But it is possible survivors are on board the life raft or made land, they add.
On Friday a third unsuccessful day of aerial searches took place, scouring the New Zealand coastline.
Six Americans aged between 17 and 73 were on board, along with a 35-year-old British man.
Some of those on board have been named: Captain David Dyche, 58; his wife, Rosemary, 60; and their son David, 17. Also aboard was their friend Evi Nemeth, 73; a man aged 28; a woman aged 18, and Briton Matthew Wootton, aged 35.
The Dyche family were said to be experienced sailors who had been sailing around the world for several years.
The 85-year-old schooner Nina left Opua on New Zealand’s North Island on May 29.
The last known communications with the crew were on 3 and 4 June – when conditions were very rough, said Rescue Co-ordination Centre New Zealand (RCCNZ), with winds of 80 km/h (50 mph) gusting to 110 km/h and swells of up to 8 m (26 feet).
Evi Nemeth called and texted New Zealand meteorologist Bob McDavitt to seek advice on how to cope with the conditions, and were advised to ride it out.
After family and friends failed to hear from the crew, rescuers were alerted on June 14. They began trying to make contact with the vessel, but were said not to be unduly alarmed as it was equipped with an emergency locator beacon which had not been activated, as well as a satellite phone and spot beacon.
But on June 28 aerial searches began, and two
extensive sea-based searches as well as two shoreline searches have yielded no sign of the vessel or crew, said RCCNZ.
Search leader Neville Blakemore said it was now logical to assume the boat sank quickly in a storm, preventing the crew from activating the devices on board – though he added it was still possible survivors could be found.
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