20442. Just explain how that was? - In order to give time after the automatic release from the bridge or by the float, so that no one coming through the door, or just passing through the door, should be injured, a hydraulic cataract cylinder, something like a gun recoil cylinder, was arranged, so that the earlier part of the drop shall be comparatively slow, depending on the leakage of a fluid back past the piston in the cylinder. To the last 18 inches or 2 feet there is a bye-pass round the piston, and the door is practically free to fall quickly just for the last 2 feet or so.
The doorplate was of cast iron of heavy section, strongly ribbed. It closed by gravity, and was held in the open position by a clutch which could be released by means of a powerful electro-magnet controlled from the captain’s bridge. In the event of accident, or at any time when it might be considered desirable, the captain or officer on duty could, by simply moving an electric switch, immediately close all these doors. The time required for the doors to close was between 25 and 30 seconds.