RN Subs

THE IMPACT OF THE TYPE 21 U-BOAT ON POST WAR SUBMARINES

From authoritative sources such as Admiral Galatin US Navy (ret) in his book "Submarine Admiral" it is clear, that in the months following the close of WWII, the US Navy became very impressed by the concept of the German Type(XXI) 21 U-Boat derived as it was from the extensive WWII experiences of U Boat commanders.

Non - US reader's should be aware of the US Navy "SUBMARINE OFFICERS CONFERENCE" founded in 1925 to advise the Chief of Naval Operations and the Secretary of the Navy. Apparently attended by submarine officers, past or present. It seems to have been an important influence in the early years after WWII

US Navy planning quickly commenced to acquire submarines that could achieve high underwater speed in the manner of this WWII German submarine that ironically never fired a shot in anger, arriving too late to effectively enter the closing sea war in the West.

Basically the twin screw Type 21 had a streamlined merged circle (figure eight) hull, large propulsion motors, a large battery and a snorkel system, coupled with a fast semi-automated torpedo reloading system. A basic propulsion concept similar to the less sophisticated Hunter Killer 'R' class, brought into service in 1918 by the Royal Navy for ASW duties and then largely forgotten in the peacetime obsession with large Fleet submarines and high surface speeds.

There was little to prevent most pre - WWII submarines being built in the general style of the Type 21 U-Boat - it simply wasn't considered an operational requirement until the experiences of WWII. It is appropriate to mention at this point that the US Navy captured the Japanese ST fast submerged submarines I-201 and I-203 that were both taken to Pearl Harbour for study. They were expended in destructive testing in May 1946. see Appendix H.

During WWII, the Royal Navy led the Allied move to quickly acquire fast submarine ASW exercise targets, reportedly acting on 'Intel' about the Type 21 program, Intercepted Japanese reports are suggested by some authorities - Hyperwar/N.S.A - Quote. On 28 December 1943 (in a message not read until June of 1944), the Japanese Naval Attaché in Berlin sent to Tokyo a description of two new types of U-boats, details of which had not yet been officially released to him by the German navy. Unquote. Appendix I

The limited conversion of six S Class submarines commenced in Oct 1944 with the Seraph. The speed was increased to 12.5 knots by extensive streamlining and moderately overloading the existing motors for 30 minutes of high speed. See the Seraph 1944 trials document from the HMS Scotsman article.

Following US Navy operating experience with the captured German Type 21 U-3008 and U-2513, the new twin screw Tang class was planned incorporating the concepts of the Type 21. The first of class was laid down in 1949.

They were also given another attribute of the Type 21 - a significant increase in maximum depth compared with the earlier submarines.

Admiral Galatin who was present during the 1952 Tang commissioning trials, reports that 725 feet was achieved. He was a keen exponent of increased depth from his time as a CO in WW II until the appearance of the Los Angeles SSN, a submarine that in this respect, disappointed him.

But the six new Tang class were expensive and there were many older style Fleet Submarines in good condition so that after a trial with a modified Fleet Submarine the Greater Underwater Propulsion Power (The Y having no meaning) programme came into being. See Appendix D Item 1

It started in 1946 with two conversions known as GI, completed in 1947, intended for use as fast targets for ASW surface vessels - USS Odax (484) and USS Pomodon (486), both Tench class. Once the concept was established by the GI submarines, the main GII conversion program commenced. Of course the greatly increased maximum depth of the Tang class could not be achieved in these older submarines, but this was apparently considered acceptable.

In 1948 the Royal Navy commissioned the drastically modified and streamlined HMS Scotsman as a non-operational experimental and trials submarine with underwater battery speeds of up to 17 knots.

It seems the Royal Navy was as impressed as the US Navy with Type 21 concept, and despite severe fiscal restraints, commenced a major conversion program in 1948 of eight T Class submarines, to produce fully operational streamlined submarines with greater submerged speeds. The programme was completed in 1956. It should be remarked that the Royal Navy had also since WWII, made a large submarine investment in continuing Dr Walter's Hydrogen Peroxide work using the captured U-Boat U-1407 that became HMS Meteor. The program expanded and finished with the construction of two experimental submarines. HMS Excalibur launched 1955 and HMS Explorer launched 1954 achieved 25 knots submerged and provided useful fast ASW targets. It is reasonable to assume this form of submerged propulsion, despite technical difficulties, would have been pursued but for the success of the US Navy nuclear submarines. However though beyond the scope of this article, the range limitations of Hydrogen Peroxide should be not overlooked.

US Navy thoughts were also moving another directions. About this time plans was progressing for a radically new type of submarine hull, that after completing extensive model testing including a wind tunnel trial, was laid down as the Albacore in 1952. An entirely experimental vessel with a form similar to an airship - the teardrop hull that with various modifications eventually became the future shape of all SSN, SSBN and SSK submarines. Interestingly, in the early planning period the new shape was known in US Navy documents as the Hilda Lyons Form after a pre-war British airship designer.

Admiral Galatin mentions that in these early days there were also ongoing shore based submarine propulsion research projects, including hydrogen peroxide and closed cycle diesel systems - neither came to fruition in a submarine, apart from the 35 ton midget USS X-1(commissioned 1955) that had a HTP engine for just over 18 months but following a fuel explosion was converted to diesel-electric.

And of course the success of the SSN Nautilus in 1955, set the path of the US Navy submarine propulsion until the present day. Currently we hear of a loan diesel-electric submarine being trialled by the US Navy, but this may simply be target evaluation in a new era.

THE CONVERSION of the WWII US Navy FLEET SUBMARINE for Greater Underwater Propulsion Power Y - GUPPY

Commander John D Alden's definitive book "The Fleet Submarine in the U.S. Navy", records the general history of the US Navy conversion of a large number of its diesel-electric Fleet Submarines of the Balao and Tench class to obtain greater underwater propulsion power for faster submerged speeds in the years immediately following WWII.

Four similar but older Gato class were also converted to the more austere version of the later GIA conversion, the GIB for transfer to Italy and the Netherlands.

Alden does not give the detail of the modifications and additions needed to achieve greater underwater propulsion power so these notes, compiled from various sources, endeavour to provide that detail and add a few other related items that might interest the reader. The emphasis is on the electrical propulsion, the heart of the Guppy conversions, indeed of any diesel-electric submarine

The main GII conversion program of 13 Balao class and 9 Tench class submarines for operational service, was completed in the period 1948 to 1951. The two GI submarines were converted to GII in 1951.Making a total of 24 GII.

