Home - Dits & Bits - BR3043 - Part One - Chapter 14

Chapter 14: Overseas Patrol Types - Oberon, Odin, Parthian and Rainbow Classes

14.1 Oberon Class

1. The L Class and L50 Classes had been designed in 1915/16 and their surface endurance was low by 1920 standards for overseas requirements. In May 1922 it was decided to develop a submarine to be known as the Overseas Patrol Type. The main requirements were:

  • Surface displacement about 1200 tons.
  • Maximum speed 15 knots with a cruising speed of 12 knots.
  • Endurance 10 000 miles at economical speed.
  • Six 21-inch bow torpedo tubes, and one 4-inch or 4. 7-inch gun.
  • W/T range of 500 miles minimum or 1000 miles If possible.
  • Habitability for operation in the tropics.

Additional requirements of importance made later were:

  • Two 21-inch stern torpedo tubes.
  • Control room to be silent with auxiliary machinery fitted elsewhere.
  • Rapid flooding and blowing tanks forward and aft for quick trimming.
  • Fewer and larger air bottles with the HP pressure increased from 2500 to 3500lb/in2
  • Bow torpedo tube shutters to be omitted.
  • Torpedo tubes to be strengthened to resist pressure due to deep diving.

2. From these requirements the design for Oberon was developed and included in the 1924 Programme. She was laid down at Chatham Dockyard on 22 March 1924 and completed on 24 August 1927.

The main advances In the Oberon over the L Class were much greater endurance, increased diving depth, improved torpedo armament and increased wireless range. This was obtained on about 75% increase in displacement but with the penalty of loss in speed both on the surface and submerged.

Vickers built Oxley and Otway for the Royal Australian Navy laying down about 18 months after Oberon but completing at about the same time. These two vessels were transferred to the RN in 1931. Oxley and Otway are usually classed with Oberon but there were certain differences, which are mentioned hereafter where known.


14.2 Design

3. The main particulars of Oberon as built were:

Length overall 269ft 8in.
Beam maximum 27ft 11½in.
Displacement surface 1598 tons
Displacement submerged 1831 tons
Speed surface, knots 13.75 (15 as designed)
Speed submerged, knots 7-8 (9 as designed)
Endurance surface, miles 11400 at 8 knots
Torpedo tubes, bow Six 21-inch
Torpedo tubes, stern Two 21-inch
Gun One 4-inch

A General Arrangement of Oxley is shown in Plate 37.

4. Oberon was of saddle tank construction but with the external tank plating extending much higher than in the L Class to port and starboard longitudinals at the superstructure sides. The pressure hull amidships was slightly elliptical. The stern arrangement of rudders, hydroplanes, etc followed the K26 pattern.

Oberon was the first vessel to be designed for 500 feet diving depth and was tested to 200 feet.

5. As designed the surface displacement of Oberon was 1480 tons; as built the displacement had increased to 1598 tons. This increase of 118 tons was due mainly to an additional 74 tons of oil fuel.

A comparison of the weights as designed and as built is as follows:

As Designed
As Built
Hull and equipment 794 805
Main engines 129 131
Main motors 35 38
Battery and cooling plant 154 148
Armament 58 69
Ballast keel 132 122
  1302 1313
Oil fuel 112 186
Lubricating oil 19 19
Compensating and trimming water 26 58
Fresh water 2100 22
Surface displacement 1480 1598
External main ballast 288.5 233
Forward internal main ballast 15.1  
Main ballast in superstructure 21.2  
Submerged displacement 1804.8 1831

In the table above the design item 'forward internal main ballast, refers to No 1 main ballast tank forward which was external to the pressure hull and the item main ballast in superstructure' to the bow buoyancy tank. Both of these tanks are included in the total main ballast water as built.

The surface weight in Oxley and Otway as built was 1636 tons, an increase of 38 tons on Oberon. The increase was made up mainly by an addition of 19 tons in hull and equipment, 9 tons in oil fuel and 16 tons in the ballast keel. There was also an increase of 19 tons in fresh water but a decrease of 28 tons in the amount of compensating and trimming water.

14.3 Dimensions

6. In Oberon the Loa was 269ft 8in, the Lbp 260ft 0in and the Lph approximately 251ft. The pressure hull amidships was slightly elliptical with a moulded beam of 16ft 0in and depth 16ft 6in. The plating was ⅞ in thick and the maximum beam of pressure hull 16ft 1¾in. The overall beam of the externals was 27ft 11½in. The moulded depth of pressure hull was 16ft 6in and DNC gave the overall depth as 16ft 8⅝in; this means at 35lb keel and that the top strake of plating was a raised strake. The depth of the ballast keel is taken as 15in as in the Odin Class.

The design draught was 14ft 9in at 1480 tons surface displacement and as built 15ft 6in at 1598 tons displacement. At the standard displacement of 1311 tons it was 13ft 8in and in the 1930's was the draught usually quoted in official publications. At 15ft 6in draught the approximate freeboard was to the superstructure deck 7ft 6in to the gun deck 12ft and to the bridge 13ft.

7. In Oxley and Otway, Vickers made some changes from the Oberon. The length of the pressure hull was increased by 4ft 6in by adding 1ft to the C02 compartment, 1ft to the magazine and 2ft 6in to the motor room. The Loa was 275ft 0in the Lbp 264ft 6in and Lph 255ft 6in.

The maximum beam of the pressure hull was 16ft 13 in as in Oberon but the breadth over externals decreased by 4in to 27ft 7⅝in.

DNC gave the overall depth as 16ft 7½in at the same time as quoting 16ft 8⅝in for Oberon which suggests that the moulded depth of pressure hull in the former was reduced to about 16ft 5in.

The surface draught was 15ft 8½in at 1636 tons displacement. The changes in form in Oxley and Otway increased the submerged buoyancy by nearly 40 tons.

14.4 Displacement and Stability

8. The design figures for Oberon were submerged displacement 1805 tons and surface displacement 1480 tons. This was when carrying 112 tons of oil fuel which was called the normal stowage, but even at this stage some main ballast tanks must have been earmarked to carry reserve or emergency oil fuel to obtain the legend endurance given in Paragraph 13.

9. On completion the displacement figures became submerged 1831 tons and surface 1598 tons carrying a total oil fuel of 186 tons. This became the normal surface condition. The difference between these two displacements of 233 tons of main ballast water includes the bow buoyancy tank of 13 tons. The reserve of buoyancy was 14.6%. The surface displacement of 1598 tons was with main tanks completely empty. Coupled with these figures DNC gave a surface displacement of 1615 tons as inclined. This means that when the main tanks were blown under ideal conditions for an inclining experiment 17 tons of water remained in the main tanks. In service, therefore, a displacement lighter than 1615 tons could not be obtained and was usually exceeded. For the purposes of comparison between designs the surface displacement quoted in Appendix II is always the submerged displacement less the total main ballast water and does not allow for water which cannot be blown from the tanks.

