The Porpoise Class

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Porpoise, the first post-war operational submarine design. For their day, they were exceptionally quiet.

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By 1948 it was recognised that anti-submarine warfare (ASW) would be the primary role of the Royal Navy’s submarine force. Subsidiary to this, it was clear that they would have to spend much operational time providing targets for the training of surface ASW vessels and aircraft and the development of ASW tactics. In addition, it was thought likely that British submarines could operate against Soviet merchant ships supporting their army in the Arctic and possibly the Baltic and Black Sea. The German Type XXI had shown what conventional technology could do when required. The concept was brilliant and original but the individual techniques used for increased diving depth, greater battery capacity, more powerful electric motors and streamlined hull form were all well known. First priority was to use such techniques in modernising existing submarines and then to build a new design diesel-electric boat.

Six Porpoise class submarines, designed by R N Newton and E A Brokensha, were ordered in April 1951, and two more in 1954. They were a little bigger and somewhat shorter than the ‘T’ conversions, but improved design methods and UXW steel together with new structural design methods gave them a considerably increased diving depth. A model of the very long engine-room collapsed during tests at NCRE and a number of extra deep frames were fitted in the final design. They were exceptionally quiet for their day, mostly by careful attention to detail in the design and support of their machinery.

The engines were Admiralty Standard Range I (ASR I) designed at AEL, West Drayton and built first at Chatham Dockyard, an unusual case of ‘in-house’ machinery development. They were to prove very successful even though some of the design aims proved over-ambitious. Great attention was paid to habitability, including air-conditioning. The six bow tubes had rapid reloading gear so that a second salvo could follow very soon after the first, but this was heavy and not very reliable. Torpedoes could be fired at much greater depth than in previous classes. Submerged endurance was expected to be 55 hours at 4kts, about three times that of any previous RN boat.

The DNC, Sir Victor Shepherd, explained in 1955 that the submerged speed had come down from 17kts to 16kts, which he blamed on the use of reduced-noise propellers of lower propulsive efficiency and on caution in estimating full size performance from model tests. Submerged endurance on batteries had been further reduced following a re-assessment of auxiliary load by the Director of Electrical Engineering. At 4kts the auxiliary load was equal to the power for propulsion. In consequence, the nominal endurance at 4kts was reduced from 55 hours to 40 hours. Problems with the UXW steel, led to a reduction in diving depth from 625ft to 500ft. In 1953 the design had been lengthened 4ft to accept an increase in machinery weight. Despite the fact that they fell short of performance targets, they were probably the best submarines of the day within NATO, and considerably superior to the Soviet ‘Whiskey’ class. They were deeper diving than either the ‘Whiskey’ or the German Type XXI (both 400ft), faster than the ‘Whiskey’ but 1kt slower than the XXI, and far quieter than either.

They had noise-reduced propellers, benefiting from trials on surface ships and on Scotsman. Initially, these were very prone to ‘sing’, in which eddies are shed from the trailing edge, alternately from one side and then the other, exciting a resonant vibration of the blade which, in turn, makes the eddy shedding worse. It is said that Rorqual could be heard leaving the Clyde on the west coast of Scotland from a listening station on Long Island. The USN had similar problems and there was much interesting discussion on both theoretical and empirical solutions either seeking to control the eddy shedding or to prevent vibration by damping. Luckily, the Porpoise propellers had been designed before Conolly’s work on propeller strength and were stronger than necessary. This made it possible to cut grooves in the blades, which were filled with a damping material which gave a complete cure.

They had a good sonar fit aided by their low noise level and proved successful in a long service life. Top speed was about 16kts submerged and they had an endurance of 9000 miles on the surface.

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2 thoughts on “The Porpoise Class

  1. Great article. Thank you very much. Three questions: 1. How did these submarines compare to the GUPPY’s of the USN and 2. How were the Porpoise class subs. deployed/what was their operational history? 3. Are there links for any museum ships?

