which drove a hydraulic pump connected to hydraulic motors on the hinge-shafts of the covers. A draining valve was fitted at the base of the funnel uptakes on the hull in case the upper funnel cover was damaged.

Scotts had much to do with the design of this funnel operating gear. It started with the Swordfish as explained in Chapter 7 Para 37. In the early boats of the K Class a very similar electric funnel operating gear was used, but for K15 Scotts designed and installed hydraulic type gear, which the firm describe as follows. 'The funnel and top doors (which were connected by gearing) were closed in ten seconds, the lower doors operating in four or five seconds. Quite a novel feature was introduced in connection with the top door in which each of the four clips was operated by a hydraulic cylinder. This arrangement was a great advantage as it enabled the clips to be used at all times with greatest ease, the action being instantaneous. Hydraulic interlocking gear, which was fully proved, was designed to prevent the funnel or top door being moved when the clips were on. Hydraulic funnel gear embodying these improvements was fitted in all the later vessels of the K Class'. A Schematic Arrangement of this gear is shown in Fig 8.3.

8.4 Auxiliary Equipment

10. Air to the boiler room was through four openings about 3ft diameter each. After various schemes had been considered it was decided to fit telemotor operated covers to these intakes. The covers were dome-shaped with lugs on the rim, which slid on vertical bars. To open, covers were raised about 10in by telemotor ram to allow air to enter. The telemotor ram also pressed down on the cover to keep it on its seating when shut. Each cover weighed about 8 cwts and this weight helped the closing operation, which was quick. Hand gear was also fitted. Air supply to the stokehold was by two turbo-fans directly below the air intakes.

11. The control gear for operating the funnels and air intakes was fitted in the turbine room, the boiler room being completely closed off on the order to prepare to dive. Electrical indicators were fitted in the turbine room to show the position of the covers and when they were closed word was passed to the Commanding Officer in the control room. The accident to K13 was caused by the air intakes being left open although word had been passed that they were closed. This showed the necessity of fitting indicators in the control room and this was afterwards done in all boats.

The time specified to close down and secure was 30 seconds.

In French submarines, in which the power had usually been smaller, the funnels telescoped into the pressure hull. Watertight covers then closed the openings, which were reasonably small.

12. Two Yarrow type boilers were fitted in the boiler room, which was entered from the engine room through an air lock. A watertight passage at the side of the boiler room connected the engine room and the beam tube space so that the boiler room could be isolated from the rest of the boat without affecting communications.

13. An interesting point is the introduction of an 800 hp diesel engine of the type fitted in the E Class, driving a 700 hp dynamo which supplied current to the main motors for surface propulsion at about 9-10 knots. It was provided primarily for the interval to get up steam after surfacing. It was also used for propelling the boat just before diving when shutting off steam and unclutching the turbines and could also be used for charging the batteries. Using the dynamo a very considerable endurance could be obtained at 9-10knots and furthermore the turbines could be reserved for the higher speeds. It was at first intended that the diesel engine should drive an independent shaft and propeller but this was discarded as being very inefficient.

14. The Loa was 339ft 0in. The Lbp is generally quoted as 334ft but this is based on a FP at the bottom of the turn of the forefoot and on AP where the LWL cut the hull aft. The actual Lbp was 328ft 6in and the Lph 317ft.

The moulded beam of the pressure hull amidships was 21ft 6in and the moulded depth 19ft 4in. The figures to outside of plating would be about 1in more. The maximum beam and depth over the externals were 26ft 6¾in and 20ft 11¼in respectively. The ¾in docking keel amidships was 1ft 3in deep.

The mean draught as designed with a surface displacement of about 1880 tons was about 16ft. Due to growth and changes the final mean draught was about 17ft 0in at 1980 tons displacement. This gave a freeboard to the main deck of about 5ft 3in and to the bridge deck of about 9ft 9in.