10. The standard displacement using the as built figures given in Paragraph 5 of 186 tons of oil fuel, 19 tons of lubricating oil, 58 tons of compensating and trimming water and 22 tons of fresh water, i.e. a total of 285 tons of liquids works out at 1313 tons. The actual standard displacement given for Oberon was 1311 tons.

11. Corresponding figures for Oxley and Otway were submerged displacement 1872 tons, surface displacement 1636 tons with main tanks empty and 1668 tons as inclined, reserve of buoyancy 14.4% and standard displacement 1354 tons.

12. All the above figures for displacement were given by DNC at the same time and after the vessels had been inclined. He also gave the corresponding stability for Oberon as GM 14.75in and BG 8.1in and for Oxley and Otway GM 18in and BG 10.6in. The higher figures in the latter were due mainly to changes in the form, an increase of 16 tons in permanent ballast and a different disposition of liquids.

14.5 Speed and Endurance

13. The legend figures for Oberon were:

  • Speed surface 15 knots at 2700 bhp
  • Speed submerged 9 knots at 1300 bhp
  • Endurance surface 5000 miles at full speed 12 000 miles at 8 knots
  • Endurance submerged 60 miles at 4 knots.

The surface speed and endurance were based on the design displacement of 1480 tons.

14. On trials the surface speed was most disappointing, only 13.74 knots being obtained at 2950 bhp with the increased displacement of 1598 tons. In Oxley and Otway of perhaps slightly improved form a speed of 15.19 knots was obtained at 3170 bhp and a displacement of 1636 tons. Vickers must have made efforts, at considerable cost, to fair the hull and appendages to achieve this speed, probably based on the knowledge of this sort of work in the L50 Class submarines built for Yugoslavia. The legend speed for Oxley and Otway had been 15.5 knots at 3000 bhp.

No records have been seen to suggest that any modifications were carried out in Oberon in an effort to improve the speed and in fact in 1938 the speed was still given as 13.74 knots.

15. Although the design submerged speed was 9 knots some Haslar model results indicated that less than 8 knots at 1300 bhp could be expected. The motors were eventually rated at 1350 bhp. On submerged speed trials Oberon obtained 6.89 knots at 1250 bhp and 294 rev/min which is equivalent to about 7.1 knots at 1350 bhp. On this trial the 1250 bhp was developed at 1650 amps. The battery discharge at the 1 hour rate was 1950 amps, which might give a speed of 7.5 knots.

For Oxley and Otway Vickers quoted 8.5 knots and taking into account the surface speed in these vessels this may have been reasonable.

In the 1930's most authorities continued to quote 9 knots for all vessels and CB 1815 (1930) gave the operational submerged speed of Oxley and Otway as 9 knots for 1 hour 45 minutes.

During the building of Oberon there was a growth in topside fittings not envisaged earlier. These had a devastating effect on submerged speed. The same thing was happening in the later classes building and modifications were made as explained later for the Odin Class. It is to be expected that some of these modifications were made in Oberon after completion, but even then a submerged speed of 9 knots for Oberon seems impossible.

16. The legend surface endurance figure of 12 000 miles at 8 knots should have been based on 112 tons of oil fuel. After completion DNC gave 11 400 miles at 8 knots for Oberon with 186 tons of oil fuel and 12 300 miles at 8 knots for Oxley and Otway with 195 tons of fuel. Vickers gave 14 000 miles at 8 knots on 195 tons of fuel for Oxley and Otway. In CB 1815 (1930) the operational endurance from trial records for Oberon with 180 tons of fuel was 4570 miles at 13.7 knots and 6800 miles at 10 knots and for Oxley and Otway with 201 tons of fuel 8450 miles at 10 knots. By 1938 the fuel had decreased to 160 tons in Oberon and 166 tons in Oxley and Otway. See Para 19. The corresponding endurance in Oberon was 4500 miles at full speed (13.74 knots) and 10 800 miles economical. These would be trial conditions. The operational endurance in fine weather and moderately clean bottom was 4330 miles at 13 knots and 8150 miles at 8 knots. These official figures are difficult to compare because of the varying amounts of oil fuel given over the years.

17. Other than the legend figure for submerged endurance of 60 miles at 4 knots the only figure seen is an operational one of 9 knots for 1 hour 45 minutes in Oxley and Otway and although given in CB 1815 must be considered optimistic.

14.6 Tanks

14.6.1 Main Ballast Tanks

18. All the main ballast water in Oberon was in the external saddle tanks except for No 1 main tank in the bows of 26.6 tons and a bow buoyancy tank of 13.1 tons. Including the latter the total main ballast water was originally 325 tons. When some of the main tanks were used for oil fuel the total capacity was 233 tons.

In Oxley and Otway the total capacity was 236 tons.

14.6.2 Oil Fuel

19. As designed 112 tons of oil fuel was to be carried in Oberon with 7 tons in two ready use tanks in the engine room and 105 tons in the externals above the main ballast tanks. Some of the main ballast tanks were earmarked as reserve oil fuel tanks to bring the total oil fuel to 186 tons. This became the normal stowage.

At the time Oberon completed oil fuel of sg 0.871 was being used and tank capacities were calculated on that basis. In the mid- 1930's the specific gravity of oil fuel was taken as 0.842 and capacities adjusted bringing the total in Oberon to 180 tons. By 19:38 for some reason this had reduced to 160 tons. In Oxley and Otway a corresponding reduction in fuel took place. Starting off with 195 tons this had reduced to 166 tons by 1938.

Trouble was experienced with leaks of oil fuel from these external tanks starting at about the same time as in X1.