Following were two less expansive electrical propulsion conversions - the GIA-1951 programme involving 9 Balao class and 1 Tench class followed by the GIIA-1953 programme involving 12 Balao class and 4 Tench class. Submarines in both these conversion programmes retained the same battery tanks but replaced the cells with units of higher capacity. Alden does not suggest that the streamlining was in anyway different from the GII. The removal of an engine in the GIIA created more auxiliary machinery space, consequently leaving needed space in the control room area. Appendix D Item 2.

A total of 50 Guppy submarines were in active service by the early fifties.

ENGINES and SNORKEL

The WWII Fleet Submarines of the Gato, Balao and Tench classes used diesel-electric propulsion on the surface. The twin shaft electric propulsion motors were of sufficient size to transmit the full electrical power output of the four diesel-generators as mechanical power to the propeller shafts. NavPers 16162. - "For surface operation, using the various combinations of armatures and taking power from the main generators, the motors developed from 20 hp to 2700 hp per propeller shaft at speeds ranging from approximately 67 rpm to 282 rpm about 19-20 knots max'. Source"

The general arrangement can be seen in (Fig 1). These large motors (total 5400 shp) adequately matched the increased battery power of Guppy I/II conversion. See Appendix C for more detail.

The earlier Fleet Submarines of the Gato and Balao classes were fitted with four sonar noisy, high speed geared single armature motors.

Responding to WWII experience and submissions by experienced captains, the later Tench class were fitted with two slow speed double armature motors per shaft in a direct drive shaft arrangement, with no change in shaft power. The propulsion circuits remained electrically the same, as four separate armatures were still employed but in a different mechanical mode. The change was not essential in the Guppy conversions, but very desirable for an otherwise quiet streamlined submarine.

The two GI Submarines were not initially fitted with snorkel until their later conversion to operational GII. Their maximum submerged speeds of 18.2 knots are reported by Alden. One can only assume their small sail reduced the drag to allow greater speed than the following GII and it is not unlikely that other streamlining measures were applied to the GI that would not be practical in the operational GII.

The US Navy type snorkel, a telescopic design, was first trialled in the Tench class Fleet Submarine USS Irex (482) in 1947. (See article - Snorkel in the US Navy - 1945 onwards).

The new Tang class, GII conversions and the snorkel development, were effectively parallel programs as a telescopic snorkel similar to the Type 21 was essential for fitting in these streamlined submarines.

Too much drag would have been created by the rather crude but effective system used by the Royal Navy, following on from the basic fold - down U- Boat snorkel that was fitted on most Royal Navy conventional WWII style submarines shortly after the war. The first Royal Navy telescopic snort came in 1951 with the converted high speed T Class. Taciturn first of class.

It must be emphasised that snorkel was also fitted to non - Guppy US Navy Fleet Submarines, initially in a specific 1951-54 program of 19 submarines that produced the so-called Fleet Snorkel class. However it seems other submarines were fitted with snorkel as required. A modern fin was fitted to house the telescopic snort and the exhaust.

Now a major part of the doctrine of the modern diesel-electric submarine operation, snorkelling to propel and battery charge submerged was a rapidly developing practice in the US Navy and Royal Navy at this time. Briefly - all US Navy snorting systems were semi-automated with the apparent intention of never allowing any sea water to get into the engine. The US Navy always used two stroke engines and this may well be the reason for the more complex snorkel system.

The Fleet Submarines including Guppy conversions fitted with snorkel, apparently had engine modifications and a consequent reduction in maximum power. See APPENDICES E and C.

The 300 kW auxiliary diesel generator was removed from all Guppy conversions.

Royal Navy Snort (US Navy Snorkel) systems (fold - down or telescopic) were quite simple and remained so even with the advent of the telescopic snort in converted T Class submarines and new - build P Class. It is assumed this was because of the continued use of four stroke engines. Certainly earlier Royal Navy submarine engines flooded occasionally with no serious consequences.

In the later, more modest Guppy conversion GIIA, the No 2 Main Diesel - Generator was removed to create space for operational equipment such as sonar. The redundant No 2 D-G switch was adapted to allow the No 1 D-G to supply the port motor to compensate for the loss of port/starboard symmetry of supply (NavPers 10490 "Basic Enlisted Submarine Text Part 1."). This loss of an engine was apparently acceptable as slower surface speeds seemed to be the trend in modern Diesel-Electric submarines, with the emphasise on snorkelling. Anecdotes report that when snorkelling usually only two engines would be used, restricted by maximum safe vacuum and maximum speed.

An informal report from a retired US Navy officer states that late in the life of the GII USS Pomodom (486), the two forward engines were removed and a small special diesel-generator was installed in the engine room, down the C/L, on sound mounts for "quiet" snorkelling - the engine itself proved unreliable. Project abandoned!

He also advised that the change to five bladed propellers was merely to reduce noise and did not contribute significantly to increased speed.

THE MODIFICATIONS TO THE ELECTRICAL PROPULSION SYSTEM

Despite lacking official archival documents, it has been possible to provide the following, reasonably accurate description of the electrical propulsion modifications.

The data has obtained from the various sources in Appendix H, but in particular US Navy battery purchase specifications, Appendix A.

GUPPY II

The main internal structural changes were a consequence of the need to extend the two existing battery tanks to accommodate the 504 cells in place of the original 252 cells. The individual new Guppy 1 cells were about 60% of the mass and 64% of the volume of the original Sargo I cells.

In early conversions, because of an existing main bulkhead, the forward tank was in fact in two parts with the larger section (184 cells) being the original tank forward of the bulkhead and the smaller section (68 cells) just aft of the bulkhead, NavPers 10490.

Rik Nilsson, who served on the Cubera. (Fig 1) tells of a different structural alteration.

There were many subtle changes made as the Guppy program progressed. In Cubera the bulkhead was actually cut and moved aft into the pump room, and the control room floor reinforced above the well so the entire battery well (252 cells) was in one compartment. A backup air conditioning plant was removed and the spare parts compartment was then relocated under the deck in the forward torpedo room to make room for this modification. I have no idea if any other Guppy-II's used this scheme.

Rik checked this detail with a fellow crew mate from the electrical propulsion section.

Contrary to some reports, the Guppy conversions hulls were not extended as part of the program to achieve greater underwater speed. However over the period 1959 to 1963, nine of the G11 boats were extended by 15 feet to give more operational space - the control room was enlarged and the sonar room moved out of the forward torpedo room where it had been placed in the GII conversion - they became GIII. They also gained a conning position at the top of the fin This further conversion resulted in a loss of underwater speed- Alden reports down to 14 knots from 16 knots. He also reports it had been intended to convert all GII into GIII but budgetary restraints intervened. Appendix D Item 6.

