Loose Servomotor Grub Screw Led To Grounding


The UK Marine Accident Investigation Branch (MAIB) issued an investigation report on the loss of control and grounding of the ro-ro passenger ferry “Hebrides”, in Lochmaddy, North Uist, Scotland, in September 2016.

Hebrides backing over mooring pontoons


On 25 September 2016, the ro-ro passenger ferry Hebrides was approaching Lochmaddy, North Uist, Scotland when control of the ferry’s port controllable pitch propeller was lost. The master attempted to control the ferry’s movements, but he was unable to prevent it from running over several mooring pontoons and briefly grounding. There were no injuries but the ferry was damaged and had to be taken out of service and repaired in dry dock.

Specifically, several mooring pontoons in Lochmaddy were upturned and damaged. The hull plating along Hebrides’s starboard side above the fat bottom was indented and the ferry’s starboard bilge keel was distorted. A length of mooring chain was also found wrapped around the starboard propeller shaft, which had damaged the outer stern tube seal and propeller.

Damage to starboard bilge keel and hull bottom

Action Taken

Rolls-Royce Marine has:

  • Issued a service letter to all users of the HeliconX propulsion control systems regarding the inspection and integrity of the jaw coupling within the linear servomotor actuator.
  • Issued a service procedure for the linear servomotor based on OEM requirements.

Damage to starboard bilge keel and hull bottom

CalMac Ferries Ltd has:

  • Issued a technical bulletin to its major vessels that requires all propulsion controls, including emergency controls, at all stations are tested regularly.
  • Conducted its own investigation of the accident. The investigation report made recommendations related to, among other things:
    1. The conduct of a failure mode analysis on the pitch control systems to identify potential for single point failure of normal and secondary emergency pitch control arrangements.
    2. The establishment of a working group of masters to assess the approach speeds at all berths included in the company issued passage plans and to provide guidance as appropriate.
    3. Measures to improve operational familiarity with propulsion emergency control systems and the application of HELM techniques in routine and emergency operations.
    4. Improving the ‘Master’s Decision Support System’ and associated management system components
  • Fitted pitch deviation alarms on board Hebrides and its other vessels using the HeliconX control system.
  • Fitted a fixed communication system on board Hebrides between the CPP control position on the gearbox and the bridge.

Mooring chain around starboard propeller


  • The loss of control was caused by the loosening of a setscrew (grub screw) within a servomotor assembly
  • Thread locking compound had not been used to secure the setscrew
  • Service instructions for the maintenance of the servomotor were not available to ship’s engineers or shore-based service engineers
  • An alarm system that had been recommended by the control system manufacturer had not been fitted
  • The bridge and engine control room teams were not sufficiently prepared or practiced to deal effectively with a loss of control in confined waters:
    1. It took the bridge team 2 minutes to realise that control of the port CPP had been lost. By then, the ferry was less than 200m from the pier roundhead and was still making good 10kts
    2. It is highly likely that stress, panic and poor communication contributed to the master’s situational awareness regarding the status of the control of the port CPP
    3. Hebrides’s speed when approaching Lochmaddy was too fast to enable the actions taken by the master to stop the ferry in safe water to be effective.

Position of jaw coupling setscrew (as found)


  • Document and process recommendations for safety critical system upgrades received from manufacturers.
  • Introduce drills and contingency plans to better prepare its crews to deal with propulsion failures.

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Source: MAIB