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Chapter 17: Minelayer Type - Porpoise Class

17.1 Introduction

1. Six of the E Class ordered in 1914 had been built as minelayers carrying twenty mines in mine tubes in the saddle tanks. The broadside torpedo tubes were omitted. E24, the first RN minelayer, was completed on 9 January 1916 and laid mines on her first patrol soon afterwards. She was lost on her second minelaying operation in March that year.

Six of the L Class ordered in 1916 were also fitted with mine tubes in the external tanks and the beam torpedo tubes omitted. L14 and L17 carried sixteen mines and L24-27 fourteen mines.

2. In July 1920 the Naval Staff commenced an investigation into the need and require­ments for submarine minelayers. This is detailed in Chapter 12 Paragraphs 12-14. The main point of controversy was whether the mines should be carried internally or externally. Eventually as an experiment M3 was converted in 1927 to carry mines externally. The satisfactory performance of M3 so converted led to the design of the Porpoise with 50 mines externally in the superstructure.

3. Six of the class were ordered starting with Porpoise in the 1930 Programme. This prototype vessel was completed in March 1933 before the next two vessels Grampus and Narwhal in the 1933 Programme were laid down. Rorqual was in the 1934 Programme, CACHALOT in the 1935 Programme and SEAL in the 1936 Programme. The builders are given in Appendix I.

Porpoise was laid down in September 1931 and completed in March 1933. The details, which follow, concern Porpoise as built. Grampus was not laid down until August 1933 and details of the design only are given. The last of the class did not complete until May 1939 and there were undoubtedly a number of changes over the intervening eight years. A General Arrangement of Porpoise is given in Plate 42.

17.2 Design

4. The hull form of Porpoise was very similar to that of Parthian with the same saddle tank arrangement and. all dimensions and displacements were very nearly the same, see Appendix IIB. The effect of carrying 50 mines in Porpoise was that the stem torpedo tubes were deleted and the main engine horsepower reduced by 25% with a loss in surface speed. The design diving depth was reduced to 300 feet.

5. For Grampus the hull form was radically changed. The saddle tanks were extended under the bottom of the pressure hull to make a double-hull except in the superstructure. The pressure hull was circular until about 2-3 feet from the keel where it joined the top of tanks In the outer hull built to pressure hull standards and which were used for oil fuel. This change was made so that the oil fuel carried externally in Porpoise could be moved to internal tanks. The argument for this was that although the external fuel tanks were welded and experience had proved that they did not leak, it was feared that if exposed to depth charge attack leaks would occur and so give away the submarine's position. This change also Increased the main ballast water carried 'by about 100 tons, and Improved the stability and reserve of buoyancy.

17.3 Dimensions

6. In Porpoise the Loa was 289 ft 0in and the Lbp 267 ft 0in. The overall beam and depth of the circular pressure hull was 16ft 9in, and the beam of the outer hull 64in and the beam of the outer hull 29ft 10¼in. Corresponding figures for Grampus as designed were Loa 293ft 0in, Lbp 6in beam and depth of pressure hull 17ft 0in and 19ft 0in respectively and h of outer hull 25ft 6½in.

Draughts are given in Paragraph 7.

17.4 Displacement

7. Both the surface and submerged displacements in these vessels with external mines varied with the number of mines on board. The two extremes with the full complement of 50 mines on board and no mines are given. The legend surface displacement of Porpoise was 1770 tons and the standard cement was fixed at 1500 tons.

After completion, displacements with 50 mines on board were submerged 2053 tons surface with main tanks empty 1768 tons. Main tank capacity was therefore 285 tons. The reserve of buoyancy was 16.1%. As inclined the surface displacement was 1782 tons so that 14 tons of main ballast water remained in the tanks. In this state the reserve of buoyancy is 15.3% and draught 16ft 0in.

Without mines corresponding figures were submerged displacement 2015 tons, surface displacement, with main tanks empty, 1730 tons and reserve of buoyancy 16.5%. As inclined 14 tons of water remained in the main ballast tanks with a surface displace-of 1744 tons and reserve of buoyancy 15.6%, at a draught of 15ft 8in.

In the low buoyancy condition the displacement was 1907 tons and draught 17ft 4in. These figures apply approximately with and without mines since the main tanks blow varied, see Paragraph 10.

8. Because of the change in form in Grampus, the submerged displacement and main ballast tank capacity both increased by over 100 tons. In the legend state with 50 mines on board the submerged displacement was 2155 tons and the surface displacement 1752 tons. The standard displacement was fixed at 1520 tons. Using these figures the reserve of buoyancy becomes 23% a considerable improvement on Porpoise. Without mines the legend figures for displacement were submerged 2117 tons and surface 1714 tons.

9. All vessels had balloon tanks, i.e. pressure tight tanks, in the superstructure forward to balance the buoyancy of the mines aft and prevent the submarine diving stern first. The capacity of these tanks is included in the submerged displacement.

