"Competition is all-consuming and it gives you a break from the issues and problems in your personal life," Iain Percy says as he explains how he has immersed himself in a renewed sporting challenge after struggling to overcome the tragedy which cost the life of Andrew "Bart" Simpson, his best friend and Olympic medal-winning partner, last May. Percy and Simpson were both on the America's Cup boat, Artemis, on a seemingly mild day in the bay around San Francisco, when devastation struck during training.

Artemis capsized and it did not take Percy long to work out that Simpson was trapped beneath the boat. When they finally found him, and Percy cradled Simpson in his arms while paramedics tried desperately to revive him, the impact felt unspeakably cruel. Percy had been close to Simpson for 30 years, since they had first met in a sandpit aged seven and made boats out of Lego at a British national sailing competition, and yet he was helpless to save the man who meant so much to him. They had won gold and silver medals together as sailors at successive Olympic Games in 2008 and 2012 and Percy had been Simpson's best man but he could do nothing when it mattered most.

Percy had to extricate himself from his friend's body, stand up and begin his search for Bart's wife – Leah Simpson. It was Leah's birthday that day, on 9 May 2013, and all these months later, Percy pauses for a long time when he reaches this point in his memory. "I realised at the time that I needed to get to Leah," he eventually says. "It was important to do that – so I could be the one to tell her."

He is an obviously intelligent and driven man but his swell of feeling is almost overwhelming as he relives the worst day of his life. Percy is back on Artemis, as the leader of the most compelling contender for the next America's Cup, but his wounds are still raw. He sounds fiercely competitive when suggesting that, on Artemis, "I'm confident we can dominate this sport for the next 10 years." And yet he is irreducibly human when admitting how the end of Simpson's life affected him.

"There was a period going into the America's Cup [last year] where my goals changed," he says. "The only thing that mattered then was to honour Bart and compete in a way which meant we got round safely and did not risk losing anyone else. After the tragedy we had to think that way. But, as time passes and you get back into competition, something inside you takes over. You get a little break from the sadness. It's coming back to me in competition. The frustration of a poor result or the satisfaction of performing well is still there – despite everything. In some ways competition has become my release."

Percy has even voiced the wish that he, rather than Simpson, should have died. Simpson was married and the father of two young boys, Freddie and Hamish, whom he cherished. He was needed – as a husband and a father – and so it has taken Percy almost 11 months to find the fire to sail on in his place and decide that the best way of "honouring" his friend is by dominating the competition which took Simpson's life.

"I think Bart would want that," Percy says. "He knew how passionate I was and sometimes he would tell me to cool off and think about other things. One of my saddest moments this year was when we launched the blue boat [as Artemis prepares to challenge the US-based Team Oracle for the America's Cup] and the sadness got me again when we went out on the water. I knew how much Bart would've enjoyed it as all the ideas and technology come together on day one. That was almost my saddest moment. We had shared some big wins but what we'd shared most was those mini-successes when the technical problems are solved. As I go on I know he'd want me to have more of those moments."

Percy remembers Simpson's generosity even when they competed against each other individually. Simpson lost the chance to sail for GB in the Finn class at the Sydney Olympics in 2000 but the first words he said, after hearing that Percy would go in his place, were framed as a question asking how best he might help his friend win the gold medal. Percy achieved that ambition, with Simpson's support, and he says: "Bart was a team player all the way through – even in an individual boat. He was just so generous and he cared more about his mates than himself."

Simpson was also open about his emotions and he would be ribbed mercilessly by Percy and Ben Ainslie when whispering "sweet nothings" down the phone to Leah. "That was him," Percy says. "He was the most honest, upfront guy. He did what he wanted to do – and that was being as sweet as possible to Leah and the boys while enjoying his sailing and spending time with his mates. It was no more complicated than that and that's why he was loved. He was also an incredible sportsman. But as our mate he was much more than that. He was always on at me and Ben and giving us counsel. We miss that a lot."

Last year, after Ainslie had also been on the water on the day that Simpson died, the most-decorated sailor in Olympic history was summoned aboard Oracle when the US team were losing 8-1. Team New Zealand needed just one victory when, in his unfamiliar role as tactician, Ainslie set about producing a sporting miracle. The America's Cup was decided in the last race and Ainslie and Oracle completed a stunning transformation by winning 9-8.

