Both yachts carry AIS transponders, which broadcast their position, course and speed to anyone listening. Shore stations pick that signal up, and we plot it here.
A boat shows on the map whenever her transponder is on and she is within range of a receiver. On her berth at Mercury Yacht Harbour the transponder is usually switched off, so you will most often see where she was last rather than where she is this second. When she is out in the Solent, or halfway across the Channel, you can watch her go.