17.5 Stability

10. Porpoise as inclined with 50 mines on board had a surface GM of 9.2in, submerged BG 8.6in and low buoyancy GM with Nos 1, 2, 5 and 7 main tanks blown 3.5in. Without mines surface GM 15.3in, submerged BG 9.8in and low buoyancy GM with Nos 1, 2 and 5 main tanks blown 8.4in. The extra main tanks No 7 were blown when carrying mines so that the displacement and draught with and without mines was approximately the same at about 1907 tons and 17ft 4in. respectively.

The low buoyancy stability with mines on board was not good and the submarine listed on surfacing. The latter was improved by cutting extra holes in the casing. The 4.7-inch gun was also replaced by a 4-inch gun to conform with the latest submarine policy. The stability eventually became surface GM 10.4in, submerged BG 9.5in. and low buoyancy GM 4.5in. when carrying 50 mines.

The stability in Grampus was much better with surface GM 12.6in submerged BG 11.7in and low buoyancy GM 6.5in with 50 mines.

17.6 Speed and Endurance

11. The design surface speed in Porpoise was 15 knots at 3300 bhp. On trials 16.16 knots at 3385 bhp was obtained. For Grampus the design speed was 15.75 knots at 3300 bhp and 16 knots at 3200 bhp was obtained on trials.

12. The design surface endurance for Porpoise was 3860 miles at full speed and 12800 miles at economical speed. About 1935 the operational endurances were given as 3860 at 16 knots, 9700 miles at 10 knots and 11500 miles at 8 knots. The Grampus endurance was less than in Porpoise the design figure being 3480 miles at full power. This endurance is considered optimistic. See Paragraph 15(b).

13. In both Porpoise and Grampus the designed submerged speed was 8.75 knots at 1630 bhp. On trials Porpoise obtained 8.95 knots at 1670 bhp.

14. The design submerged endurance was 64 miles at 4 knots. Submerged endurances quoted for Porpoise after completion in (a) war condition with auxiliary load of 125 amperes, and (b) peace condition with auxiliary load of 175 amperes were:

(a)  (b)
9 0.9 0.9
4 16.4 15.3
2.5 37 32

17.7 Tanks


  • (a) Porpoise had 285 tons of main tank capacity in external tanks and in the bow buoyancy tank. With the changed form in Grampus this was increased to 403 tons which improved the reserve of buoyancy and stability. This increase was in part at the expense of oil fuel,
  • (b) Porpoise carried 155 tons of oil fuel mainly in external tanks. In Grampus the oil fuel was in internal tanks as mentioned in Paragraph 5 with a capacity of 119 tons which was sufficient for the endurance asked for of 6000 miles at 12 knots. However the after main tanks No 8 Port and Stbd were arranged to carry oil fuel in an emergency to bring the total to 135 tons to give 10 000 miles at economical speed.

17.8 Main Machinery

16. Two Admiralty designed vertical 4-stroke blast injection six cylinder diesels developed a total of 3300 bhp at 400 rev/min. On trials 3385 bhp was recorded.

17. Tandem sets of motors on each shaft developed a total of 1630 bhp at 272 rev/min. This would be for one hour. The continuous rating was 1320 bhp at 253 rev/min. The motors were guaranteed to give 1800 bhp for ½ hour and also 940 bhp continuous with fans and coolers stopped.

Auxiliary drive motors were not fitted. With the armatures of the main motors in series, the bhp per shaft could be varied from 225 to 40 bhp over a speed range of 177 to 94 revs/min at 200 - 220 volts.

18. Three batteries each had 112 cells of normal capacity of either Chloride or Tudor type. The weight of 336 cells was 139 tons.

17.9 Armament

19. Six bow 21-inch tubes with twelve torpedoes.

One 4.7-inch gun was originally fitted in Porpoise but this was changed later to a 4-inch gun which had been chosen as the standard gun to be fitted in RN submarines. This saved about 7 tons in weight.

20. An endless chain type minelaying unit was fitted outside the hull in the super structure casing to carry 50 mines. The mines and minelaying gear weighed approx 54 tons.

17.10 The M3 Trials

21. The M3 trials as a minelayer showed the minelayer gear and compensating arrangements to maintain trim to be satisfactory except that the mine launching gear, which was of the jigger type, required excessive upkeep to ensure its efficiency. In Porpoise this gear was changed to a chain and rack system

M3's diving qualities had however been adversely affected by the conversion and this applied especially to the time to dive, which took about 5 minutes in calm weather and 13 minutes or more in rough weather. This was due to the long time it took to flood the mine casing. By careful design arrangements in Porpoise, the time to dive with mines on board from full buoyancy to periscope depth was reduced to 1 min 32 secs and using Q tank to 1 min 14 secs.


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Chapter 16: Small Patrol Type: Swordfish and Shark Classes