Grave concerns for seven missing in yacht off N.Z.

New Zealand rescue authorities expressed "grave" fears Thursday for six Americans and a Briton whose historic yacht has been missing for more than three weeks.

The 21-metre (70-foot) wooden schooner Nina, build in 1928, set off from the North Island bound for Newcastle, Australia, on May 29 but has not been heard from since June 4, the Rescue Coordination Centre New Zealand said.

Rescue coordinator Kevin Banaghan said friends and relatives raised concerns on June 14 and all attempts to contact the vessel had failed.

He said the yacht had a satellite phone, a tracking beacon and an emergency beacon, which had not been activated.

A New Zealand air force Orion conducted extensive searches off the waters between the North Island and the Australian coast this week.

"Unfortunately, no sign of the vessel has been found," Banaghan said, adding that conditions were "very rough" when it went missing, with winds gusting at 110 kilometres per hour (70 mph) and swells of eight metres (26 feet).

"We do hold grave concerns for the Nina and her crew but remain hopeful of a positive outcome."

He said the missing crew comprised six Americans -- three men aged 17, 28 and 58, and three women aged 18, 60 and 73 -- and a 35-year-old British man.

The yacht's last known position was 370 nautical miles off the New Zealand coast.

"No sign of the vessel has been reported by any other vessel in the area since June 4," Banaghan said, adding that New Zealand and Australian rescuers were coordinating on the search.

While the rescue centre did not name anyone aboard the yacht, the Northern Advocate newspaper reported earlier this year that the Nina had participated in a local regatta and was owned by an American named David Dyche III.

It said Dyche, his wife Rosemary and son David, 17, left Florida in 2008 to circumnavigate the world, travelling through the Bahamas, Jamaica, Columbia, Panama, Costa Rica and French Polynesia before arriving in New Zealand.

A Facebook profile in the name of David Dyche III has pictures of the Nina and a post from May 21 apparently referring to the upcoming voyage to Australia with his son.

"Dave is leaving and going to college in the States in July. This is our last trip together crossing the Tasman Sea," it reads.

On May 26, he said he expected rough weather, posting: "The Tasman Sea is shooting gales out like a machine gun, living up to it's reputation.

"We are shooting at leaving out after the first one this week. No doubt we will be dancing with one or two of them."

The Nina is well known in yachting circles and in 1928 because the first US vessel to win Britain's famous Fastnet race, according to an entry on the website by Rosemary Dyche.

In it, she says the schooner was the flagship of the New York Yacht Club after World War II and her husband bought it in 1988, describing its restoration as "a labour of love".