Penny Palfrey's attempt to become the first known swimmer to conquer the 75 miles between O'ahu and Kaua'i was cut short Saturday by a wall of Portuguese man-of-war.

Palfrey, a 47-year-old grandmother from North Queensland, Australia, got about halfway, then stopped about an hour later with excruciating pain from stings on her arms and legs.

"It's so frustrating," she said. "All the hard work and organizing. I was in good shape. I wasn't sunburned. All the muscles were working well. I was making great time and then the jellyfish absolutely stopped me in my tracks."

Palfrey promised to try again.

"I think I can do it," she said. "I think I am capable of it."

She started at 8:08 a.m. Saturday about a mile south of Ka'ena Point and ran into the man-of-war about dusk. She continued until 8:10 p.m.

"It was like putting my fingers into a power socket and turning on the switch," Palfrey said.

"My muscles were completely shaking out of control."

Like most marathon swimmers, Palfrey is accustomed to jellyfish.

"I get stung all the time, almost every time I go to the sea," she said. "Usually within half an hour I don't even remember it."

But these stings were like nothing she has ever encountered, and within minutes, Palfrey was having dry heaves.

"Usually I can swim through that, but in this case, it was so violent I couldn't even keep my face in the water," she said.

Palfrey's escort crew started preparing for worst-case scenarios, including shock, unconsciousness and difficulty breathing, said Forrest Nelson, a veteran long-distance swimmer from Los Angeles who was on Palfrey's escort boat.

"When she got on the boat, all of her limbs, every square inch of her skin was sensitive," Nelson said. "She screamed whenever we touched her."

The escort boat headed for Kaua'i as fast as possible. For most of the three-hour trip, Palfrey lay wrapped in a blanket in a fetal position occasionally crying out in pain, Nelson said.

She did not require medical treatment, though.

Palfrey said she felt good about every other aspect of her swim.

By the time she had to stop, Palfrey had covered about 38 miles in 12 hours and 2 minutes. Until the stings, she was swimming with such strength that her escort crew felt she would finish well under the original prediction of 30 to 40 hours.

Palfrey's attempt was part of a unique story arc in the world of marathon swimming.

The Ka'ie'iewaho Channel between O'ahu and Kaua'i is believed to be the only channel in the main island chain that has not been crossed by a swimmer. The only other known attempt was by Jonathan Ezer in 1975.

Ezer, a Kalani High School graduate who now lives in Texas, had done only one other marathon swim — crossing the 26-mile Kaiwi Channel between Moloka'i and O'ahu in record time in 1974.

That record was broken Saturday by Palfrey's 52-year-old husband, Chris.