The ranger told me that since they have done away with the tax disc, it has become much harder to check boats out on the water. The quick glance of the right colour disc as you passed is now a read the number and enter it into the database etc.
He said that whereas they used to visually check all the boats that passed it is now down to one in ten, hence the keenness for fully visible numbers.
Oh, by the way, their super duper database that "is absolutely up to date as of 08.00 this morning when I left the office" wasn't !!