Two US women who were rescued by a navy ship after spending five months adrift at sea had an emergency beacon aboard their sailboat, but never activated it, the coast guard says.

Key points: The women said they were sailing from Hawaii to Tahiti when their engines failed, leaving them adrift for five months

The women said they were sailing from Hawaii to Tahiti when their engines failed, leaving them adrift for five months The Coast Guard says the women never activated their EPIRB because they did not feel truly in distress

The Coast Guard says the women never activated their EPIRB because they did not feel truly in distress Experienced rescue officer says their account of failed communication devices doesn't add up

Jennifer Appel and Tasha Fuiava said they were sailing from Hawaii to Tahiti when their engine broke down in bad weather in late May.

The pair, who were travelling with their two dogs, were rescued by the US Navy after a Taiwanese fishing vessel spotted them about 1,500 kilometres south-east of Japan, well off their planned course, and alerted the US Coast Guard.

Ms Appel later said the women were "just incredibly lucky" to survive, adding their boat had endured a tiger shark attack and the pair had been running out of food when they were found about 1,500 kilometres south-east of Japan.

Jennifer Appel said there were six communication devices on board, all of which failed. ( US Navy: Jonathan Clay )

But experts have cast doubt on the women's version of events, saying some of the details of their story do not add up.

The Coast Guard conducted a review of the incident, and said the women revealed they had an emergency position indicating radio beacon (EPIRB) aboard, but never switched it on.

During the post-incident debriefing by the Coast Guard, Ms Appel was asked if she had the emergency beacon on board. Ms Appel replied she did, and that it was properly registered.

While Jennifer Appel was an experienced sailor, Tasha Fuiava was a novice. ( US Navy: Jonathan Clay )

"We asked why during this course of time did they not activate the EPIRB. She had stated they never felt like they were truly in distress, like in a 24-hour period they were going to die," Coast Guard spokeswoman Petty Officer 2nd Class Tara Molle said.

Lieutenant Scott Carr said the Coast Guard made radio contact with a vessel that identified itself as the Sea Nymph — the name of the womens' boat — in June near Tahiti, and the captain said they were not in distress and expected to make land the next morning.

That was after the women reportedly lost their engines and sustained damage to their rigging and mast.

EPIRBs 'rarely fail', retired Coast Guard officer says

Sorry, this video has expired Sailors lost at sea for five months found by US Navy

A retired Coast Guard officer, who was responsible for search and rescue operations, said if the women used the emergency beacon, they would have been found.

"If the thing was operational and it was turned on, a signal should have been received very, very quickly that this vessel was in distress," Phillip R Johnson said.

EPIRBs activate when they are submerged in water or turned on manually and send a location to rescuers within minutes.

The beacons are solid and built to be suddenly dropped in the ocean.

"Failures are really rare," Mr Johnson said, but added that old and weak batteries also could cause a unit not to work.

The women were travelling with two dogs, Zeus (pictured) and Valentine. ( US Navy: Jonathan Clay )

It was not clear if the women had tested it before the journey.

Last week they said they had six forms of communication on board that all went dead.

"There's something wrong there," Mr Johnson said.

He said he knew of cases in remote Alaska where a ship in distress just using one form of beacon brought a fairly quick response from nearby fishing boats and the Coast Guard.

"I've never heard of all that stuff going out at the same time," he said.