A US registered yacht capsized near Fastnet Rock off the Co Cork coast this evening.
Some 21 people were onboard the Rambler 100, which was participating in the Fastnet Race from Cowes to Fastnet and back to Plymouth, when it capsized in force five winds at about 7.30pm.
The Department of Transport said all 22 crew had been accounted for. As many as 10 people were feared missing earlier.
A department spokeswoman said 16 people were now sitting on the hull of the ship and the remainder were on life rafts.
The Irish Coast Guard and Baltimore Lifeboat are attending. Two Irish Coast Guard helicopters from Shannon and Waterford were also dispatched.
Naval patrol ship LE Ciara was in the area and is assisting.
The search operation was hampered by misty and rainy conditions with visibility down to less than a mile. Winds were gusting to Force 6 and a five-metre swell was reported.
The remainder of the fleet was unaffected except for a number of nearby competitors that suspended racing to assist in the search and rescue operation.
The Fastnet Race, regarded as the one of the oldest offshore yacht races in the world, first took place in 1925.
The 608-mile race passes Land's End before turning north-north-west and across the Celtic Sea towards West Cork. Participants round the famous lighthouse three miles off Cape Clear Island before heading to their ultimate destination of Plymouth.
The majority of the crews participating are amateur and club racers.
A total of 314 boats started the race, now sponsored by Rolex, off Cowes on the Isle of Wight on Sunday, beating the previous record turnout of 303 entries registered for the tragic 1979 installment.
A total of 15 people died that year when the fleet was caught in violent weather and sea conditions between Lands End and Fastnet.
A memorial stone listing the 15 names was erected on Cape Clear's North Harbour in time for the 25th anniversary which was held on the island in 2004.
Prior to 1979 the Fastnet Race had a good safety record with one crewman lost overboard in 1931.