T Class Conversion
With associated notes on the new-build 'P' and 'O' Classes that followed the Conversions into service.
This is a summary of the key points in BR1965 the Hand Book (Electrical propulsion Equipment) for the T Conversion Class and the First of Class Trials of the first conversion, HMS Taciturn, to FOSM May 1952 - that enable a satisfactory understanding of the electrical propulsion conversion of the T Class to meet the object stated in FOSM letter, 546/SM.472 20th May 1948:
A fully operational submarine with maximum possible submerged speed and endurance, capable of carrying out a war patrol continuously submerged
Included are two unofficial circuit sketches drawn after a careful study of the detailed textual description of the propulsion circuits, as the document drawings of the actual propulsion electrical circuits are missing. Also included are two unofficial circuit examples of basic submarine switches. Also attached are BR1965 drawings, one of the general propulsion layout and three showing the cubicle panels control and indication equipment.
To meet the main object of maximum submerged speed, it was required to increase electrical propulsion power involving a hull extension and streamline the outer hull. As the itemised summary is limited to describing the converted electrical propulsion system, a few brief notes on the streamlining and the hull extension may be of value.
Streamlining included the reluctant but necessary removal of all stern facing torpedo tubes, added externally to the original T Class after war experiences, causing the FOSM to press for gyro-angled torpedoes and associated control to make these submarines efficient in anti-submarine operations.
Roger Fry (submarine author) had contact in later years with a retired draftsman formerly employed at the Royal Dock Yard, Chatham who was involved with the conversions and he sent Roger a handbook on the subject of streamlining and advised that the techniques of noise reduction and related hull resistance were initially not well known and the result was that the earlier boats were relatively noisy with greater hull resistance - such resistance as described might well have taken away a knot at full submerged speed. The first conversions were apparently improved in future refits.
Roger Fry's article archived RNSM (1992/026) published in the SHIPS MONTHLY February 1990, describes much of the hull extension method employed, he was later able to locate more definitive information that was unfortunately not published.
In brief, the first four conversions had 14 feet added to the engine room, amidships, allowing the existing engines and electric motors to be moved forward to create space aft for a further pair of motors. No hull extensions were needed for the additional battery section fitted under the Control Room.
However in the last four conversions, two hull cuts were made about 1.25 feet into the control room and about 1.25 feet into the Engine Room thus removing the ER/CR bulkhead and rebuilding the bulkhead complete with original door into a new hull section of about 20 feet. Again adding 14 feet to the ER but increasing the length of the CR by about 3.5 feet. All the main control room equipment, masts etc were moved aft, in particular the new conning tower hatch. Apparently to make the usual bad weather water pouring down the hatch more manageable in terms of equipment damage, but it seems the main gain was in improving the surface instability suffered by the earlier conversions due to loss of buoyancy and the need to remove ballast, thus moving the center of gravity. Extensive later use of aluminium in free flood structures also helped reduce weight.
Despite serious earlier doubts that these older, T Class 220 volt 725 bhp motors, could have been reliably run with a nominal 440 volts applied, developing a nominal 1500 bhp (3000 bhp per shaft), BR1965 confirms in detail that all four tandem motors (typical double armature submarine units) were of the original T Class design (Laurence Scott and Electromotor T/A. 4212) but strengthened as noted below.
The existing unmodified engines and the strengthened existing motors, were moved forward into the space created by the hull extension in the forward engine room effectively lengthening the motor room sufficiently for an additional strengthened motor on each shaft. The tail clutch was removed on each shaft and strengthened thrust blocks installed. A new clutch was inserted between the propulsion shafts of the original motor and the new one.
The shunt fields were arranged by series/parallel switching, so they were always supplied with 110 volts.
The motors were strengthened by:
- Rebanding the armatures to withstand the increased centrifugal forces.
- Redesigning the commutator VEE rings and bolts to prevent eccentricity of the commutators caused by accelerated heating of the segments during high speed running.
- Strengthening the interpoles field and readjustment of the interpole gaps to ensure good commutation.
- Strengthening the series field to facilitate good load sharing.
The new high capacity individual battery cells were type 6560, (6560 Ampere Hours at the 5 hour rate), designed to give a very heavy discharge over a half hour period. The number of cells was increased by the addition of another 112 cell battery under the control room deck giving a total of 448 cells in four sections. The dimensions and mass of the new cells were similar to the original as-built cells of 4750 A/Hr and the later 5350 A/Hr.
