Chapter 16: Small Patrol Type: Swordfish and Shark Classes
1. About 1928 the need for a new type of small submarine became apparent. The existing types would soon be over- age and it was necessary to consider replacements for the purposes of training and for patrols in the vicinity of our bases and in confined waters in war. In February 1929 RA (S) remarked on the type of small submarine required in the sense of the following:
'The present type of vessel (Odin) would be too large to use in the Baltic unless the conditions obtained denied the operating submarine the use of a base at a distance not greater than say 500 miles.'
'The tonnage of the small submarine should permit:
- (a) Sufficient oil fuel to give an endurance to proceed to a patrol position 500 miles from base at not less than 10 knots, remain on patrol 10 days and return to base 500 miles. This to include fuel for charging batteries.
- (b) Armament of six bow tubes and six spares; two stern tubes is also desirable if thereby the design is not adversely affected.
Out experience tends to show that the above could be embodied in a ship of 600 tons and that the habitability would be good enough to permit the work at (a) being carried out.
2. Although these requirements could be embodied in less than 600 tons, it was recommended that the displacement should not be less than 600 tons. This on account of sea keeping qualities and bearing in mind that patrols in the H Class were limited to 5 or 6 days in the latter part of the war because of the hardship entailed in patrols of longer duration. The 500 miles range was chosen because this covered the distance between certain bases and patrol areas which were named and furthermore 'because of the great increase in size of W/T equipment, etc above that range and its effect on the size of the boat.
3. Forcing an entrance to confined waters was of importance and RA (S) continued 'subject to the remarks above I would fix the tonnage at that of the present L Class of 760 tons (the figures for displacement being quoted by RA(S) are standard displacements) but I would emphasize that the forcing of an entrance under the conditions that prevailed in the last war would be attended by considerable risk to any submarine and therefore the use, of those of the smallest tonnage is advisable observing that their attacking qualities would be as great as those of an Odin Class submarine provided that the surface speed is not unduly diminished.'
4. To enable the small submarine to fit into the war plan D of P asked for an endurance to steam 1200 miles to its billet and back and remain on the billet eight days with a passage speed of not less than 9 knots. The Naval Staff gave the following additional requirements:
- W/T range 500 miles (600 miles if possible).
- At least four bow tubes (six if possible).
- A maximum full speed of 12 knots (More is possible).
- A Battery endurance (proportional) of the L Class.
5. DNC was asked to prepare two designs to cover the limits of 600 tons and 760 tons standard displacement to see if anything was to be gained by going for the larger tonnage.
Out of these discussions the sketch design for Swordfish was started early in 1929 and was approved by the Board on 28 November 1929.
6. Swordfish and Sturgeon were in the 1929 Programme and building should have started early in 1930 but it was decided to wait until the London Treaty had been ratified (October 1930). In this waiting period RA (S) raised objection to oil fuel being stowed under and at the sides of the battery tanks. New legends and designs were prepared by the end of July 1930. Swordfish and Sturgeon had been ordered on 1 July. Even after building had commenced the moulded diameter of the pressure hull was decreased from 15ft 0 in to 14ft 11 in primarily to save 6 tons in displacement but also to give more tank space in the externals without increasing the displacement. At the time, the designers were worried that from the weight calculations no adjustable weight was available in the ballast keel. During the building of the early boats of the class it was a continuous struggle to keep down weights and having to accept items such as conning tower castings which had been cast excessively heavy and could not be scrapped on the grounds of economy. This was all a reaction to the London Treaty and cost, and brought troubles as explained later.
|Length overall||202ft 6 in.|
|Maximum beam||24ft 0-3/8 in.|
|Displacement standard||640 tons|
|Displacement surface||730 tons|
|Displacement submerged||927 tons|
|Speed surface||14.2 knots|
|Speed submerged||10.0 knots|
|Endurance surface at 8 knots||5750 miles|
|Torpedo tubes, bow||Six 21-inch|
8. As mentioned in Paragraph 6, the design of Swordfish had been prepared under difficulty and weight was a problem. In consequence the stability was lower than hoped for. In addition the first four boats on completion were complicated and maintenance was difficult. Efforts were made after completion to simplify fittings and although much thought was given to the problem little could be dispensed with or changed except a few small items.
A number of changes were made in the Shark and Sealion in the 1931 Programme which followed. The length was increased, as was the displacement by 30 tons. The layout was simplified and the stability was improved. The restrictions imposed on the original Swordfish design were forgotten in spite of the London Treaty.
