The report by the Marine Casualty Investigation Board into the capsizing of the ocean racing yacht Rambler 100 has been published.
The accident occurred on 15 August 2011 when the keel broke off the vessel as it rounded the Fastnet Rock.
The boat then rapidly keeled over until the mast and sails hit the water. It stayed on the water before capsizing completely.
It took less than 60 seconds for the boat to lose the keel and capsize.
The radio operator tried to send a 'mayday' call, but it was not heard as the mast and aerial were in the water.
There were 21 crew members on board. The Baltimore Lifeboat rescued 16 who were on the hill, while five more were picked up from the water by the dive boat, Wave Chieftain.
One crew member, Wendy Touton, who had been in the water for some time, was suffering from severe hypothermia and was airlifted to Tralee General Hospital where she made a complete recovery.
The report said that the lifeboats on the rear of the yacht could not be accessed because the boat was upside down.
The automatic emergency beacon did not trigger either but remained in its bracket.
The report states that the accident occurred due to the separation of the keel from the vessel. However, the cause of that is still being examined.
Analysis of the structures is continuing.
The report points out that the lifeboat crew found it very difficult to see the upturned hull of the vessel until they were very close due to poor visibility.
A number of safety recommendations are made in the report, including the location of safety equipment onboard vessels such as the Rambler 100.
It is suggested that the underwater part of the hull should be painted a highly visible colour to make it easier to find.
One other recommendation suggested that major yacht racing authorities develop regulations concerning the standard of welded keel fins.