Chapter 9: Monitor M Class Types
9.1 Introduction
1. The idea of this class, basically of a monitor submarine, was due to Commodore Hall. The decision to design the M Class was made by a committee under Admiral Tudor as Chairman. The concept was a distinct change in submarine development by fitting a 12-inch gun in addition to four bow torpedo tubes.
M1
2. A précis of statements made by the First Sea Lord and RA(S) in August 1918 is as follows:
The object behind the design was to supplement torpedo attack against surface ships which was found difficult against high speed vessels steering a zigzag course and which could often elude a torpedo when sighted by changing course. A 12-inch projectile at fairly close range should be difficult to elude. Furthermore 50 projectiles could be carried instead of a few torpedoes.
'The boat and gun were designed so that the gun could be kept loaded submerged. A round already aimed could be fired at a small target in 30 seconds from periscope depth or about 20 seconds after breaking surface. The vessel could be at periscope depth (24ft) again in 15 seconds, i.e. before the smoke of discharge had cleared away. To reload it was necessary to surface but it was recognised that if the shot found the target, the size was such that it would destroy the destroyer or light cruiser struck. In addition it could be used for bombardment. There is little doubt that if the Germans had such boats they would have caused much havoc to our coastal towns.'
During building the vessels were delayed 'as it was recognised that if the enemy employed similar submarines he could do more damage to us than we could do to him.' M1 was later accelerated after some delay.
3. After the completion of M1 in April 1918 the question was raised as to future employment. There were now no targets for her, the enemy had not initiated anything bold in submarine policy and it was not good policy to give him the lead in such a weapon. It was suggested that M2, M3 and M4 should be completed as overseas patrol vessels without the 12-inch gun. The First Sea Lord turned this down and ordered M2-4 to be completed as originally designed. M1 joined the XI S/M Flotilla for ordinary patrol duties 'until any special operations for her arose.'
4. K18-21 had been ordered in February 1916 but before building had a chance to commence it was decided to substitute four M Class vessels instead. In the same month an order was placed with Vickers to build a boat to the new design. M1 took the place of K18. A second vessel M2 (ex K19) was ordered from Vickers in May 1916 and two more M3 (ex K20) and M4 (ex K21) from Armstrong Whitworth in August 1916. M1 completed in April 1918 but M2 and M3 were not completed until 1920. M4 was scrapped before completion.
9.2 Design
5. The main engines, motors and batteries in the M Class were the same as being fitted in the L Class building at the same time but they had only four 18-inch bow torpedo tubes as against four bow and two beam 18-inch tubes in the latter. The best comparison to find the effect of fitting the 12-inch gun is between M1-2 and L1-8.
Particulars of these two classes as completed are:
| M1-2 | L1-8 | |
| Length overall | 295ft 9 in. | 231ft 1in. |
| Beam maximum | 24ft 8 in. | 23ft 5½in. |
| Displacement surface, tons | 1594 | 891 |
| Displacement submerged, tons | 1946 | 1074 |
| Speed surface, knots | 15 | 17.2 |
| Speed submerged, knots | -9 | 10.5 |
| Bhp surface | 2400 | 2400 |
| Bhp submerged | 1600 | 1600 |
| Torpedo tubes, bow 18-inch | 4 | 4 |
| Torpedo tubes, beam 18-inch | 2 | |
| Guns | One 12-inch | One 3-inch |
| One 3-inch | or 4-inch |
The building weights in tons were:
| M1-2 | L1-4 | |
| Hull | 662 | 354 |
| Main engines, motors and batteries | 248 | 248 |
| Auxiliary machinery | 33 | 33 |
| Equipment and services | 119 | 87 |
| Torpedo armament | 23 | 29 |
| Gun armament | 146 | 6 |
| Spares and sundries | 12 | 12 |
| Ballast keel | 140 | 44 |
| 1483 | 813 | |
| Oil fuel and lubricating oil | 87 | 56 |
| Fresh water | 17 | 9 |
| *Auxiliary ballast water | 114 | 42 |
| 1601 | 920 |
*The amount of auxiliary ballast water shown in the above table represents the full capacity of the tanks. This water or the ballast keel would be adjusted to bring the total weight to the surface displacement shown.
The above shows that carrying a 12-inch gun increased the weight of the submarine by about 650 tons with the additional penalty of loss of speed (and endurance) both on the surface and submerged.
