Paxman Submarine Engines

Paxman's involvement in building diesel engines for submarines goes back to before World War 2. Immediately prior to that conflict the Company was making engines for both British S Class and U Class submarines. A 1945 Paxman publication says the Company constructed "the engines for well over half the British submarines built during the German War". (1)  A knowledgeable ex-RN submariner has recently estimated that Paxman engined something nearer two thirds of the British WW2 submarine fleet.

S Class

Totalling 60 boats or thereabouts, this was the largest single class (by number) of British submarines ever built. The engines, two per boat, were built to a standard Admiralty pattern using a fabricated steel main frame generally along the lines of the Paxman RXS. Each 8-cylinder, 14½" bore x 15½" stroke, unit had a rating of 960 bhp at 460 rpm. Paxman built a number of these engines but was not the only business contracted to make them for the S Class. Engines installed in S Class submarines built by Cammell Laird, for instance, included some made by Paxman, some by Peter Brotherhood of Peterborough, and others made by Cammell Laird itself. The electric motors in the power train were built to an Admiralty design and were rated at 1,300 bhp.

S Class submarines supplied with Paxman-built engines are recorded in the Paxman order book as Satyr, Scorcher, Scythian, Seadog, Sea Rover, Sentinel, Shalimar, Sidon, Sirdar, Solent and Syrtis. However, Ian Buxton has come across Cammell Laird records of engine shop trials which show that Scorcher's engines were built by Cammell Laird and Sidon's engines by Peter Brotherhood. The same records show that the Paxman engines originally destined for Sidon and Scorcher were installed in the S Class boats Springer and Sanguine respectively.

During speed trials one standard S Class submarine, "Sahib" (lost during the War), achieved a surface speed of 14.56 knots at 464 rpm, with fuel consumption of 86 gallons per hour. From 1944 six S Class boats were streamlined and modified. They retained the same engines and motors but were fitted with bigger pitch T Class screws. In this form the S Class achieved a surface speed of 16.75 knots at 460 rpm, with a consumption of 92 gallons per hour.

Like other WW2 Royal Navy submarines, with the exception of the U and V Classes mentioned below, the S Class had direct drive. The power train arrangement was as follows:

Clutch Drive

The electric motors could be used as generators when the diesels were propelling in direct drive, either to charge the batteries or float the hotel (auxiliary) load. Also, in harbour with the tail clutch out, the diesels and electric motors could be used as stand-alone generating sets.

U and V Class

The U and V Class boats were the first British submarines with diesel-electric drive. They were also the Royal Navy's only diesel-electric submarines until the post-War P and O Classes and then the Upholder (now Victoria) Class boats.

The first three Group One U Class boats were ordered by the Admiralty in November 1936 and the other twelve in September 1939. The first ten Group Two boats were ordered in March 1940, another batch of twelve in August 1940 and a final batch of twelve in July 1941. (Total 49 U Class)

A document produced by Paxman in 1945 says the Company manufactured all the diesel engines for U Class submarines. There is evidence that four of the engines, those for "Una" and "Umpire", were built by Chatham Dockyard as is noted in the relevant entry in the Paxman order book of the time. There is also a file in the Public Records Office about Paxman's demands for licence fees for these which caused some consternation among civil servants at the time.

The engines were 6-cylinder RXS types, specially developed for service in submarines. They had fabricated (welded) steel frames which were better than cast-iron for withstanding the shock of underwater explosions and lighter in weight. Each 6RXS produced 400 bhp at 825 rpm and was coupled to a 275 kW generator by a Wellman-Bibby flexible coupling. The engines were made in pairs, left and right handed, so that all controls were between the two engines. Paxman started delivering them (2 engines per boat) in 1938, before war broke out. A listing of the orders for these engines can be found on the page Paxman-Ricardo 6RXS Engines

The 6RXS also powered the V Class submarines, an improved version of the U Class and very similar in design. The first eight V Class boats were ordered by the Admiralty in December 1941, a further 12 in May 1942 and the final two in November 1942. (Total 22 V Class)

Altogether seventy one U and V Class submarines were completed and accepted for service in the Royal Navy and seven different Allied navies. They served with distinction during World War 2, particularly in the Mediterranean where they disrupted reinforcements and supplies to the Axis forces in North Africa.