8.5 Displacement and Stability

15. Vickers gives the submerged displacement as 2566 tons and a figure of this order is generally quoted. This does not include the controlled free flooding spaces of 56 tons capacity. It was originally intended to carry 531 tons of main ballast water in the externals and 127 tons in internal tanks. This would have given a surface displacement of about 1908 tons. However, when the reserve oil fuel tanks were introduced as mentioned in Paragraph 21 the internal main ballast capacity decreased to 79 tons. The surface displacement should then be 1956 tons, but whilst agreeing with the figures for tank capacity Vickers gave a surface displacement of 2000 tons which meant that only 35 tons of internal water ballast was used. These results applied to K3-4. A restriction is confirmed by later figures by Vickers, presumably for a later boat, of a surface displacement of 1976 tons which means that 59 tons of internal water ballast was used. It is to be expected that the figures would vary between boats. Furthermore, when the alterations mentioned in Paragraph 29 were made there would have been a saving in weight. A figure of 1980 tons has been given by DNC and this is taken as a good average for the class. The reserve of buoyancy would be 32.5%.

When external emergency oil fuel was carried in Nos 5 and 6 Port and Stbd main tanks the amount of external main ballast water decreased by 113 tons. The surface displacement therefore increased to 2093 tons and the reserve of buoyancy fell to 25.3%. The reserve of buoyancy figures included the controlled free flooding spaces of 56 tons.

16. Figures for stability are given by Vickers for K3-4 as GM 36in and BG 8in and for K8-10 as GM 38in and BG 9.75in. These would be as built figures. When K4 was inclined in November 1918 the BG was 9.75in, which suggests that the upper deck torpedo tubes had been removed, see Para 29. With subsequent changes and additions it is to be expected that the stability would decrease. In 1918 DNC was quoting figures of GM 27in and BG 9in for the class, which were probably when carrying emergency fuel.

The ballast keel in K3-4 is stated by Vickers to have weighed 179 tons and in K8-10 it had increased to 202 tons both figures including a 20-ton drop keel. Vickers gives a later figure for these five vessels as 235 tons including 27 tons of lead and 56 tons of steel billets.

8.6 Speed and Endurance

17. The design figures were:

Speed surface 24 knots at 10 000 shp
Speed submerged 9 knots at 1400 bhp
Endurance surface 960 miles at 24 knots
Endurance submerged 13.5 miles at 9 knots

The above speeds, both surface and submerged, continued to be quoted some years later and were undoubtedly achieved on completion. K3, the first boat completed, achieved a speed of 23.84 knots at 405 rev/min on the measured mile at appreciably greater draught than designed due to some of the tanks being flooded. K9 on trials in April 1917 obtained a mean speed of 23.5 knots with 10 900 shp at 400 rev/min. The boat however was at a mean draught of 18ft 3in over 1ft deeper than the normal draught due to leaky main tank vents. When carrying emergency oil fuel, with increased surface displacement, the speed would undoubtedly drop by perhaps 0.5 knot.

18. One great disadvantage of steam was the greater fuel consumption than diesels and its effect on endurance. 'At full power the turbines used 1.25lb per hp hour as against 0.43lb per hp hour with diesels. At lower speeds the disadvantage in lost endurance is still more marked'. Using these consumption figures and 197 tons of oil fuel the surface endurance at 24 knots would be about 800 miles. With emergency fuel this would increase to about 1200 miles. Using the diesel engine the endurance at 10 knots would be 12700 miles and over 19000 miles with emergency fuel. Some years after completion DNC was quoting the former as 12500 miles.

19. The main motors were rated at 1440 bhp for 1½ hours and 2040 bhp for 20 minutes. At 1400 bhp the submerged speed was just over 8 knots and there is no doubt that 9 knots could have been obtained at a higher rating. Although submerged endurances of 13.5 miles at 9 knots and 83 miles at 1.75 knots were given in 1918, the actual submerged endurance was disappointing. This may have been due to the unreliability of the electric generating plant when required to run for lengthy periods as mentioned in Paragraph 32. Figures given by DEE were 8 miles at 8 knots, 30 miles at 4 knots and 45 miles at 1.5 knots, which are extremely low at the slower speeds.

