Water and air in Hood's torpedo-protection experiments

Were Hood's crushing tubes for torpedo protection filled with water or air?

Both arrangements were explored at different points; he recalls water-filled tubes in at least one configuration but treats the exact sequence cautiously.

The sequence is recalled cautiously because the reference books were unavailable. Water-filled tubes appear to have been used at one stage, while earlier arrangements may have used air. A later clarification describes experiments that exchanged air and water between the tubes and surrounding bulges, partly to study energy absorption and trim. The system changed as part of an evolving underwater-protection programme.