A fishing boat which ran aground and sank off Scotland's west coast swerved unexpectedly into danger, a new report says.

The St Apollo became stuck on rocks after getting caught in a squall in the Sound of Mull on August 24 last year.

The scallop dredger's five crew escaped on a liferaft before it went down and were rescued by Oban RNLI.

An investigation by the Marine Accident Investigation Branch (MAIB) has found the accident was caused by navigational error, an equipment malfunction, or a combination of the two.

The MAIB said it is possible the St Apollo turned towards the rocks because of a problem with its satellite compass, but investigators also noted its watchkeeper did not try to stop the boat or steer it clear.

They reported: "As St Apollo grounded very quickly after the sudden and unexpected change of heading was noticed, it is likely that the vessel was already significantly to the north of the planned route.

"Nonetheless, the watchkeeper still had enough time to reduce the engine speed and to alert the skipper.

"That he did not attempt to turn St Apollo towards safe water or stop the vessel by putting the engine astern, indicates that he panicked to some degree."

The St Apollo was probably already north of its planned route with automatic steering engaged when it turned unexpectedly towards the rocks, the MAIB said.

The watchkeeper alerted the skipper, who was asleep in his bunk, but the dredger ran ground before he arrived.

The crew could not refloat the St Apollo and abandoned the boat when it began to list. It sank in shallow water and was later salvaged, although it was beyond repair.