Members of the National Union of Rail, Maritime and Transport Workers (RMT) who work on the Orkney-Scrabster ferry and the Shetland-Aberdeen ferry will walk out on December 14, 21 and 28.
Workers on the Aberdeen-Orkney-Shetland route will strike on December 16, 23 and 30.
Members voted nine to one for action earlier this week.
The union is in dispute with ferry operators Serco, a multinational outsourcing firm, saying that jobs are under threat from cuts.
RMT general secretary Bob Crow said: "Serco are fully aware that we will not accept attacks on our members" jobs and conditions and what is now an all-out attempt to bulldoze through cuts to core staffing numbers and the casualisation of these skilled shipping grades with ad-hoc agency arrangements and de-facto zero hours contracts.
"This strike action, in the face of this deliberately provocative action, shows that our members are prepared to fight for their future livelihoods and instead of issuing provocative and misleading press statements Serco should get round the table and resolve this sensibly."
Steve Todd, RMT national secretary, said: "This dispute can be resolved by Serco simply playing fair by the staff who generate their profits, and the ball is now firmly in their court.
"The public will understand that it is our members" jobs today and will be cuts to services and increases to fares tomorrow.
"Serco should withdraw this threat immediately and take up the offer of resuming talks through the proper consultation procedure as a way of resolving this dispute."