Ok, I think I've solved the problem. The mission objectives for each of the tugs is different. The towing tug only deals with the yachts, whereas the the other tug (attached to the oiler) shows the other objectives. This tug is really only providing extra power, so most of the time you are working with the towing tug and missing most of the icons on the chart.
While discovering this, I wasn't concentrating fully on the game and caused a container ship to veer to starboard and almost collide with Erasmusbrug.
It then seemed to have an AI memory lapse (container alzheimers?), turned around and sailed straight into Veerhaven ending up imbedded in the wharf.
I wonder if I will lose my licence from this!