Figures assessed as reasonable for speed and endurance are given in Appendix IIIB.

8.7 Structure

20. The designed diving depth was 200 feet. The moulded beam of the pressure hull was 21ft 6in as against 15ft in the E Class. The section was elliptical in the former and circular in the latter. However, the frame spacing returned to 18in as in the D Class as against 21in in the E boats. The pressure hull plating was similar to that in the E boats, 25lb undoubtedly the keel strake and 20lb generally reducing to 18lb at the ends. The frames were of 6½in x 3in x 15lb bulb angle bars. The external hull plating was a variation of 20, 18, 16, 10 and 8lb plating.

Although an Admiralty design, frame numbering reverted to the old practice of starting from aft. In K26 the frames were again numbered from forward.

There were eight internal main watertight bulkheads making nine main compartments all tested to 35lb/in. The boiler room and turbine room were the largest compartments of about 240 and 275 tons respectively without deducting the equipment therein. With over 500 tons of buoyancy on the surface even when carrying the emergency oil fuel the subdivision was reasonably good.

The structure was tidy with no discontinuities in the pressure hull except at the ends of the external hull. The externals were full of main ballast tanks tested to 15lb/in2. All other tanks were in the pressure hull and tested to 50lb/in2 other than cable locker 75lb/in2.and periscope wells 10lb/in2. Stern tubes were tested to 100lb/in2.

8.8 Tanks

8.8.1 Main Ballast Tanks

21. The external space between the inner and outer hulls was completely occupied by main ballast tanks. As designed the externals had baling flats each side just above the LWL as in J Class and it was intended to carry main ballast water below the flats with controlled free flooding spaces above. This meant long leads of piping for venting the tanks below the flats and also the need for scoops for the upper tanks. The complication of this arrangement was not very satisfactory so the baling flats were made non-watertight by cutting holes in them. A WT longitudinal was fitted on the middle line between the pressure hull and the upper deck along the whole length of the external hull. This made 10 external main tanks each side with a total capacity of 531 tons. Two telemotor-operated vents were fitted at the top of each tank, one at the forward end and the other at the after end as near the ML as possible. Each tank was fitted with a Kingston. Later, after the completion of one or two boats, the Kingstons were abolished. Holes cut in the bottom of the tanks to get more rapid flooding and therefore diving time, except that the two central tanks each side retained the Kingstons and were arranged to carry oil fuel to increase endurance if desired. They were called emergency oil fuel tanks and could carry 102 tons of fuel of sg 0.93 as against 113 tons of main ballast water. In actual fact they carried 96 tons of oil possibly to keep the oil level above the Kingston valves.

Originally there were eight internal main ballast tanks named A, B, C and D forward and Q, X, Y and Z aft with a total capacity of 127 tons. Three of these tanks were under the battery tanks but two with a capacity of 48 tons were fitted as reserve oil fuel tanks during building. The internal main ballast capacity was therefore 79 tons.

A total space for 658 tons of main ballast water was therefore allocated originally but deducting the reserve oil fuel tanks the final tankage available was 610 tons. When emergency oil fuel was carried the main ballast tankage was reduced to 497 tons. All this space could not be used as mentioned in Paragraph 15. Three controlled free flooding spaces, two in the bows and one right aft had a total capacity of 56.46 tons.

8.8.2 Oil Fuel Tanks

22. 156 tons of fuel was carried in sixteen internal tanks. There were two types of fuel of sg 0.93 and 0.896, only two tanks carrying a total of 16 tons of the latter grade. The fuel was self-compensating. In some of the later boats, e.g. K8-10, No 11-fuel tank of 4.7 tons capacity was converted into a store.

As mentioned in Paragraph 21 two of the internal main ballast tanks were fitted as reserve fuel tanks. These tanks had a capacity for 44.5 tons of fuel of sg 0.93 but Vickers give amounts carried of 40.2 tons in K3-4 and 41, 6 tons in K8-10. The tanks therefore were not completely filled with oil probably for the same reason as mentioned in Paragraph 21 for the emergency oil fuel tanks. The total fuel for the purpose of endurance was approx 197 tons in the early boats reduced in the later vessels, and perhaps retrospectively in the others, to about 192 tons.