14.6.3 Other Tanks

20. Named tanks followed the normal practice except as follows:

  • An important change from previous practice was the use of auxiliary ballast tanks in lieu of forward and after trimming tanks, which were deleted thereafter. Auxiliary ballast tanks A and B were fitted forward and X and Y tanks aft. Each tank was fitted with a telemotor operated vent and Kingston which could be opened down to maximum diving depth, and could be blown direct from the HP line. In principle, A and Z were designed for quick blowing and B and Y for quick flooding. The final policy adopted is explained in Chapter 23 Paragraph 20. The total capacity of these four tanks was 41 tons.
  • Two compensating tanks H forward and X aft were fitted inboard and oil fuel compensating tanks called O compensating tanks fitted port and starboard sides amidships in the externals. The amount of compensating tank space in Oberon was excessive and H and X tanks were deleted in the Odin Class, which followed. In Oxley and Otway the amount of compensating trimming water carried was 28 tons less than in Oberon.
  • Originally three fresh water tanks with a total capacity of 20.5 tons were provided and distilled water tanks fitted in the battery spaces. A distilling plant worked off the main engine exhaust but it was found that this caused objectionable heat in the engine room. DNC suggested that if this plant was removed sufficient fresh and distilled water could be obtained by fitting additional distilled water tanks in the battery spaces and dividing the 0 compensating tanks so that part could be used for fresh water. Although the latter was done in the Odin Class it is uncertain whether this modification was made in Oberon. It is however possible that the distiller was removed, because in 1935 the original No 4 store aft was converted into a fresh water tank to bring the total stowage to 29 tons. It would appear that additional fresh water tanks were fitted in the externals in Oxley and Otway since the fresh water stowage rose to 41 tons but the fresh water in the externals would have to be discharged overboard when diving in fresh water at the beginning of a patrol.
  • Two innovations were a torpedo warhead stowage compartment and a bath drain tank of 1 ton capacity.

14.7 Main Machinery

21. The two Admiralty designed six cylinder diesels in Oberon developed 2700 bhp at 400 rev/min and were made by Chatham. Later on the engines were uprated to 2950 bhp at full power.

Vickers redesigned the engines for Oxley and Otway by increasing the bore and stroke of the cylinders and they developed 3000 bhp at 400 rev/min. On full speed trials 3170 bhp was obtained.

22. The two twin armature main motors developed a total of 1300 bhp at 300 rev/min. They were eventually rated to give 1350 bhp but bursts of higher power could be achieved. A big advance was the introduction of closed type motors.

The battery of 336 cells was fitted in three battery compartments with 112 cells in each. Cooling plant was fitted.

14.8 Armament

23. The torpedo armament consisted of six 21 inch bow tubes and two 21 inch stern tubes with 16 torpedoes.

One 4-inch gun was mounted as built and it was intended that it should eventually be replaced by a 4.7 inch gun. This was never implemented since the policy changed and the 4-inch gun was adopted as the standard gun for submarines. They carried 120 rounds of ammunition.

14.9 Odin, Parthian and Rainbow Classes

24. Oberon, the post- war concept of an overseas patrol submarine, had been laid down in March 1924 and was completed in August 1927. In 1925 the need for a programme of new construction of overseas patrol submarines was approved and discussion took place on the Staff Requirements for such vessels. These discussions are given in detail in Chapter 12.


14.10 Design

25. The original sketch design for the Odin Class was a vessel of Lbp 273ft, surface displacement 1620 tons, main engines of 4000 bhp to give a surface speed of 17 knots, main motors of 1320 bhp to give a submerged speed of 9 knots, an endurance of 10 000 miles at 8 knots with 125 tons of oil fuel, six bow and two stern 21 inch torpedo tubes and one 4.7 inch gun.

From the Oberon, the Lbp had been increased by 13ft, as follows:

  • (a) 1ft in the C02 compartment.
  • (b) 1ft in the magazine.
  • (c) 2ft 6in in the motor room to put in an auxiliary motor.
  • (d) 1ft in the forward torpedo tube compartment to admit lengthened tubes for AIV gear.
  • (e) 7ft 6in. in the engine room to obtain 4000 hp compared with 2700 hp in Oberon.

Items (a), (b) and (c) had been included in the Vickers built Oxley and Otway. Later on in the Odin Class a further 9in was added to the engine room at the expense of the motor room so that the engine room became 45ft 3in long as compared with 37ft in Oberon and only 1ft 9in was added to the motor room.

For stability reasons, the moulded diameter of the pressure hull was increased to 16ft 9in as compared with a depth of 16ft 6in and a width of 16ft 0in in Oberon. Later on this circular section was changed to one based on a 9in square with 8ft radii from the corners of the square, which enabled longer stroke pistons to be fitted to the main engines with an increase in power from 4000 hp to 4400 hp. The legend speed was increased to 17-17.5 knots.

26. The design was developed and approved by the Board on 12 August 1926. In the development the surface displacement had increased to 1710 tons, the submerged displacement being 2010 tons. Although the surface endurance was still given as 10,000 miles at 8 knots the oil fuel carried had increased to 145 tons.

Although proposals had again been made to abolish the stern tubes and increase the speed to an operational 18 knots, which was equivalent to a trial full power for 20 knots they were not adopted.

A 'considerable increase in free living spaces necessary so that the health of the crew during wartime cruising of 2-3 months should not be impaired' was also raised. Although the suggestion of 'living spaces in the upper deck easing to be used in harbour and at sea, when the nature of operations will permit', was not pursued, efforts were made to give good accommodation. To increase habitability the flat between frames 39-90 was lowered 6in which meant abandoning a scheme of being able to pass one cell over the top of others which itself was a very good idea. Amongst other changes the Commanding Officer's cabin was placed in the bridge.

27. The Odin Class was of the saddle tank type similar to the Oberon but considerably attention was paid to the design of the form to ensure that the legend speed would be obtained. This had been a failing in most previous classes although the bad results in Oberon were not at the time neither available nor anticipated. The oil fuel was carried in the externals and trouble was experienced with leaks when completed, but it was necessary to have external fuel tanks to obtain the high endurance called for.

The main characteristics of the Parthian Class and Rainbow Class were practically the same as the Odin Class although the shape of the bow was altered in Parthian from that in Odin which increased the Loa. The approved legend for Rainbow was essentially a repeat of the Parthian but was later modified. The length of the pressure hull was reduced by two feet, made possible by a rearrangement of the engine room which involve moving the starting platform and engine controls from the forward to the after end of the machinery space. The frame spacing between frames 90 - 109 was rearranged but the frames aft of frame 109 were not disturbed but renumbered one less. The positions of some of the divisional bulkheads and tanks in the externals were modified. This decreased in length would have decreased the buoyancy of the pressure hull by about 10 tons and the capacity of the externals by about 5 tons.

The designed diving depth was 500 feet and each vessel did a deep diving trial to 300 feet.

28. Six Odin Class submarines were ordered in the 1926 Programme, six Parthian Class in the 1927 Programme and six Rainbow Class in the 1928 Programme. Subsequently Rupert and Royalist of the 1928 Programme were cancelled. The shipbuilders are given in Appendix I.

