Home - Dits & Bits - BR3043 - Part Two - Progressive Development of Design and Equipment - Chapter 24

Chapter 24: HP Air Services

24. 1 HP Air Compressors

1. In the Holland boats anhp air compressor of American make was sited over the main motor and geared to the main shaft gearing so that it could be driven by the main engine. The same gearing also operated the main bilge pump. Driven at 500 rev/min the compressor output was 8ft4of air per hour at a discharge pressure of 2000lb/in². The separator and its tanks and the cooler for the compressor were placed below the engine room flat on the port side of the engine.

2. Starting with A1 motor driven four-stage inverted water cooled air compressors were adopted. In A1-13 and B1 the two compressors were on a common shaft driven by a 25hp motor and at 500 rev/min discharged 12ft4of air per hour at 2000lb/in². This same motor also drove one of the bilge pumps. In B2-11 and the C Class the one compressor only was driven by its own 30hp motor and had the same capacity of 12ft4per hour at 500 rev/min. The discharge pressure rose to 2500lb/in².

In D1, which was ordered and completed before C19-38, two of the C Class type compressors were fitted and the motor of one of the compressors also ran a bilge pump. In D2, similarly with two compressors, the motor of each compressor also ran a bilge pump. The output of each compressor was 13. 5ft4per hour.

3. Up to now the air compressors had been sited in various positions in the vessel as follows:

  Number of
Holland 1 Near main motor
2 Control room Pt
C Class 1 Engine room
D1 2 Torpedo tube flat
D2 2 Control room Port and Stbd

4. In D3-8 the two compressors were placed aft of the main motors and driven through gearing and clutch gear from the main shafts, one from each shaft. The output was 22. 5ft4per hour at 2500lb/in² at 350 rev/min. Compressor cooling water was circulated direct from the sea. This size of compressor was fitted in the E Class, H Class, K Class, L Class and M Class and Nautilus and it is to be expected for the sake of consistency in the F Class and J Class. They were of the C Class type in the V Class.

The two compressors were mostly gear driven and in the machinery spaces. But there were exceptions as in the K Class with one motor driven compressor in the officers quarter's forward and the other aft in the generator room which it is stated was driven direct from the end of the generator shaft. The discharge pressure remained at 2500lb/in2.

5. After the L Class, driving the compressors through gearing from the main shafts was dropped. In the R Class the one compressor was sited on the port side of the engine room driven by a motor which also worked a bilge pump. The twin shaft H boats probably had the same arrangement.

In X1, one compressor was abreast the officer's quarters forward and the other aft in the motor room. Each compressor motor also drove a bilge pump.

Taking the Odin Class as typical of the post-war designs, the two compressors were of the two-crank four-stage motor driven type each giving 15ft4of air at 3500lb/in2 in 80 minutes. It is likely that the increase in pressure from 2500lb/in2 to 3500lb/in2 started in X1 ; it was definitely used in Oberon.

24. 2 LP Blowers

6. The first LP blowers were fitted in E5 and were motor driven to discharge at 15lb/in above atmosphere. They were fitted in all subsequent boats. In the L Class the two machines each gave 750ft4of air per minute to a gauge pressure of 11lb/in2 with valves arranged so that each main tank could be blown independently of the others. As typical of the 1920 designs the two rotary LP blowers in the Odin Class each delivered 800-850ft4of air per minute at 11lb/in2 above atmospheric pressure for 30 minutes and at 15lb/in2 above atmosphere for at least 15 minutes. The after blower was driven by a combination motor and reducer which could be used as a reducer when the blower was not working.

The LP blower was nearly always combined with some other service. As examples, in X1 the forward blower motor also worked the A end for the capstan and the hydroplane housing gear, the second LP blower was driven by the auxiliary drive motor and the third LP blower motor drove a bilge pump. In Perseus one blower was coupled to the capstan motor and the other to the auxiliary drive motor.

24. 3 Air Bottles

7. Some details of the number of air bottles carried, their total capacity and pressure are shown in Appendix V. The figures given are the best assessment that can be made of the boats as finally finished. For example for the Holland Class Vickers state that 47 bottles with a total capacity of 60ft4were fitted. Plate 1 shows 47 bottles as drawn although the group numbers have been altered to a total of 53. DNC later gave 53 bottles with a capacity of 60ft4. It is obvious that six bottles were added at some time to make the total of 53 bottles with a capacity of about 68ft4. The capacity of each bottle was 1. 28ft4 and the maximum storage pressure 2000lb/in2.

8. In the early boats air bottles were stowed horizontally mainly in groups of five or six over the side tanks in way of the batteries except for a few generally in the extreme forward end.

In A1 the size of the air bottle was increased to about 1. 7ft4. Each bottle was approximately 6½ft long by 1ft diameter.

This size of bottle was retained in the classes that followed until it was changed for the E Class. In the A Class, B Class and C Class the bottle stowage of the Holland pattern was followed of groups above battery flat level at the sides with two groups near the torpedo tubes. In general all bottles were forward of amidships.

A1 had 48 bottles with a total capacity of about 82ft4. From the records it would appear that it was intended originally to fit only 34 bottles in A2-4 but this was increased later to 48 bottles as shown in Plate 3. This must have been a period of experiment with the actual quantity ofhp air required. Only 41 bottles were fitted in A5-13 and a note by DNC suggests that eventually the number in A2-4 was reduced to 41. This was logical, if only to save weight, which was critical in all these boats.