Nos 5 and 6 Pt and Stbd main ballast tanks could be used to carry emergency oil fuel. The reserve of buoyancy fell by 7% if these tanks are used as such. They carried 96 tons of fuel of sg 0.93 which meant approximately 50% increase in endurance.

8.8.3 Other Tanks

23. The arrangement of other tanks was generally as in previous classes except as follows:

(a) There were no named compensating tanks, the two tanks shown in Fig 8.1 having been converted into oil settling tanks. All compensating water was carried in eight auxiliary ballast tanks with a total capacity of 56.5 tons.

  • A statical diving tank was not fitted which now seemed to be the policy.
  • Store rooms come more into the picture. They started with a boatswain's store and a provision room and soon an oil fuel tank was converted into a store even though there were complaints later that endurance was low.
  • A drop keel gear tank was fitted for the first time. It was in the turbine room about 50ft aft of amidships and was tested to 50lb/in2.
  • Feed water and reserve feed water tanks, essential with a steam plant, of 23 tons capacity.

8.9 Main Machinery

24. Twin sets of geared steam turbines together developed 10500 shp at 380-400 rev/min. In the trials in K9, mentioned in Paragraph 17, it is stated that 10900 shp was developed at 400 rev/min. Parsons type turbines were fitted in the VickerS Boats and Brown-Curtis type in the remainder. The 2 Yarrow's type boilers worked at a pressure of 235lb/in2.

The 800-hp diesel engine and its duties are mentioned in Para 13. It drove a dynamo to supply current to the main motors and was not connected to either main shaft.

25. Submerged, each shaft was driven by single armature motors in tandem developing 360 bhp each making a total of 1440 bhp. Helical gearing was fitted from the motors to the propeller shafts, instead of the motors being on the shafts as previously. At 20 minutes battery discharge rate, the main motors could develop a total of 2040 bhp.

The battery consisted of 336 Exide cells split into three sections of 112 cells each stowed in two battery tanks well forward of amidships. The working voltage was 220.

8.10 Armament

26. Although 21-inch torpedo tubes were included in the original design to adopt them would have meant an entirely new design of tube. Details of a four 18-inch bow tube arrangement had been worked out for the J Class. For quick building it was necessary to adopt 18-inch tubes with four in the bows and four beam amidships. In the final design, twin 18-inch deck tubes, to be used on the surface at night were included, but were subsequently removed where fitted. A total of sixteen torpedoes were carried.

Originally two 4-inch guns were fitted on the upper deck and one 3-inch HA gun on top of the superstructure. Later on one of the 4-inch guns was removed and the other placed on top of the superstructure with the 3-inch gun. They carried 175 rounds of 4-inch and 100 rounds of 3-inch ammunition. Some of the class were fitted with depth charge throwers for anti-submarine surface work.

8.11 Telemotor System

27. A much more extensive system was fitted than in earlier classes. It included the operation of vent valves periscopes, W/T masts, boiler room ventilator covers and in later boats the funnels and funnel covers.

Telemotor operated vent valves were being fitted in the J Class, Nautilus and Swordfish, but at this time there had been no experience with them in service. Since there were forty vent valves scattered throughout the boat to fit telemotor control from the control room was the obvious thing to do. A system of similar valves in a large floating dock then building for the Admiralty was operated from the deckhouse at one end of the dock by a Westinghouse hydro-pneumatic system. A similar system was fitted in the K Class. The power was from two units, each of a 5 hp motor and a Williams Janney gear (in the VickerS Boats) and a Hele-Shaw pump (in the others).

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

8.12 Operational Features

29. The first boat K3 was laid down by His Majesty the King and was completed at Vickers in September 1916 within 15 months of being ordered. On trials the only difficulty experienced was the very high temperature in the turbine room and this led to the fitting of additional fans which had the desired effect.

