Media playback is unsupported on your device Media caption A look around the Astute's control room

HMS Astute is half submerged, its dark fin looming above the waters of the Clyde.

Even from a distance, the UK's newest and most powerful attack submarine looks formidable - a vessel you would rather not encounter in the murkiness of the ocean depths.

Even if you did, you would be unlikely to know it was there until it was too late. HMS Astute, the first of its class, marks a step change in capability for naval defence in anti-submarine and anti-surface warfare.

It is the stealthiest sub ever built in the UK, able to sit in waters off the coast undetected, listening to mobile phone conversations or delivering the UK's special forces where needed.

The 39,000 or so acoustic panels which cover its surface mask its sonar signature, meaning it can sneak up on enemy warships and submarines alike, or simply lurk unseen and unheard at depth.

The submarine can carry a mix of up to 38 Spearfish heavyweight torpedoes and Tomahawk Land Attack Cruise missiles, able to target enemy submarines, surface ships and land targets, while its sonar system has a range of 3,000 nautical miles.

When we arrive on the jetty at Faslane, the most essential supplies are being loaded onto HMS Astute ahead of its next sea trials.

HMS Astute Specifics Cabling and pipe work would stretch from Glasgow to Dundee

First submarine to have an individual bunk for each crew member

Manufactures its own oxygen from sea water as well as drinking water

Crew of 98 are fed by five chefs who, on an average patrol, will serve up 18,000 sausages and 4,200 Weetabix for breakfast

Faster under the water than it is on the surface, capable of speeds in excess of 20 knots - although its top speed is classified

Box after box of chocolates, rice, and the ingredients for curries and other meals are being hauled on board by crane for the crew of 98, to keep them going through their long days and nights at sea.

HMS Astute itself should never need refuelling over the next 25 years, thanks to the latest nuclear-powered technology which means it can circumnavigate the world submerged.

It even creates the crew's oxygen from seawater as it sails, meaning that the air on board is no longer heavy with diesel fumes, as submariners used to complain of older vessels. The only limit to how long it can stay underwater is the amount of food on board, enough for 90 days at sea.

Hi-tech specifications

"She brings a whole new capability for the 21st Century. The technological difference to her predecessors is fantastic, so we have a huge sense of pride serving on her," says the commanding officer of HMS Astute, Commander Andy Coles, who used to command the Trafalgar-class HMS Turbulent, which will be decommissioned next year.

We climb down a steep vertical ladder to enter the control room of HMS Astute.

It is packed full of the latest technology, although much of it is switched off for security reasons while we film inside.

So we see rows of blank screens, one of which is apparently the 21st Century version of the periscope. HMS Astute is the first Royal Navy submarine not to have a traditional periscope, instead using electro-optics to capture a 360-degree image of the surface.

Chief Petty Officer Gavin Clelland, 46, has been in the Navy for 30 years, and is in training for his new role looking after the nuclear reactor.

"We do a lot of simulator training, and we are there to deal with things should they go wrong." If for any reason they do, he says, there is a diesel back-up so that the boat can still make it home to safety.

HMS Astute is the length of a football pitch, just under 100 metres or 323ft long.

Bunk space

Yet while it may be the UK's biggest attack submarine to date, space is still at a premium inside. Even the captain's cabin is hardly luxurious, although at least he has it to himself.

It is the first submarine in which all crew members have their own bunk to sleep in during their "six hours on, six hours off" shift pattern, rather than having to "hot-bunk". Traditionally, two submariners on opposite shifts often had to share the same bunk.

Engineering technician Jamie Bell, 25, shows us his sleeping space. It is one of three bunks stacked from floor to ceiling, in a tiny room barely big enough to squeeze through to the next row.

What we have today is a world-beating piece of technology, which gives us a fantastic capability of huge utility to the UK over the next thirty years Commander Andy Coles

A small curtain offers each submariner some privacy - vital when more than 20 men share this one room.

Jamie says he has no problem with the six months they may spend at sea without daylight or fresh air, often with no access to communications with their families back at home. The only way to tell the time of day is by the meals being served.

"It's not too bad once you get into a routine," he says. "You just concentrate on your time off, and work hard when you're on duty. It's busy, and then you enjoy the six hours off."

He has never suffered from claustrophobia, he says, "although you do find out at an early stage if you do."

Morale booster

Four meals a day are prepared for the crew, with one at midnight for the night shift, in a kitchen that is just as economical with space.

Chef Mark Laing is one of three going out on the current sea trials.

"We do a roast on a Sunday, and we have theme nights such as Mexican nights. Food is very important for morale on board, and you have to keep changing the menu.

"Everybody wants to be the chef's friend," he smiles. "It's a good job."

The official commissioning of Astute into service last week, overseen by the boat's patron, the Duchess of Cornwall, was also something of a morale boost for a Royal Navy that is likely to face steep cuts as part of the current defence review.

HMS Astute is the first of four in its class, with the initial three now expected to cost £3.9bn, a hefty chunk of the annual £38bn defence budget.

As the base port of all the Navy's submarines from 2016, Faslane will be home to the whole Astute class, which will also include Ambush, Artful and Audacious, already under construction by BAE Systems at Barrow-in-Furness. The Royal Navy would like another three.

Costly contract

Yet HMS Astute's long journey to its berth on the banks of the Clyde has not always run smoothly.

Image caption The submarine's 98 crew members are catered for by five chefs

The initial studies for what would become the Astute class were given the go-ahead in 1991, and in 1997 the MoD agreed to place a £2bn order for three submarines.

But technological and programme difficulties left the project running more than four years late and more than £1bn over the original budget, although the work on the four submarines currently guarantees almost 6,000 UK jobs.

The contract has, however, ensured that the know-how for building such a complex attack submarine was not lost to UK industry, as it might well have been without that investment.

BAE Systems had to re-establish the UK's strategic capability to design, build, test and commission nuclear-powered submarines following the 10-year gap between the Vanguard and Astute classes.

As we climb the ladder to leave this billion-pound underwater world, blinking at the daylight outside, the crew are keen to focus on the positives.

"What we have today is a world-beating piece of technology, which gives us a fantastic capability of huge utility to the UK over the next thirty years," says Commander Andy Coles.

"Before they were first built in the UK in 1901, submarines were condemned as 'underhand, underwater and damned un-English'. But the critics soon changed their minds when the war started."