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Some Aspects of Modernising T Class Submarines

by Ian Buxton

Paul Kemp in his book "The T Class Submarine" discusses the modernisation from an operational and appearance viewpoint, but this account focuses more on technical performance.

Peter Hulme in his article T Class Conversion gives the technical detail of the electrical propulsion part of the conversion.

In May 1947, consideration was given to converting existing T Class or Amphion Class submarines to high underwater speed. The German Type XXI U-boat had shown that a speed of about 17 knots was possible using high powered batteries and a streamlined form. The Germans had also experimented with the High Test Peroxide turbine driven Type XXVI. Britain had been experimenting with one of these, U.1407 renamed HMS Meteorite. On the basis of this experience, two experimental submarines, E14 and E15 were ordered in 1947, later named Explorer and Excalibur. But it would take several years before they would be ready (the first was not completed until 1956), while as unarmed target vessels, they would not be operational. Even if successful, it would take several more years for British industry to build such machinery for operational boats, while a large HTP production industry would have to be created.

S Class Scotsman had already been approved for conversion to a high speed target, and work had started at Chatham. Her existing twin diesels were to be removed, and replaced by a smaller diesel-generator solely for battery charging. Two further batteries were to be fitted. The two original S Class motors were to be replaced with Amphion Class motors. Such all electric drive would produce 3600 horsepower, over double that of a normal S Class. With streamlining, about 15 knots was anticipated, almost double the speed of an existing boat.

The Director of Naval Construction, with his engineering and electrical counterparts, considered it possible to apply similar techniques to some of the Amphion Class. There were eleven all-welded boats which could be lengthened to accommodate a second electric motor in tandem on each shaft. Adding a fourth set of batteries under the control room in place of oil fuel tanks would double the voltage to a nominal 440V (and hence double power with the same current from batteries in series) which could be applied to each motor. With a slight overload, the four motors could produce 6000 shp in place of the former 1450. It was estimated that this would give a speed of 17 knots for 20 minutes, about double the 8-knot speed of existing boats.

Such a conversion was also considered for the newer Amphion Class submarines. But there was no space for two additional batteries. It would be necessary to add 35 ft to their length to accommodate both this battery and extra electric motors, which would add to cost and conversion time, so this conversion was taken no further, although several boats were streamlined in the 1950s.

Flag Officer Submarines foresaw that in a war that might take place in the 1950s, such modernised T Class boats would be far more valuable fighting units than existing boats. By that time the Russians would have built up a fleet of fast underwater boats based on German technology. Tactical requirements called for high underwater speed , rather than high surface speed, especially as with the snort, batteries could be recharged without surfacing. Not only would such streamlined boats be faster, but also quieter, and would produce less self noise, enhancing sonar (then called Asdic) performance. There were also plans for a new design of fast battery submarine, which would become the Porpoise Class. But much development work was needed on both their ASR1 diesels and their electrical propulsion system, such that 1951 was the earliest such boats could be ordered.

The only realistic short term option was to convert T Class boats, taking the first in hand in 1948, as they could be completed in about two years. Only the welded boats were considered. Not only was the flush constructed pressure hull easier to extend, preserving its structural integrity, but their diving depth was 350 ft compared with 300 in the riveted boats. The latter were constructed of thinner lower quality steel susceptible to corrosion and thus had a potentially shorter life.

Once the plan was approved, design work and model testing was put in hand. Improved communications were proposed, with a new layout of periscopes, radar and snort masts. A mock-up was built to test the layout of a new control room. Discussions were held with the two manufacturers of Amphion Class electric motors, Metropolitan-Vickers (Metrovick) of Manchester and Laurence Scott (LS) of Norwich. But they advised that it would take at least two years to build new motors, each weighing 17 tons. This would delay the conversion programme, but it was considered possible to remove motors from a few existing boats which were not fit for further service, as well as utilise the five spare motors. It was desirable to keep the same make of motor in each boat. It was found that four of the spares were LS motors and that three of the unfit boats (Templar, Tantivy and Tantalus) also had LS motors. So five LS boats could be converted without new motors having to be built. Of the eleven welded boats (twelve had been built but the Dutch had Tijgerhaai ex Tarn but their Zeehond ex Tapir was due to be returned in 1953) six had LS motors and five Metrovick. So in deciding which boats to convert first, LS ones were chosen, the first being Taciturn and Turpin (the original plan was for Trump). By adding eight frame spaces at the forward end of the machinery space, the necessary 14 feet for the additional motors and larger switchboards could be accommodated.

