Victorious America's Cup skipper Brad Butterworth will be keeping a close eye on Sir John Kirwan when the All Blacks legend enters the water for a 20km ocean swim.

Butterworth was set to jump on board his 10m launch to lead the fleet responsible for supporting about 100 people swimming from Waiheke Island to Judges Bay, Auckland, for tomorrow's Chopper Swim Challenge.

The former Alinghi skipper and Team NZ tactician's role would be to make sure swimmers, including Kirwan, were safe in the open water.

"I think it's great he is getting involved and that's probably one of the reasons why I'm out there ... because it would be nice for him to stick around a little bit longer, I don't want to see him go down in the swim," he said.

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"So we'll be watching out for him for sure."

Although Butterworth was keen to be involved in the event, aimed at raising funds for Auckland Rescue Helicopter Trust (ARHT), he was not so interested in getting in the water for the swim, which has been likened to running two marathons.

"I think it's amazing they can swim from Waiheke to Judges Bay but I'm not sure I'd make it," he said.

Butterworth said ARHT provided a vital service, especially to people on Waiheke. Almost a third of their 1038 missions last year were to the Island.

"It's very important for Waiheke to have this facility in terms of people getting sick or somebody hurt.

"As a professional sailor I've seen a lot of rescues over the years. I've never been involved in one, which is very nice for me but without the rescue helicopter I think a lot of people would've died."

Kirwan and his mate, Adam Clark, were set to tackle the challenge by swimming 10km each. He has been training four times a week including in the ocean.

"It's a long way. It's open ocean and so it brings some anxiety with it. I'm looking forward to it and trying to turn that anxiety into excitement," he said.

Kirwan has taken part in ocean swims before in Italy – the furthest of which was 10km.

Kirwan got involved for a number of reasons, including to help raise money for ARHT and to have a good reason to train. He was also a member of the ARHT board.

"I need some pretty strong goals to get me out of bed and so this was a real good one. And then I tell people so I can't get out of it," he said.

The event, now in its third year, has become increasingly popular with about 82 participants signed up this year – compared to 44 in 2017.

As of Saturday, the event had raised more than $75,000 for ARHT.

Founder of the event Olaf Adam expected the final total would surpass the $82,000 raised from the previous two challenges combined.

"[The swim] takes a long time and depending on the conditions it can be really rough. It's something where you push yourself to the limits. It's a pretty big undertaking," he said.

It takes between 4 and 7½ hours, and is about 6km shorter than the journey across Cook Strait.

The Chopper Swim Challenge kicks off at 8am on March 26, starting at Matiatia Bay on Waiheke and finishing at Judges Bay, Auckland.

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