A General Arrangement of Olympus is given in Plate 38 and of Rainbow in Plate 39.

14.11 Dimensions

29. In Odin the Loa was 283ft 6in. The FP where the LWL cuts the curved bow was 10in aft of the foremost point of the stem bar. The AP was 9ft 8in forward of the after end and this was common in all three classes. The Lbp was therefore 273ft 0in. The Lph was 266ft 8in.

In Parthian the FP was kept in the same position as in Odin but the bow was reshaped to extend further forward above the LWL. The Loa was increased to 289ft 2ins the Lbp remained as in the Odin at 273ft 0in.  No 1 main tank was decreased in length by 1ft 6in so that the Lph became 265ft 2in. In the Rainbow 2ft was taken out of the pressure hull and the Loa was 287ft 2ins, the Lbp 271ft 0in and the Lph 263ft 2in.

The moulded beam of the pressure hull in all classes was 16ft 9in, the plating was ⅞in. thick so that the beam to outside of plating was 16ft 10¾in. The designed moulded beam to external plating was 29ft 10in and in the O and P Classes the external plating was 14lb (⅜ in). In the R Class it was increased to 20lb (½ in). The design maximum beam was therefore 29ft 103 in for the O's and Parthian Class and 29ft 11in for the R Class. Due to leaks in the oil fuel tanks considerable modification and reconstruction of the externals took place and rubbing strakes were fitted. DNC gave 29ft 10½in, 29ft 10⅞in. and 29ft 11⅜in respectively some years after completion and are probably typical final figures.

The moulded depth was as the beam 16ft 9in. DNC gave the overall depth of the pressure hull as 16ft 11⅞in in the O's, 16ft 11¾in in the Parthian Class and 16ft 11⅞in in the R Class. This means a 40lb strake of plating in the keel and a raised strake of plating at the top. The ballast keel was 15in deep amidships.

At the normal surface displacement the surface draughts were 16ft 1¼in in the O's, 15ft 11in in the Parthian Class and 16ft 1in in the R Class. The draught at the standard displacement of 1475 tons was of the order of 13ft 8in in Odin to 13ft 10in in Rainbow. The freeboard to the superstructure was just over 7ft in all classes.

14.12 Displacement

30. The legend displacements of the Odin Class were submerged 2010 tons and surface 1710 tons when carrying 145 tons of oil fuel. Before completion of the first boat, but when the terms likely to be included in the London Treaty were known, figures given were submerged displacement 2030 tons, surface displacement 1724 tons with 137 tons of oil fuel and standard displacement 1475 tons. The latter figure was undoubtedly given whilst the Conference was in session and this standard displacement of 1475 tons was thereafter retained for the Odin Class, Parthian Class and Rainbow Class.

31. After completion DNC gave displacement figures for Odin as submerged 2038 tons, surface with main tanks empty 1781 tons and surface as inclined 1795 tons. The reserve of buoyancy was 14.4%. In the surface condition the vessel carried 174 tons of oil fuel, 31 tons of lubricating oil, 38 tons of compensating and trimming water and 39 tons of fresh and distilled water - a total of 282 tons of liquids. By the rules of the London Treaty the standard displacement by this time was 1499 tons but such minor changes were ignored.

The surface displacement will depend on the amount of water remaining in the tanks on the surface after blowing and the as inclined figures given are the lightest possible. However to obtain a comparison between designs the reserve of buoyancy is based on the main tanks being empty.

The submerged displacement of the Parthian Class was 2040 tons with 280 tons of main ballast water and the surface displacement 1760 tons with main tanks empty and 1765) tons as inclined. The reserve of buoyancy was 15.9%.

32. In the approved legend for the Rainbow Class the submerged displacement was 2045 Tons. Later on, when 2ft was cut out of the length of the boat, the submerged displacement fell to 2016 tons. Due to the addition of buoyancy tanks after completion (see Para 40) and other small modifications the final submerged displacement was 2030 tons. They carried 267 tons of main ballast water and 158 tons of oil fuel. The surface displacement was 1763 tons with main tanks empty and 1777 tons as inclined and the reserve of buoyancy 15.1%.

33. It was in the Parthian Class that the practice of inclining in the low buoyancy condition was commenced. The standard laid down in the Rainbow Class and R Class was with Nos 2 & 7 Port and Stbd main tanks blown. In this condition in the Parthian Class, the displacement was 1980 tons at a draught of 19ft 4½in and in the R Class a displacement of 1970 tons at 18ft 9½in draught.

14.13 Stability

34. The designed stability of the Odin Class was surface GM 15.5in without emergency fuel and 13.85in with emergency fuel and a submerged BG with emergency fuel of 9.3in. In the following statement all conditions are with the full stowage of fuel, which became the normal condition.

"The original inclining and trimming experiment of Osiris, the first of the class, was carried out at Vickers in December 1928 and a cheek trim made during trials in February 1929. As a result of these trims 90 tons of ballast was added in excess of the designed keel weight. This extra weight accumulated as a result of savings during building and the general tendency to be preferably high rather than low in working out design weights. With this added ballast the stability of Osiris worked out as surface GM 20.5in and submerged BG 12.95in".

35. After completion a considerable number of modifications took place including the stiffening of the external oil fuel tanks, fitting rubbing strakes and engine outboard induction, and the installation of escape hatches, Davis Submerged Escape Apparatus (DSEA), indicator buoys and salvage blows. By early 1931 Osiris had become 20 tons heavy; this amount of ballast was removed and the stability then became GM 18.9in and BG 12.0in

There is always variation in stability between ships of a class especially between those building in different yards and due to special equipment. After the modifications mentioned above had been made throughout the class the stability figures were:

Surface GM
Submerged BG
OSIRIS 18.9 12.0
OSWALD 18.7 12.0
OTUS 19.0 12.0
ODIN 17.0 12.0
OLYMPUS 15.7 11.5
ORPHEUS 15.3 10.7

Orpheus was the only one of the class with a major difference being fitted with Vulcan clutches.

36. The stability of the Parthian Class was less than in the Odin Class and this applied especially to Parthian fitted with a high capacity battery and Perseus originally with a 4.7-inch gun. In the latter the GM was 12.4in and BG 9.25in. Eventually figures for the class were given as GM 11.3in and BG 9in carrying a 4 inch gun. The ballast keel was about 40 tons less than in the Odin Class. The stability was good and no complaints had been received about surfacing or excessive rolling. However, at this time interest began to be taken in the stability of a submarine during the period of surfacing and also in a misadventure condition.