U and V Class submarines fitted with Paxman-built engines, as recorded in the Paxman order book, were:

HMS Ulex
HMS Unbeaten
HMS Undaunted
HMS Unique
HMS Union
HMS Universal
HMS Unruffled (P46)      
HMS Unruly
HMS Unswerving
HMS Untiring
HMS Upas
HMS Upholder
HMS Upright
HMS Upshot
HMS Upstart
HMS Urchin
HMS Urge
HMS Ursula
HMS Urtica
HMS Uther
HMS Utmost      
HMS Utopia
HMS Vagabond      
HMS Vampire
HMS Vandal
HMS Vantage
HMS Varangian
HMS Variance
HMS Varne
HMS Vehement
HMS Vengeful
HMS Venom
HMS Venturer
HMS Verve
HMS Veto
HMS Vigorous
HMS Viking
HMS Vineyard      
HMS Virile
HMS Virilent
HMS Virtue
HMS Visigoth
HMS Visitant
HMS Vitality
HMS Vivid
HMS Volatile
HMS Voracious      
HMS Vortex
HMS Votary
HMS Vulpine
HMS Curie
HMS Pipinos

Funds for the building of HMS Unruffled were raised by the people of Colchester during Warship Week 1942. For more information about her links with Paxman and Colchester, and her wartime exploits in the Mediterranean, see the page HMS Unruffled (P46).

A detailed history of the development of this Class and its operation during World War 2 is A History of the British 'U' Class Submarine, by Derek Walters, Pen & Sword Maritime - an imprint of Pen & Sword Books Ltd, Barnsley, S Yorks. 2004. ISBN 1 84415 131 X.

[ 6RXS - engine description and technical details ]

Valiant and Churchill Classes   (SSN 02 to SSN 06)

The Class:  Five early nuclear submarines.

The Boats:  HMS Valiant, Warspite, Churchill, Conqueror, and Courageous.

Paxman Machinery:
Application:  Auxilaries - power generation.
Type:  12 cylinder A12YHAZ.
Additional Details:  Two engines per boat. The normally aspirated engines were each rated at 310 kWb (415 bhp) at 1,200 rpm. Generator output: 290 kW.

[ YH - engine description and technical details ]

Resolution Class (Polaris)   (SSBN 01 to SSBN 04)

HMS RepulseThe Class:  Four submarines designed to carry Polaris missiles. Laid down in 1964-65, they were commissioned between 1967 and 1969. After 28 years of unbroken Polaris patrols, the last two boats in service, Renown and Repulse, were decommissioned in 1996.

The Boats:  HMS Resolution (01), Renown (02), Repulse (03), and Revenge (04).

Paxman Machinery:
Application:  Auxilary - power generation.
Type:  12 cylinder A12YHAZ.
Additional Details:  Two engines per boat. The A12YHAZ was an aluminium vee form 7" bore engine. In the Resolution Class each naturally aspirated engine had a normal load rating of 422 bhp at 1,200 rpm and a maximum rating of 464 bhp. The Laurence Scott generators had an output of 290 kW.

In addition to providing the emergency back-up supply, the YHA engined gen-sets supplied power during reactor start-up, for battery charging when the reactor was shut down, and in situations of reduced shore supply capability.

The YHA was a tried and tested engine by the time it was installed in the first Polaris submarine. It was developed not many years after the end of World War 2. Conscious of the heavy wartime shipping losses due to magnetic mines, the Admiralty was very keen for Paxman to develop an engine with a low magnetic signature. The aluminium Admiralty version of the YH was introduced in 1952 and adopted as an Admiralty Standard Range (ASR) engine. It was supplied in large numbers to the Royal Navy as a propulsion engine for inshore minesweepers. The YHA was also used for auxiliaries in Leander Class frigates and fitted in other vessels.

Swiftsure Class   (SSN 07 to SSN 12)

The Class:  Six SSN (Ship Submersible Nuclear) Fleet submarines built between 1973 and 1981.

The Boats:  HMS Swiftsure (07), Sovereign (08), Superb (09), Sceptre (10), Spartan (11) and Splendid (12).

Paxman Machinery:
Application:  Auxilary - power generation.
Type:  16 cylinder Ventura - 16YJAZ.
Additional Details:  One engine per boat. Output: 896 bhp at 1,200 rpm. Generator output: 615 kW.