As a comparison, the later and larger, Oberon Class submarine had 448 cells of 7420 A/Hrs at the 5 hour rate arranged in two batteries.
There is no mention of using air agitation to mix the electrolyte in the individual cells and avoid layering, that when fitted, minimised the need for extending battery charges to create gassing, thus reducing the risk of excessive hydrogen. This was an important feature of the US Navy Guppy conversion as were water cooled cell interconnections also apparently not fitted in the T Class converted to Fast Battery Submarines. All features of the later Royal Navy Porpoise Class and Oberon Class. A brief anecdotal reference to battery cooling water and difficulties in the freezing temperatures of Northern Seas, suggests that perhaps these features were fitted after 1953, the date of the this BR1965. Possibly only to the later conversions. A former Royal Navy S/M CO advises that gassing became unacceptable in Royal Navy submarines due to the increasing risk of hydrogen gas explosion, this being most definitely the case in the last Royal Navy Diesel-Electric submarine, the Upholder Class.
Each battery section was protected by a hand closed, electrically tripped Battery Circuit Breaker fitted with overload tripping and trip buttons on the starboard motor switchboard. The breakers were fitted close to their respective battery sections. These replaced the battery fuses in earlier submarines. Each Battery Circuit Breaker could electrically opened by pressing a Push Button fitted on the Starboard Forward Motor Switching cubicle.
Battery currents were monitored in the control room due to the heavy discharge and short endurance at maximum submerged speed.
It is quite clear that two 112 cell battery sections were exclusive to the port side propulsion and two to the starboard, with no possibility of the four sections being ever connected together. The unacceptably high fault current is given as one reason but it also made switching much simpler by having two quite independent electrical propulsion systems - port and starboard.
Provision was made to feed port and starboard electrical propulsion from two battery sections, but this emergency linking arrangement prevented any possibility of connecting the four sections together.
All the following notes and drawings refer to the starboard side but apply equally to the quite separate port side. Only Fig 1 shows the two basically identical but quite separate sides.
It is important to note that the Aft Motor could not be used for generating. Generating was exclusive to the Forward Motor. In direct drive with all clutches engaged, the Forward Motor could be used to 'float the load' or charge when the diesel engine was propelling the submarine on the surface. The Aft motor was not excited in this situation. It is notable that the battery charging capacity remained the same as the unconverted T Class and therefore considered adequate to charge the much larger batteries.
Interlocks prevent the Forward Motor from been used alone for propulsion. It would seem the ASTERN markings on the Forward Hand Control Wheel were merely for indication when in combined control. However it would be possible to disengage the interlocks to allow the Forward Motor to propel alone, in either direction, in an emergency.
The Motor Shaft Clutch disengaged and Engine Clutch engaged, allowed the Forward Motor to be used as a generator, charging the batteries and supplying propelling electrical power to the Aft Motor in a diesel-electric mode. Interlocks ensured this mode could only achieved when the two Main Batteries were connected in parallel - that is a battery bus of 220 volts. In this mode it should noted that the T Class motors were ill-matched to the engine and unable to transfer the full power of the engine to the screws. The efficiency at full power in the diesel-electric mode was 78% according to the Taciturn trial document. However this mode provided operational advantages by always having the aft propulsion motor on the battery bus.
Though it appears to have been technically possible to snort in direct drive and gain greater speed, clearly it was intended from comments in BR1965,that snorting would always be carried out in the diesel-electric mode. Interlocks prevented the Aft Motor being used as a generator in direct drive.
Attached at the end of this document are two edited tables from BR1965 to give some idea of the numerical data for charging while snorting in diesel-electric drive, the preferred, if somewhat inefficient mode.
Table references to 325 volts have been edited out as comments in BR1965 make it plain that this regime involving gassing, was not likely to be allowed in future practice. (see earlier comments on gassing and air agitation of the electrolyte). Hydrogen management was increasingly important in modern submarines, especially as batteries increased in size.
The use of direct drive was an approved mode when on the surface, taking advantage of the full engine power. Anecdotes say diesel-electric mode was used in bad weather to reduce screw and engine racing and in harbour manoeuvring.
It is noteworthy that all unconverted T Class submarines snorted using the same engines in direct drive, there was no practical option. However the unconverted hull likely restricted submerged speed.