This led to Phoenix being inclined in the low buoyancy and misadventure conditions as a representative vessel of the class. She had a 4-inch gun; 6.5 tons of ballast was added 7ft above the datum line to represent the extra weight of a 4.7-inch gun. The vessel was first of all inclined in the low buoyancy condition with Nos 2 and 7 main tanks blown. Afterwards 27 tons of ballast was added 4.5ft above the datum line to represent the conning tower and gun access trunk flooded, and the drop keel was dropped in the basin. The vessel was again inclined. The results were:

(a) In low buoyancy condition Nos 2 and 7 main tanks blown

GM with 4.7-inch gun 6.10in.
GM with 4-inch gun 7.07in.

(b) In low buoyancy condition with conning tower and gun access trunk flooded and drop keel released

GM with 4.7-inch gun 3.86in.
GM with 4-inch gun 4. 82in.

The results were really quite satisfactory. However, a 4-inch gun was eventually fitted in all three classes instead of a 4.7 inch gun but this was done for other reasons as much as stability. This trial started the practice of inclining in the low buoyancy condition during inclining experiments.

37. In the Rainbow Class two feet had been taken out of the length in an effort to decrease the size of the ballast keel nearer to the Odin original design figure. In addition the plating in way of the fuel tanks was made heavier. During building the various modifications mentioned in Paragraph 35 were incorporated with the result that the total amount of ballast carried was less than anticipated and this had a detrimental effect on stability.

On Regent's trials, the first of the class, heavy weather was experienced and when in a following sea with the crest of the wave amidships, the vessel would roll over to about 100 and hang for a relatively considerable period. This had not been experienced in older submarines. This hang, which afterwards proved to be a 'baulked oscillation', increased the uncertainty in the minds of the personnel as to the stability of the vessel, an uncertainty imparted by an unofficial source at Barrow before the original inclining experiment to the effect that the vessel was unstable. The actual inclining experiment had proved this to be entirely false, the results having been surface GM 11.42in, submerged BG 7.05in and low buoyancy condition GM 2.5in.

38. The low buoyancy stability of 2.5in after blowing Nos 2 and 7 main tanks was low and after Regent reached Portsmouth complaints were received that when surfacing she appeared to be 'tender'. The 4.7in gun was removed and a series of surfacing experiments carried out at sea with RA (S) and a representative of DNC on board. With Nos 2 and 7 main tanks blown completely the vessel rolled irregularly (maximum roll 12°) and DNC's representative decided that it was unsatisfactory. Surfacing on Nos 2 and 7 tanks with No 1 tank vent left open was satisfactory. In consequence of these trials inclining experiments were carried out at Portsmouth with the following results:

As inclined GM (in.)
with 4.7-inch
gun added
gun added
(a) Nos 2 and 7 tanks completely blown. No 1 vent open. Bow buoyancy vent open. 4.5 3.3 4.2
(b) Nos 2 and 7 tanks completely blown. Bow buoyancy vent open. 3.7 2.5 3.4
(c) Nos 1 and 7 tanks completely blown. Bow buoyancy vent open. 4.3 3.1 4.0
(d) Nos 2 and 7 tanks partially blown for 12 seconds. Bow buoyancy vent open. 4.2 2.9 3.8

It is seen that the worst stability condition is with Nos 2 and 7 tanks completely blown. This condition was fixed as the low buoyancy condition at which all future submarines should be inclined i.e. with two tanks completely blown.

The original surfacing trials showed that the easing was not draining efficiently and many extra holes were cut. This lack of draining in itself had had a detrimental effect on surfacing.

39. About this time the Controller visited Regulus at Chatham and was informed that in a following sea great care would have to be exercised in turning the vessel to prevent an excessive angle of heel. After a consultation, between RA (S) and DNC, it was decided to send Rover and Pheonix out in rough weather to compare the rolling of the two vessels. Rover had a 4.7 inch gun, Pheonix a 4-inch gun. These trials showed that the rolling in Rover was no worse than in Pheonix, but did confirm that she possessed the peculiar feature of having a 'baulked oscillation' every now and again. This made the movement in Rover uncomfortable and feel more than it actually was.

40. Trials carried out in Regent at sea showed the vessel to be satisfactory in a low buoyancy condition, with Nos 2 and 7 main tanks blown and the vent of No 1 main tank open. It was agreed to modify the class so that a minimum metacentric height of 4.5in was obtained under the worst conditions of blowing.

Buoyancy tanks were fitted in the superstructure forward and aft, the 4.7 inch gun was replaced by a 4 inch gun, and one towing cable and hawser was removed (this was later replaced). The vessels were re-inclined in surface, submerged and low buoyancy conditions and also in a low buoyancy condition with the drop keel released and ballast added to represent the conning tower, captain's cabin and gun access trunk flooded.

The results obtained were as follows in Regent:

(a) Surface condition 11.42 10.42
(b) Submerged condition 7.05 8.0
(c) Low buoyancy condition Nos 2 and 7 main tanks blown. Bow buoyancy vent open. 2.5 4.6
(d) As condition (iii) with drop keel released and conning tower, captain's cabin and gun access trunk flooded - 3.2

It will be noticed that the surface stability was less than before modification. The submarines after modification were accepted as satisfactory.

41. By July 1932 the stability of all the vessels of the class after other small adjustments to ballast had become:

Low buoyancy
REGENT 10.2 8.0 4.6
REGULUS 10.8 8.8 4.6
ROVER 10.2 8.2 4.6
RAINBOW 9.7 7.6 4.5

The low figures in Rainbow were due mainly to the fact that she carried a heavier high capacity battery. Later on in all vessels of the class the fuel tanks were reconstructed and the ballast keels adjusted for a sg of oil fuel of 0.842 instead of 0.871 as previously used. As typical of the class the stability figures for Rover when inclined then became GM 12.9in, BG 10.2in and low buoyancy 5.6in. By now the stability of the class was very good

14.14 Surface Speed

42. The legend speed for the Odin Class was 17-17.5 knots at 4400 bhp at 400 rev/min. On trials on the measured mile Oswald obtained the following results:

Bhp Rev/min Speed (Knots)
- 171.3 8.15
982 242.6 11.35
2539 322.7 14.20
2888 341.2 15.43
3755 372.4 16.82
4420 388.7 17.56

Bhp Rev/min Speed (Knots)
Single engine runs: 421 221.2 7.99
738 261.2 9.48

In June 1929 on speed trials Otus at 1725 tons displacement and draughts 15ft 2½in. Fwd and 15ft 9in. Aft obtained 17.53 knots at 396 rev/min. Osiris on trials got 17.8 knots. These trials were undoubtedly carried out at a displacement between 1720 and 1730 tons with the legend stowage of 145 tons of fuel. At the later normal displacement of 1781 tons there would have been some slight reduction in speed. Odin on trials obtained only 16.7 knots but this low speed could not have been due to increased displacement alone. However, although the shop trials of the engines for Osiris gave 4415 bhp at 400 rev/min the hp for the class was later given as 4520 bhp. It can therefore be taken that 17.5 knots was achieved.

