The select few chosen to serve on the Royal Yacht Britannia had to learn when to nod and not to nod as part of their duties on one of the Royal Navy’s most sought-after postings.

What to do should a member of the Royal Family happen to pass while the deck was being scrubbed was another of the challenges that faced the crew.

The protocols, rules and accepted modes of conduct were laid down in the “precedent book” for crew members, now published by the Ministry of Defence.

A nodding acquaintance with members of the Royal Family on board was one of the stipulations - although in this case “the nod” required the following manoeuvre: “Lower your head forward until your chin touches your chest momentarily and then raise your head to an upright position. The whole sequence takes about one second.

“There is no need to bend at the waist in the best Sir Walter Raleigh style,” the protocol memorandum said under the heading: “Forms of address and nodding by Royal Yacht officers and Royal Yachtsmen.”

Under a sub-heading, the memo, dated May 19, 1982, said: “To nod or not to nod?” The answer is that a nod was required when anyone entitled to this form of obeisance “enters or leaves the Wardroom, Royal Drawing Room or on to the Verandah Deck but not [underlined] when they enter or leave Church in the Royal Dining Room”.

Additionally: “Nod when on deck with a cap off and anyone entitled approaches. If you have a cap on, salute in the normal way but do not nod.”

It was deemed appropriate to “nod with cap on when receiving a handshake” from a member of the Royal Family when arriving on board or disembarking at the “Royal Gangway”.

All adult members of the Royal Family were entitled to the nodding routine. Younger ones were to be treated differently. “Private secretaries will advise on the address of younger members who have not left school. Normally they will be addressed by their Christian names and not accorded a nod.”

The precedent book was nearly 200 pages long and included an insight into the type of officer and yachstman considered to be suitable for the privilege of sailing with the Sovereign around the globe.

No more, of course. Britannia was taken out of service in 1997 and resides as a tourist attraction just outside Edinburgh; the Major Government had announced that it would be replaced but Tony Blair scrapped that decision.

Yacht officers were not required to be Eton-educated, but “they should be professionally thoroughly sound and proven in their specialist fields, men of well-rounded and pleasant personalities, of good appearance and bearing, above average social attributes and able to get along with their fellows without friction”.

Divorce was not a bar to selection “unless the officer has been the subject of divorce proceedings of the most unattractive kind involving unsavoury behaviour and/or publicity”.

Conduct on board was carefully regulated: as a mark of respect “if any of the Royal Family or Household passes a Yachtsman at work he is to stand to attention momentarily and then carry on”. Whistling was banned “anywhere on board at any time”.

Behaviour ashore was also strictly defined. It had to be “impeccable at all times”, and if a member of the Royal Family passed in a car Britannia officers and yachtsmen were to stand to attention and salute.

“Watch your language ashore, too,” said the memo.

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