Shown on Fig 3b left hand side top, are the Generating Ammeters and Voltmeters (scale maximum 2000 amps per armature, machine and battery volts 300 full scale) for the Forward Motor only when used as a generator. When motoring there were four motoring ammeters (2500 amps) for both Forward and Aft Motors on the RH side top. Fig 3a shows the two battery dual ammeters (scale maximums - 2000 amps charge, per each of the two battery section and 10,000 amp discharge per battery section).
Each of the Tandem Motors (dual Armatures) had a Motor Switching Cubicle suspended on resilient mounts with flexible cable connections.
Each cubicle had a handle operated Armature Grouping Switch and a handle operated Field Switch. The forward cubicle had a large Tandem Motor Grouping Switch operated by a substantial hand wheel while the aft cubicle had a large Battery Grouping Switch also operated by a substantial hand wheel. There were various indication and control items. Fitted within each cubicle were the Armature fuses, one Circuit Breaker and one Motor Controller Unit.
These major items of motor control switchgear are described in ITEMS 6 (Grouping) and 7 (Motor Controller).
The Forward Motor Switching Cubicle Fig 3a was joined to the similar Aft Switching Motor Cubicle by a recessed Control Panel Fig 3b . The Port and Starboard cubicle sets formed the main passage between the engine room and the aft compartment where the stoker mechanics messed. A little imagination evokes the chaos on the sounding of the klaxon as the stokers tumbled through the motor room towards their diving stations just as the electrical propulsion watch keepers were setting the switchboard for diving! Fig 1.
The grouping switches that provided for series/parallel connection of the Batteries and the Tandem Motors were in principle, similar to the armature grouping switches on the preserved Alliance see Fig 2c , but in the conversion fitted as complete units within the motor switching cubicles with external operating handles and hand wheels.
The Battery Grouping Switch was in the Forward Motor Switching Cubicle and the Tandem Motor Grouping Switch in the Aft Motor Switching Cubicle, there being only one each of these items per side. These two large grouping switches, while not identical in detail, were basically similar in construction and both were operated by a substantial hand wheel. See Fig 3a . The main difference being that the Battery Grouping Switch had an additional set of contacts that ensured the Motor Fields were always supplied with 110 volts for each field regardless whether the battery bus was 220 volt or 440 volt. The basic switch circuit is shown in Fig 2c .
The smaller Armature Grouping Switches were virtually identical to those found on the preserved Alliance and the unconverted T Class, Fig 2c but with only the operating handle in view. The same applies to the usual Field Isolating Switches, one for each Tandem Motor, Fig 3a .
Each switch is complete as a unit before installation in the cubicles.
Main Motor starting and the Ahead or Astern switching was in principle similar to the preserved Alliance and unconverted T Class, but was implemented using a remarkably compact rotary Motor Controller unit, one Controller unit for the Forward Motor mounted in the its associated switching cubicle and one for the Aft Motor mounted in its associated switching cubicle.
Each Motor Controller unit consisted of a series of knife switches operated by cams on a shaft and positioned by Geneva wheels, connected to a substantial hand wheel mounted at the front of the control panel by a system of rods and gears. See Fig 2a & b . For the basic circuit principles of the reversing switch see Fig 2b , however it should be noted the Controller is constructed using two sets of four separate knife blades to achieve the same reversing circuit. A further four blades perform the starting function, see Fig 2b , where a Motor Controller unit is shown in simple electrical schematic form.
The gear and rod drive was, in essence, a more complex version of that found in the preserved Alliance, controlling the field regulators.
The Forward Motor Controller unit was controlled by a hand wheel smaller than the large Aft hand wheel because when the motor propulsion shaft clutch was engaged and both motors were being used for propulsion with the Engine Clutch out, both the Forward and Aft motors would then be controlled by the larger Aft control wheel alone, by engaging a control rod - Combined Operation Hand Clutch, thus connecting the Forward and Aft Motor units controller rod systems together. See Fig 3b and the magnified Fig 3c .
The choice of Ahead or Astern was determined by the initial direction of the control hand wheel closing the appropriate Ahead or Astern cam operated knife switches, then as the control was turned further, a mechanically operated, latching, electrically tripped, double pole circuit breaker was closed by a mechanical connection from the Controller unit (one pole for each Armature, fitted with over current tripping).
