Martin Gibbs, from Somerset in the UK, was sailing in the Bristol Channel when the wind changed causing the boom to swing into him. The 55-year-old was knocked unconscious and into the sea near Barry, south Wales.
Thomas Hendy, a sailing friend who described himself as Mr Gibbs' deck hand, said he was 'very competent, he would not take risks'.
Mr Hendy said they had their life jackets on during a race on 7 September around Flat Holm and Steep Holm islands, but they were not clipped to the boat - a 26ft (7.9m) sloop called Gunsmoke. At the inquest he told the hearing they were near Barry when the wind began blowing from behind them.
'That's the most dangerous point of sailing because if the wind gets the wrong side of the main sail the boom can come slamming across,' he said. 'An extra large wave hit the side of the boat, I lost my balance a bit and to my horror I saw the boom was coming over. It just slammed across, knocking Martin.'
Asked by Cardiff coroner Mary Hassell if Mr Gibbs had seen it coming, he said: 'I don't think he did. I think it was travelling too fast to have done anything anyway.'
He said Mr Gibbs, a member of his local yacht club with more than 30 years experience, appeared unconscious as he fell.
Mr Hendy said the boat was travelling too fast for him to reach his friend, but he could see he was face down in the water and if his lifejacket had inflated he would have been on his back. He said he turned the yacht around but lost sight of Mr Gibbs, so he radioed for help.
Tim Thompson, a search and rescue winchman who pulled Mr Gibbs from the water, said the only thing keeping him afloat was his puffa jacket.
Attempts were made to resuscitate Mr Gibbs, but there were no signs of life and he was pronounced dead at the University Hospital of Wales, Cardiff.
Gas cylinder:
Hugh Davies, of Barry Lifeboat, who attended the scene, said being hit by the boom was the most common cause of injury on a yacht.
Life jacket expert Joe Bottomley said Mr Gibbs' buoyancy aid should have inflated within 10 to 15 seconds of him hitting the water. He said the only explanation for the jacket not inflating was that the gas cylinder inside had become unscrewed and the piercing pin had not been able to reach it. The cylinder can become loose over time and that manufacturers recommend life jackets are serviced once a year after purchase. Records showed the jacket was just over two years old but had never been serviced.
A post-mortem examination found that Mr Gibbs died after suffering a blunt head trauma and submersion in water.
Pathologist Dr Richard Jones said if Mr Gibbs had received the blow to the head on dry land he might have survived as he had no evidence that it caused anything except concussion.
Coroner Mary Hassell said: 'Looking at all the evidence I have no doubt in my mind he drowned. He was knocked overboard unconscious and couldn't protect his airways.'
Ms Hassell recorded a verdict of accidental death.
