"...At high tide, she will be guided through a narrow exit in the harbour. This has been widened but still only allows 14 inches either side to spare.
Recent dry weather in the Highlands and Cairngorms has meant less rainfall in the Forth - when she squeezes through the exit, the man at the steering wheel on the bridge - Chief Petty Officer Sticky Vercoe - will only have 50cm beneath the keel and the river bed.
The flight deck will overhand buildings either side. If the wind is too strong she risks bouncing off the harbour walls which could damage her before she's even reached open water. Once clear she will be pulled into the middle of the Forth where she will drop anchor for a few hours.
At low tide, she will make her approach towards the three Forth bridges close to midnight, sailing at 3-4 knots; any faster and suction will pull her downwards into the river - an effect known as 'squat'. The first two road bridges can flex by up to three metres in high winds but the third, Brunel's iconic Forth Bridge, poses its own challenge - height.
A radar mast will be hydraulically lowered to make room for the ship to pass underneath. Even so, were a person to stand on the highest point of her, and reach up, they would be able to touch the Forth Bridge as they sailed through. 'It is like driving your car into a car park you go 'oh my god' and dip in your seat. It is exactly the same feeling as that,' said Captain Jerry Kyd, who has previously commanded the UK's last two aircraft carriers HMS Ark Royal and HMS Illustrious.
The whole process could take more than 10 hours...."
UK's largest warship sets sail