A boat that capsized off the Vietnamese coast, killing two friends from Northwood and Sarratt, was “unstable” and had been illegally modified, an inquest has heard.

Karen Puddifoot, from Northwood, and Tim Roney, from Sarratt, were in their cabins when the Halong Party Cruiser, which was “obviously” listing to one side, tipped over.

The pair, a Frenchman and two Vietnamese nationals – a crew member and a tour guide – all died on September 24, 2009.

Tim, 21, and Karen, 27, were taking part in an around the world bike ride to raise money for the British Lung Foundation.

They had completed a tour of Europe alongside Karen's brother Chris earlier that year, before Tim and Karen travelled to Asia in September, on their way to Australia. They intended to move onto America afterwards.

“They seemed to be the most energetic and resourceful youngsters,” said Hertfordshire Coroner Edward Thomas. “They seemed to be going everywhere.”

The pair arrived in Bangkok before flying to Hanoi and then Halong Bay, the inquest heard.

After meeting a pair of French tourists on a bus, the four travellers decided to take a boat trip and joined the Halong Party Cruiser at about 5.30pm that evening, the inquest was told.

Also among the 36 people on board were visitors from South Africa, an Australian family and Vietnamese nationals.

However, in evidence read by Mr Thomas, one passenger had noticed the boat “appeared to be listing to the right”.

The boat, described as a “party dancing boat”, set off from the dock shortly afterwards. Other passengers who had joined earlier in the day were taking their seats for dinner in the restaurant while Tim and Karen were in their cabins, which was located below the dining area.

However, at about 6.30pm, the four-storey boat was hit by a “squall”, a sudden gust of wind, from across the bay.

One passenger described the boat capsizing “in slow motion”, the inquest heard.

Some passengers were able to escape through the dining room door while others went underneath the water, Detective Chief Inspector Graham Weston said.

However, Tim and Karen were unable to escape and were found in their cabins.

DCI Weston, from Hertfordshire Police, told the inquest there was a squall, but “nothing to indicate a significant increase in wind or particularly bad weather”.

In Vietnam, the cause of the incident was recorded as a “natural disaster”.

Further investigation by DCI Weston discovered the boat, sailing in shallow water, was only permitted to sail between 12pm and 4pm, while there was 50 tonnes of cement on board, acting as ballast.

The Halong Party Cruiser had also been modified without permission, measuring a length of 32.6m when it was designed to be 25.87m.

Speaking at the inquest, Michael Puddifoot, Karen's father, said the design of the modified boat had made it “unstable”.

He added: “The feeling [is] these authorities seem to be manufacturing a storm when it was only a squall. If there was a storm, why didn't any other boat go down?”

DCI Graham Weston also revealed the Halong Party Cruiser is now “up and working again”.

“They have changed the layout on the bottom deck and have the crew down there,” he said.

Mr Thomas said the last text messages sent by Tim, from The Briars, Sarratt, and Karen, from Grove Park Farm, Northwood, to their families “said how much they were enjoying themselves”.

“They packed a lot in; skiing, cycling, all sorts of things. They did a lot of good work [for charity]. They must be dreadfully missed.

“I can imagine getting on that boat and thinking I have done quite well here. Clearly it was unstable. There was something wrong with the ballast. This should not have happened with a gust of wind or whatever you call it. It wasn't a cyclone.”

At Hatfield Coroner Court, Mr Thomas recorded a narrative verdict after a limited post mortem noted the cause of death as drowning.

“They died through drowning after the boat in which they were in capsized,” he said.

“This boat had recently been redesigned without permission being granted and it appeared the ballast may have been inadequate. Photographs showed in calm water there was still a list.

“It was travelling outside the time permitted by the Vietnamese authorities.”

Two months after the death of his sister, Chris Puddifoot was killed in a car crash in Hampermill Lane, Oxhey.