At this point the maximum value of resistance, limiting starting current, was in circuit. As the hand control wheel continued to be turned, the cam operated knife switches reduced the starting resistance value, in progressive steps until finally the whole resistance was cut out and both Motor Armatures were directly across the supply. See Fig 2a & b .
To stop the Motors, the Push Button on the control cubicle was pressed to open the Circuit Breaker, then the control hand wheel was required to be turned to neutral before any further action could be taken. As with older submarines, the field regulator(s) were first run to maximum field current, reducing the armature current to the minimum for the particular group, before disconnecting the Armatures from the supply.
Thus the Circuit Breaker was used to make the initial starting circuit and break the running current when stopping, reducing the sparking at the other switches involved in the control process. In speed ranges G1/G4 (Item 7) either of the Circuit Breaker Trip Buttons will trip both Forward and Aft Motor Breakers.
When the Forward Motor was acting as a generator it was desirable that the Circuit Breaker of the Forward Motor be tripped from the Engine Control Platform, for this purpose a Push Button was provided that was disabled when the Engine Clutch is out. Also when charging, if the generating Motor Circuit Breaker tripped, the associated Engine was automatically shut down.
An extensive system of interlocks prevented incorrect operation of any of the controls. The round Emergency Interlock Disabling Knobs can be seen on Fig 3c . Similar knobs can see on the preserved Alliance switchboards.
The combination of the Battery, Armature and Motor grouping switches, enables four groups detailed below. G1 had the two Batteries in series, two Armatures in parallel and the two Tandem Motors in parallel, that was all four Armatures in parallel. This gave the maximum nominal 440 volts across each Armature and thus the maximum speed range. (475 to 615 rpm), 3000 bhp at maximum rpm. Pessimistic comments on usefully maintaining 615 rpm for speed trial purposes are noted in the Taciturn First of Class trials report Page 10. 15 knots at 590 rpm at 90 feet, (5760 bhp - both shafts), was regarded as maximum speed for further trials.
G2 had the two Batteries in parallel, two Armatures in parallel and the two Tandem Motors in parallel, that was all four Armatures in parallel. This gave a nominal 220 volts across each Armature.(260 to 474 rpm.) 1340 bhp at maximum rpm.
G3 had the two Batteries in parallel, the Armatures in series and the two Tandem Motors in parallel. This gave a nominal 110 volts across each armature (135 to 260 rpm). 224 bhp at maximum rpm.
G4 had the two Batteries in parallel, the two Armatures in series in, two Tandem Motors in series, which were four Armatures in series giving a nominal 55 volts across each Armature (70 to 135 rpm). 32 bhp at maximum rpm.
The shunt field regulators would be used, as in older submarines, to vary the speed within the range of a particular grouping (see bracketed rpm). Additional variable resistors were provided to balance the field currents. The group switching circuitry is shown in block form in Fig 2a .
In the preserved Alliance there is a rotating machine called a REDUCER that served to provide a back EMF to maintain the Constant Pressure ring main at 220 volt as the supply voltages rise during battery charging. This machine was also found on the unmodified T Class but in the conversion was replaced by two Auxiliary Motor-Generators, one controlled from the Starboard controller panel and one from the Port, to supply constant voltage auxiliary supply power to the CP ring main. A number of alternative connection options were available for auxiliary supplies.
As in the preserved Alliance there was a Group order receiver but extended to order G1, G2, G3, G4, but in addition in the same instrument case, there were clutch orders IN and OUT for the Engine and Motor Clutches. There was also a separate charge order indicator: CHARGE - CHARGING and BREAK CHARGE - CHARGE BROKEN.
The usual telegraph indicated FULL, HALF, SLOW - AHEAD and ASTERN, but unlike the Alliance there was no reference to the engine clutches.
There was the usual press button for the Control Room order acknowledgment gong.
There was also an order indicator for rpm along with the usual Main Motor Tachometers. Only the rpm of the After Motor(s) was indicated in the Control Room.
There were four battery section pilot cell temperature indicators in the Control Room monitoring the batteries that would get very hot when maximum current was being drawn at full speed in electric drive.
There was one further order instrument - it appears to have been a command to the Engine Control Platform from the Motor Room when the diesel engine was operating solely as a drive for the Forward Motor working as a generator with the Motor Clutch OUT - a straight forward diesel-electric generating set! STOP - SPEED 1 - SPEED 2 - SPEED 3 - SPEED 4, then INCREASE - STEADY - DECREASE.
