Collecting coins was said to be the “hobby of kings”, while the weakness of Prince Alfred, Queen Victoria’s second son, for stamps turned a long line of royals on to philately.

Now it seems that the anorak gene flows as dominantly as ever through British blue blood, thanks to the Princess Royal’s own unusual hobby: lighthouse-bagging.

In the same way that some walkers set out to climb every one of Scotland’s 284 Munros, the Princess has made it a personal mission to visit each of the 209 lighthouses that illuminate the Scottish Coast.

Last week she ticked more off her list, with a private visit to the Inner Hebrides. Taking pride in her role as patron of the Northern Lighthouse Board (NLB) she visited Pladda, off the Isle of Arran (latitude 55? 25.5’N; longitude 05? 07.3’W; three flashing lights every 30 seconds). The nearby Holy Island inner and outer lights (latitude 55? 31” 4’, longitude -5? 3” 37’, two white flashes every 20 seconds) were also scored off. She is expected to return again next week to “bag” another couple off the Isle of Skye. One on her list is Rubh Reidh Lighthouse, Melvaig at Gairloch.

Until capturing the Princess Royal’s attention, pharology - for that is what her hobby is called - had featured rarely in the passions of princesses, but when it comes to bagging scores, few amateur pharologists can compete with her. The Princess is now thought to have passed the critical halfway point, having logged about 80 through her work with the board. She has visited another 20 during private yachting excursions with her husband Vice-Admiral Tim Laurence. The couple keep a yacht at Ardfern on Loch Craignish in Argyll and Bute.

Roger Lockwood, chief executive of the NLB, said that the Princess’s visit had been a private one. “She visited three lighthouses and is now a long way along the way of seeing them all,” he said. “She must have visited over 80 with us and takes her role as patron very seriously.

“It is not just about ticking off another light on the list – she also likes to see the places and conditions in which the technicians have to work.

“The Princess has done all the major lights now and it will not be easy to do them all because there are many that are scattered all over the place. But it will be a remarkable feat if the Princess ticks them all off. Other than some of our technicians I doubt if anybody else has been to all of them.”

Her only challengers, he said, would be the engineers who have spent years working across Scotland’s 10,000km of coastline and 790 islands.

The Princess has been interested in lighthouses since she was a child. In 1956, when she was only 5, Princess Anne accompanied the Queen on a visit to Tiumpan Head in Lewis - although it was her big brother, Charles, who got to blow the foghorn. In 1998 she was present for the final shift of Scotland’s last manned lighthouse in Fair Isle. A guide at the Museum of Scottish Lighthouses in Fraserburgh, Aberdeenshire, told The Times that the Princess Royal was included in the visitor book.

She was not alone in her quest to bag all 209. “We see a lot of these anoraks, sorry, pharologists, who show a lot of interest in the lights. They are a far more sensible lot than the twitchers and the trainspotters,” he said. “Generally, the more remote the lighthouses are, the happier they are to have visited them. Some have great difficulty visiting these far-flung corners of the Hebrides, and Orkney and Shetland. That adds to the appeal.”

To the lighthouse

- Pharology is derived from the Pharos lights of Alexandria, one of the Seven Wonders of the World

- Legend has it that the light from Pharos could burn enemy ships before they reached shore

- It was designed by the Greek architect Sostratus in about 270BC

- The Tower of Hercules, a 2nd-century Roman lighthouse near La Coruña in Spain, is modelled on the Alexandrian Pharos

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