[ Ventura - engine description and technical details ]

Trafalgar Class   (SSN 13 to SSN 19)

The Class:  Seven SSN (Ship Submersible Nuclear) Hunter-Killer Fleet submarines built between 1978 and 1990.

The Boats:  HMS Trafalgar (13), Turbulent (14), Tireless (15), Torbay (16), Trenchant, Talent and Triumph.
Trafalgar was launched in 1981 and is due to be decommissioned in 2009.

Paxman Machinery:
Application:  Auxilary - power generation.
Type:  12 cylinder Ventura - 12YJAZ.
Additional Details:  Two engines per boat. Output (each): 474 kW (635 bhp) at 1,200 rpm.

Upholder (now Victoria) Class

Upholder/Victoria Class SubmarineThe Class:  Four Type 2400 SSK 'Hunter/Killer' submarines, built between 1986 and 1992. With an overall length of 70.3 metres, each relies on two 16 cylinder Paxman Valenta engines for its diesel-electric propulsion system. Built for the Royal Navy, with plans originally for ten in the Class, these boats were victims of defence cuts made by HM Government in 1993 when they were taken out of service. All four were sold in 1998 to the Canadian Navy which has designated them the Victoria Class.

A key feature of the Upholder boats is their exceptionally quiet running and small acoustic signature made possible by the Paxman powered diesel-electric arrangement. In a review of the Upholder Class at the time the boats were retired from the Royal Navy, Commander John Powis RN had this to say about the propulsion system: "A large double-armature motor on a single shaft provided propulsion through a large modern propeller. A 9,000 amp/hour battery capable of rapid charging along with two Paxman Valenta diesels (with twice the power of those in the O class) resulted in an extremely flexible propulsion system. Efficient hull design kept propulsion loads - and discharge rate - low, despite the increased operational electrical load. Patrolling at slow speeds would require 40 to 60 minutes of snorkeling per day; a transit at 8 knots would require snorkeling approximately 30% of the time. Top speed submerged matched that of any similar SSK and could be maintained for more than 90 minutes from a fully charged battery. Further, once a submarine reached the end voltage, it still had considerable capacity remaining for operationally useful speeds."

The Boats:  HMS Unicorn, Unseen, Upholder, and Ursula. Each has been renamed by the Canadian Navy as follows: Windsor, Victoria, Chicoutimi, and Cornerbrook.

Paxman Machinery:
Application:  Main Propulsion.
Type:  16 cylinder Valenta - 16RPA200SZ.
Additional Details:  Two engines per boat. Each has a maximum continuous rating of 1,500 kWb (2,010 bhp) at 1,350 rpm. Maximum overload rating: 1,530 kWb (2050 bhp). Maximum engine speed: 1,418 rpm. The Valenta engines are supercharged (not turbocharged), the superchargers being driven through a step-up epicyclic gear train via a hydraulic clutch.

The two GEC brushless generators each have an output of 1.4 MW. Propulsion is through a single 5400 shp (4 MW) electric motor turning a single propellor.

[ Valenta - engine description and technical details ]

Vanguard Class (Trident)   (SSBN 05 to SSBN 08)

Vanguard Class SubmarineThe Class:  Four SSBN (Ship Submersible Ballistic Nuclear) submarines - the latest ballistic missile submarines of the Royal Navy. The first of the Class, Vanguard, was commissioned in 1993 and the last, Vengeance, in 1999.

These submarines provide the United Kingdom's strategic and sub-strategic nuclear deterrent and with their Trident missiles now form the cornerstone of our Defence policy. That fact underlines the importance of high levels of performance and reliability in the machinery specified for them.

Main propulsion is through steam turbines fed from a pressurised water reactor (PWR).

The Boats:  HMS Vanguard, Victorious, Vigilant, and Vengeance.

Paxman Machinery:
Application:  Auxilary - power generation.
Type:  12 cylinder Valenta - 12RPA200SZ.
Additional Details:  Two engines per boat. Output (each): 905 kWb (1,215 bhp) at 1,200 rpm.

One engine drives a Brush alternator with a rating of 850 kW. The AC output from this provides auxiliary power for the ship's service (hotel) load. The other Valenta is coupled to a Brush generator with a rating of 750 kW, providing DC output for propulsion in the event of failure or non-availability of the main propulsion system (emergency "get you home").

