Royal Navy's largest ever warship, HMS Queen Elizabeth, to set sail

The largest warship ever built in the UK is due to set sail for the first time today.

HMS Queen Elizabeth, the Royal Navy's new aircraft carrier, will leave her dock in Scotland around lunchtime to start two years of sea trials.

She is named after Elizabeth the first and is the second ship to carry the name - the first was a First World War battleship.

:: Over 3 football pitches long: HMS Queen Elizabeth facts

More than 700 crew are onboard, from seamen to aircraft engineers, dentists to force protection. The oldest crew member is 58 and the youngest 17 although the average age is in the early twenties.

In recent weeks they have been practising drills and familiarising themselves with their new ship. Their first task will be to successfully sail her out of the basin she was built in.

It will need precise calculations and nerves of steel. 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, the 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.

The following six weeks will be spent in the North Sea and Moray Firth "proving systems" - she will finally sail into her home port of Portsmouth sometime in the autumn.

Flight trials will begin off the eastern seaboard of the United States in October 2018.

HMS Queen Elizabeth has taken more than eight years to build and is the first of two new carriers for the Navy; a second, the Prince of Wales, will follow in two years' time.

Together they will cost over £6bn, a price tag which the ship's first commanding officer defends as value for money.

"You look at all the premier nations around the world and why is it that every nation in the top tier are investing billions of dollars in aircraft carriers? Is it just us or has everyone got it wrong here?" Cptn Kyd points out.

"The reason being is they provide the government, very simply, with an incredibly flexible tool that is not just about war fighting.

"This is about deterrence, coercion, political signalling and providing a huge sea base - don't forget it is not a ship - it is a sea base for disaster relief, humanitarian assistance, defence engagement and selling British industry abroad."