GUPPY IIA and GIA

Conversion were similar in principle to the GI but retained the original 252 cell space in the existing forward and aft battery tanks. (Fig 4) but changing the cell types from Sargo I cells to physically identical but higher capacity Sargo II. APPENDICES A & B

THE BATTERIES

The Fleet Submarine at the end of WWII was using the Sargo I cell (Exide and Gould) (Fig 3), this is confirmed by Alden. The submerged submarine was capable of about 9 knots for one hour at 2400 shp - a fairly typical submerged speed for submarines of this era. It is important to appreciate that with the batteries permanently connected in parallel in the unconverted Fleet Submarine (Fig 4), it was only possible to deliver less than the half the voltage of the diesel-generators thus severely restricting the submerged power and speed that was further reduced by the inherent high drag of the un-streamlined submerged hull (Fig 2).

The Guppy conversions involved raising the nominal battery voltage and currents to values similar to the output of the diesel-generators of 415 volts at 10,600 amps to allow the propulsion motors to develop power ratings approaching or even exceeding their nominal 5400 shp that previously was only possible on the surface.

This was achieved in all three conversions by connecting the forward and aft batteries in Series for high submerged speeds, with the facility to return to the original Parallel connection for the slower speed ranges. The Series/Parallel control was in the submarine's manoeuvring room as detailed in Item E, MOTORS AND CONTROL

The total Ampere Hour capacity of the batteries when in the original Parallel connection, was halved in the Series connection thus it was a requirement to significantly increase the battery AH capacity with additional cells as in GI/GII or replace the original cells with similar units of the same dimensions but with increased capacity as was the case with the GIIA/GIA

In all the conversions, cells with more and thinner plates were used allowing increased capacity in the same volume and mass, together with much improved battery ventilation, air agitation of the electrolyte and water cooled cell terminals described in BATTERY MANAGEMENT IMPROVEMENTS. Cells so designed had a shorter life cycle. Appendix D Item 4

The three cells used in the original submarine and the following conversions that were very similar in general appearance with four terminals per pole, eight per cell.

Fleet Submarine - Sargo I cell
GII Submarine - Guppy 1 cell
GIA/GIIA Submarine - Sargo II cell
Specification notes - Appendix A

BATTERY MANAGEMENT IMPROVEMENTS

The original Fleet Submarine individual cell ventilation was replaced by open tank ventilation in the Guppy conversions, with variable speed ventilation blowers that exhausted into the engine room, one for each well. Open tank ventilation was the usual practice in Royal Navy submarines of this period.

The original filling cap of the Sargo I for individual cell ventilation was replaced on the Sargo II by one with a spark or flame arrester. (Fig 3)

There were also recirculating fans in the battery wells. There were other additional hydrogen management devices such as extra hydrogen meters, one for each well (max 3% concentration) displayed in the manoeuvring room and air flow meters. Charging rates followed the strict US Navy procedure - Temperature - Voltage - Gassing curves, to ensure there was no excess emission of hydrogen or raising of cell temperature.

The individual cell terminals were water cooled to allow the very heavy sustained current draw - offs to be maintained without overheating the cell plates and electrolyte. The closed cooling system used water from the battery water system and was itself cooled by a heat exchanger using pumped seawater as the cooling agent.

In each cell was installed a simple air lift pump supplied with air through piping from dedicated blowers and filters. This system was used to mix the electrolyte by a flow of air bubbles to prevent density layers forming. Basically air agitation avoids the long period of gassing at the finishing rate of charge that was previously used to mix the electrolyte thus reducing temperature rise and excessive production of hydrogen. This was very important, particularly when charging during snorkelling.

There is no indication from drawings that the Type 21 U-Boat cells had air agitation or water-cooled connections. This seems the same for the Royal Navy Taciturn in 1953 documents, but anecdotes suggest that water - cool connections did come later.

This photograph (Fig 5), shows the cells in the preserved USS Clamagore (SS 343) with the array of tubes to supply agitation air and cooling water. (Fig 6) shows a similar view of one battery tank of the USS Tang (SS 563), believed to be identical cells to those used in GII submarines, see Appendix A

The battery systems of modern diesel-electric submarines are similar. It is notable that taking the charge beyond the point where the cells emit hydrogen gas was not allowed at sea in the British modern Upholder Class due to the serious risk of explosion.

The existing Fleet Submarine Individual Cell Voltmeter (a light wire from each cell back to a monitor) was modified to provide continuous scanning of all the individual cell voltages and initiated an alarm if a "low" cell was detected during heavy high speed battery discharge. It should be noted that cell voltages can drop as much as 33% from start to finish if the maximum current is sustained for half an hour.

Maximum high speed for the full half hour was unlikely to be used in practice but monitoring provision had to be made to ensure that in any circumstance no individual cell voltages fell below the critical level where cell reversal could occur.

The Type 21 had two large panels that monitored the cells. There are no circuit details but there is clearly an indicator lamp for each cell.

Captain Coote Royal Navy (Ret) in his book "SUBMARINER" reports that the Royal Navy fast T Class conversion he commanded, had fitted in the control room, an instrument that recorded the instantaneous battery current and volts. Where the needles intersected indicated on graph lines drawn on the scale showed the charge left in the batteries.

It is possible that at least some of the Guppy conversions had a similar instrument but there are no reports so far.

MOTORS and CONTROL CIRCUITS

All of these items were applicable to the GII, but it is possible some parts of A,B,C and D may not have been applied fully to the GIIA and GIA where the motor currents were less. The essential Item E was applied to all the Guppy conversions

Electrical propulsion control cubicles (Fig 1) were in the submarine manoeuvring room, just above the main motors. Photograph (Fig 7) of the later split cubicle of the Fleet Submarine

A) It is recorded (Navpers 10490) that the existing Fleet Submarine propulsion motors were extensively tested to ensure that they were adaptable to the increased battery voltage. That they were is hardly surprising as the voltage output of the normal diesel generators used for surface running, had output voltages of about the same order as the new Series battery connection. However it is possible this brief section of the training manual is mistaken and should refer to increased current.

Though trial data is lacking, there is little doubt that nominal motor armature currents were exceeded in the GI and GII to maintain maximum submerged speed over the half hour period as the battery voltage progressively dropped to the final limiting value and the current was progressively increased by lowering field current to maintain power and thus speed - the rpm displayed in the manoeuvring room. Large motors of this era, typically had a 20% overload rating for one hour and this period would not be dangerously exceeded due to the limiting discharge capacity of the batteries. Note Items C) and D) below, also see Appendix B & Appendix C

B) Additional insulation was provided where needed for the higher battery voltage appearing in the cubicles. NavPers 10490. This modification was probably needed because the unconverted battery circuits were only exposed to the high end of the charging cycle - 345 volts.

C) The main motor overload relays were removed because of unwanted tripping and the battery overload relays were readjusted and relied on to protect the main motors. (NavPers 104900.Relevant to Item A above)

D) The main motor and battery contactors were increased in size with improved arc chutes. NavPers 10490, Relevant to Item A above)

E) The key control element of the conversion was the addition of a manoeuvring room switching facility to allow the two batteries - forward and aft, to be connected in Series applying a high voltage to the motors to achieve greater power for faster submerged speeds and in Parallel for lower voltages for the slower speed range.