43. The legend speed of the Parthian Class and Rainbow Class was 17-17.5 knots as for the Odin Class. The engines of the latter developed 4400 bhp and of the former 4640 bhp. There is therefore no reason to doubt that the Parthian Class and R Class obtained at least 17.5 knots when first commissioned. In fact 17.94 knots in Parthian and 17.86 knots in Rainbow have been stated and over 18 knots claimed.

However in the early 1930's the powers were re-rated as 4520 bhp in Odin, 4340 bhp in Parthian and 4100 bhp in Rainbow, but the speeds seem to have remained above 17.5 knots, see Para 54.

44. The effect on speed of time out of dock was considerable and the following records for Olympus are typical. Speeds were taken on the measured mile.

Number of days out of dock Speed Bhp Rev/min
6 17.46 5800 396
72 16.20 6108 390
275 14.60 6239 381

14.15 Submerged Speed

45. The legend speed for all three classes was 9 knots at 1320 bhp. On submerged speed trials in Oswald in May 1929 the speed obtained was disappointing:

Grouper Up Bhp Rev/min Speed (Knots)
Super full speed 1388 247 7.73
Full speed 1090 230 7.10
Half speed 731 202 6.34
Slow speed 267 145 4.42
Grouper Down
Full speed 502 176 5.42
Half speed 271 146 4.50
Slow speed 101 97 2.97

Efforts were made to improve the speed by stripping or cleaning up topsides and the following results were then obtained:

Grouper Up Bhp Rev/min Speed (Knots
Super Full speed 1395 256 8.35
Full speed 1100 234 7.74
Half speed 776 209 7.00
Slow speed 281 149 4.94
Grouper Down
Full speed 517 180 5.99
Half speed 261 145 4.88
Slow speed 84 96 3.16
Auxiliary drive, full speed 43 69 2.22

At 1320 bhp therefore the speed was about 7.6 knots as built and 8.2 knots with the topsides stripped. With revised topsides fitted the speed was 7.9 knots.

46. A mass of topside fittings had accumulated during the building of the Odin Class and as mentioned above an effort was made to clean up the topsides to improve the submerged speed. As an example amongst other items the stanchions and bollards were made of the disappearing type. In the Parthian Class and Rainbow Class further small differences from Odin were made as they were built, for example signal yards were altered and much more attention paid to streamlining fittings. Experiments were carried out about this time with canopies over the guns but after trials the idea was not adopted. All the bridge cab arrangements of the O's and Parthian Class were modified later, not because of submerged speed but at the request of RA (S) to improve navigation facilities.

47. The actual results of subsequent trials in the various classes were:

Speed in knots at: 1320 Bhp 1500 Bhp 2000 Bhp
OSWALD 7.91 8.25 -
PERSEUS 8.17 8.51 9.38
POSEIDON 8.22 8.54 9.31
PARTHIAN 8.27 8.57 9.32
REGENT 8.25 8.59 9.64
*REGULUS 8.08 8.45 9.49
* Anti-submarine net gear was rigged

All these trials except in Regulus were made with both periscopes raised which at full speed made a difference of about 0.4 knots.

The legend speed in all three classes had been 9 knots at 1320 bhp. After completion DNC was quoting the hp of the main motors as 1390 bhp in the Odin Class, 1635 bhp in the Parthian Class and 1670 bhp in the Rainbow Class. These powers were fixed for a speed for one hour of approximately 8 knots in the Odin Class, 8.6 knots in the Pathian Class and 8.8 knots in the Rainbow Class. Rainbow herself with a high capacity battery achieved 9 knots.

14.16 Surface Endurance

48. The legend specified 10 000 miles at 8 knots with 145 tons of oil fuel. Later on DNC quoted 11 400 miles at 8 knots with 174 tons of fuel.

Fuel trials were carried out in Otus in July 1929. The ship was in main ballast trim and the fuel available for calculating endurance is taken as 43 500 gallons equivalent to about 169 tons or approximately 97.5% of the total fuel stowage of 174 tons. The weather conditions are not known but these could have an adverse effect on both speed and fuel consumption. The speed was taken by log and also from a trial curve of speed against revolutions and the latter is used in calculating endurance. The auxiliary load was floated.

The results of the trials were:

  Rev/min Speed (Knots) Gallons
per hour
By Log From Curves
Both engines 170 7.8 8.2 33.4 10 680
190 8.6 9.1 41.5 9537
260 11.6 12.1 72.9 7217
One engine 300 10.5 10.9 62.5 7586
260 9.0 9.4 44.0 9292

A later return from Otus gave fuel consumptions at various speeds per 24 hours for the ship six months out of dock in main ballast trim. The endurance column has been calculated based on using 169 tons of fuel.

Speed Oil fuel.
Tons per
24 hours
8 2.9 11 200
9 3.8 9600
10 4.6 8800
11 5.5 8100
12 6.9 7060
13 8.8 6000
14 11.2 5060
15 13.2 4600
16 16.0 4050

Although six months out of dock the bottom must have been reasonably clean.

49. These figures must be taken as approximate but give an indication of endurance using 97.5% total fuel capacity. Vickers gave the endurance as 9700 miles at 8 knots. This would have been with 145 tons of fuel and compares favourably with the figure given by DNC some years after completion of 11 400 miles at 8 knots carrying 174 tons of fuel under trial conditions.

The Parthian Class carried 159 tons of fuel and the Rainbow Class 158 tons. After completion endurances quoted by DNC were 10 750 miles at 8 knots and 10 900 miles at 8 knots respectively.

50. It would be expected that the endurance figures would drop with years of service. In 1938 CB 1815 quoted endurances worked out on 95% of the fuel stowage in fine weather and moderately clean bottom as follows:

Oil fuel
At full speed
18 knots
At 8 knots
ODIN Class 170 2690 9720
PARTHIAN Class 159 2880 9870
RAINBOW Class 156 2750 10 350

In the Odin Class the endurances expected six months out of dock were 3030 miles at 16 knots and 8020 miles at 8 knots. The latter was using one engine only.