6. There were differences between M1-2 and M3. In particular, the length was increased by 10ft in M3 to allow 21-inch torpedo tubes to be substituted for the 18-inch bow tubes. However all further information given in this chapter is based on M1 with a reference to M2 regarding an increase in oil fuel and its effect.
A General Arrangement of M1 is shown in Plate 30.
7. The Admiralty design was governed by the 12-inch gun and mounting and ancillary spaces the details of which were supplied by Vickers. It was a partial double-hull boat with the double-hull extending for about 65% of the length of the boat. The bottom space in the double-hull was used for tanks other than main ballast tanks all tested to 50lb/in2 and built to withstand full diving depth. The space above these tanks was used for sided main ballast tanks divided by a longitudinal bulkhead at the middle line at the top. The outer hull was continued forward to the bow as three controlled free flooding spaces and aft to the pressure hull as one similar tank. The top of these tanks formed a wide practically horizontal deck from the bows curving down slightly at the after end. The 12-inch gun as shown in Fig 9.1 dictated the superstructure and bridge.
Fig 9.1
9.3 Dimensions
8. The Loa was 295ft 9in, the Lbp about 280ft and the Lph about 270ft 9in. The Lbp is often given as 290ft but this is the length on the waterline.
The beam and depth of the inner hull were 20ft 2¾in and 13ft 0½in respectively moulded and of the outer hull 24ft 8in and 18ft 8in maximum. The ballast keel amidships was 13¼in deep.
Vickers gave the mean surface draught as 15ft 11in. with a displacement of 1610 tons. This displacement is discussed in Paragraph 9. At a displacement of 1594 tons the draught would be about 15ft 9 in., the freeboard to the main deck of the order of 4ft, to the superstructure deck 7ft and to the bridge 11ft 8 in. When on the surface the trim by the stern is stated to have been as much as 17 in.
9.4 Displacement and Stability
9. As designed and first constructed the tanks affecting displacement and reserve of buoyancy were:
- Twenty external main ballast tanks with a total capacity of 375 tons. These tanks were numbered 1 to 10 Pt and Stbd.
- Two internal main ballast tanks with a total capacity of 15 tons.
- Four controlled free flooding spaces; three in the bows with a capacity of 27.5 tons and one aft of 15 tons.
10. When M1 was first inclined on 17 February 1918, the submerged displacement given by Vickers was 1931 tons. The BG submerged was only 2.75in and this had to be improved. Furthermore it is obvious from the changes eventually made that the vessel was heavy aft. This was overcome by converting the controlled free flooding space aft of 15 tons capacity into a buoyancy tank which increased the submerged displacement by 15 tons. In addition No 9 Port and Stbd main tanks were made into buoyancy tanks of 38 tons capacity which did not affect the submerged displacement. Additions were made to the ballast keel of 59 tons. When inclined in April 1918 the BG had risen to 6.6 in. The original No 10 Port and Stbd main tanks were renumbered No 9.
The submerged buoyancy was therefore 1946 tons. The total main ballast water with 337 tons in external tanks and 15 tons in internal tanks was 352 tons. The surface displacement was therefore 1594 tons and the reserve of buoyancy with the controlled free flooding spaces 23.8%. Using the same submerged displacement of 1946 tons and the same main tank capacities Vickers gave the completion surface displacement as 1610 tons. This suggests that either the internal water ballast could not be used or that 16 tons of water remained in the externals on the surface. The as built weights show that the ballast keel weighed 140 tons and that over 100 tons of auxiliary ballast and trimming water was carried in the surface condition. There is no reason why this ballast should not have been decreased to allow the internal ballast water to be used and retain the reserve of buoyancy. It is considered that because of the shape of the main tanks and the position of the flooding holes 16 tons of water remained in the external tanks after surfacing. This is called the service displacement. The reserve of buoyancy decreased to 22.6%.
11. In M2 No 9 Port and Stbd external tanks (originally No 10 tanks) were used to carry oil fuel. In this case the service displacement would have been 1649 tons and the reserve of buoyancy 19.7%; with all main ballast tanks empty the surface displacement 1633 tons and reserve of buoyancy 20.8%.
12. The design figures for stability given by DNC in 1918 were GM 2 in. and BG 7in. The actual inclined figures were GM 24in and BG 6.5in.