Model tests showed that removal of the bridge (and replacing it by a streamlined fin), the gun, the periscope standards and the external torpedo tubes, plus fairing the casing and smoothing the hull generally would reduce drag by about 40%. On this basis, a new Staff Requirement was drawn up in January 1949. Pressure hull length would be increased 14 feet to 258 ft, surface displacement would go up from 1319 to 1565 tons and submerged displacement from 1571 to 1680. Only the six internal torpedo tubes (each with one reload) would be retained, all external tubes being removed. The existing surface speed of 15 knots was expected to be maintained, but underwater speed would go up from about 8.5 knots to 17 for 20 minutes, 14.25 for one hour and 8 knots for 11 hours. Snorting speed, i.e. at periscope depth on diesel power, would be 9.5 knots. There would be four groups of 112 cells instead of 3 x 112. The existing shafting could be used, although new thrust blocks would be required. A clutch would be required between each motor on a shaft, allowing the forward motor to act as a generator driven by the diesel, and the after motor for electric propulsion. For full underwater power, only the four motors would be used. The existing propellers could produce the requisite increased thrust but running at 620 rpm instead of 470 (surface). But the existing manganese bronze propellers would be too highly stressed at these higher revolutions, so would have to be re-cast in a chromium alloy steel.

Although informal discussions were held with the three non-dockyard submarine builders, Vickers Armstrong, Cammell Laird and Scotts) regarding undertaking the conversion work, Controller preferred to have all the work done at Chatham Dockyard at roughly six-month intervals. Taciturn (a LS boat) was thus taken in hand on 29 November 1948, followed by Turpin on 13 Jun 1949. At the time, all ten RN boats were planned to be converted by 1956, i.e. excluding Tapir. Taciturn was inclined at Chatham on 17 February 1951 to check her displacement and stability. Her surface displacement came out at 1588 tons and submerged at 1696 tons. 80 tons of permanent ballast gave a surface metacentric height of 8.8 inches (11 inches submerged), and allowed her to dive in water of specific gravity of between 1.009 and 1.0275, i.e. salt or brackish water, not fresh water whose SG is 1.0. (Metacentric height GM is a measure of a vessel’s ability to return to the upright after being heeled due to wind or waves. M can be considered an imaginary point of suspension, so G centre of gravity must be appreciably below M for positive stability).

She carried out comprehensive speed trials in the Clyde in May 1951. She made 13.2 knots on the surface with direct drive from her diesels (2220 bhp, not far off the maximum of 2500), but using battery power and all four motors gave 17 knots surfaced. Snorting on diesels gave 9.25 knots. Periscope depth on electric drive gave 13.64 knots. Submerged trials were carried out at 90ft depth, but she only made 14.84 knots, about two knots less than expected. Her motors produced 5754 bhp, close to the maximum of 6000, but that reduction would only account for a loss of about 0.2 knot. Propeller rpm was 598, less than the designed 620. Her trials report gives no explanation for this shortfall in speed. Some may be due to the full scale predictions from the model tests being too optimistic (there was little full scale experience as yet to give reliable correlations) with the external hull and appendages possibly being more resistful than expected. The hull surface might not have been as smooth as possible it was to be some years before the adverse effects of even microscopic amounts of hull roughness due to steel irregularities and paint smoothness could be reliably quantified. Finally the propeller efficiency might have been overestimated, coupled with being unable to absorb the power at the designed 620 rpm. However in later service it was claimed that some boats could reach over 17 knots, but the official maximum underwater speed was listed in Admiralty Particulars of War Vessels as 15 knots.

As further boats were converted, it was found that stability surfaced had deteriorated, and the already modest metacentric height further reduced, with heavy rolling. As it was difficult to lower the centre of gravity without increasing weight, the best solution for future conversions was to add more buoyancy. Two extra frame spaces (42 inches or 3ft 6in) were added at the aft end of the control room, which improved the layout of that space. Thus the boats from Truncheon onwards were 290ft 6in in overall length.

By the time the fourth boat Totem had been completed, it was recognised that the new Porpoise Class would be due to come into service at the same time as the last two Amphion Class were being completed in 1957-58, so the conversion of Talent and Teredo was cancelled (believed to have Metrovick motors). They were however streamlined at Chatham in 1954-56, and got new sonars and torpedo control system, but were not given the extra battery and motors, so could only make 9.25 knots submerged.

All eight boats were converted at Chatham. Started & Completed, more or less, in pairs as shown.

Taciturn November 1948 to March 1951
Turpin June 1949 to September 1951
Thermopylae September 1950 to September 1952
Totem circa March 1951 to May 1953
Truncheon circa September 1951 to October 1953
Tiptoe circa August 1952 to September 1954
Tabard circa March 1953 to May 1955
Trump circa March 1954 to June 1956


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The Five Streamlined T Class Submarines of the early 1950sUpholder/Victoria Class Propulsion