Hobby: Princess Anne has now seen half of Scotland's 215 lighthouses

As hobbies come, it is an unusual choice, especially for a Royal.

In the same way that dedicated walkers set out to conquer all of Scotland's Munros, Princess Anne has made it her personal quest to visit every lighthouse that dots the Scottish coast.

Last week, the 57-year-old made a secret trip to tick-off three more lighthouses from her list and will return again in a few days to 'bag' another couple off the Isle of Skye.

She has passed the halfway mark in her missions to see every one of the 215 lights that illuminate Scotland's rugged shorelines.

On Wednesday, as patron of the Northern Lighthouse Board, she ticked-off Pladda and Holy Island Inner and Outer lights off Arran.

She also veered away from her Scottish quest to visit two more on the Isle of Man a day earlier.

The Princess Royal will be back on board the 84-metre-long NLB ship, MV Pharos on July 22 and 23 to climb two more lighthouses on Skye.

For most ships, one Royal visit is a rarity, but next week will be the princess' third on Pharos, having only commissioned the vessel a year ago.

She has told officials of the NLB - of which she is patron - her ambition is to visit every one of the 215 north of the border.

The princess's hobby is so consuming that she also visits many lights secretly on private yachting jaunts with her husband Vice Admiral Tim Laurence.

The couple keep a yacht at Ardfern on Loch Craignish in Argyll and Bute.

Passion: Princess Anne with officials from the Northern Lighthouse Board at Copinsay in the Orkneys

It total it is believed Anne has now logged more than 100 Scottish lights - some 80 alone on trips with the NLB.

It is thought her interest  in pharology - the technical term for the study of lighthouses - was sparked as a five-year-old when she accompanied the Queen on a visit to Tiumpan Head in Lewis.

Among the lighthouses Anne has visited in the past is arguably the world's most famous - that of the Flannan Isles in the Outer Hebrides where on Boxing Day 1900, it was discovered that the three keepers James Ducat, Thomas Marshall and Donald McArthur had vanished.

Roger Lockwood, chief executive of NLB, said Anne's visit on Wednesday was 'private'.

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.'