The gap could be so small, even with the mast lowered, that he joked he might be able to run his fingers under the bridge. And what will he do if he gets the calculation wrong?

“Duck,” he said.

Watch: Britain’s newest aircraft carrier

Work began in 2009 on the £3.5 billion (US$4.4 billion) carrier, which has been dogged by delays and overruns in cost and questions over whether there will be enough money to put a full complement of planes aboard.

After about six weeks of sea trials in the North Sea, the plan is for the carrier to return to Rosyth for adjustments before sailing later this year to its home port, Portsmouth.

The first of the planes is planned to arrive next year and the carrier is scheduled to be operational in 2020, bound for anywhere from the Persian Gulf to the South China Sea.

China has two aircraft carriers: the Liaoning, which is in service. China’s first domestically built aircraft carrier, the Type 001A, was launched in Dalian in April.

HMS Queen Elizabeth and a second carrier, the Prince of Wales, also being built at Rosyth and still covered in scaffolding, will together cost more than £6 billion.

The carriers, along with the Trident nuclear programme, account for a huge chunk of the defence budget.

Critics within the military complain such high-profile projects have been at the expense of surface ships, soldiers and the air force. They also question whether aircraft carriers are anachronistic and vulnerable to attack from increasingly sophisticated missiles.

Asked by the Guardian whether the carrier is a white elephant, Kyd unsurprisingly, disagreed.

“These assets give you a global presence, a serious punch, anywhere you want, at immediate notice,” he said.

“I think it is a pretty good investment at £6 billion. In 50 years from now, we will look back and say that was extremely good value and they will be used a lot.”

Each carrier can hold 36 planes and four helicopters. The navy is hoping to have 24 F-35s by 2023 and a further 24 by 2025. In addition, the US marines will fly their own F-35s off the carriers, though the number is still under discussion.

The carrier has a crew of about 700 which could theoretically double depending on the number of planes aboard. One of the biggest fears is from fire and, to counter that, it has 750 doors that can seal off compartments.

About 15 per cent of the 700 crew members are female, compared with a navy average of about 9 per cent. Three of the crew are Muslim. One of them, Mohamed Khan, the head chef, would normally be off at present because of Ramadan but he did not want to miss out on the preparations for going to sea.

“Normally I take the whole month off, but this is the biggest ship ever and I wanted to be a part of that,” said Khan, 42, who has been in the navy for 16 years.

He added that, to compensate, under Islamic law he will have to fast for 30 days before the next Ramadan and make charitable payments.

Among improvements for the crew are bigger bunks, 90 cm wide compared with almost 70 cm before.

The captain too gets a bigger bunk but not a double bed, as do his American counterparts. The British navy is too puritanical for that, Kyd laughed.