Last week, the Royal Navy aircraft carrier HMS Prince of Wales was at 30 days’ notice for sea. That means exactly what it sounds like: if such a ship is ordered to sail, it is expected that it will take the captain and ship’s company 30 days to get ready. For once they can relax – go on leave, go on a course, commit to some deep maintenance, send items away to have things done to them. They can chill out, up to a point.