Chapter 19: Diving Depth
1. Varying figures can be given for the diving depth of a submarine depending on whether it represents (a) the design depth which the hull and fittings are designed to withstand or (b) the operational diving depth which is less than the designed depth and is laid down by the Admiralty and/or RA(S) as the normal limit to which a submarine may dive; this depth can be exceeded in an emergency and any defects which result and the cost involved in repairs accepted, or (c) a limitation on depth due to age and generally due to a reduction in the thickness of hull plating which is a decision made by DNC.
2. The American specification for the Holland boats stated that the whole structure was tested to withstand a crushing pressure of 100 feet of water corresponding to 43. 5lb/in2 pressure. Whether this meant that the submarine as a whole was actually tested to this pressure externally in America is not known but it would not have been done in this country. There is no doubt that the Holland boats were designed to withstand 100 feet depth of water and so were the A Class, B Class and C Class. The normal maximum depth of submersion, i. e. operational depth, was taken as 50 feet consistent with Vickers' guarantee of safety to a depth of 100 feet. Some A Class and B Class did go to 100 feet depth at the ballast tanks and B1 reached 95 feet on the diving gauge. However, all classes were ordered not to go below 50 feet.
3. All these classes were tested internally to 35lb/in2. The shell was tested by filling the boat with water and applying a pressure of 35lb/in2. This was the common practice for testing hull structure, it being much easier to locate water leaks than air leaks.
4. The D Class and also the E Class were designed structurally to withstand 100 feet depth of water. As Watts states 'they (E Class ) were designed, as previous vessels, to have a reasonable factor of safety at 100 feet beyond which it was considered a submarine would never descend voluntarily. The events of war soon proved this to be wrong'. There is no doubt that in those days the calculation of the strength of the hull submerged was rudimentary by modem standards. The E Class were the first RN submarines fitted with internal main WT bulkheads which strengthened the hull considerably. but they had not been taken into account in calculating the strength of the hull. When, as a result of war experience, it was considered desirable for the E boats to dive below 100 feet, the designers stated that they now considered the boats capable of diving to 200 feet with the proviso that it was impossible to be quite sure on this matter without actual trials. The E Class were however classified as 200 feet diving boats and many must have reached this depth. Circa 1917/18 E40 struck the bottom in 53 fathoms of water the only damage being leaks in the engine room hatch and stern tubes.
5. Of the double-hull type submarines the builders claimed that the V Class were designed for 150 feet depth as against 100 feet in the conventional submarines to that date. Although the pressure hull sections were far from circular and the plating was thinner than in the circular spindle hull boats the strength of the hull lay in the external framing between the inner and outer hulls. This was the first increase in the diving depth as designed. The same figure of 150 feet probably applied also to the S Class and the Swordfish which were Italian designs and the W Class a French design, all double-hull with discontinuities in the pressure hull and which were probably not as strong as the conventional circular hull boats being built at the time. The designers claimed a 200 feet diving depth for Nautilus, the G Class and the K Class. It was increased to 250 feet in K26 although no reason is apparent for the 50 feet increase over the K Class. In the M Class the depth was 200 feet. A statement is made which tends to confirm this, of an exceptional circumstance when one G boat dived to 170 feet when chased by destroyers.
6. The design depth of the H Class, R Class and L Class was originally given as 250 feet. Compared with the original 100 feet in the E Class this figure is high in spite of the gradual increase in the number of main WT bulkheads. On the other hand war experience of previous designs would have given the designers a rather optimistic assessment of diving depth. The operational diving depth would have fixed at 150 feet. It is known that all these classes exceeded 150 feet on service and that L2 on one occasion submerged by accident to 300 feet and except for a few defects withstood the pressure.
A design depth of 200 feet has been seen for the L50L50 Class and there is no reason for any difference from the L Class. The doubt therefore remains whether the actual design diving depth accepted for all these classes was not 200 feet.
7. The Admiralty in 1925 stated that the M Class, K26 Class, H Class, R Class, L Class and L50L50 Class were designed for a depth of 150 feet and tested to 100 feet. Recorded maximum depths on service are said to have varied between 250 and 350 feet for the last two classes, but these must have been a few and exceptional cases. The term 'designed depth' here is being used loosely and refers to a figure considered suitable taking into the vessels and that most were of wartime construction. In 1930 the test pressure of fittings at refits of all these classes was taken as equivalent to 150 feet depth
8. The depth in X1 laid down in the Staff Requirements was 500 feet but it could not be realised because of the structural weight involved. The design depth was changed to 350 feet and the depth to which tested 200 feet.