"It was the first time I'd heard lots of my non-sailing friends saying they were staying up to watch the racing." Percy says. "That was echoed around the world. The racing became so technical and so fast. These boats could be going at 50mph and turning on a sixpence. That made for an amazing spectacle and we had the fairytale of Oracle coming back from such a large margin. I was really proud of Ben and I was just pleased the sport was shown in such a great light."

Yet the sense of loss could not be curbed on the night of Ainslie's greatest victory. Delirious celebrations unfolded around him but Ainslie went to find Percy so that, together, they could sit apart and drink a couple of beers while remembering their friend. "It says a lot about Ben that he'd won a big competition but his thoughts were all about Bart. Quite frankly, while it was nice that those guys had won, it paled into insignificance for me and even Ben. Anyone who has lost someone close to them would understand. Competition is great and it's incredibly exciting but it's insignificant compared to your best mate and his family."

This year's competition to win the right to challenge Oracle in the America's Cup has assumed added intrigue. Ainslie has left Oracle and set about fulfilling his ambition to lead a British boat. This means that Ainslie and Percy, who leads the Swedish entry, Artemis, are now in direct competition. "It's fantastic Ben's taken it on and I'm proud of him," Percy says. "It's not a small undertaking and you have to have the balls to take it on. They're obviously our competitors but on a personal level I think he's done amazingly well and been really brave.

"Ben's working hard to secure the funding of a British team to compete in the competition for which we're writing the protocol – it's going to be great. Britain is the home of the competition and this adds another dimension. Like me, Ben's facing the same challenges of running a team, and he's also the owner."

Percy leaves little room for doubt that, at least in his mind, Artemis will not only beat Ainslie's boat and their main rivals, Team New Zealand, but they will overcome Oracle and control the America's Cup for the next decade. "That's our intention," he says calmly. "There's no reason to doubt our potential to do that. You need something special and we probably haven't seen that in our sport since Alinghi [the Swiss-owned team who won the America's Cup in 2003 and 2007] dominated.

"We can match that. This team enjoys working together in a very modern, collaborative style, and that's fairly unique across the Cup world. We've got incredible talent in guys like Nathan Outteridge and Iain Jensen [the Australian pair who won gold at the 2012 Olympics]. They remind me a lot of myself and Bart and it's very hard to shortcut the eight years they've had together. We've got great skill on the design side and an army of unbelievably skilful builders. When you put all that together this far out you'll realise why I'm confident we can dominate this sport for the next 10 years.

"We had to go through an incredibly tough period after losing Bart but we held our heads high. It wasn't easy suffering the way we did but we earned the respect of our peers. Now I think we're in the strongest position of all the challengers. Everyone has one reason for being here – and that's to win."

Percy's motivation is searing – but he also tries to keep Simpson's name alive in practical ways. He helped set up and lead the Andrew Simpson Sailing Foundation, which offers young people a chance to compete on the water. "The foundation is really going somewhere. We're all amazed by how many people have supported it but that's how Bart was in real life. He was one of the most loved people I've ever come across. So the outpouring of support has come because people know what Bart stood for. And Leah has been inspirational in her response. We've lost one of the most special men in the world but Bart was her husband. There's no solution to this one and no point searching but Leah inspires us."

Simpson may leave behind him another positive legacy – which transforms the dangers of America's Cup sailing. Percy argues that, just as Ayrton Senna's death forced Formula One to address a previously cavalier attitude towards risk, the loss of Simpson could save many other lives. "Big steps have been take to address the dangers – because in the writing of the [America's Cup new] rule process we were consulted and I made a point of wanting to front-foot the safety side. It has to be the legacy of what happened. These boats are travelling incredibly fast above water. We need to do everything we can to minimise the risks and Bart would be proud that's happening now.

"Improving safety is no different to improving speed in the sense that it requires brilliant thinking and ingenuity and in the design teams we have a lot of that. It was great that in the aftermath of the accident all the teams worked together on safety but it needs to continue to happen. That would be the best legacy of all for Bart."