Good point, Doug, now I may be wrong here, it does happen, but, in the 70's once out past Wroxham and then Wroxham Broad, was it not 7mph? Till the 5mph as you entered Horning. Did the slower speeds not come into force when a certain Authority took over? This was to protect the river banks from boat wash. It certainly was after Stracey's Arms full speed ahead to GY.in the 70's
Iain