One can only imagine the conversation between the Princess Royal and her mother. “I’ve just been on a lovely cruise, Mum. Around Scotland. You’d love it. Loads of chintz and tartan. A bit like Balmoral but on a boat.” And next thing we know the Queen is booking it to celebrate her 80th birthday.

Although Her Majesty is no stranger to life on the ocean wave, she has not combined a Highlands and Islands cruise with her traditional summer stay at Balmoral since the Royal Yacht Britannia was decommissioned in 1997.

But her summer 2006 jaunt is just as private, for the Hebridean Princess — a former MacBrayne ferry refitted as a luxury cruise ship — has been chartered for the exclusive use of the royal party. Up to 49 of her friends and family will be accommodated in 30 cabins.

Both Buckingham Palace and the ship’s operator, Hebridean Island Cruises, remain tightlipped about arrangements for the charter, saying that it is the Queen’s personally funded, private holiday and not to be gossiped about by the likes of us. But conservative estimates put the cost at about £125,000 for the week.

From experience of the ship’s facilities, it is clear she has chosen a “country house-style” vessel which is, effectively, a more stylish and considerably less draughty version of Balmoral at sea.

Though huntin’ and fishin’ may not be on the agenda, it will be possible to go shootin’ — at clay pigeons, anyway — from the stern of Skye Deck. And the Queen’s love of all things Scottish can be amply catered for. Guests on most regular cruises not only tuck into haggis and neeps on gala dinner evenings — they’re also played in to the ship’s dining room by a Scottish bag piper in full fig, and treated to Burns’s address to the “Great Chieftain o’ the puddin’ race”.

While the royal party will probably not require anything quite so full-on, there may be some takers for the 15 different types of single malt whisky available in the Tiree Lounge, which is dominated by a large inglenook fireplace and surrounded by deep armchairs just made for nestling down in with a cheering dram and the latest edition of Horse & Hound.

One deck down — in the ship’s wood-panelled Columba restaurant — shots of malt are even available at breakfast time, as the traditional accompaniment to a full Scottish morning repast of oatmeal and kippers or smoked haddock.

The Isle of Arran Suite — the ship’s top accommodation, a few steps away from the restaurant — subtly maintains the Scottish feel with tartan curtains in its separate day room and bedroom.

And while rumour has it that the Queen takes her own lavatory seat with her on her travels, nothing else in the suite’s splendidly marbled Victorian-style bathroom — complete with gold-plated taps, fluffy white towels, and Molton Brown toiletries — should need replacing.

Nor will the onboard entertainment need to be adjusted to royal tastes because — quite frankly — there isn’t any. Other than a small gym, a library, a few board games and a television in every suite, the ship offers no distractions from the beauties of the Scottish landscape and the peace of the country’s coastal waters.

Hebridean Island Cruises (01756 704704, www.hebridean.co.uk). One-week cruises start at £2,990 per person for two sharing, including excursions, house wines, spirits and tips.