Major alterations made subsequent to the commissioning of the first boats of the class were:

  • The upper deck torpedo tubes were removed.
  • The bow superstructure was raised to form a large flared bow to permit the boats to make better headway in heavy weather.
  • Of the two 4-inch guns on the upper deck one was removed and the other raised to the top of the superstructure that is to the same height as the 3-inch gun.

30. Lipscombe states that the K Class by the time they were completed 'never had an opportunity to test themselves as a tactical unit of war with the Fleet. So far as keeping up with the big ships and taking up tactical position they were an unqualified success. When it became clear there would not be an opportunity for another Fleet action such as Jutland some of the class were fitted with depth charges and relegated to the anti-U boat war'.

It is stated that 'the K Class were scrapped because they became too expensive a unit of the Fleet to maintain due to the war-time material used in their construction and also because it was hoped the internal combustion engine would soon give the requisite speed'. Only six of the seventeen boats built were in commission for six years or more, and the maximum time in service was nine years.

31 Some interesting information available from the mid-1920's when the future Fleet submarines were being discussed is a comparison between the operational features of the K Class and K26, based on the Spring Manoeuvres in 1925 when K26 was in company with K2, K12, K14 and K22. In these exercises very bad weather was experienced at times and the exercises were eventually cancelled. In any adverse criticism of the K Class some allowance must be made in that they were all completed in 1917 and were of wartime construction whilst K26 was completed in 1923.

32. The adverse points made about the K Class were:

  • 'They are not sufficiently seaworthy to keep up with the Fleet'. They were then getting old with an average age of 8.1 years and this lack of seaworthiness had not been commented upon during the war when the vessels were new and the bow had been raised. In fact in September 1918 two K Class submarines overtook the Grand Fleet which was hove to in a heavy gale in the North Sea. This did show their seaworthiness but did not prove that they could keep up with the Fleet in moderate weather and at reasonably high speed.
  • However, this criticism was amplified by a statement to the effect that 'the poor sea keeping qualities were due to the small reserve of buoyancy with the normal war stowage of oil fuel'. This must mean that the emergency oil fuel was carried which brought the reserve of buoyancy down to 25%, which however was a reasonable figure even by modern standards.
  • 'The hull structure was too weak causing excessive working in a seaway'.
  • 'Fuel stowage in external tanks was bad, both as to placing and lack of specially built tanks'.
  • 'Bow tube shutter gear weak and unable to withstand heavy bow seas'. This is not really surprising with power for a speed of 24 knots.
  • 'Poor habitability'.
  • 'Unreliability of electric generating plant when required to run for lengthy periods'.
  • Other disadvantages mentioned were:
  • 'They took about 5 minutes to dive.'
  • 'Not so handy underwater as other submarines.'
  • 'Had only 18-inch torpedo tubes.'
  • 'Had comparatively low endurance.'
  • 'The tendency to ship water in the boiler room (presumably through the air vents). The vessels usually operate with a particularly small amount of buoyancy (this presumably on war patrol).'

33. Some interesting points mentioned in the 1925 exercises were:

  • 'The 36ft periscope in K12 rendered depth keeping in a heavy sea much easier than was the case in K2 and K22 fitted with the normal size (approximately 30ft)'. This is as would be expected.
  • 'In calm sea and maximum visibility the invisibility of the K Class when at 6 miles range was most marked. Occasional puffs of smoke and the sun shining upon salt encrusted funnels were the things that principally revealed their presence'.
  • 'K2 had her after funnel wrecked by a heavy sea and could not dive until the wreckage had been cleared'.
  • 'K12 showed the best sea keeping qualities owing to the construction of the fore gun platform and to carrying less fuel, thus being more buoyant'. The statement regarding the fore gun platform suggests that the forward 4-inch gun which was originally on the upper deck had been raised to a superstructure forward of the bridge on the lines of the arrangement in K26.

