 For a few years after it was opened, the centre span could be lifted at a speed of approximately 10 metres per minute to allow ships of up to 44.5m in height to pass underneath.However, this only occurred a few times, and after 1970 it could no longer be opened.The official reason for this was that the 30 minutes needed to raise the bridge was causing unacceptable delays, and that in any case silting of the river had made it impassable for large ships.