The enormity of the challenge Penny Palfrey has embraced can stretch the imagination, but before the Australian grandmother is finished, it may also test the limits of human endurance.

She wants to wade into the ocean at Ka'ena Point on O'ahu and swim to Kaua'i.

No one has ever done that. It would be the longest ocean swim in history.

The amount of time Palfrey estimates it will take is as staggering as the distance: Up to 40 hours, her arms churning like a metronome of muscle, as she claws her way across 75 miles of ocean.

"Why?" Palfrey said, laughing a bit maniacally after a quick swim at Ala Moana Beach Park last week. "Well wouldn't it be great if I get there? I hear there is a sandy beach at the other side."

Palfrey, who lives in North Queensland, is one of the world's best marathon swimmers. Doing the unthinkable isn't something she dwells on.

Since she started marathon swimming in 1993, Palfrey has completed 23 swims longer than 12 miles. Last year alone, she swam the 30-mile 'Alenuihāhā Channel between the Big Island and Maui in March and then in September swam 40 miles from Santa Barbara Island and Pointe Vicente in California.

Swimming to Kaua'i will be something else altogether.

"It's about twice as far as my longest swim," said Palfrey, who has swum about 43 miles a week since last September to prepare. "I don't know if I can do it. I'm not saying I can do it. I'm going to attempt to do it. I hope I can make it."

Of the eight channels in Hawai'i, all but the one that separates O'ahu and Kaua'i — the Ka'ie'iewaho Channel — have been crossed multiple times.

Palfrey had heard swimmers talk about it, and the difficulty it presented was all she needed to spark her fascination.

"It wasn't considered one of the channels to swim because it was considered unswimmable," she said. "People were talking about doing it as a relay but that never eventuated. But the seed was sown in my mind that I would eventually like to have a try."

The challenges are easier to describe than the motivation.

The rules of channel swimming mandate that once Palfrey starts, she cannot touch any floating object until she is done. She will have to eat and drink while treading water.

She'll have to worry about sharks, jellyfish and sunburn. Even hypothermia.

And the length of time required means Palfrey will swim at night, possibly twice, risking vertigo and seasickness as she makes her way over water 10,000 feet deep.

"So that's out there in the dark with the black night and the black sea and you really can't see anything, so your senses are deprived," she said. "There's a lot there that's pushing the body to the absolute limits."

ONE KNOWN ATTEMPT

The Ka'ie'iewaho Channel is so daunting, only one person has ever tried to swim it.

Jonathan Ezer, a 19-year-old Kalani High School graduate who had already set a record for crossing the channel between Moloka'i and O'ahu, thought he could do it in September 1975.

Conditions when he began were almost ideal — 3-foot swells and light winds — but they steadily grew malevolent during the early hours of the swim, said Ezer's older brother, Scott, who was in the escort boat that day.

Soon, the channel roiled with 6-foot whitecaps.

"The wind was coming across the side of his face where he breathed, so he was getting slop," said Scott Ezer, a 57-year-old urban planner in Honolulu. "You could tell it was difficult. And then there was the question of what happens when the sun goes down, and it gets dark?"

Ezer swam for 12 miles before reluctantly giving up. Though he promised another attempt, he did not try again and now lives in Houston, his brother said.

"He was disappointed , certainly," Scott Ezer said. "He may have reconsidered whether that particular effort was in the ability of human endurance. I think there is a huge question mark as to that still."

Palfrey has assembled a competent escort crew that includes veteran channel swimmers Forrest Nelson and Bill Goding, as well as paddleboarder Jeff Kozlovich.

Nelson, a 44-year-old writer for radio from Los Angeles, and Goding, a 56-year-old Honolulu lifeguard, are among an elite group of 12 people who have crossed the Kaiwi Channel between Moloka'i and O'ahu. Nelson has even swum it in both directions.

Nelson was part of Palfrey's escort crew when she successfully swam from Santa Barbara Island to the California coast.

"I have seen Penny swim for 18 straight hours and come out of it beautifully," he said. "But this is a huge leap for her. I trust her motivation and I trust her training."

'A TOUGH 40 HOURS'

Palfrey will need 48 hours of calm seas, and that may be hard to come by, Nelson said. On most days, the major channels in Hawai'i are too dangerous to swim in. If swells reach 6 feet, a swimmer 20 to 30 feet from an escort boat is now separated from observers by two swells, he said.

"There have been drownings of endurance swimmers that are fully trained and their support team lost them," Nelson said. "If you lose track of your swimmer for more than five seconds, there is the potential you won't see your swimmer again."

At night, Palfrey will wear a 6-inch-long, bright yellow glow stick and the escort boat will be covered with them.

The safety plan also includes Goding and Kozlo-vich taking turns escorting Palfrey on a paddleboard. Kozlovich, a 50-year-old veteran of long-distance paddleboard races and a city lifeguard, does not expect it to be easy.

"The paddler will have to work twice as hard as normal to stay close, which means sometimes dragging your feet to go slower," he said. "It is one of those things that will take concentration. No doubt about it, it is going to be a tough 40 hours."

Goding believes the distance across the channel will be longer — maybe 10 miles — because of currents. But the journey will be difficult right from the start.

"We hiked out to Ka'ena Point to scope it out and it was scary out there," he said. "It's got waves and there is no real calm area. It's pretty hairy."

Palfrey has adopted a simple plan that she hopes will get her across the channel.

"I want to swim at a pace that's comfortable to me," she said. "It's a long way and I need to swim at a pace that suits me."

She'll talk to herself, as well, keep her mind off the pain by thinking about people who are working a night shift somewhere or that it's later back home in Australia.

She'll think of the swim — the enormous blue quest — as a series of 30-minute segments that come with chocolate, cappuccino or sloppy porridge.

She'll look for the sign board notes from the escort boat that will come every six miles until she gets to the halfway point. Then she gets a smiley face.

The distance is no siren's song. It is the beauty of the challenge.

"I don't think I can be completely prepared for everything," she said. "I will do the best I can and roll with the punches — and hopefully swim to Kaua'i."