Some of the points mentioned are of course obvious, as for example the better sea keeping qualities with the greater buoyancy and the longer the periscope the better the depth keeping in a heavy sea.

34. Experience with the K Class resulted in the building of K26. The design of K26 probably started at the beginning of 1918 and at that time many of the points made in Paragraph 32 would have been known by the designers.

8.13 K26 Introduction

35. The design of K26 was based on that of the K Class modified to eliminate or at least reduce known defects in the latter and incorporating one Major change of fitting six 21-inch bow torpedo tubes in lieu of four 18-inch bow tubes. The hull shape and lay-out of main compartments were similar. The main machinery, main motors and batteries and the beam tubes were the same.

8.14 Design

36. In general the layout in K26 was practically the same as in the K Class except in way of the bow tubes and the latter were mainly responsible for an overall Increase in length of 12ft. This increase was made up by changes in the main compartments compared with those in the K Class as follows:

  • From the after bulkhead of the torpedo room to the bow, was increased by 15ft. An additional WT bulkhead was fitted at the after end of the tubes to divide the torpedo room from the tube space.
  • The officers' quarters and control room were reduced by a total of 1½ft and a switchboard compartment included in these spaces.
  • The beam tube space and the boiler room remained the same whilst the turbine room and the engine room were each reduced in length by 1½ft although the same machinery was fitted.
  • The after crew spaces were increased by a total of 3ft.
  • The after main ballast tank was made into a steering compartment and lift in length saved at the after end.

The lay-out of compartments and tanks in K26 is shown in Fig 8.4 and a General Arrangement in Plate 29.


Fig 8.3

Fig 8.4

37. The other main structural changes were in the external hull and superstructure. The same general form of outer hull was adopted but instead of the tops of the external tanks going to a middle line bulkhead as in the K Class they stopped at longitudinal bulkheads port and starboard sides of the middle line. This was probably done because the externals, if taken to the middle line, would have been about 8in. deep at the top. The main deck forward of the bridge was given much more flare to the bow bringing the forecastle 13ft above the LWL as against 9ft originally in the K Class. The forward end of this structure was made into three buoyancy tanks. As mentioned in Paragraph 29 the bow superstructure was raised in the later boats of the K Class.

The main superstructure amidships was increased in height by 3ft to 8ft above the main deck and this gave much better protection to the boiler room air intakes and funnels. This superstructure was also extended forward of the bridge sufficient to mount one of the 4-inch guns. The enclosed bridge was raised the same amount and in addition an open bridge was fitted on top of the closed bridge.

38. The main change in principle in K26 is that most of the oil fuel was carried in the external tanks and much more of the main ballast water carried internally. The K26 arrangement would certainly improve diving time. Two of the complaints about the K Class in Paragraph 32 are fuel stowage in external tanks and time to dive. It would appear that the latter has been given priority in K26.

8.15 Dimensions

39. A comparison of the main characteristics of K26 and the K Class as built are:

K Class K26
Length overall 339ft 351ft
Beam pressure hull 21ft 7in. 21ft gin.
Beam maximum 26ft 6¾in. 28ft 0in.
Depth pressure hull 19ft 5in. 19ft 1in.
Depth maximum 20ft 11¼in. 19ft gin.
Displacement surface, tons * 2093 2140
Displacement submerged, tons 2566 2530
Speed surface, knots * 23-1 2 23 1/2
Speed submerged, knots 8+ 8+
Torpedo tubes, bow Four 18-inch Six 21-inch
Torpedo tubes, beam Four 18-inch Four 18-inch
Guns Two 4-inch Three 4-inch
One 3-inch
* With emergency oil fuel

K26 was laid down in June 1918 and launched at Vickers in August 1919. In 1920 she was towed to Chatham for completion and was completed in June 1923. K27 and 28 were ordered from Vickers at the same time as K26 but were later cancelled and broken up after commencement.

40. The Loa was 351ft 0in. The Lbp is usually given as 347ft but this is the length on some arbitrary WL, which cuts the underside of the stern 4ft from the after end. The actual Lbp was about 339ft and Lph 326ft.

