Chapter 5 - Double-Hull Submarines
1. By 1907 a number of foreign countries were building double-hull or partial double hull type boats which were defined as 'submersibles' as distinct from the single hull boats called 'submarines'. The main difference between these two types was that the submersible had a ship shape form, with a high reserve of buoyancy of between 30-40%, as against 6-10% in submarines at that time. The submersible also had greater longitudinal stability and with the greater freeboard was more seaworthy than the submarine. Hoare states as one disadvantage 'it has a short rolling period on the surface whilst the submarine has an almost entire lack of rolling' meaning that the submersible is much more uncomfortable on the surface. Hoare also claims that Lake in the USA was the first to advocate the ship shape hull with forward and after hydroplanes and although the latter is undoubtedly correct, the Lake design was not really a ship shape form nor double-hulled. The particular feature of his design was the watertight superstructure with controlled flooding. The USN adopted the Lake type of submarine in 1911.
2. The French Navy was the first to adopt a double-hull type submersible with any real potential, in the Narval of 106 tons displacement. Laid down in 1897 to a design by Laubeuf, the ballast tanks completely surrounded the pressure hull except at the keel and were carried up to the top of the superstructure. The thin external plating was shaped into the keel to give a good ship shape form.
The term 'submersible' soon disappeared and the double-hull types became submarines.
3. By the turn of the century France was involved in building a large number of submarines, many of a newer Laubeuf type, and held the lead for a number of years. The new Laubeuf design was a truly double-hull type with the pressure hull completely surrounded by external tanks. In 1905 the German Navy placed an order for its first submarine U1, of the Germania-Krupp type, which was a combination of the old Laubeuf externals and a controlled flooding superstructure. Italy built her first submarine in 1905-08, a Flat-Laurenti type double-hull characterised by a non-circular pressure hull. Sweden's first submarine, completed in 1905, was a Holland type redesigned by C Richson, an engineer on the Naval Engineering staff of the Swedish Navy, but in 1907 that country concluded a contract with Fiat, San Giorgio of Spezia for a Fiat-Laurenti double-hull boat, Hvalen (Whale) of 185 tons displacement.
4. The advantages claimed for the double-hull type of construction were:
- The external hull could be made to any shape and similar to that of a surface ship and so give better speed than the single circular hull form.
- A greater reserve of buoyancy and hence, better seaworthiness.
- The spaces between the two hulls could be used for water ballast.
- Since the external shape was that of a surface vessel, surface stability was similar and far more than in single hull boats.
- Strength of the pressure hull increased since web frames of appreciable depth could be fitted between the hulls.
- With above since all the frames were outside the pressure hull there was more space inside.
- The deck top of the outer hull provided a space for exercise for the crew.
5. The disadvantage was that the double-hull boat as then designed took longer to dive. This disadvantage was not appreciated at the time and in fact the question of quick diving appears to have been given little consideration in the earlier classes of submarines but even looked upon as a danger. The need for quick diving was soon made evident by war experience in 1914.
6. In the French Laubeuf design the ballast tanks extended from the keel to the top of the boat and were flooded through Kingstons. In the Italian Flat-Laurenti design the main tanks also extended from the keel to the top of the boat but were cut in two by WT longitudinal flats just above the surface waterline. The bottom spaces flooded through Kingstons and those above the longitudinal flats through valves or doors fitted above the flats. War experience showed that it was necessary to remove these valves or doors and to replace them by open holes to make the above water portion free flooding. They therefore became controlled free flooding spaces which means spaces which are wholly or mainly above the normal surface waterline, which are open to the sea at the bottom and which are vented by opening vent valves at the top.
7. Controlled free flooding spaces by the very nature of being controlled had an adverse effect on the time to dive. The control meant hand or telemotor operated vent valves, the number of these that could be fitted was limited by complication and cost. In our own submarines, even as late as in the 1930's, we were very conscious of the hold up in flooding free flooding superstructures. The number of holes for venting was drastically increased as, for example, in Oberon in which the diving time to 35ft was decreased from 3 min 10 sec to 1 min 50 sec simply by cutting more holes. It would have been impracticable in practice to increase the venting area so drastically in a controlled free flooding space.
5.2 Overseas and Coastal Type Requirements
8. As mentioned in Chapter 1, there was a general feeling in 1910 among RN submarine officers that the double-hull type of submarine should be considered; also as previously mentioned the restrictions in the contract for building with Vickers made some other source of design necessary.
