Modifications made to a fishing boat that capsized with the loss of two lives had reduced the vessel's stability, a report has found.
The prawn trawler Nancy Glen capsized "rapidly" as it turned to starboard in darkness and it sank in Loch Fyne, Argyll and Bute, on January 18 last year.
The Marine Accident Investigation Branch (MAIB) found that it overturned after one of its nets became fouled with mud and seabed debris during a turning manoeuvre.
Investigators found that various modifications, culminating in the replacement of a crane with a heavier model, had "significantly" increased its vulnerability to capsizing.
The MAIB also noted that the owners and skippers of small fishing vessels can be unaware of the risks of not carrying out stability assessments, and it recommended that the Maritime and Coastguard Agency ensures the stability of such boats is assessed regularly.
The report further suggested that the cumulative effect of long working days on the crew could have impacted on their ability to react to an emergency.
The Nancy Glen capsized in Lower Loch Fyne near Tarbert at around 5.50pm, sparking a major search and rescue response.
Duncan MacDougall, 46, and Przemek Krawczyk, 38, who lived in the village, were on board when the trawler sank and lost their lives.
Mr MacDougall, the skipper, and Mr Krawczyk, a Polish national who had lived in Tarbert for a number of years, were both described as experienced fishermen.
A third crew member was pulled from the water by the crew of a passing boat and survived.
The report on the MAIB investigation into the tragedy stated: "Nancy Glen was trawling when its starboard net became fouled with mud and debris from the seabed during a turn to starboard, and the vessel capsized rapidly.
"One of the crew escaped but the skipper and the other crewman were trapped inside.
"The missing crewmen's bodies were recovered when the wreck was salvaged."
Underwater surveys following the sinking revealed that the boat's port net was empty, while the starboard net was full of mud and was largely buried beneath the seabed surface.
Investigators assessed that the rapid capsizing of the boat, the inrush of water to the wheelhouse, the displacement of gear on board and disorientation "denied the skipper and second crewman the chance to escape".
The MAIB concluded that the boat's stability had been "insufficient" to overcome the effect of the increased load on the net and the turn to starboard.
It identified various modifications made to the 13m-long boat since 2002 and noted that its "reduced stability can be directly attributed to the effect of the modifications".
The report stated: "The MAIB's investigation established that through life modifications to Nancy Glen culminating in the replacement of the crane with a heavier model, had reduced the vessel's stability, significantly increasing its vulnerability to capsize.
"Despite the skipper's attempt to bring the situation under control, the combined effect of the increased towing load from the fouled net, the turn to starboard and the limited stability meant that Nancy Glen was unable to recover from the rapid heel to starboard."
Investigators said the trawler's owners had made their own stability assessment and, in some cases, had taken professional advice on the effect of modifications before they were implemented.
But the MAIB found that there was no requirement for the skipper or crew of small fishing vessels to undertake stability awareness training, which limited their ability to conduct effective stability assessments.
The report also said there was insufficient evidence to conclude that fatigue had contributed to the accident, but added: "However, the cumulative effect of long working days ... could have impacted on the crew's ability to react to an emergency."
The MAIB's chief inspector of marine accidents, Andrew Moll, said: "The capsize and sinking of Nancy Glen, which resulted in the tragic loss of two respected Tarbert fishermen, has again demonstrated the consequences of not knowing how stable a boat is."
He added: "The MAIB has recommended that the Maritime and Coastguard Agency implements changes to legislation to require both new and existing small fishing vessels to assess their current stability and to continue to monitor this throughout the life of the vessel."
The MAIB's findings were released on Thursday after it emerged that the criminal investigation into the sinking of the trawler has been dropped. A fatal accident inquiry will be held in due course.