Cal Mac crews are to strike next week.
Three days of industrial action will affect Cal Mac ferries servicing the Western Isles.
There will be an overtime ban all day Wednesday and Thursday next week.
If the ferry is in port then RMT members are instructed not to turn to for any duty on Friday 26th June.
RMT seafarers are also asked not to break leave to return to their ship to cover for any staff shortage.
The union says the ferry network is being "set up for takeover by the profiteering private company Serco" which, along with Caledonian Macbrayne, is in the running to provide the lifeline service to the west coast islands.
It demands assurances that staff are rightly demanding in their fight for workplace justice while the billion pound contracting process is kicked around behind closed doors.
RMT members have voted by well over 90% for action in a ballot that covers both strikes and action short of strikes.
General Secretary Mick Cash said: "RMT members on CalMac rightly feel that they are caught in the crossfire of an unnecessary and damaging tendering battle that leaves jobs, conditions and pensions hanging by a thread. That is simply intolerable.
"As a result of the failure to give staff the most basic assurances we are now using the massive mandate secured in the ballot and confirming a programme of industrial action.
"RMT wants cast iron assurances and we want them now and we are prepared to engage in meaningful talks around that agenda as we prepare for the first phase of industrial action."
Cal Mac ferry crews to strike next week
17 June 2015