9. Oberon Class, Odin Class, Parthian Class and Rainbow Class were designed for 500 feet. The Oberon Class were tested to 200 feet and in the three later classes the were to 300 feet although it is known that some boats went deeper.
10. It is relevant at this point to review the policy regarding deep diving. Increasing the diving depth meant thicker scantlings in plating and/or framing and increase in weight and displacement. The London Conference was in being and the London Treaty was ratified in 1930. The total tonnage of submarines was being limited. The requirements for future submarines was discussed in 1928 and the remarks of DTD are interesting. He stated:
'The ability to dive to 500 feet was introduced principally in order that pressure hulls of these submarines should be more capable of withstanding the effect of the explosion of a depth charge. Submarine officers do not visualise any intentional diving to such depths as 500 feet though the ability to do so is an asset in the event of an involuntary deep dive which might cause the submarine to go much deeper than ever was intended. '
RA(S) laid down 300 feet as the test diving depth of boats designed to withstand 500 feet depth of water. The need to go deeper was not of paramount importance and should a submarine be forced by exceptional circumstances to go deeper the risk of defects and the subsequent cost of repairs was accepted.
11. In the Thames Class, taking into account the need to keep down displacement to obtain the required speed, the designed depth was taken as 300 feet. The object of improving protection against depth charges had been achieved in the Odin Class by using 35lb pressure hull plating but in Thames a reduction to 25lb plating was accepted. The Thames was therefore designed for 300 feet depth with an operational depth of 200 feet as were the Swordfish, Shark and Porpoise Class.
12. On the deep dives to 300 feet in the Odin Class - the first boats to have an official deep diving trial to this depth and at which deflections of the hull were recorded-structural defects found were:
- The deflections of the elliptical hull sections aft were excessive and the hulls had to be stiffened. Otus which inadvertently dived to 360 feet opened up a buttstrap. Three additional pillars were fitted aft.
- Although the cable lockers were designed to withstand 225lb/in², they were tested to only 100lb/in2 because one boundary was the pressure hull designed to withstand an external and not internal pressure of 225lb/in2. It was found necessary to fit additional stiffening in the cable lockers to prevent mal-alignment of the torpedo loading winches.
- Consequent on excessive deflections the gun tower and torpedo hatch forward were stiffened.
13. An interesting point arose in connection with the strength of the torpedo tubes which passed through A, B, Y and Z tanks. These tanks were designed for full diving depth but an internal test of 225lb/in² would throw unfair stresses on the pressure hull plating so the test pressure was fixed at 75lb/in2. As an experiment to test the strength of the bulkheads in B auxiliary in Odin a pressure of 150lb/in² was applied. As was expected, the pressure hull showed signs of distress at this pressure and the test was not taken any further. It was also found that the torpedo tube joints in this tank leaked and in consequence the question was raised at a Torpedo Tube Design Committee in November 1928. The trouble was that owing to the deflection of the bulkheads the torpedo tubes acted as ties and took a heavy pull. This point had not been appreciated in the design of the torpedo tubes. Since it would necessitate extremely heavy bulkheads to eliminate the deflections, stays were fitted to No1 main tank and in B and Y auxiliaries in the Odin Class, i. e. in tanks in which there were torpedo tube flanges, to ease the pull on the torpedo tubes. A number of tubes and rods already passed through the tanks and the stays were arranged in conjunction with them to interfere with access as little as possible. In the Parthian Class stays were not fitted except for two in Y auxiliary; the interlocking rod castings wore made strong enough to do the work. In the Rainbow Class the torpedo tubes were made strong enough to take the pull required of them and no stays were fitted.
14. In rather similar manner to the tanks mentioned in Paragraph 13, the external O compensating and fresh water tanks in the Odin Class, Parthian Class and Rainbow Class wereonly tested to 100lb/in². During deep dives it was the practice of Commanding Officers to back up these tanks withhp air. Instructions were issued in 1931 that this was unnecessary although it may be advisable to back up with a little air when going very deep towards the 500 feet mark. Otus dived to 360 feet satisfactorily without doing so.
15. In the Oberon Class certain operations had to be carried out during diving to depth which gave a sort of 'staircase descent in steps'. This was not necessary in the Odin Class onwards except to close the torpedo tube bow caps at 200 feet since the torpedoes would not stand a greater pressure. This restriction was removed when torpedoes designed to withstand greater diving depths were introduced.