The maximum beam and depth of the pressure hull were 21ft 9in and 19ft 1in respectively and of the external hull 28ft 0in and 19ft 9in. The mean draught was 16ft 10in for a surface displacement of 2140 tons. This gave a freeboard of 4ft 2in to the upper deck and 12ft 2in to the bridge deck. The ballast keel was 15in deep.

8.16 Displacement and Stability

41. After completion of K26, DNC gave figures of displacement as submerged 2530 tons, surface 2140 tons and Washington 1786 tons. From these displacement figures the quantity of main ballast water used was 390 tons. The Washington displacement agrees with the surface displacement by deducting 300 tons of oil fuel and its compensating water of about 31 tons and 23 tons of feed water.

This Washington displacement was quoted for some submarines after the Washington Treaty was ratified in 1923 and until the standard displacement was introduced as a result of the London Treaty in 1930. The standard displacement of K26 was 1710 tons. See Chapter 12, Para 7.

The K Class had 56-1 tons of controlled free flooding space in the superstructure and this was not included in the submerged displacement but was included for the reserve of buoyancy. In K26 the bow was raised from the K Class arrangement and three large buoyancy tanks fitted in the forward superstructure. Their capacity was not included in the submerged displacement of 2530 tons and therefore originally must have been considered as controlled free flooding spaces. In the later half of the 1929 s a change in policy regarding bow buoyancy tanks took place. Although controlled free flooding tanks they were fitted with HP blows and became main ballast tanks and their capacity included in the total main ballast water used and also in the submerged displacement. It is considered that this policy was adopted later on in K26. In CB 1815 (1930) the submerged displacement was given as 2770 tons an increase of 240 tons from the figure previously shown. The surface displacement was given as 2148 tons. Although seemingly rather high this 240 tons must have been the capacity of the bow buoyancy tanks.

Two sets of figures for submerged displacement can therefore be found - as built 2530 tons and later on 2770 tons. In both cases the surface displacement was about 2140 tons and the reserve of buoyancy 29.4%. In Appendix IIB the submerged displacements shown up to and included the L50 class (and including K26) are based on the old policy.

From X1 onwards the capacity of the bow buoyancy tanks is included in the submerged displacement.

8.17 Speed and Endurance

42. The main machinery of K26 was the same as fitted in the K Class and with similar forms speed should be similar. The design surface speed of K26 was only 23.5 knots as against 24 knots in the K Class. There is no doubt that K26 obtained 23.5 knots on trials in 1923. At the time it was suggested that the reduction in speed in K26 was due 'to the after hydroplanes being in the wake of the propellers and to increased draught'. The increase in displacement was the reason. K26 had a displacement of 2140 tons with 300 tons of oil fuel. The 24 knots for the K Class was at 1980 tons displacement carrying the normal load of 197 tons of oil fuel; with emergency fuel the displacement was 2093 tons and the full speed would have been of the order of 23.5 knots.

The surface endurance expected in K26 was of the order of 1200 miles at full speed, and 14500 miles at 10 knots using the diesel engine.

43. The designed submerged full speed was 9 knots at 1440 bhp. Although 9 knots was probably obtained with motors overloaded to 2040 bhp for 20 minutes both the submerged speed and endurance were disappointing as they were in the K Class.

44. In 1930 the operational figures for K26 taken from CB 1815 were:

(a) Surface speed 22.5 knots
(b) Surface endurance 930 miles at 22 knots
12 670 miles at 10 knots
(c) Submerged speed 8 knots
(d) Submerged endurance 8 miles at 8 knots
30 miles at 4 knots
45 miles at 1.5 knots

These figures relate to the vessel in calm water with reasonably clean bottom. The state of the bottom has a considerable affect on speed and with the ship only two months out of dock it is possible for the maximum speed to be reduced by as much as 1 knot. This suggests that the trial speed of 23.5 knots was obtained.