9.5 Speed and Endurance
13. The design figures were:
| (a) Speed surface | 16 knots at 2400 bhp |
| (b) Speed submerged | 10 knots at 1600 bhp |
| (c) Endurance surface | 2500 miles at 16 knots |
| 4000 miles at 11 knots | |
| (d) Endurance submerged | 10 miles at 10 knots |
| 80 miles at 2 knots |
- Reference (a) above, speed trials at Stokes Bay gave 14.7 knots. The water was shallow for the size of the vessel and 15.4 knots was afterwards obtained. New propellers were ordered with which it was hoped to achieve 16 knots. A later record gives 15 knots at 2600 bhp and this was the likely maximum surface speed attained.
- Reference (b) above, the designed speed was optimistic and before completion DNC gave 9.5 knots; 9 knots was also mentioned. The 1600 bhp was for 11 hours and undoubtedly higher powers could be obtained for bursts of speed and perhaps 9 knots achieved. 8 knots has been mentioned by RA(S) and this is taken as the service full speed.
- Reference (c) above, figures of 2000 miles at full power and 4500 miles maximum were also given early on using 76 tons of fuel in M1. This endurance would appear to have been obtained in M1, increased by about 45% for M2 with 110 tons of oil fuel.
- Reference (d) above, 8 to 9 miles at full speed was probably obtained. No actual figures have been seen although the endurances must have been better than those given for M2 below when fitted with a seaplane hanger in place of the 12-inch gun.
14. After conversion to a seaplane carrier CB 1815 (1930) gave the following figures for M2:
| (a) Speed surface | 14.1 knots |
| (b) Speed submerged | 6.4 knots |
| (c) Endurance surface | 2350 miles at 14 knots, 3700 miles at 10 knots |
| (d) Endurance submerged | 1.7 hours at 6 knots, 6 hours at 4 knots |
| 12 hours at 1.9 knots | |
| (e) Fuel carried | 97.5 tons |
M2 recovering plane
9.7 Structure
15. The diving depth was 200 feet. The pressure hull was of 25 and 22lb plating amidships which would decrease towards the ends. The outer hull was a variation of 30, 25, 20, 18, 15 and 10lb plating - an extraordinary mixture - but as mentioned in Paragraph 7 much of the lower portion of the outer hull was built to pressure hull standards. The frames were changed from the K Class 15lb bulb angles to 12.8lb bulb angles spaced 21in. and 18in. apart. The frames were numbered from forward.
There were eleven internal main WT bulkheads making twelve internal main compartments, all tested to 35lb/in2. The largest compartment was the engine room of 216 tons capacity without deducting equipment therein. The sub-division was probably not as good as in the K Class which had a much larger reserve of buoyancy.
The standard for testing tanks was maintained as in the K Class. The oil fuel tanks were tested individually to 10lb/in2 but each group of tanks Nos 1-7 Pt and Stbd was tested to 50lb/in2.
9.8 Tanks
9.8.1 Main Tanks
16. As designed there were twenty main tanks in the externals with a total capacity of 375 tons. As explained in Paragraph 10 two of these main tanks were converted into buoyancy tanks before completion to improve the stability submerged. They therefore completed with only eighteen external tanks of capacity 337 tons total. To assist in quick diving, scoop valves were fitted to the external tanks just below the surface waterline and additional Kingstons fitted as compared with a conventional Submarine. The scoops and the vent valves were telemotor operated from the control room. Two internal main tanks A and Z with a total capacity of 14.9 tons were sited one forward and one aft.
In M2 the original No 10 (now No 9) external tanks were used to carry oil fuel and the capacity of the external main tanks fell to 298 tons.
The whole of the superstructure from the bows to the after end of the torpedo room was built as three controlled free flooding spaces with a total capacity of 27.5 tons.
9.8.2 Oil Fuel Tanks
17. In M1, 76.5 tons of oil fuel of sg 0.896 was carried in fifteen external tanks. Individual tanks were tested to 10 lb/in2 but each group of seven tanks port and starboard sides was tested to 50lb/in2. The number of tanks fitted all about 5 ton capacity each and all adjacent seems excessive.
In addition in M2 No 9 Pt and Stbd external main ballast tanks were fitted to take 34 tons of oil fuel so that M2 carried about 110 tons of fuel. This was of course at the expense of reserve of buoyancy and seaworthiness.