The modification was done by adapting the two existing switch lever operated battery isolating contactors and reconnecting the battery circuits.

The switch levers were mechanically interlocked to prevent simultaneous use. Other interlock provisions were made to suit the new operation. The physical layout of the modification circuit shown in drawing (Fig 4) is merely suggested. The actual layout to achieve the required series/parallel circuitry would depend very much on the very heavy cable termination and bus work of the existing control system in a confined space.

Pat Householder, SubVet - advised that the change from a single cubicle to increase security (Fig 7), was designed by Admiral Rickover of US Navy nuclear fame early in his submarine engineering career, and more closely follows Royal Navy practice of separation of the port and starboards systems. Tony Phillipson, General Manager, Rahmi M Koçç Müüzesi, Turkey, responding to a personal request, kindly investigated this important switching feature in the preserved TCG Uluççalireis (S 338) former US Navy Thornback (SS 418) GIIA, now in his charge. He sent close - up photographs of the two inboard levers on each cubicle section one labelled SERIES and one PARALLEL. Removing any doubts about how this part of the conversion was achieved!

The basic battery series - parallel system with more modern switch gear, was also used in the new US twin screw Tang class and the later single screw Barbel class. The 1950 - 1956 conversion of eight Royal Navy submarines, involved the same system and continued in the new-build twin screw Porpoise Class & Oberon Class and later single screw Upholder Class.

AUXILIARY SUPPLIES

In the Fleet submarine, the auxiliary power supplies with a range of 175 to 345 volts supplying the some 50 or so auxiliary motors and other equipment, were fed by two switchboards, one forward supplied by the forward battery and one aft supplied by the aft battery. The Guppy II modification involved supplying the power from No 2 forward battery and No 4 aft battery. (NavPers 10490).

J. Christley - 'U.S. SUBMARINE FORCE INFORMATION BOOK', reports information supplied by R. Sminkey in regard to the GIIA conversion of USS Quillback (SS 424)

The electrical system was beefed up by doubling the capacity of the AC motor-generators to handle lighting as well as the previous load, and 120-volt direct current for other purposes was provided through rectifiers instead of rheostats. Two 400 - cycle motor-generator sets were also added to meet the needs of new electronic equipment.

It can be safely assumed that all the electric motors for auxiliary machinery were still supplied directly from the batteries as stated above.

The battery voltage varied from near discharged on load at 175 volts to end of charge at 345 volts. This applies to all diesel-electric submarines and constant voltage during charging for lighting etc has in the past has been supplied by various methods, with basic motor-generators becoming more common after WWII.

Modern power electronic systems are available for duties like this and likely used in the very latest submarines.

The 345 volts is associated with the high end of the gassing part of battery charging cycle and was not likely to be reached in practice in the Royal Navy, at least, after about 1953. There are some notes on this subject here.

PERFORMANCE AT HIGH SPEED SUBMERGED

Lacking speed trial data, the simple calculations in Appendix B give a good approximation of the submerged speeds/power achieved by the two basic Guppy conversions when the batteries were switched in the Series connection. The results are listed below. The half hour rate and one hour rate shown are the maximum for the two speed ranges used in the Series connection.

For the more normal speeds below these ranges, the original parallel connection (Fig 4) would be used, see Appendix E.

For a given voltage applied to the motors, speed adjustments could be made by the shunt field regulator within the speed range up to maximum armature current.

Calculated Results from Appendix B

GIA/GIIA - 14.5 knots at the half hour battery discharge rate, propulsion power 4345 shp
GIA/GIIA - 13.1 knots at the one hour battery discharge rate, propulsion power 3213 shp
GII - - 16.0 knots at the half hour battery discharge rate, propulsion power 6100 shp
GII - - 14.25 knots at the one hour battery discharge rate, propulsion power 4306 shp

The maximum speeds for each conversion are close to those stated by Alden. From these figures it can be taken that the streamlined "drag" of the GIA/GIIA and the GII was similar, as anticipated. However note Rik Nilsson's comments Appendix E

Lacking US Navy Guppy speed trial data, the official speed trial figures of 1952 for the first British fast streamlined submarine Taciturn give confidence that the results calculated in Appendix B are reasonably accurate.

Speed trial at 90 feet. Endurance 34 minutes. 5754 shp, 15 knots.

Speed trial at PD Endurance 33 minutes. 5718 shp, 13.7 knots.

Submerged displacement of the Taciturn was 1,680 tons compared to the Guppy with 2400 tons. Appendix F for more Taciturn detail and the then new Porpoise Class that had some similarity to the GII submarine and the Tang class.

The rather better Guppy speed performance figures, as stated by ALDEN, can perhaps be explained by a more streamlined double hull of less drag than the streamlined saddle tank Taciturn.

It might also have been a matter of screw efficiency. At maximum surface speed the shaft rpm of the Fleet Submarine was 282, while the Taciturn was 450 rpm We can also assume that the high submerged speed of the Guppy II was achieved with motors and screws operating at about the same rpm achieved in the original submarine for surface operation, whereas the Taciturn screws were rotating 31% faster than maximum surface speed.However these are the speculations of an interested amateur in what is a specialised field and should be treated with reservation.

The propellers of the Guppy submarines were 93.5 inches. The T Class class were 64 inches. There are of course other important propeller dimensions but these will serve to demonstrate the significant difference. The propellers of HMS Porpoise were 84 inches. The Type 21 U-Boat were 84.6 inches. The Soviet Project 641 Foxtrot were 67.6 inches two outer and 66.1 centre. (Fig 10) show the screws of HMAS Ovens at Fremantle Australia. There is no obvious reason to believe the Royal Navy P Class differed from the similar O Class.

SUBMERGED ENDURANCE AT SLOW SPEEDS

An obvious consequence of increasing the battery capacity in the Guppy conversion for high submerged speed was more endurance capacity at very slow speeds.

Unfortunately there is no detailed anecdotal information available to give an indication as to whether it was considered that the GII offered a useful tactical advantage over the GIA/GIIA

The one Cold War event involving the endurance of a US Navy submarine is the well known story of the then modern Tang class USS Gudgeon (SS 567) that was forced up by Soviet ASW ships when the boat's endurance finally ran out after 39 hours, more detail is available at - Page 80, HUNTERS OF DEEP Appendix I. One could come to the conclusion that the heavy discharge of the initial high speed run reduced the charge state of the large battery with no tactical gain

The other reported events involved Soviet Foxtrot submarines being forced up by US Navy ASW escorts, during the Cuban missile crisis, but nothing is known of the circumstances on the submarine.

