Kapitanleutnant Walthur Schweiger of U-20 reported he only fired one torpedo. He also reported that the resulting detonation was 'unusually heavy'. From the Lusitania's perspective there were two explosions in quick succession, both reportedly showing effects external to the vessel, indicating that the second explosion also blew out the shell plating.
On the matter of condensation inhibiting the precipitation of coal dust, only one surface of the bunker compartments was shell plating and could therefore be assumed cooled by the external seawater; the compartments were otherwise bounded by tank top, deckhead, transverse w/t bulkhead and the starboard internal longitudinal w/t bulkhead, this last pierced by the coal-passing apertures fitted with w/t doors. These would necessarily have been open at the time to work the ship. The compartments would therefore have been exposed to stokehold temperature and can therefore be assumed largely free of bulkhead condensation.
It is therefore entirely feasible that the shock wave of the torpedo explosion could instantaneously have precipitated an explosive mixture of coal dust and air. This could in turn have been ignited by a flashback from the furnaces, the communicating doors into the stokeholds being open.
The main steam pipe from the boilers to the stop valve would be near the centreline of the vessel, shielded moreover by the longitudinal w/t bulkhead, and therefore it is unlikely for it to have been damaged by the torpedo explosion. There were other adjacent steam lines, to ash ejectors etc. but any pressure released therefrom would hardly account for damage to shell plating.
NFJ