9.8.3 Other Tanks
18. Other named tanks were generally as in previous classes with the following points of interest:
- (a) The large number of nine compensating tanks and twelve auxiliary ballast tanks with total capacity of approximately 42 tons and 105 tons respectively. Most of the compensating tanks were sided, the auxiliary ballast tanks were not. The large number of tanks and their capacity seems excessive in comparison with the K Class which had only 56 tons of compensating and auxiliary ballast tank capacity in eight tanks. The only exceptional feature in the M Class was the 12-inch ammunition of a total weight 29.0 tons. To allow for diving in a range of sea water densities from sg 1.025 to 1.00 required 40 tons of compensating water, but this large range was not a design requirement until many years later.
- (b) Two drop keel gear tanks apparently one at each end of the drop keep which weighed 20 tons.
- (c) The compartments in connection with the 12-inch gun e.g. magazine, shell room, hydraulic machinery, loading chamber and directing tower.
- (d) A binnacle inspection chamber from the bridge to the control room is shown for the first time and tested to 35lb/in2.
9.9 Main Machinery
19. Two Vickers twelve cylinder diesel engines each of 1200 bhp at 380- 400 rev/min one on each of the two shafts. They were of the same type as being fitted in the L Class at the time. On test these engines developed 1300 bhp and some authorities quote the total power as 2600.
The main motors were also as in the L Class of double armature type of 400 bhp each at 300 rev/min giving a total of 1600 bhp.
Three battery tanks contained a total of 336 Exide cells. Two of these tanks were forward of the 12-inch magazine and the third aft of the magazine under the control room.
9.10 Armament
20. The 12-inch BL gun was the main feature of the design as already mentioned. The gun and its loading chamber and director tower were on the fore side of and incorporated in the bridge. There was a common access compartment to the tower and the bridge. The 12-inch magazine with passageways each side, the 12-inch shell room and the hydraulic machinery compartment with hydraulic tank occupied about 30ft of the length of the pressure hull directly below the gun.
The 12-inch guns came from the Majestic Class battleships. The weight of gun and mounting was about 120 tons and the 40 rounds of ammunition weighed 29 tons. The buoyancy of the gun and mounting is given by Vickers as 10.0 tons and of the director tower and loading chamber of 34.0 tons both about 9ft above the LWL. It was this buoyancy so high up which made these vessels such quick diving boats.
21. The following extract from a letter of October 1967 from an ex-employee of Vickers to the Managing Director of Vickers is a link with these boats when building:
I spent many months on M2, measuring up and making notes. The gun looked neither ungainly nor out of place. As far as I remember it was an old 12-inch Mk 9 of 40 calibre and weighing about 50 tons. It was probably selected because of its considerable girder strength. There was no turntable, but its high angle elevation made it possible to fire it when the submarine was dived to some fifteen feet or so. It had to be loaded on the surface and laid too, although there were several good ideas put forward about doing the latter when submerged. Firing was done from inside, its curious bead sight being lined up with a periscope, rather like a huge rifle with aperture sights. The idea was to submerge, cruise around looking for a suitable enemy, and they let him have an 850lb thump from nowhere when he was not expecting it. This could be done at a considerable range far beyond that of a torpedo.
All this was very slow. In a battleship the Mk 9 could get off about 2 rounds a minute, but in M1 everything depended on weather, range, type of target, etc, and coming to the surface to reload and relay was a great nuisance. As the war was over we never knew whether the experiment was worth while. I believe the Admiralty gave up this type of thing not because of lack of faith in it but because the Washington Treaty forbade heavy calibre in submarines.
22. Further to what has already been said the gun could be trained through 15° elevated 20° and depressed 5°. The loading and firing operations were carried out from the loading chamber behind the gun. The gun could be loaded at low buoyancy if desired.
23. In addition to the 12-inch gun, a 3-inch HA disappearing type gun was mounted aft on the superstructure. A separate magazine aft stowed 72 rounds of 3-inch ammunition.
24. The torpedo armament in M1-2 consisted solely of four 18-inch bow tubes and they carried eight torpedoes. In M3 21-inch tubes were substituted for the 18-inch tubes. No bulkhead was fitted to separate the tubes from the torpedo room.
9.11 Subsequent Service
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