One factor in determining endurance at very slow speeds submerged is the electrical load apart from propulsion known in modern parlance as the HOTEL load.

This load can reach a value equal to the slow speed propulsion power, depending on the submarine's circumstances - evading attack in "Silent Routine" or simply patrolling with full sonar, heating, cooking etc. Main motor field currents still have to be supplied - one of the benefits of using a much smaller, separate "creep" motor.

The modern R.A.N. COLLINS class apparently uses a completely separate hydraulic drive with its own screw

The Soviet Foxtrot of the same submerged displacement as the Guppy submarine, a three shaft boat, had a single centre shaft "creep" motor of 140 shp.

This submarine also could make 6 knots at 2 x 200 shp on the two outer main motor shafts. (Fig 9). and Appendix G.

A Red Navy officer, the source of much information on the Foxtrot class, says that 6-8 knots was the usual speed range submerged. with a maximum of 14 knots with a range of 30 miles (2.07 hours. At slow speeds this would translate into a very useful submerged endurance! Using the calculated results in Appendix B, the Guppy II could only manage 14.5 knots for one hour!

The US Fleet Submarine (including Guppy) like the British A Class built in WWII, had a switching facility to allow all four armatures (two per shaft in each tandem motor) to be connected in series for very slow speeds. A similar system was used in the T Class Conversion and more detail can be seen in those diagrams.

HOTEL loads eventually limit the endurance of the submerged battery submarine at very low 'creep' speeds. That is apart from the condition of the crew.

It is of some value to try and compare the slow speed performance of the GII and the GIA/GIIA but without official speed trial documents another educated guess is required.

The speed trial documents of the Royal Navy Taciturn suggest using the 20 hour discharge rate as a starting point. The calculation details are shown in Appendix B.

RESULTS

The GIA/GIIA 5.9 knots x 20 hours = 118 miles endurance at 5.9 knots.
GI I = 5.9 knots x 25 hours = 147.5 miles endurance at 5.9 knots.

Note.

Obviously in practice, the submarine will be using various speeds to make an attack and escape ASW retaliation. The charge left in batteries for extended "Silent Routine" will not be always be sufficient for 5.9 knots at 20 hours and a lower speed will be used mixed with short periods of higher speeds but clearly the GII had significantly greater low speed endurance than the GIA/GIIA.

STREAMLINING

As the focus of this article is on Power as in the acronym Guppy, no serious attempt has been made to obtain official US Navy details of the streamlining and consequent noise reduction of the exterior hulls of the Fleet Submarines that likely varied as the program progressed and experience was gained. This was certainly the case with the British converted T Class as reported by a retired Dockyard draughtsman to Roger Fry.

In any case the techniques are now well described in submarine technical literature. A photograph of USS Cubera's forward 5-inch/25 calibre gun in 1946, prior to conversion (Fig 2), clearly shows the "clutter" that was cleared away to reduce drag. Alden discusses the development of the sail and these can be seen in diagrams in his Appendix A As can the other obvious changes. There are many photographs on the web.

The US Navy made the "mistake" later repeated in the first Royal Navy converted T Class, of having the conning position in a "cab" lower down in front of the fin. As was the case with the later converted T Class, the conning position was later raised to the top of the sail or in British parlance, the fin.

When evaluating the "educated" guessing of speeds, the reader should be aware that during the Taciturn speed trials, opening the bow shutters at 11 knots at 90 feet reduced the speed by 0.6 of a knots. About the same for fore planes turned out. A graph shows the raising of the snort induction and exhaust had little effect below 7 knots and at 8.5 knots the loss of speed was 0.4 knots, however this latter result should be considered along with general loss of speed at periscope depth.

Of course it has to be assumed all speed trials are done with clean hulls as was the case with Taciturn!

NOISE REDUCTION

Although streamlining in itself provides a significant reduction in noise there were always continuing efforts to reduce noise. A retired US Navy officer advises that quite some time after the original conversion, his GII Guppy submarine had an engine removed and a large Prairie-Masker compressor and air distribution system fitted.

This system produced an envelope of bubbles that surrounded the engine rooms and also created a masking effect for the screws. It was quite effective in attenuating diesel sounds and screw turn count. This latter equipment was not universally fitted in Guppy conversions and appears to have been simply part of later ongoing experiments and equipment trials. Prairie Masker was used more extensively on US Navy surface ships.

On commissioning in 1948, the experimental Royal Navy submarine Scotsman spent a great deal of her early life (from 1948) testing a screw noise reducing system. Albert Birchnall, former Stoker Mechanic directly involved in this project, reports that there was literally a nest of tubes around the screws with air jets fed by the on-board storage bottles and to be later replenished by submarine's compressor. Research revealed that the code name for this project was NIGHTSHIRT and though the system does not seem to have been applied to other Royal Navy submarines, it was more widely applied to Royal Navy destroyers and frigates. Appendix I

REVIEW

The Guppy conversion was basically the doubling of the battery voltage applied to the motors thus substantially increasing the shaft power to the propellers and the maximum speed to something in the order of 15 to 16 knots submerged. This was achieved by placing the two main battery groups - one forward - one aft, in SERIES. (Fig 4)

The Type 21 U-Boat did not switch the batteries, instead the designers chose to have the batteries permanently connected to give a nominal high 360 volts for supply to its twin 2500 shp motors. Lower speeds were obtained by using a substantial "creep" motor on each shaft using 'V' belt drive.

Unlike US Navy practice of the times, these were direct drive submarines similar to WWII Royal Navy submarines, the mechanical difference being both motors and engines were geared to the twin propulsion shafts

The Soviets submarines built after WWII certainly increased submerged speeds but other electric propulsion influences by the Type 21 design seem to be limited to the adoption of the large hand wheel style, single cubicle motor control units of the Type 21 (one for each shaft) and the use of 'creep' motors, but otherwise continued with their longstanding use of direct drive with clutches in conventional double hull submarines. Like the Germans, they apparently did not use the series/parallel battery arrangement but more on this subject in Appendix G

Unlike the Royal Navy they used geared high speed diesels in direct drive submarines like the Zulu and Foxtrot and likely the Whisky/Romeo.

Geared diesels are a separate discussion but domestic supply can be a problem and the high speed locomotive style diesel is smaller than the submarine direct drive unit of similar BHP at lower rpm. The Royal Navy Dockyards built their own direct drive diesels or they were constructed by the submarine builder, but the modern Upholder Class utilised a commercial rail diesel engine for its diesel-electric generators. There are indications that the Soviet engines were also rail adaptations.

The story of the problems of the US Navy with submarine engines and the naval encouragement of the commercial move to diesel-electric locomotives on the US railroads prior to WWII is well known, and hence the four D-Gs in the Fleet Submarine.