The daily fuel consumption on patrol allowing 12 hours diving, 12 hours steaming at slow speed and 8 hours charging for all classes is given as 2.1 tons per day except in Orpheus the consumption was 2.6 tons per day. CB 1815 also states that the endurance figures for Orpheus were 4% less than those for other vessels of the class. The only reason for these reductions in Orpheus would appear to be that she was the only vessel of the Odin Class fitted with a Vulcan clutch.

14.17 Submerged Endurance

51. The only figure seen, which would have been quoted before the first Odin was completed is 60 miles at 4 knots and this is taken as the legend endurance. One hour at the full speed of 9 knots would have been expected.

Actual endurance figures for the three classes from trials soon after completion are 1 hour at 8 knots in the Odin Class, 8.6 knots in the Parthian Class and 8.8 knots in the Rainbow Class. At 4 knots corresponding figures were 13 hours (52 miles), 17½hours (70 miles) and 15 hours (60 miles). These were for normal peace conditions and increased under war conditions with a reduction in auxiliary load. The auxiliary load taken had a considerable effect on endurance especially at the lower speeds. Its value taken over the years varied between classes and between authorities making the calculations and also varied with speed. Later on it was standardised for a class for all speeds. For operational purposes it was 250 amperes auxiliary load in peace condition and 150 amperes in war condition at all speeds for all three classes.

In those vessels fitted with high capacity batteries the endurance was of course higher than the values mentioned above, for example, in Rainbow the endurance at 4 knots was 75 miles.

14.18 Tanks

52. In September 1926, one month after the Board had approved the Odin design, RA (S) put up some proposals on principles regarding the arrangement of tanks and essential pipe lines in new construction. These principles are given in quotes as follows:

  • (a) 'The first principle laid down for design is to move a large or small quantity of water quickly at any depth to which a submarine is designed to dive'. This principle had already been adopted to a considerable extent in X1 and Oberon.
  • (b) 'It is desirable that main ballast tanks shall be capable of being completely flooded in 20 seconds'. In the older classes many of the tanks took at least 11 minutes to flood. In X1 the time was reduced to 35 seconds. It was hoped to improve on this in Oberon but this was not achieved in all tanks. In Osiris the maximum time to flood any tank was 30 seconds with an average of 23 seconds. This was possible because no main tank extended above the normal surface waterline.
  • (c) 'Compensating tanks should be reduced in number to a minimum and in capacity as far as is compatible with dealing with all weight differences and density problems. Tanks must be strong enough to blow at maximum diving depth and be fitted with HP blows and Kingston valves'. This seems obvious but the tanks in Oberon were rather excessive and he proposed deleting H and X tanks and repositioning A and B together right forward and Y and Z together right aft. O and Q (quick diving) tanks to remain.
  • (d) 'Fuel tanks are required to be placed symmetrically round their central O compensating tank'.
  • (e) 'The LP airline to be abolished except to WRT's. As it is necessary to retain pumps and a bilge line, suctions from compensating tanks to the bilge line should be retained'.
  • (f) 'The number of ballast pumps can be reduced from three to two, one of these two being the deep diving pump No l.
  • (g) 'All compensating tanks to be fitted with an accurate means of measuring'. A measuring gear called the Martins gear was being developed at this time.

53. The arrangement of tanks in the three classes were very similar and details of individual types of tanks in the Odin Class are given below with major changes in the Parthian Class and Rainbow Class where applicable.

14.18.1 Main Ballast Tanks

No 1 tank was in the bows and Nos 2-9 Pt and Stbd tanks in the lower half of the externals. They were flooded through holes cut in the lower plating except that Nos 4, 5 and 7 Pt and Stbd tanks had hand worked Kingstons as a safety precaution against the accidental opening of the vents. They had telemotor operated vents - one 12in in No 1 tank and one 7in in each remaining tank. Those tanks not fitted with Kingstons could be blown direct from the 3500lb/in2 HP main. All tanks could be blown at 15lb/in2 from the LP blower. The tanks were tested to 20lb/in2.

To amplify these particulars in the Rainbow the number of main tanks was reduced to six each side in the externals from eight in the Odin. Two 17in diameter hand operated Kingstons were fitted to each of Nos 3, 4 and 6 Port and Stbd tanks. Two flooding holes 12in x 22in with semi-circular ends were cut in each of the remaining tanks. HP direct blows were fitted to No 1 main tank, No 2 Port and Stbd tanks (common line), No 5 tank Port, No 5 tank Stbd and No 7 Port and Stbd tanks (common line) that is to all main tanks not fitted with Kingstons. All tanks were on the LP blower line.

The main tank capacity in the Odin was 257 tons, in the Parthian 280 tons and in the Rainbow 267 tons. All these figures include the bow buoyancy tank.

14.18.2 Bow Buoyancy Tank

This tank had flooding holes in the side plating, was fitted with a telemotor vent and could be blown direct from the HP air main. The tank was tested to 10lb/in2. In addition buoyancy chambers wore fitted forward and aft in the superstructure in the Rainbow Class. They were watertight and could not be flooded.

14.18.3 A. B. Y and Z Auxiliary Tanks

Each tank was fitted with an 8in telemotor-operated Kingston, a 4in-telemotor vent, a direct lead from the HP air main, a 25lb/in2-sea relief valve and 2½in suction from the main line. They were tested to 75lb/in2. These tanks made the old type of trimming tanks unnecessary. The Kingston was of the 3 way type described in Chapter 23 Paragraphs 20.

14.18.4 O Compensating Tanks

One tank was each side in the externals and each fitted with a 4in telemotor-operated Kingston and a hand worked vent, a direct lead from the HP air main, a 25lb/in2-sea relief valve and 3in suction from the main line. They were tested to 100lb/in2. These tanks were oil fuel compensating tanks and were sited amidships between the forward and after groups of oil fuel tanks.

14.18.5 Q Diving Tanks

One tank fitted port and starboard sides and as for O compensating tanks except that each had an 8in Kingston and there was no suction from the main line. The HP direct blow was common to Port and Stbd tanks. They were tested to 100lb/in2.

14.18.6 WRT Tanks

These tanks were flooded through a 3½in sea valve, vented inboard, had an LP blow from the 50lb auxiliary service and a 2½in. suction from the main line. The tanks were tested to 75lb/in2. The forward WRT had a 6in overflow to a TOT (torpedo operating tank) with similar venting, blowing and pumping services, but tested to 50lb/in2 only. An AIV automatic inboard venting) tank aft was tested to 20lb/in2 only, had a 2½in suction, would be flooded, through the suction and was vented inboard. These were in all cases the ship services and were additional to the normal piping from the torpedo tubes.