The Electrical propulsion of the New-Build Porpoise Class & Oberon Class of Submarines in service with the Royal Navy and others.
The recent acquisition of a Canadian Navy training notebook describing the electrical systems of the Oberon Class submarine has enabled this section to be usefully enlarged. The Canadian notebook while valuable, lacks a lot of the detail contained the comprehensive BR1965 manual used to prepare the main article and therefore it is still beneficial to refer to the detail of the Converted T Class to get a more full understanding of the Porpoise Class & Oberon Class electric propulsion.
It is apparent there was little, if any difference, between the electrical propulsion circuits of the Porpoise Class class from the very similar, but improved Oberon Class and they will be treated as one class in this section. It is notable that these were the last twin screwed diesel submarines in the Royal Navy.
There is little or no discussion here of the generator indication etc, auxiliary controls and indication devices that are also part of the motor room or the quite complex auxiliary and emergency system involving the several motor generators and motor alternators. It is noteworthy that the batteries are centre tapped for 220 volt for emergency use.
Numerical data for the new Porpoise Class & Oberon Class that followed the T Class conversions in service with the RN, is shown in Appendix F. This Guppy article is mainly devoted to the USN early Cold War work to achieve faster submerged speed, but it is also useful to compare the electric propulsion detail of Porpoise Class & Oberon Class to these conversions.
The Porpoise Class & Oberon Class propulsion system was basically similar to USN Fleet submarines in that there were diesel generator sets to supply electric power to the main bus bar to either charge the battery or supply the twin main motors or both. In the case of the Porpoise Class & Oberon Class, individual circuit breakers were used to connect each item to the main bus. Only the main electric motors were connected to the propeller shafts driving the screws. The situation in the converted T Class was similar, but being a conversion was also able to set-up direct drive between the diesel engines and the propellers, as described in the main article. This has no relevance to the Porpoise Class & Oberon Class.
Technically oriented readers will have noted the Converted T Class is quite a complex electro-mechanical mix of the original submarine with simple direct drive and the Conversion where an additional double armature motor was added aft on both shafts. The forward motor on each shaft could used as a motor or a generator, while the Porpoise Class & Oberon Class diesel-generators are quite mechanically separate, only being used to generate electric power as is the case with all modern submarines, however the detail of the various switching arrangements of the converted T Class aids understanding of those used in the new-build Porpoise Class & Oberon Class.
The new Porpoise S01, was completed in 1958, but according to the official Particulars of Vessels (1959) the programme year was 1951, as it happens the same year the first T Class conversion, Taciturn, was completed. It is perhaps not unreasonable to assume that design aspects of the T Class Conversion were applied to the Porpoise Class class where appropriate.
From the First of Class speed trials document (1952) for the Taciturn, the T Class conversions of the main article, developed 5780 shp and achieved 14.84 knots for 56 minutes at 90 feet. With the comment recorded that 15 knots could be regarded as the maximum submerged speed at 90 feet.
While first of class trial data for Porpoise or Oberon Class is not available, the estimated figures in the Particulars of Vessels (1959) in regard to Porpoise, states 6000 shp at an achieved 17 knots (other sources give an endurance 20 to 30 minutes) in temperate waters with a clean hull, while the more precise mean figures on trial, are given as 5331 hp for 16.45 knots. When considering these figures, readers should recall the cube law, to double the speed of any particular hull requires the power to be increased eightfold.
As the above figures show, the new Porpoise and the following similar Oberon Class had a superior submerged performance to that of the smaller converted T Class with similar electrical propulsion power. One can reasonably assume this was attributable in part to the developed streamlined hull of the new-build Porpoise Class & Oberon Class. The Oberon Class being recognised as the quietest design of its era, it is useful to quote from the book by Burcher and Rydill; "Concepts of Submarine Design" page 104. A minimum drag body for high speed will be compatible with that for quiet operations at slower speeds.
Another influence on improved efficiency, were the much larger screws of the Porpoise Class & Oberon Class at maximum 2 x 3000 shp, 7ft at 400 rpm compared to the Converted T Class, 5ft 4inches at 590 rpm. This is a specialist area of naval architecture and obtaining actual calculations of the improvement in efficiency is beyond the scope of these notes.