POSTSCRIPT

Simply as an amateur researcher I came to think that the high speed policy might be flawed by excess to the detriment of the overall submarine.

But further to this, I have recently become aware that the hull noise at the higher speeds of the Guppy II conversion, was found to badly affect the submarine's sonar listening sensitivity, that was constantly improving in the early post war period. Though the stated reason for changing the conversion program from the GII to the slower GIIA/GI conversion, was said to be a desirable reduction in overall conversion cost. Change in submerged performance as detailed in the main article.

Encouraged by reading authoritative views that more or less concurred with my thoughts, I have added this postscript that has a look at some aspects of the quest for high submerged speed in the diesel-electric submarine.

U.S. NAVAL TECHNICAL MISSION IN EUROPE TECHNICAL REPORT No. 287-47 September 1945-Dr M. S. Livingston - Technician

TACTICAL PLANNING FOR HIGH SPEED U-BOATS -

Summary

Report of interrogation of various German Naval personnel regarding the tactical use of high speed U-Boats. The type 21 was used for implementing the following tactics:
(a) Submerged approach to a favourable attack position,
(b) High quiet-speed run to the centre of convoy for the first attack
(c) Position-keeping directly beneath a ship in the convoy for protection while reloading torpedo tubes
(d) Retirement at high speeds to evade attacks by escorts.

Note. One of those interrogated was Admiral Godt who was on the U-Boat staff at OKM and was primarily interested in tactics. He had interviewed 1300 Commanding Officers on return from patrol. Apart from detailed comments about the fear of air attacks on U - Boats he made statements that expand on the above summary. He also stated that the primary protection of the Type 21 U-Boat was its high underwater speed. Interestingly, he also stated that tactics were general, and much was left to the judgement of the U-Boats C.O.s.

From the gist of the full report one can assume that the Type 21, a substitute with lower performance for the faltering Hydrogen Peroxide program, was influenced by the perceived needs of U-Boat C.O.s to counter the growing Allied ASW threat. Godt was Chief of Staff to Admiral Doenitz during the war.

Would the unique merged circle hull (figure of eight as is usually called) have been used for mass production if it had not already been designed for Hydrogen Peroxide storage, as has been suggested or was it considered a requirement for increasing the maximum depth? The US Navy didn't think so in their 700 ft Tang class nor the Royal Navy in their Oberon Class.

The XXI U-Boat entered the war too late to have any impact but there seems little doubt it was intended to revive the U-Boat campaign against Allied shipping but what sort of submarine campaign tactic did the US Navy envisage for the Guppy conversions emulating the Type 21?

Was the Western quest for greater underwater speed driven in the first place simply by WWII perceptions of the future of submarine? Or were the US Navy Guppy to be primarily fast targets for US Navy escorts who might soon have to face the Soviet hordes of fast diesel-electric submarines? Hordes that incidentally failed to materialise on the scale envisaged by the analysts.

In 1950 through 1953 US Navy converted 7 Fleet Submarines into Hunter Killers for the ASW role with no increase in submerged speed, together with new build small and slow Hunter Killer K-class submarines. The Hunter Killer was concept abandoned by 1960 with the arrival of the SSN.

Shortly after WWII the British declared that the submarine prime role was ASW, a role that in fact took quite some time to evolve, with the development of the 'new' torpedoes being far slower than anticipated, in fact being far outstripped by the improving submarine platform and its sonar.

Commander Royal Navy (ret), Compton - Hall's book "Sub vs Sub" deliberately mixes fact and postulation to present a picture of how a Sm versus Sm campaign may have progressed in a war that thankfully never happened. In terms of the Guppy conversions it is well to recall that this book was published some 40 years after the first Guppy appeared, however the Soviet diesel-electric Foxtrot and Tang discussed earlier, appear in his scenarios!

Recently reading Friedman's "THE POST WAR NAVAL REVOLUTION". Page 190 - it was intriguing to note that the British Director of Plans apparently commented in 1952,

It seemed to him that the Porpoises owed too much to Type 21 ideas. Surely the Royal Navy did not plan to fight convoy battles in the open ocean. Did it really need high speed, which (in the Director's view) had been adopted to overhaul convoys?

Friedman goes on to briefly offer various scenarios involving a submarine war off the Norwegian Coast and near the Arctic.

Equally intriguing, he states on page 192 in a caption attached to a photograph of one of the streamlined T Class submarines with a very modest increase in power.

"However streamlining was considered more important as a means of eliminating flow noise. In the post war ASW role, US and British submarines found high underwater speed less valuable than had been supposed".

It is difficult to determine how valuable the high speed of the Guppy was in practice over the long years they were in service without the views of the many generations of Commanders but from SubVet anecdotes, it would seems likely that the Guppy high speed facility was not often used to its fullest extent, placing as it did a very heavy demand on the battery with short endurance. Stable depth keeping was difficult and demanding at the highest speeds, with the attendant risks that a hydroplane or handling failure could quickly result in the submarine reaching its crush depth. Pat Householder – Appendix E

A former US Navy submarine officer advised that early in the conversion program, the size of the rams and hydraulic pressures of the aft hydro planes of the Guppy II had to be increased to maintain depth control at these new high submerged speeds.

The inevitable generation of high levels of Sonar "noise" at high speed was obviously undesirable. But as always, one must keep in mind that the SSK and ASW practices in 1950, may well have been quite different in 1960 and so on.

Dave Perkins describes an attack by Royal Canadian Navy O Class, capable of 17 for 20 minutes. It appears that 12 knots was considered a high submerged speed for this very quiet submarine? Appendix E

Turning to more recent times, in their book CONCEPTS IN SUBMARINE DESIGN, (Cambridge University Press, 1994), Academics and submarine designers Burchall and Rydill, associated with Vickers (VSEL) submarine builders, cover the subject of submerged speed from two aspects - the impact on the design of the submarine of high submerged speed requirements by the operator and the tactical limitations of the high speed battery submarine.

The opening open words on page 101 convey the first point -

The maximum speed of a submarine can be the most difficult and contentious aspect of the dialogue between operator and designer. This is because it is difficult to find a logical reasoning to arrive at a required maximum speed. Yet it is the probably the most expensive requirement to meet in the submarine. The propulsive power requirements of a submerged Submarine of a given displacement vary as the cube of the speed.In essence to double the maximum speed of a submarine from 20 Knots to 40 Knots would require eight times the propulsive power; in fact the submarine would have to be considerably larger to accommodate an eightfold increase in power plant, the speed would fall short of the 40 Knots and so even more power and size would called for.

The designer could be placed in a position of having to oppose unsubstantiated requirements for high maximum speed.

After discussion on submarine operational needs they conclude -

A more logical approach would be to try to establish the minimum/maximum speed capability which would permit the submarine to perform its required functions.

