Q: Are you going to war? A: We are a warship

It was a lovely day to go to war. HMS Ark Royal cruised out of Portsmouth harbour yesterday to lead the largest British naval force since the Falklands, and thousands of people stood along the seafront to say goodbye. They waved and cheered, and some wept.

"It's all very exciting, this big farewell, but it's when you get home and there are two coffee cups on the kitchen table," said Sue Kelly, whose partner Joel Kennett was on board. "That's when it gets really hard."

She had been hoping to get a last glimpse of Aircraft Engineer Mechanic Kennett, as Ark Royal left the harbour accompanied by dozens of small boats and two helicopters. But hazy sunshine and light glancing off the sea made the grey ship appear ghostly, and it was hard to tell which of the figures standing to attention on the flight deck was her lover. The people of Portsmouth have been seeing off ships since before the very first Ark Royal sailed with the Armada, but that doesn't make each new departure any easier on the families.

"Not when they are going away for such a long time," said Sue, holding tight to her six-month-old son Alfie. "Not when they are going off to war."

There were a few flags but little of the jingoistic fervour that has encouraged previous incarnations of Ark Royal out to sea. This time nobody in the crowd or even the crew really knew what the ship was going to do, and when it might come back. The 20,000-ton aircraft carrier is officially taking part in a long-planned exercise that ends in the summer. But no British aircraft carrier has completed a deployment as scheduled in the past 10 years, thanks to crises in Kosovo, Sierra Leone and Afghanistan. This one is due to meet up with the helicopter carrier HMS Ocean in a fortnight, as well destroyers, frigates, landing ships, minesweepers and auxiliary vessels with a combined crew of 5,000. They will also carry 3,000 Royal Marines with the ability to strike at targets on land.

War was the unspoken dread as Sue Kelly and Joel Kennett spent Friday night together, their last for a while, at home on the Isle of Wight. After watching an episode of Footballers' Wives over a takeaway curry they played games with family and friends. Then on Saturday morning the sailor caught the 7.50am train to Ryde harbour and a ferry into the docks. "He phoned me several times from on board this morning to say he was getting ready, putting his blues on for the departure," said Sue. "He had a cheeseburger for lunch." She gave a weak smile and rubbed her eyes. Soon her partner will be out of range. "Sorry, you'll get me started again. You forget that the Navy is bigger than anyone's family. It calls and he goes. Like the other women, I suppose."

Susannah Black had joined the large crowd at the Round Tower to wave off her boyfriend, Lieutenant Webster. "I like to think I saw him, though I'm not sure really," she said. This goodbye was different. "There is so much uncertainty. On previous occasions I have known exactly when he will be coming back; this time I have no idea."

The previous morning Admiral David Snelson had told the press conference on board Ark Royal that the purpose of the ships under his command could be "to produce a credible force to influence Saddam's thinking". As he spoke cranes were lifting supplies aboard including sacks of potatoes, a million cigarettes (all to be consumed in the one tiny smoking room in the bow) and 144,000 chocolate bars.

"Mars is the most popular, but the five-finger Kit Kat is coming up behind, " said the Naafi manager who was smiling. But there were military vehicles in desert colours parked by his sacks of Desiree spuds, and as the wind lashed across the flight deck he muttered, "Be a lot warmer where we're going in more ways than one."

A third of the 650 crew members is under 21 years of age. Frances Guy, a 19-year-old trainee officer from Worcestershire, joined Ark Royal four days before the ship sailed. She was proud to be on board and already well rehearsed in the official line: the crew was well taught and ready for anything but if the ship does go into battle there is one question that every officer on board, trainee or otherwise, can expect to be asked: "Why are we going to war this time?"

"I can't honestly answer that," said Midshipman Guy after a long and awkward pause. "I would hope that when I am eventually in command of people I will know more about the situation we are going into."

So what would she say to her parents if they posed the same question? There was another silence. "My parents don't ask. They understand that I'm going away, and that's all they want to know."

Her boss, Captain Alan Massey, said the order would come from above, if it came at all, and would be obeyed. He described the long exercise as though it were an opportunity for his young crew to see the world, and even talked about the two electric guitars he had brought in the hope of joining the ship's band.

But the joking and smiling stopped when he was asked if Ark Royal was preparing for war. The answer was careful and precise, and came to mind yesterday as it left harbour. "We are a warship."