A lack of instructions for the fitting of a replacement part on a ferry led to a loss of control of the vessel before it crashed into pontoons and rocks.
Caledonian MacBrayne's MV Hebrides was damaged in the incident at Lochmaddy on North Uist in September last year.
According to an investigation, the part was replaced in the boat's propulsion control system six months earlier.
Investigators said instructions were not provided and engineers were unaware a locking compound had to be used.
No-one was hurt in the incident which saw the ferry's master attempt to bring the boat under control after the mechanical failure.
CalMac said actions have been taken since to avoid any repeat of the incident.
'Highest possible standards'
In a report on its investigation, the Marine Accident Investigation Branch (MAIB) said the mechanical failure resulted from a part called a setscrew becoming loose after not being secured in position with a thread-locking compound when it was replaced.
There were no instructions available that a locking compound should be used, the MAIB said.
It has made a recommendation to Rolls-Royce Marine, the provider of the propulsion control system fitted on board the MV Hebrides, to ensure instructions are always available to engineers.
It has also made a recommendation to CalMac that it ensures its engineers have the instructions they need.
A spokesman for CalMac said: "We note the MAIB's report which largely confirms the findings of our own investigation into the incident.
"Since the grounding in September last year we have put in place a number of processes to mitigate the issues raised and will continue to monitor these areas to ensure our procedures meet the highest possible standards and give our customers assurance that we take their safety very seriously."
The MV Hebrides had 32 crew and 45 passengers on board when the mechanical failure occurred.
In its report, the MAIB said the crew's response to the loss of control was "well-intended but was uncoordinated".
Investigators said this was because the crew "were not sufficiently prepared or practised to deal quickly and effectively with the loss of pitch control in the confined waters".
The ferry required repairs at a dry dock and was out of action for a number of weeks.