Regardless of the performance difference, both these submarine classes went on to give long service in the Cold War period. Amongst other duties at home and abroad, they were involved in the long 'observation' trips known in the Service as 'Mystery Trips', close to the Northern Soviet bases. Confirmed by personal anecdotes of crew members and those specialists carried for the radio 'listening". They were usually submerged for most of the outward and inward passages. (Ref: Snorting in the RN)
Motor Control Porpoise Class & Oberon Class Submarines
Port and Starboard Motor Controls are fitted on one single panel located in the Motor Room. (See small image of all the panels below).
|An annotated image of a typical panel is shown above. Move your mouse over the image for more information. The other panels not directly associated with the main motors are not shown|
Before making comparisons between the main motor controls of the Porpoise Class & Oberon Class and the converted T Class described in the main article, it should be appreciated that the Porpoise Class & Oberon Class had two motor armatures per shaft, while the converted T Classes, due to needs of conversion, had four per shaft and two quite separate main motor control system, port and starboard. Any comparison to the converted T Class while discussing the Porpoise Class & Oberon Class main motor control system, refers to one side only with four armatures, as described in the main article.
The Porpoise Class & Oberon Class motor controls for the port and starboard motors were mounted on one single panel, fitted with the usual shaft speed tachometers, traditional telegraph receivers and reply buttons to sound gongs in the control room. In general one can say the electrical propulsion control system was quite straight forward compared to the duplicated converted 'T. Fig 1 , Fig 3a & Fig 3b . However the basic principle used in the RN remained from before WWII, armature groups and battery groups were able to be arranged by switching into series or in parallel, to obtain different speed ranges. What did change after WWII was the use of both battery and armature grouping on the same submarine to obtain high submerged speeds.
Official information (A basic BR 4005 training manual) obtained about new-build Royal Navy Porpoise Class class, indicates the motor control had the same type of camshaft and Geneva stepping wheel main motor starting controller as that of the converted T Class described in the main article, Fig 2b but driven by a cam shaft electric motor rather than the mechanically interlockable hand wheels and drive shafting of the T Class conversion. No detail of the starting process is available, but all indications are it was basically similar to that employed in the converted "T". Fig 2a and 2b .
Two robust hand pistol grip switches were fitted on the motor control panel, one controlling the port starter cam shaft drive motor and one for starboard cam shaft motor. Three control positions, FORWARD, OFF, REVERSE.
In case of a cam shaft motor failure, an emergency hand wheel was provided for manual operation of each cam shaft, port and starboard.
No details are available, but it reasonable to assume the cam starting switches would be similar in construction to those in the converted T Class as those described in the main article and illustrated in BR1965, but with more clearance space due the voltage increase to 880 volts from 440 volts.
The traditional knife blade Armature Grouper Switches for both port and starboard sides in the older submarines and used in the converted T Class, shown in the main article, were discarded in the Porpoise Class & Oberon Class to be combined in one switch mechanism for both port and starboard armatures, manually controlled through rod and gears by a single large handwheel mounted on the front panel presumably similar in construction to the starting switches and stepped/locked into each group position by a Geneva wheel.
It is not clear if there were two adjacent rotary cam switch mechanisms, for port and starboard, driven by the single handwheel through gears and rods or if there was a single rotary mechanism mounting all the grouping contacts on one rotor. The important operating point being that port and starboard group changing was carried out simultaneously by a single handwheel on the motor control panel.
This combination of port and starboard armature grouping, follows the operational practice of older submarines in that had though they had independently operated port and starboard shaft grouping switches of the open knife style, they were always manually placed in the same position, there only being one grouper telegraph. Group Up and Group Down would be the normal speed ranges used on the Porpoise Class & Oberon Class
The single Grouper Handwheel had five positions OFF, SHAFTS in SERIES, GROUP DOWN, GROUP UP, BATTERIES in SERIES.
Table showing the simple circuit diagrams from BR 4005.
It should be noted that on the Porpoise Class & Oberon Class there was an auxiliary method of propelling the submerged submarine should the Main Motor Control Panel require isolation for emergency repair. A system of links and switches disconnected the port forward armature from the Main Control Panel to be then connected to the controlled output of the No 2 Ships Supply Constant Pressure Motor Generator with all the Ship's load taken by No 1 CPMG. The maximum shaft speed was 120 rpm.
These MG sets normally convert the battery voltage that can vary over a large range while charging or on heavy load, into a constant 220 volts for equipment requiring constant pressure. Most of the large auxiliary motor were directly connected through auxiliary switchgear to the battery, being designed to operate over the whole battery voltage range.
