Presumably the early Sunderlands took off from the Medway next to the Rochester works (I know much of the volume production was built in Belfast).
Every time we visit Rochester Castle I stand and look down on the river and wonder just how they got in and out of that stretch of water. It's not very straight and Rochester bridge would have restricted them to the upstream direction. The M2 vaduct (let alone the Channel Tunnel train viaduct !) was not built at that time, of course, but it still seems all so tight.