A dinghy which capsized in rough seas off Anglesey, drowning a boy of eight and his grandfather, did not meet stability and buoyancy standards.
Ben Tinsley, eight, and his grandfather William both died
An accident investigation report found the crew of the Mollyanna were also unaware of deteriorating weather.
Dinghy owner William Tinsley and his grandson Ben, both from Buckley in Flintshire, died off Puffin Island in July 2005.
Ben's brother and father, who were also on board, survived the accident.
The Marine Accident Investigation Branch report, published on Wednesday, said the Mollyana was not stable, did not meet the required standard and was no longer made.
The Polish-made Mollyanna capsized in rough seas and strong winds on 2 July last year, said the report, and Mr Tinsley's son Ian twice tried to right the dinghy but without success.
The boat did not meet the required standards, say experts
William Tinsley died about 10 minutes after the initial capsize, but the three others held onto the upturned hull and an hour-and-a-half later were spotted by a passing fishing boat.
Both boys were airlifted by helicopter and Ben was pronounced dead on arrival at hospital.
The MAIB report said its investigation highlighted a number of contributory factors.
The dinghy could not be righted followed the capsize and did not meet the boat design's stability and buoyancy requirements.
Calculations made in 2001 on the boat's stability and buoyancy contained errors and were possibly incomplete, the report also found.
It said the crew were not aware that deteriorating weather conditions were forecast, they were inexperienced dinghy sailors, their clothes would have given little protection and the owner's lifejacket was not securely fastened.
The MAIB said it sent letters last November to the dinghy's suppliers and manufacturers advising them that it did not meet requirements on stability and buoyancy.
It strongly recommended that customers who had already bought the dinghy the should be advised of the limitations.
It said the Maritime and Coastguard Agency, the Royal Yachting Association, and the British Marine Federation had undertaken to introduce measures to encourage boat users to undergo appropriate sailing courses, and to help them understand the technical information provided with each craft.