As in all diesel RN submarines of the time and earlier, Field Regulators controlled the speed within a chosen grouping, by varying the current through the series connected shunt fields of each armature pair, one hand wheel Port, one Starboard. These regulators were actually quite large variable resistors, taking a significant rear of panel space in the motor room. On the Porpoise Class & Oberon Class, there were also Field Trimmers, smaller variable resistors to trim the shunt fields and hence balance the twin armature currents when the armatures were in Group Up, drawing heaving currents, particularly with Batteries in Series. There were of course Field Supply switches.
Batteries in Series - Highest submerged speed.
As was the case in the main converted T Class article, the increase in maximum voltage applied to the Porpoise Class & Oberon Class main motor armatures for the highest speeds for relatively short periods, was obtained by operating a battery grouper switch to connect the two banks of cells in series. When in lower speed groupings the maximum motor current would be divided between two parallel banks of cells, whereas in series each cell had to provide all the current. This cell increase in current when in series was obviously generated by increased chemical action within each cell and had to be catered for by appropriate cell design as briefly described below for the 7420 Ahr cell of the Porpoise Class & Oberon Class. Hydrogen gas production was increased and battery ventilation effective. Catalytic Hydrogen gas eliminators were fitted overhead in compartments, these were not available in WWII submarines. They are not mentioned in regard to the converted T Class, but were likely retrofitted.
The Porpoise Class & Oberon Class motors followed the general basic pattern of earlier British submarine motors as shown in the main article having two armatures per shaft in one casing. Fig 1 . The maximum current per armature was 1500 amps, a typical upper value for all British submarines of the era, however unlike the converted T Class where the screw design required a substantial increase in normal T Class speed (600 rpm) to develop the high propulsion power, the Porpoise Class & Oberon Class motors and screws were matched to provide similar power at lower RPM, about 400 rpm. Note the tachometers fitted on the Porpoise Class & Oberon Class had a maximum scale reading of 450 rpm.
Forced cooling was fitted as was heating when the main motors were idle and a bearing oil pump/sump fitted.
It should be noted these heavy currents caused a very significant drop in the nominal cell voltage and cells of all submarines have a lower voltage when heavy currents are being drawn by the main motors at high speed and of course power = voltage times amps.
If the specified lower cell voltage is reached, then the current draw-off must be reduced to prevent any cell voltages going below the firmly specified figure. Monitoring the battery or even individual cell voltages was an essential operational requirement.
This series /parallel switching of batteries was not the method used in the famous WWII German XXI Electroboat. Here a fixed battery voltage of 360 volts was used to supply the main motors. This was a significantly higher nominal voltage than earlier U-Boats.
A Brief Comparison of the Battery Cells in British Submarines.
An unconverted T Class from WII had three banks, each of 112 cells permanently connected in series. The three banks were connected in permanent parallel. Post war cells improved from 4750 A hr to 5350 Ahr at the 5 hour rate. 435 kg. Nominal voltage 220.
The converted T Class as detailed in the main article, had four banks of 112 cells connected in parallel, the cells in each bank connected in permanently in series. The four banks were used as two permanently separate pairs that could connected in series or parallel. Each cell was 6560 Ahr at the 5 hour rate, 457 Kg per cell. Nominal voltage in series 440. Each cell of similar dimension to the original T Class cells above.
The P&O class cell was 7420 Ahr at the 5 hour rate, 508 Kg per cell, 448 cells in two banks of 224. Nominal 880 volts in series. These cells had individual air agitation of the electrolyte and water cooled links between cells. A description of the later improved Oberon Class battery ventilation in the RCN, is shown in Snorting in the RN. Similar changes are reported in some later RN and RAN Oberon Class submarines. Charging these large and numerous cells required great care to avoid the accumulation of excessive hydrogen.
Whether cells with electrolyte agitation were later fitted to converted T Class is not known.
To recap: In the converted T Class and the Porpoise Class & Oberon Class, the nominal voltage to the main motors was doubled for high speed by switching to connect two battery banks in series. In the converted T Class this was achieved for each battery pair by a heavy switch controlled by a large handwheel. Fig 3a . One for each side. However on the Porpoise Class & Oberon Class, the two large battery banks were switched from parallel to series, by contacts on the motor grouper switch system as described earlier, controlled by a single hand wheel on the motor control panel.