The surface endurance is based on using 95% of 267 tons of oil fuel and is equivalent to 1120 miles with 300 tons of fuel and at 23.5 knots (the trial speed using the same power for 22 knots). The endurance expected was of the order of 1200 miles. The endurance quoted of 12 670 miles at 10 knots would have been that obtained using the diesel engine.

The submerged speed of 8 knots is for the 1 hour rating of the motors. This is much lower than the expected 9 knots at 1440 bhp for 1½ hours.

The submerged endurance at slow speed was disappointing. At medium speed 50 miles at 4 knots had been expected.

8.18 Structure

45. From the displacement figures there can have been little change in hull scantlings from those in the K Class even though the designed diving depth increased from 200 feet to 250 feet.

The pressure hull was slightly more elliptical by 2in increase in beam and 4in decrease in depth. The overall beam of the externals increased by 17in but the depth from the top of the pressure hull to the external plating decreased to 8in from 18in in the K Class. It is reasonable to assume that the plating between the Port and Stbd longitudinals forming the tops of the external tanks would be made portable and not fitted at all within the superstructure.

The design changes, mentioned in Paragraph 36, mean that two additional main watertight bulkheads were fitted making ten in all in K26. The watertight sub-division was good.

46. There were ten main ballast tanks in the externals and fifteen internal tanks. Of the 390 tons total of main ballast water it is estimated that approximately 220 tons was carried externally and 170 tons internally. Although the internal tanks would have Kingstons, the externals were flooded through holes in the bottom of the tanks - a policy that had been adopted in the K Class. Later on RA (S) suggested that scoops should be fitted to these tanks to improve flooding time but it is most unlikely that this was done in K26 although such scoops had been fitted in the M Class.

47. Except for about 25 tons of fuel in four internal tanks at the after end of the turbine room the remaining 275 tons of fuel was carried in twelve tanks in the externals. The total fuel was 300 tons. Four oil fuel-compensating tanks were fitted in the turbine room, no tanks being named as such in the K Class at least originally. There may have been some changes after completion since CB 1815 (1930) gives 267 tons of oil fuel.

8.19 Main Machinery

48. The main machinery, main motors and batteries were as in the K Class although space allocated was a little less, 1½ft having been taken out of both the turbine room and the engine room. A great improvement was the fitting of battery compartments instead of the previous battery tanks with portable battery boards.

8.20 Armament

49. Six 21-inch torpedo bow tubes brought in the six-bow tube arrangement for the first time. A main WT bulkhead, which became standard practice later, separated the torpedo room and the torpedo tube space but space must have been cramped. The torpedo hatch was in the officers quarters aft of the torpedo room. Six spare 21-inch torpedoes were carried.

In addition four 18-inch beam tubes were fitted amidships with eight 18-inch torpedoes. It was later approved for the beam tubes to be removed and this was done about 1929.

Three 4-inch guns were sited on top of the superstructure, which had been raised 3ft from that in the K Class.

8.21 Operational Features

50. The disadvantages of the K Class based on comparison with K26 during the Spring Manoeuvres in 1925 are mentioned in Paragraph 32>. The advantages of K26 were given as:

  • 'Far more rapid submersion'. This had been achieved in the design by removing a proportion of the main ballast water from the externals to internal tanks below the water line.
  • 'The design and height of the casing affords excellent protection to boiler room intakes and better protection to funnels' and 'the tendency to ship water in the boiler room has been overcome. The height of the casing had been increased by 3ft over the K Class and the freeboard to the casing top increased by 2½ - 3ft.
  • 'Stowage of the battery in a battery room is a great advance'.
  • 'Design and position of the forward hydroplanes a great improvement'. As far as is known the only change was that the planes were about 16ft further aft from the bows but with the much larger buoyant bow they would be less susceptible to damage in heavy seas.

The only disadvantage mentioned was a decrease in speed. This was not a fair comparison as explained in Paragraph 42.


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Chapter 7: Double-Hull Overseas Types - Nautilus, Swordfish (1913), G & J ClassesChapter 9: Monitor M Class Types