Diesel-Electric Propulsion

In terms solely of efficiency, direct (mechanical) drive has the advantage over a diesel-electric arrangement. The latter incurs energy losses in both generator (or alternator) and electric propulsion motor, mainly through the heat each produces but also through friction in the bearings. Power output from the engine is thus reduced twice, first by the generator and then by the drive motor, before reaching the propeller shaft.

While direct drive may be more efficient, this is outweighed by the disadvantages. Options for the positioning of engines in the hull are severely limited by the need to line them up with propeller shafts. Engine speed has to be increased or decreased in direct proportion to the propeller speed required at any particular moment. Large diesels are happier running at a constant speed which minimises thermal cycling and stresses associated with frequently varying speeds. Diesels are also more fuel efficient when run at a constant speed. Furthermore, the ideal combination for a submarine is two or more high speed engines driving a propeller at low speed. With a direct drive system this can only be achieved using a reduction gearbox. Apart from its size and weight, a gearbox tends to be an additional source of noise - something highly undesirable in submarine warfare.

A diesel-electric arrangement offers an attractive solution to these issues. Compact, high power to weight ratio diesels of the Paxman type, coupled to alternators, can be located virtually anywhere in the hull. Engines can be run at a fuel efficient constant speed and no gearbox is required. Propeller speed can be closely controlled by systems feeding power to the propulsion motor. Each gen-set can be operated independently as required, whether to provide power for a propulsion motor, for battery charging, for the hotel load, or a combination of these. A well engineered diesel-electric system makes it possible to achieve a very low acoustic signature, a matter of critical operational importance.

Generator Efficiencies

The power output of a generator or alternator is always less than that of the engine driving it. The reason is energy losses due to heat and friction in the electrical machinery. As a rough rule of thumb, Paxman works on the basis of a 100 kW generator being about 92 - 93% efficient. Generators of 1 MW and above are about 96% efficient. Thus, making allowances for energy losses, an alternator with a rating of 100 kW needs an engine capable of producing, say, 110 kWb.

Noise and Vibration

In marine applications, particularly naval, of large high speed diesels, noise and vibration are major issues. Few things are more important in submarine and anti-submarine warfare than remaining undetected. This becomes increasingly difficult with the sensitivity of modern listening devices.

Paxman's experience of dealing with noise and vibration in marine applications goes back to the early 1930s. Perhaps it was the vibration problems encountered in the first diesel-electric vessel with Paxman engines, MacBrayne's "DEV Lochfyne" (1931), which initially focused the Company's attention on the subject. Paxman supplied propulsion engines for two later MacBrayne ships, "MV Lochnevis" (1934) and "MV Lochiel" (1939). In these the engines were mounted on specially designed bedplates incorporating springs to reduce the transmission of vibrations to the hull. The concept was taken further in the design of the engine mountings used in British built tank landing craft in World War 2.

Paxman subsequently acquired a great deal of expertise in torsional vibration. Leo Crawley, for example, who worked for the Company for over forty years until his death in 1990 was recognised far beyond the confines of the Paxman business as an authority on the subject. Such specialist in-house knowledge has played an important part in the Company's work on engine design, development, and applications engineering.

There are at least three areas in which Paxman engineers have worked to minimise noise and vibration. Firstly, in the design and development of the engines themselves and large components such as crankshafts. Secondly, there was the applications engineering input on engine mountings to minimise the transmission of vibrations. Thirdly, there was the specification and fitting of acoustic enclosure around engines to reduce noise. The use of Paxman engines and Paxman applications engineering skills in vessels such as Upholder Class submarines, Type 23 Frigates and the new Royal Netherlands Navy frigates bears testimony to the technical capabilities of the Company in the field of noise and vibration.

1. Visit to the Works of Davey, Paxman & Co Ltd, Colchester by the Contractors to M.O.S. 23rd June 1945. Paxman Publication 1155A.

Acknowledgement: A major debt of gratitude is owed to Peter Hulme, an ex-RN submariner now living in New Zealand, both for his encouragement and providing much useful background information about submarines and propulsion systems. Thanks are also due to Guy Cooper, another ex-RN submariner, who helped with identification of photographs, and to Vic Borley, formerly of MAN B&W Diesel Ltd, Paxman for his invaluable assistance.

© Richard Carr 2003

Page updated: 15 JUL 2016