The S Class continued to be built for many years with many changes. The following paragraphs deal only with the first four boats of the Swordfish Class with variations in the Shark type as applicable.
9. They were saddle tank boats of the L Class type of construction. The pressure hull was circular amidships of 14 ft 11 in moulded diameter. The plating amidships was 15lb.
The designed diving depth was 300 feet and they were tested to 200 feet.
10. Particulars of the Swordfish Class and Shark Class are given in Appendix IIB. The increase in length of 6 ft in Shark was mainly in the pressure hull amidships which increased the displacement by 30 tons. All other dimensions were as in Swordfish and the capacity of the saddle tanks was the same within two tons. In spite of an increase in weight of some of the equipment in Shark, the increase in the waterplane area and the ballast carried was sufficient to have a marked improvement in stability over that of Swordfish.
On completion the displacements in Swordfish were submerged 927 tons and surface with main tanks empty 730 tons. The capacity of the main tanks was 197 tons. The reserve of buoyancy was 27% the highest yet achieved except in double- hull vessels.
The surface displacement as inclined was 736 tons and the reserve of buoyancy 26%. In the low buoyancy condition with Nos 2 and 7 main tanks blown the displacement was 873 tons and the draught 14 ft 3 in.
12. In the Shark corresponding figures were submerged displacement 960 tons, surface displacement 761 tons with tanks empty. The reserve of buoyancy was 26.2%. In the low buoyancy condition with Nos 1 and 5 main tanks blown the displacement was 909 tons and draught 14 ft 4½ in.
13. As designed, Swordfish had a surface GM of 16.45 in, submerged BG 6.31 in and low buoyancy GM 4.67 in with Nos 2 and 7 main tanks blown. Due to the conditions during building as explained in Paragraph 6, which were to the detriment of the designed stability, Swordfish when inclined on completion had surface GM 13.9 in, submerged BG 4.9 in and low buoyancy GM 2.9 in. Trouble was experienced with listing on surfacing. When coming to the low buoyancy condition the vessel listed as much as 200 at times. Although the stability in the low buoyancy condition was not as much as had been desired, this listing arose primarily from three factors. The submarine had a permanent list of 30 which increased as the stability decreased and was accentuated when surfacing by the HP blows having a greater effect on the tanks on the 'up' side. The bridge and casings drained too slowly. In addition compensating tanks under the batteries were often partially filled and had a large free surface effect.
Extra holes were cut in the bridge and superstructure and it was found that by blowing quickly the submarine could be surfaced much better in spite of the permanent list. However, the permanent list was corrected. The change in the 3 - inch gun, mentioned in Paragraph 24, also had an appreciable effect. The high bow was modified to prevent yawing at anchor and this saved weight. After these modifications the stability improved to surface GM 17.1 in, submerged BG 6.9 in and low buoyancy GM 3.7 in. The surfacing was greatly improved but the early vessels were inclined to roll considerably when surfacing beam on to a heavy sea.
14. Shark onwards had better stability, actual figures for Sealion being surface GM 19 in, submerged BG 9.1 in and low buoyancy GM 6.6 in with Nos 1 and 5 main tanks blown. In this low buoyancy condition, even with the drop keel released and the conning tower and gun access trunk flooded, the GM was 5.4 in. They were, if anything, a little too stable on the surface and had a quick rolling period. This could be remedied by flooding down.
16.6 Speed and Endurance
15. A high surface speed was not called for in this class in fact the requirement was 'a speed of 12 knots. More if possible.' The legend speed was 13.75 knots at 1550 bhp. On trials Swordfish obtained 14.3 knots at 1641 bhp (but this power reading is thought to be high) and 14.2 knots at 1575 bhp has also been given. Shark on trials obtained 14.7 knots at 1576 bhp.
16. The design was 3800 miles at 9 knots with 44.5 tons of fuel in Swordfish and 3800 miles at 9 knots with 40 tons of fuel in Shark. The Operational endurance given in 1938 with 38.6 tons of fuel was 1880 miles at 14 knots, 3600 miles at 10 knots and 5750 miles at 8 knots for both types.
17. The legend-submerged speed was 10 knots at 1300 bhp. Swordfish obtained 9.7 knots at 1350 bhp. At the 1 hour rating of the motors the total bhp was 1440, so there is no doubt that at this rating over 10 knots could be obtained in both Swordfish and Shark.
