Britannic struck a mine in the Kea Channel after the field had remained undetected for a month. The explosion opened the forward compartments and damaged the firemen's passage leading toward Boiler Room 6, allowing water into that space. Boiler Room 5 then flooded when its watertight door failed to close. Blast damage is proposed as an explanation for the door failure, while the suggestion that it had been wedged open for ventilation is questioned. Britannic was designed to remain afloat, though motionless, with six compartments flooded. Nurses had opened lower-deck portholes to ventilate the wards despite standing orders, and these admitted water as the ship settled and listed. Flooding then spread farther aft and exceeded the subdivision's intended tolerance.
Chapter · 40:10-48:11
Why Britannic's mine damage defeated her subdivision
Mine damage, a compromised boiler-room passage, a failed watertight door, and open portholes combined to overwhelm Britannic's nominal flooding tolerance.