9. The quantity ofhp air carried naturally increased with the increase in size of succeeding classes as shown in Appendix V. The storage pressure was governed by thehp compressor at 2000lb/in2 until the compressor discharge pressure changed in B2 onwards to 2500lb/in2. In spite of this, the bottle storage pressure in B2-11 and C1- 18 remained at 2000lb/in2, presumably because the bottles had already been ordered to the lower pressure or stronger bottles could not be made available in time. There was an increase in weight of 2. 2 tons in thehp air service in C19 i. e. an increase of 33% over that in C18 undoubtedly due to the heavier bottles and fittings required. From C19 the air bottle storage pressure was 2500lb/in2 and remained so until the 1920's.

10. So far some of thehp air bottles had been stowed against the frames above the main flat using valuable space. In the D Class the number of bottles had increased to 108 but the amidships portion of the boat was kept clear by stowing 32 bottles in the externals port and starboard amidships. This cannot be accepted as a good way of obtaining more space in the pressure hull and could bring difficulties operationally and in maintenance.

11. In the E Class the bottles were increased to 2. 74ft4each as against 1, 7ft4in the A to D Class and the number reduced to 57. Of these 51 were stowed vertically in the spaces abreast the battery tanks and the remaining six under the torpedo tubes forward. In the later boats of the class an air bottle space was built below the torpedo room flat to take 12 bottles the remainder being abreast the batteries. A tidy arrangement had now been achieved in the lower half of the boat freeing space above for other essential equipment and living space but this was at the expense of ease in removal of the bottles.

12. The pattern had now been set, which later classes in general adopted, of keeping air bottles below the main flat and out of the normal working and living spaces. In some vessels special air bottle storage compartments or tanks were allocated to take some of the bottles; in the Swordfish (1913) some of the bottles were stowed inside the battery compartments.

The E Class bottle with a capacity of 2. 74ft4was retained in all classes until after 1920 except in two cases. In the V Class the bottles originally fitted or intended to be fitted were only about 2. 0ft4capacity and there were 35 of them; they were later changed to the standard type. The bottles in the W Class had the same capacity as those in the Holland boats. Being a French design this was probably the standard size adopted by that country and the bottles, as for the engines, supplied by France. This class was sold to Italy soon after completion. Swordfish (1913) used many different sizes of bottle as mentioned in Paragraph 17.

13. In the 1920's the working pressure of the bottles was increased to 3500lb/in2. The bottles were of forged steel of ultimate tensile strength not less than 45 tons/in2, had a capacity each of 8. 375ft4and weighed 13. 56 cwt and were tested to 5600lb/in2.

24. 4 Piping Systems

14. Fig 24. 1 shows the arrangement of air blowing pipes and connections in A5-12 and the scheme shown applied in principle to the A Class, B Class and C Class. Pipes were led from the air bottles to manifolds with about five or six bottles to each manifold as far as practicable. There was only one valve on each manifold so that if one bottle or its connecting piping leaked all bottles on that manifold were out of use. The manifolds connected to a small ring main. A simple schematic arrangement for A2-4 when originally carrying only 34 bottles is shown in Fig 24. 2.

15. LP air was obtained from thehp line through reducing valves at 50lb/in2 to the gasoline, trimming, firing and WRT tanks and at 10lb/in2 to the main, auxiliary ballast and buoyancy tanks. By-pass valves were fitted to the 10lb/in2 reducing valves to the main ballast and buoyancy tanks to allow the blowing pressure to be increased in an emergency. A shore charging connection allowed for charging the air bottles through thehp line.

16. Typical piping in the system was as follows:

  • 2000lb/in2 main line and connections to bottles. ½in.
  • Blows to main and auxiliary ballast tanks, trimming tanks and the buoyancy tank. ¾in.
  • Auxiliary 50lb/in2 service to gasoline tanks and tank jackets, torpedo tube firing tanks and WRT tanks. 1in.

17. From the simple arrangements shown in Fig 24. 2 thehp air services obviously increased over the years in size and complexity. As a matter of interest thehp air service in Swordfish (1913) was certainly the most extensive of its time when built and perhaps since.

The twohp compressors in this boat were fitted in the engine room. A total of 151 air bottles were stowed in the forward half of the boat mainly horizontally but some vertically in seventeen groups. There were eighteen different sizes of air bottle ranging from 10ft 3in long by 8⅞ in diameter to 3ft long by 107/16 in diameter. Each of thehp air bottle group connections was brought to a master valve box in the control room and from this box the various connections were tapped off for blowing and other purposes. The blowing valves for all main ballast and compensating tanks were in the control room and these tanks could also be blown from the forward or after ends of the boat. Each air bottle was water tested to 4000lb/in2 and tested in situ by being pumped up to 2500lb/in2 and allowed to stand for 24 hours in which time the loss in pressure was not to exceed 50lb/in2. Allhp fittings in the system were tested to 4000lb/in2 and LP fittings to 120lb/in2.

18. In the K Class the three blowing position idea was continued, one at the forward end of the control room, a second in the broadside tube space and a third in the motor room. Each station had its own valve panel for venting and blowing the tanks concerned. Electrical order boards were fitted at each station for the transmission of orders from the Commanding Officer in the control room. This would appear to mean splitting up the boat into sections for separate blowing of the tanks in each section rather than the more complicated arrangement in Swordfish of being able to blow all tanks from each position.

Fig 24. 1

Fig 24. 2

19. In the L Class blowing was centrally controlled with leads from the 2500lb/in² system to the external main tanks loaded so that the pressure in the tanks could not rise above 15lb/in2 and to the internal tanks loaded so that the pressure in the tanks could not rise above 50lb/in2.

20. By the time the Odin Class were completed the blowing of tanks from thehp line was limited to main tanks without Kingstons, to the auxiliary tanks (which had taken the place of trimming tanks), to Q diving tanks and to O compensating tanks in the externals. The LP air line had been abolished except to the WRT's. Further details of the changes at this time are given in Chapter 14 Paragraphs 52 and 53.