14.18.7 Fresh Water Tanks

There were three internal tanks all tested to 20lb/in2 and fitted with 1in vent cocks for venting inboard. In addition to the normal filling and suction piping of the fresh water system the tanks could be pumped and flooded from the main line by hose.

An external tank was also fitted port and starboard sides in the externals and alongside the O compensating tanks. The reason for fitting these tanks is explained in Paragraph 20(c). The fresh water in these external tanks and also the water in O compensating tanks had to be discharged overboard when diving in fresh water. The requirement to be able to dive in fresh water was not introduced until late 1928 when the building of the Odin Class was well advanced and allowance had not been made in the design for the necessary amount of compensating water required to meet such conditions especially at the beginning of a patrol.  This is discussed further in Chapter 18 Paragraph 25. About 40 tons of fresh water was carried.

14.18.8 Oil Fuel Tanks

In all classes the oil fuel tanks were in the upper half of the externals except for the oil fuel gravity tanks which were in the engine room in the Odin Class and Parthian Class and in the externals in the Rainbow Class. Six tanks were fitted each side in the latter class and eight tanks in the two former classes. The tanks were self compensated being flooded by circulating water from the main engines, which pushed fuel to the gravity tanks. Each tank was fitted with 1in petcocks for venting and could be blown at 15lb/in2 from the LP line. The external tanks were tested to 20lb/in2. Two expansion tanks were fitted in the superstructure, one forward and one aft.

Originally in Odin the normal stowage of oil fuel was stated to be 137 tons and the total stowage 174 tons. In service the full stowage was always used. On completion 174 tons of fuel was carried in Odin, 159 tons in Parthian and 158 tons in Rainbow.

14.19 Main Machinery

54. All three classes were fitted with four cycle blast injection eight cylinder diesels designed to develop a total on the two engines of 4400 bhp in Odin Class and 4640 bhp in the Parthian Class and Rainbow Class, all at 400 rev/min. The two later classes were fitted with Vulcan clutches, as was Orpheus only of the Odin Class. The building Yard made the main engines in each case.

As typical of the Odin Class, the 8- hour full power shop trials of the Osiris engines gave 4415 bhp at 401 rev/min with a fuel consumption of 0.406lb per bhp hour. The 72 hours 80% full power trial gave 3573 bhp at 375 rev/min with a fuel consumption of 0.394lb per bhp hour. On speed trials Oswald obtained 4420 bhp at 389 rev/min. Actual powers recorded on speed trials in the Rainbow Class were Rainbow 4326, Regent 4180, Regulus 3965 and Rover 4270 but the figures for the last three boats do not include the auxiliary load. The corresponding speeds were 17.72, 17.94, 18.0 and 17.87 knots. Allowances have to be made in these figures for errors in recording the power and for displacement and weather.

After completion DNC gave the bhp for the classes as Odin 4520, Parthian 4340 and Rainbow 4100. The lower figures for the last two classes may be due to losses in the Vulcan clutches but the difference between the Parthian Class and Rainbow Class is difficult to understand in view of the speeds obtained being similar and the boats practically identical in form and displacement.

The exact date that DNC gave the hp's mentioned is not known. It is known that because of recurring piston seizures at the higher powers the full power rating of the engines was reduced to 90% in 1936 in all three classes. This problem may have been solved later.

The main machinery weights increased from 183 tons in Odin to 191 tons in Parthian to 199 tons in Rainbow. The estimated weight of the Vulcan clutches in the machinery tenders for the Parthian Class was 16.8 tons plus 2.6 tons for oil in the clutches.

55. The twin armature main motors on each shaft were designed to give a total of 1320 bhp at 240 rev/min for 1 hour. They were of the closed type and main motor coolers were provided.

At 1320 bhp the legend speed submerged was 9 knots but on trials in the Odin Class the speed obtained was disappointing. As mentioned in Paragraph 46, modifications were made to topside fittings. The power at the 1 hour rate of battery discharge was fixed at 1390 bhp with an auxiliary load of 150 amperes.

A serious redesign of the main motors for the Parthian Class and Rainbow Class must have taken place in so far as space limitations would allow. The main motors weight increased from 39 tons in Odin Class to 44 - 50 tons in Parthian Class and Rainbow Class. Although the legend power was given at 1320 bhp the subsequent figures quoted were 1635 bhp for the Parthian Class and 1670 bhp for the Rainbow Class.

An auxiliary drive motor was fitted with maximum load of 188 amperes at 220 volts, which would give a shaft speed of 60 rev/min.

All classes carried 336 cells in three battery tanks with 112 cells each, of either Exide 3820 LS or 3830 1 type. See Chapter 26. Parthian AND Rainbow individually had high capacity type cells.

14.20 Armament

56. In discussions on the torpedo tubes for the Odin Class at a conference in November 1925 it was decided that the tubes should be generally as for Oberon but:

  • They should be suitable for 'disc' discharge.
  • Quick closing rear doors should be developed.
  • Bow shutters not to be fitted.
  • To be designed for 500 feet depth.

In lieu of bow shutters the whole of the fore end was plated in and holes cut representing the projections of the torpedo tubes on to the plating with an inch or two clearance all round. Guides were fitted from near the lip ends of the tubes to the shell plating. As already mentioned the forward torpedo tube compartment was increased in length by one foot over Oberon to allow AIV gear to be fitted.

All three classes had six bow 21-inch torpedo tubes and two stern 21-inch tubes. Spare torpedoes were carried for the bow tubes only so that the total allowance was fourteen Mark IV* torpedoes at first which were changed to Mark VIII-VIII* later.

57. The Odin Class carried one 4 inch Mark IV gun forward of the bridge with the gun deck about 3ft 6in below the bridge deck. The gun access trunk was over 7ft high so that a man could stand in it with considerable advantage in passing ammunition. It had been the intention to change to a 4.7-inch gun later but as explained earlier the 4-inch gun became the standard weapon for all these classes. Perseus and some other vessels had a 4.7-inch gun when completed but they were replaced later by a 4-inch. There was magazine stowage for 120 rounds of ammunition.

In the Parthian Class definitely and it is reasonable to assume in the other classes a ready use ammunition locker was positioned in the superstructure under the gun.

The gun platform in the Rainbow Class was 3ft 2in lower than in the earlier classes, the bridge was also lowered. This was to cut down silhouette and improve stability. However, the change made the bridge and gun platform much wetter and flared plating was fitted around the gun platform to reduce spray.



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Chapter 13: Cruiser-Submarine X1Chapter 15: Fleet Type Thames Class