In another section, page 244-245, it is demonstrated in a simple graphic example, that for a given submarine, target interception may not be achieved at 20 knots, but likely at 15 knots over a longer period. Miller and Jordan in their "MODERN SUBMARINE WARFARE" give similar information and a brief but useful description of Lines of Submerged Approach diagrams and text from the ASW point of view.

This is all clearly related to the finite amount of stored power and the speed/power cube law. For example - reducing the speed from 20 Knots to 15 Knots, reduces the power output by just less than half. There would some other gains in the batteries operating with less current and thus lower temperatures, with less loss of water from the electrolyte.

The Royal Navy's last diesel-electric submarine, the very modern diesel-electric Vicker's type 2400, with a modified tear-drop hull, that became the Upholder Class four submarines.

Commander Powis Royal Navy in a published article stated that his Upholder Class, Unseen (designed late 1970's) was capable of maintaining the top submerged speed (20 knots?) for 90 minutes from a fully charged battery. Further once the submarine reached the end voltage it still had useable capacity remaining for operationally useful speeds. Very impressive, but the principles stated above still apply.

The last diesel-electric submarines built for the Royal Navy, they are now owned by Canada, leaving the only SSN.

With benefit of hindsight one could put forward the argument that in 1948, a more modest initial program such as the G1A/GIIA together with improved sonar and other operating facilities, may have been a more efficacious expenditure of the considerable sum spent on the GII conversions.

In those early years of the Cold War the money saved could been spent on more conversions in total. It is also clear that valuable space in the original Fleet Submarine was lost that in the end had to be recovered with the considerable cost of the 1962 GIII program and the inevitable reduction of maximum submerged speed down to that of the GIA/GIIA.

For myself, in retirement, I think about the period in which I served as a young man far, far removed from the decision making, in the motor rooms of the submarines of the early Cold War.

Was it the right decision in 1948, for the Royal Navy to have spent very scarce money and resources on the expensive and lengthy conversion of the eight T Class? The only gain being about 6 Knots submerged for 30 minutes and little else? Indeed there was the consequent serious loss of all stern torpedo tubes (a concern expressed by FOSM in 1948) and the motor room crew doubled to handle the extra switching - a serious consideration in a submarine that had had no improvement in accommodation since the original pre-war design long before the extra bodies came along for radar and the like.

Would it not have better to have urgently proceeded with the less expensive streamlining and noise reduction program of almost new A Class and T Class submarines in the Fleet, instead of delaying this successful program to the latter part of the fifties. There are unsourced reports that delays in the new P Class was one of the reasons given for the T Class conversions proceeding, but this ignored the needs of the day in those early years of the Cold War - a period longer than WWII. See more background here and here and Appendix D Items 7,8 and 9

The opposing Soviet new - build of this era were the eventually numerous, but small, Project 613 Whisky (1350 ton submerged), that seem to have had an overall performance rather inferior to the larger GIA/GIIA conversions at 13 knots submerged with less forward torpedo tubes.

The long range contemporary but far less numerous Project 611 Zulu (2400 tons submerged) had about the same speed as the GIIA/GIA and similar torpedo tubes, but with greater endurance.

The Zulu had hull problems and later reappeared as the very similar, but successful project 641 Foxtrot (2400 tons submerged) in about 1958 and was the main Soviet long range diesel-electric submarine. Propulsion detail Appendix G

Where the US Navy would have gone in the future with the diesel-electric submarine, is a matter of speculation as the last operational diesel-electric attack submarine was commissioned into the US Navy in 1959 - the three vessel Barbel class. But like the Guppy, they all remained in service for many years. A significant number of US diesel-electric submarines being transferred to other navies, in particular, the Turkish Navy.

The other Navies of the world appear to have followed the path set by the USS Barbel, particularly the Dutch and Japanese who based their successful designs on this innovative US submarine. The Russian Navy finally moved to the single screw, tear-drop hull with their relatively new Kilo class but with unique influences from the well established Russian design Bureau.

Regardless of any doubts about the advantages and disadvantages, there are even higher submerged speeds in some modern designs, with diesel engines sizes determined largely by the snorting battery charging needs. The fast WWII surface speeds are inappropriate with the tear-drop hull design and the concept of always operating submerged.

Hopefully we will never have another global war to test the concept of the fast battery submarine.

12 July 2007. Revised 29 Sept 2007.
Peter.D.Hulme, Taupo, NZ.

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ADDENDUM 2018

With particular reference to the US SUBVETS anecdotes, Appendix 40, describing noise in Guppy Submarines at high submerged speed.

It is over a decade since I put together this article on the USN GUPPY conversion of the Fleet Submarine. The aim of the article was to describe the overall conversion of existing USN Fleet Submarines to substantially increase their maximum submerged speed using battery drive, with the article focus on the changes to the electrical propulsion system. I think this was achieved, but one thing remained unanswered despite my making inquiries and that was the cause of the excessive hull noise at high speed as described in anecdotes in Nautilus, sent to me by indvidual USN Subvets and there matter lay until recently a posting was made on the Marine History List, Canada.

This posting drew attention to the book the "Silent War" by John Pina Craven, a biography of a Naval scientist. Readers of submarine books who have read the popular 'Blind Mans Bluff' will recall John Craven and underwater scientifically based exploits such as locating the sunken USS Scorpion (SSN -589). He died on Feb 12 in Hawaii,he was 90.

According to the New York Times obituary:

Dr. Craven's naval career began in 1951, when, as a civilian, he investigated how to improve ships and submarines. He was promoted quickly after correctly predicting and helping to fix a structural problem with the Navy's first nuclear-powered submarine, the Nautilus. From 1959 to 1969, he was chief scientist of the Special Projects Office

In 'Silent War' he tells of these experiences with the then new nuclear submarine USS Nautilus seeking solutions to the problems of extreme noise and vibration at high submerged speed that appeared to originate in the external ballast tanks. He also tells of his considerable efforts to determine the cause and that he beleived it was cavitation due to open ballast tank inlet valve to allow the M.B.T. s to be flooded to submerge the submarine.

I took notice of this and surmised the hull of the Nautilus would simply be a larger version of the hull of the earlier diesel Fleet Submarine converted for higher submerged speeds by the Guppy conversion and that the noise and vibration described in the USN SubVet's anecdotes, Appendix E of this article, was in fact caused by the same phenomenon experienced in the later Nautilus and had been thoroughly investigated by Craven, apparently quite seperately from those conducted on the Guppy. A problem that was to plague Dreadnought in the years ahead was that her hull vibrated at critical speeds when submerged.