9. An opportunity was given by the Fiat San Giorgio Co of Spezia for British officers to inspect vessels of the Laurenti design building for the Italian Government, and Admiralty representatives who inspected them in August 1911 were favourably impressed. An order was given in 1911 to Messrs Scotts, under license from the Flat Co, to build in this country one vessel (S1) of this design, so that its advantages and disadvantages could be compared with our existing vessels. Before any comparison could be made a further two vessels were ordered in 1913. The same officers visited Messrs Schneider's works at Toulon where vessels of the Laubeuf type were inspected. Although interesting in many points, their design was not recommended as they were too long, too slow and had torpedoes in frames outside the hull; in fact the design was out of date. In spite of this, two of them, W1 and W2 were ordered from Armstrong Whitworth & Co in 1913 and two more in 1914.
10. As a result of the inspections of the French and Italian designs, the First Lord ordered a conference of submarine officers to recommend requirements for future designs. This Submarine Committee of February 1912 made the following recommendations for an 'Overseas' type:
- Large reserve of buoyancy.
- Surface displacement about 1000 tons.
- Double-hull (complete or partial).
- WT subdivision of hull for safety and improved habitability.
- 20 knots surface speed approximate.
- Submerged speed high for a short distance.
- Large surface and submerged endurance at economical speed.
- To submerge in 3 minutes from surface condition.
- Good accommodation for officers and men.
- Armament two bow, two beam and two stern tubes.
11. In defining this 'Overseas' type, the Committee had in view a double-hulled ship shape vessel of sufficient speed to accompany the Fleet and to keep the sea for extended operations in all conditions of weather. Germany and France were producing vessels on an E Class displacement having a good speed, etc, but they were not fitted with beam tubes and this of course enabled a better ship shape form. On the other hand, the tactical value of the beam tubes could not be disputed.
12. The first result of these recommendations for the 'Overseas' type was the acceptance by the Admiralty of a Vickers' design for the Nautilus. Although much larger than was originally intended she represented the Vickers' concept of the problem put before them by the Committee and was laid down in 1913.
13. The Committee laid down the requirements for a Coastal type as follows:
- Larger reserve of buoyancy than C Class, probably necessitating WT superstructure.
- Surface displacement 250-300 tons.
- Double-hull (partial or whole).
- WT subdivision.
- Twin screw heavy oil engine.
- Speed not under 14 knots, endurance of 1200 miles.
- Good accommodation.
- Two periscopes.
- Lifting bolts built into the vessel to facilitate salvage.
- Two bow tubes and if possible one stern tube.
14. These requirements in many ways followed the Flat S type design then being progressed at Scotts. The V Class was Vickers' idea of a submarine to these requirements. V1 was ordered in 1912 and V2 and V3 in 1913.
15 As a result of the failure of the Nautilus design to offer the speed of 20 knots laid down in the Submarine Committee's requirements. Coupled with misgivings about using the high power diesel engines proposed, enquiries were made of the Flat Company to see if that firm and M Laurenti could offer a design to meet the requirements. The Italians were wary about using diesels and Laurenti prepared a steam driven design. Although there was considerable criticism, especially regarding the use of steam, Scotts developed the design, which became the Swordfish ordered in 1913. This was the last foreign design to be adopted.
16. In 1913 it was decided to produce a standard Admiralty double-hull Coastal type and the F Class was designed with rather similar characteristics as the V Class but with a number of improvements including a stern torpedo tube.
17. At the end of 1913, it was found that the Germans were spending nearly the whole of the money available for submarines on 'Overseas' vessels of double-hull type to approximately E boat displacement. It was decided that the Admiralty should prepare a design of partial double-hull construction of about E boat size. The G Class evolved from this exercise.
18. Late in 1914 a report was received that the Germans had some submarines with a surface speed of 22 knots. In Nautilus and Swordfish, then building, only 17 and 18 knots respectively were expected. An Admiralty design - the J Class - was prepared by the end of January 1915 and the first boat, J1 completed by April 1916. They were double-hulled boats on the Fiat design principle with a speed of 19-19.5 knots. The German report mentioned was actually unfounded.
19. In 1915 the demand arose for submarines to accompany the Fleet; the speed had risen to 24 knots. The K Class evolved from this request. They were not actually of double-hull construction although the M Class design was.
20. The various types of double-hull and partial double-hull submarines, mentioned above, are dealt with individually later. It is relevant to mention that the two classes built to foreign designs, S1-3 and W1-4, were sold to the Italian Government soon after that country entered the war. They were not really a success operating in home waters. The Swordfish was not completed until July 1916 and spent little time as a submarine before being taken in hand for conversion to a Patrol Boat in June 1917. Similarly Nautilus, completed in June 1917, spent most of her life as a Depot Ship for instructional purposes.
21. The undoubted disadvantage of the double-hull boat was its excessive time to dive. The problems of stability and reserve of buoyancy in the single hull boats could be solved adequately by using saddle tanks and this was the policy adapted in this country after the war.