The mast was carefully examined by naval engineers and scientists working at the Navy’s metallurgical laboratory, who put it through a series of rigorous tests in a bid to discover why it had broken away from the hull. The cause soon became apparent and a report circulated on 17 July states that it was
principally due to a brittle fracture taking place in the circular tube of the snort, with its origin at or near the longitudinal weld between the tube and the port fairing plate [a section which secures the snort to the hull]. . . . The fracture would be followed by a shearing of a weld in the starboard fairing plate and, finally, by the failure of the elliptical tube and a weld in the port fairing plate in a ductile manner.
The report stated that the whole sequence of events leading to the snort becoming detached would have taken place ‘with great rapidity, in fact almost simultaneously. The position of the failure corresponded with the point of greatest weakness of the mast.’
Metallurgists confirmed that materials used in the mast were mild steel ‘of satisfactory chemical composition’. However, they discovered that the circular tube
possessed an exceptional coarse microstructure indicative of gross overheating and had not been heat treated after being welded at a very high temperature. As would be expected from such a microstructure, mechanical tests on specimens from the tube showed the material to possess little resistance to shock at normal operating temperatures.
In addition to the principal cause of failure, transverse welds which had sheared in the fairing plates were found ‘to be of poor quality and in view of their location at the point of greatest bending moment of the structure, must have constituted a secondary source of weakness’. In other words, metal fatigue was the cause of the problem.
The experts were unable to state if the fracture had been caused by ‘normal stresses in service or by the snort mast receiving a blow’ and added that this was considered ‘a matter of little significance’. The report added: ‘The real point at issue is that the complete failure of the mast was due to the combination of two factors, namely the metallurgical condition and secondly the presence of indifferent welds at the point of greatest weakness.’
It was judged that ‘the complete and sudden failure of the mast could not have occurred if the circular tube of the structure had been in the correctly heat treated condition; in all probability total failure would not have taken place if poor transverse welds had been absent at the critical area.’
While tests on the snort mast were undertaken at Emsworth, the divers and crew of Reclaim were allowed a long period of leave to recover from their round-theclock work. When they returned to sea, it would be their job to try and ascertain if anyone in the submarine had attempted to shut off the big induction valve fitted at the point where the snort trunking entered the pressure hull. If divers discovered this to be shut, then there must have been some other cause for her sinking. If it were open, then a real possibility existed that the snort was the main culprit. It was vital, therefore, that the true position of the valve be confirmed. But the valve was located in an almost inaccessible position and difficult for divers – or a television camera – to get anywhere near it without resorting to using explosive charges that might destroy the evidence needed.
When Reclaim put to sea again – this time without Crabb on board – she encountered bad luck almost every day, none of it of her own making. The weather for the remainder of the summer was the worst for years with gale force winds making any further diving attempts impossible. When the gales subsided, a period of strong tides meant that the divers could only go over the side for short periods.
It was during one of these short dives that a diver noticed that Affray had increased its list to port on the seabed and there were fears that she might topple over completely, making further work on the wreck a potential death trap.