Three men who died when their fishing boat sank worked to exhaustion in the days before the accident, it has emerged.

Martin Johnstone, Christopher Morrison and Paul Alliston died when the Louisa went down in the Outer Hebrides in April last year.

The Marine Accident Investigation Branch (MAIB) found a series of mistakes contributed to the sinking of the boat, while the emergency response was delayed by almost an hour.

The Louisa's four-man crew were asleep when the creel boat began taking on water at anchor off Mingulay.

They had worked 20-hour shifts for four days before the sinking, sleeping between stringing creels and eating snacks as they went.

An alarm intended to warn them about flooding had been disabled and they did not wake up until the Louisa was already sinking rapidly.

The men abandoned the boat but were unable to inflate their faulty life raft.

There was then a 49-minute delay in the search and rescue response due to confusion between people coordinating the operation. Lives may have been saved if rescuers had arrived on the scene sooner, the MAIB said.

Authorities were alerted by an emergency beacon aboard the Louisa at 2.30am but lifeboat crews were not launched until 3.40am.

Lifeboat crews arrived at 4.15am and found the bodies of skipper Paul Alliston and crewman Christopher Morrison together near their uninflated life raft a few minutes later.

Martin Johnstone was found dead close to shore, then the body of skipper Paul Alliston was lost as rescuers tried to recover it. His body remains missing.

At 5.40am, a lifeboat crew rescued 27-year-old Lachlan Armstrong - the only survivor of the sinking - who had swam to shore and climbed on to rocks.

The Louisa was later raised by investigators from the MAIB in an attempt to establish the cause of the sinking.

They found she had foundered after her hold flooded with water, probably from a hose left running by the exhausted crew.

The fishermen, who were paid based on the size of their catch, had been eager to land as many fish as possible before returning to port, the MAIB said.

"The work ethic that resulted in the skipper driving himself and the crew to a level of tiredness that compromised their safety can be linked to the share remuneration scheme, which encouraged them to grasp every opportunity to continue fishing and maximise the catch whenever suitable conditions prevailed," it noted.

The investigators also uncovered a series of issues with the lifesaving equipment on the boat.

A series of tests on the life jackets used by the crew raise questions about their usefulness and the effectiveness of safety tests, the MAIB said.

"Paul Alliston, Christopher Morrison and Martin Johnstone had all undertaken sea survival training during which they were taught how to don lifejackets correctly," it reported.

"That all three were found unresponsive and face down in the water indicates that their lifejackets were incapable of turning them onto their backs and keeping their airways clear."

"This questions the suitability of both historical and extant lifejacket water performance test protocols," the MAIB added.

A CO2 cylinder inside the Louisa's life raft intended to help it inflate in an emergency was empty and had not been refilled during a recent refurbishment.

MAIB chief inspector Steve Clinch said: "The scope of this safety investigation has been necessarily broad.

"After abandoning the vessel, the skipper and crew depended on their life jackets for survival.

"A life jacket should turn an unconscious person onto their back and keep their airway clear of the water.

"It is therefore of concern that the skipper and two crew were tragically found unresponsive and face-down in their life jackets when the rescue services arrived on scene.

"The results of lifejacket trials undertaken following the accident strengthen that concern."

Mr Clinch said he would urgently ask authorities to carry out an audit of life jacket safety tests.

A spokesman for the owners of the Louisa, Duncan and Murdo Kennedy, said they hoped the issues raised by the MAIB would help improve safety at sea.

"The tragic events of April 9 resulted in the loss of three fine fisherman and our thoughts remain with the families of Martin Johnstone, Chris Morrison, Paul Alliston and surviving crew member, Lachlan Armstrong."

"We have fully cooperated with the MAIB inquiry and are committed to implementing the report's recommendations," they added.