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    • 1.The ‘Guppies’ came quiet close to Tang performance. A 1953 comparison shows the latter no faster submerged, and actually slower on the surface, but its specialised design (and shorter hull) showed in much better manoeuvrability and better longitudinal stability. These qualities made for improved depth control and easier rapid depth changes.

      The US fleet submarine was so large that battery capacity could be more than doubled, and motor power greatly increased, without any change to the outer hull. British submarines were smaller, and their equivalent battery section was also added; underwater speed wa approximately doubled, to 15 knots. They were, then, rough equivalent (albeit much mailer) of the US ‘Guppies’. At the same time, the Royal Navy streamlined the riveted ‘T’, replacing the original batteries with new higher-capacity one , but without adding underwater power. Here the primary motive was to reduce cavitation noise by reducing underwater resistance, ie propeller loading. Trials in October 1952 showed an increase of 1.4 knot underwater. In both case, streamlining entailed removal of the usual British external torpedo tubes, so that four forward and two after tubes remained. Twelve ‘S’ class submarines received an even more austere conversion, with snorkels (snorts), and improved radar and sonar (asdic). Most had their 4in gun removed a a rudimentary form of streamlining. The other major surviving wartime class, the large ‘A’, was also rebuilt. It was long enough to fit enlarged batteries, and reportedly the original electric motors were not replaced. Like the fast ‘T’, the rebuilt ‘A’ had an underwater speed of 15 knots, and a much-reduced torpedo battery of six tubes.

      Britain and France built their own Tang equivalents, the Porpoise and Narval classes. Unlike the Tangs, the British submarines carried their sonars in big dome above their bow . The American boats employed the passive fixed BQR-2 array in a chin sonar dome as their primary sensor, whereas the British relied on a trainable searchlight sonar, Type 187.

      Work on Porpoise began early in 1949, after the British concluded that they would not have an operational High Test Peroxide or HTP (Walter) submarine soon enough. They initially considered building a slightly modified Tang, but instead developed their own design. But US-type (‘Guppy’) batteries were adopted to simplify wartime supply. The basic design requirement was for a maximum underwater speed (at the half hour rate) double that of the existing’ A’ class, ie about 16 knot. Draft staff requirement called for a speed of 17 knots to be sustained for 20 minute. ASW was to have been the primary mission, and the six bow torpedo tubes (12 reload torpedoes) were designed to be fired at great depths. Ultimately, however, like the Tang, the Porpoises were fined with a pair of stern tubes (primarily for short ASW torpedoes) in addition to their bow armament.

      Like Tang, the new British submarine was intended to dive considerably deeper than her predecesor. In mid-1949 it was expected that he would operate at 650ft, with a safety factor of 1.75 (collapse depth 1140ft); the designers hoped for 750ft. This compared with 550ft for the experimental HTP submarine , Explorer and Excalibur. In fact materials development lagged, and by 1953 the new Porpoise class was rated only at 500ft. Yet, at least from 1949 onwards, the British submarine designers aimed at a 1000ft operating depth, corresponding to a collapse depth of I750ft. This compare with a US policy which appears to have kept to the 700ft of the Tangs until the design of Thresher in 1957-58. Thus, like the US Navy, the Royal Navy made considerable efforts to develop new high-yield submarine steels. By mid-1949, for example, it had a UKE steel with a yield strength of 54,000Ib/sq in, and expected soon to have 70,000 or even 90,0001b steels. Reportedly the modified Porpoise (Oberon) class could operate below 1000ft, which suggests that the 1949 goal was soon met.

      2. British Porpoise-class submarine
      https://en.wikipedia.org/wiki/British_Porpoise-class_submarine

      3. The Oberon-class submarines, which were almost identical to the Porpoises, and the first of which was commissioned in 1961, survived their predecessor only a little longer, all being decommissioned in the early 1990s.
      HMS Otus (S18)
      http://www.hms-otus.com/englishpage/indexenglish.php

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