18. For Swordfish the submerged operational endurances in 1938 with auxiliary load (a) under war condition of 68 amperes and (b) under peace condition of 110 amperes were:
|2 knots||4 knots||9 knots|
- (a) Swordfish had thirteen main tanks, No 1 in the bows and Nos 2 to 7 Port and Stbd in the externals. A bow buoyancy tank was fitted.
- Nos 3 and 5 Port and Stbd tanks each had two 14 in hand operated kingstons. Nos 2, 4, 6 and 7 Port and Stbd tanks each had two flooding holes 18 in x 12 in with circular ends. No 1 tank had flooding holes of total area 500 in2. HP direct blows were fitted to the bow buoyancy tank, No 1 main tank and Nos, 2, 4, 6 and 7 Port and Stbd main tanks.
- The total capacity of main tanks and the buoyancy tank was 197 tons in Swordfish and 199 tons in Shark.
- (b) Only two auxiliary tanks were fitted, A forward and Z aft. Each tank had one 8 in diameter telemotor operated 3 - way Kingston and a HP direct blow.
- (c) Sided Q tanks amidships each had a 6 in telemotor operated 2 - way Kingston and a HP direct blow.
- (d) As built Swordfish carried 44. 5 tons of oil fuel in eight internal and two external tanks. Soon after completion the external tanks were abolished and the total capacity fell to 39.5 tons and later to 38.6 tons. See Chapter 25 Paragraph 50.
16.8 Main Machinery
20. All vessels had two sets of Admiralty design vertical 4-stroke blast injection, six cylinder diesels each developing 775 bhp - a total of 1550 bhp at 420 rev/min. Higher powers were obtained on trials, although RA (S) rated the engines at 760 bhp each. The, last two or three boats of the Shark type, e.g. Sunfish were fitted with eight cylinder diesels with jerk injection and developed a total of 1900 bhp at - 184 rev/min.
21. In Swordfish the two main motors developed a total of 1300 bhp at 380 rev/min at continuous rating. They were also guaranteed for 1440 bhp for 1 hour. Working voltage 190.
An auxiliary drive motor was not fitted and slow speed was obtained with the port and starboard motor armatures in series to give a maximum of 70 bhp at 130 rev/min.
The Shark and later vessels had tandem motors. The normal power output and revolutions remained as in Swordfish but the tandem motors were guaranteed to give 1600 bhp at 410 rev/min for 1 hour. They were also guaranteed to give a total of 800 bhp continuous with coolers and fans stopped. The weight of the motors per shaft increased from an average of 10.75 tons in Swordfish Class to 13.25 tons in Shark Class.
22. The 224 cells were fitted in two battery tanks of 112 cells, each battery being arranged in two sections of 56 cells for grouping. The cells were of high capacity type.
23. Six bow 21-inch torpedo tubes with six spare torpedoes. The torpedoes were of the G type as used in the Rainbow Class.
24. A 3-inch disappearing gun was included in the original design sited on the super structure forward of the bridge but before the design was completed the gun platform was raised 2 ft 9 in at the request of RA (S). The gun was of the disappearing type to cut down resistance but was very heavy. On trials it was found that the resistance of the gun was less than originally thought, in fact the slight increase in submerged speed with the gun housed did not warrant the complication of a gun and mounting of this type. It was further found that the RA mounting was far from satisfactory.
At the same time trouble was being experienced with the submarine listing in the low buoyancy condition when surfacing. In consequence of these various factors the 3-inch disappearing gun and its raised platform were removed and a 3-inch HA fixed mounting placed on the superstructure casing. With this change the loss of speed was negligible and 6 tons in topweight was saved.
16.10 Further Development
25. The four boats of the Swordfish Class, Swordfish, Sturgeon, Seahorse and Starfish and the eight boats of the Shark Class, Shark, Sealion, Salmon, Snapper, Seawolf, Spearfish, Sunfish and Sterlet, mentioned in Appendix I were the only S type submarines ordered before the war and were all in service at the outbreak of the war. The first of the T Class was in the 1935 Programme and of the U Class in the 1936 Programme and at the time it was intended that no more of the S type should be built. However, operational experience with the twelve existing boats at the beginning of the war showed that they were very suitable for North Sea work, over the years since completion, minor troubles had been eliminated. Early in 1940 the design was reviewed and many more S boats were built. Many changes were made during the subsequent building programmes including welded construction, fuel increased by being carried in the externals, and a stern firing external torpedo tube or a 4-inch gun in lieu of the 3-inch gun. The Loa was finally increased to 217 ft 0in and the surface displacement to 814 tons.