Having read Craven's description of the noise problem, I did a web search and came across a detailed book by Rodney P Carlisle 'Where the fleet begins: A History of the David Taylor Research Center, 1898-1998'. Though not relevant to this ademdum to my Guppy article, page 260 of this book and onwards gives a detailed picture of the increasing investigating into nuclear submarine noise'.

Here is an edited extract from the book Pages 246-247:

while at New London on his six month tour. Lasky learned of severe noise problems with Guppy conversions. On relatively quite electric motors running at fifteen knots, the overall noise noise generated would reveal the sub's location. If the Soviet W class met or exceeded Guppy characteristics, the Guppy would not qualify as a killer submarine directed at Soviet W's unless it could be quietend down. The US Navy Underwater Sound Laboratory and EES contributed to the sound studies, along with the DTMB. By the end of 1949 W L Pryor at the Sound Laboratory after trials with Dogfish (SS350)

Pryor concluded that the one could not identify the snorkel as the major culprit in noisy Guppy. Nevertheless, it remained Guppy Submarines even when operating on battery power underwater, could not evade detection.

Authors Note:
It later turned out the the maximum submerged speed of the numerous Soviet W was only 13 knots, however the Soviet Fast Battery submarin project 614 'Foxtrot' appeared with a performance in excess of the Guppy, 15 knots.

My conclusion is that the excessive noise/vibration at high under water speed as described in the SubVets Guppy notes in Appendix E, had the same cause as was determined on the later Nautilus by Craven, that is:

cavitation ( or other phenomena) in the main ballast tanks and their free flooding entry and that this noise was also present in the Guppy submarines, but despite serious investigstion, the cause not determined. My conclusion is simply based on the premise that had the cause of the noise been determined at some stage in the Guppy conversions, before the hull construction of the Nautilus, the design of her main ballast tanks would have been such as to eliminate the cause of the noise.

In my earlier research into the 1950s conversion of eight of the Royal Navy T Class into Fast Battery Submarines, I became aware of the difficulties of eleminating noise at high speed submerged and the adoption of differently shaped free flood holes often with bars and grids at the sea inlets. In my RN submarine files I have just come across an E-Mail (28-11-2006) from Michael Young, who served as a submarine officer in the RN and RCN, responding to some speed related query of mine. Peter,

I too raised my eyebrows at the 20 knots! I was navigator in Oberon Class and I don't recall ever getting over about 17-18 knots in BIS (Batteries in Series) on trials. Same in 'Onondaga', but I was the Torpedo Officer then. We made so much noise and vibrated madly as well!

Regards Michael

To give some perspective to this annecdote, the Oberon Class were required for official acceptance trials to reach 17 knots for 20 minutes and for more about reasonable maximum speed, read Dave Perkin's anecdote, the last one in Appendix E, where he states that about 12 knots is the maximum practical speed for the then modern British built Oberon Class. Dave is sadly deceased and I lost a submariner and author who was very well informed on diesel submarine topics. The Oberon Class was considered a very successful submarine.

Very recently I came across this quotation from Cold War Submarines: The Design and Construction of US and Soviet Submarines, by Norman Polmar, Kenneth J. Moore page 15:

Guppy submarines could reach 14-16 knots submerged, compared to ten knots for non-converted fleet boats. The Guppys suffered major vibrations at higher speeds; one submariner recalls "almost losing my teeth to the vibrations." Some efforts were undertaken to quiet the machinery noises in the Guppys, because the (captured) Type XXI was found at 12 knots to be quieter than the best U. S fleet submarines at 6 knots.

Authors Note:
I had difficulty grasping John Craven's detailed explanation of the problems found in the ballast tanks of the USS Nautilus and referred to a book I rely on greatly matters in regard to submarine design, here below is an extract discussing what appear to be the same phenomenon experienced aboard Nautilus. The authors, by any standard, are experts in modern submarine design. Sadly Professor Roy Burcher passed away in 2001.

Concepts in Submarine Design by Ray Burcher and Louis Rydill. Published 1994. Extracts from pages 108 and 109:

there is still a requirement for rapid drain down of free flood volumes at the top of the hull to avoid stability problems and to allow water exit from and ingress into the MBTs. Both involve providing quite large openings in the otherwise smooth hull of the vessel. Special attention has to be paid to their shaping and alignment to avoid additional drag. The main cause of drag due to flow across a hole is the induced fluctuation of flow in and out of the hole caused by pressure differences and instabilities, which is exacerbated by the forward-looking edge at the aft end of the hole acting as a splitter plate and causing fluctuating eddies to be shed. It is a process similar to that use deliberately to create sound in a flute or organ pipe. The phenomenon can happen in a submarine at speed if the connecting tank or internal volume experiences a resonant condition with eddy shedding. Not only is energy lost from the flow contributing drag, but noise may be radiated as well. In some extreme instances the resonance of the structure has resulted in fatigue cracking.

I deleted a reference by the authors, to the use of doors as being not relevant and the extract continues:

a simpler solution is to fit a flat bar grill across the opening with the bars across the flow direction and angled away from the flow. These effectively inhibit the oscillatory eddy shedding whilst imposing little restriction of the necessary flow of water through the hole.

The last Commanding Officer of USS Nautilus (SSN 571) 1970, made a presentation at Nautilus Golden Anniversary Lecture Series, Submarine Force Museum, Wednesday, November 17, 2004. USN Rear Admiral (ret) Dick Riddell is the Director for Undersea Warfare Programs at General Dynamics Corporate Headquarters in Falls Church, Virginia. He joined General Dynamics Corporation in October 1998.

Here is an extract from this speech relevant to noise and vibration:

It was "hard" to hear (a target)with the ship's sonar when Nautilus was making more than about four knots. Any sort of speed caused such rattling of the ship's superstructure that the resulting noise drowned out any sonar contacts. In spite of a lot of effort in looking for loose pieces of superstructure, the rattling persisted.

I have been unable to find a satisfactory drawing or photograph of the ballast tank structure of the Appendix E converion or T Class'. As there is an anecdote in Appendix E in the main article regarding the British built Oberon Class making noise at speed and the E-mail from Michael Young, I thought it suitable to show the photograph (Arry Hartnett) of an Oberon Class being scrapped about 1970, clearly showing the main ballast tank of the submarine in the scrap yard and the struts between the relatively thin ballast tank panels only subject to modest pressures when surfaced and the heavy gauge pressure hull steel plate subject to pressures at up to 700 feet.

Together with a shot of one of the several 15 x 24 inch flood holes, provide by John Eade taken of an RAN Oberon Class

And a simple hull cross section sketch from the Oberon Class submarine manual of the Canadian Forces (RCN).

Note that in British built submarines the steel hull frames are inside the hull, while on USN submarines they are outside, however the general principals of ballast tanks on submarines of this era remain the same, that is fitted to the outside of the pressure hull, supported by tank dividing plates and struts.

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