The difficulties have been prolonged because of "complications" with the repair of MV Clansman's propulsion system, the west coast ferry firm admitted.
CalMac has been forced to juggle vessels to cover the gap on the ferry's Oban-Coll/Tiree route.
Sailings on two routes have been suspended or altered, and capacity on three others reduced.
The Mallaig to Lochboisdale on South Uist route will be halted, with sailings from Oban instead.
The summer route between Ardrossan and Campbeltown will now not start until at least 31 May.
There will also be less space for vehicles than normal on three other routes:
- Ardrossan to Brodick on Arran - one of CalMac's busiest
- Kennacraig to Port Ellen/Port Askaig on Islay
- Mallaig to Armadale on Skye
The Skye route has already suffered some five cancelled sailings a day, with Skye, Lochaber and Badenoch SNP MSP Kate Forbes claiming these have significantly hit visitor numbers.
CalMac said that to minimise further disruption, the Maritime and Coastguard Agency had agreed to extend its operating licence for MV Hebrides, which normally operates the Skye-North Uist/Harris route, by another three weeks before it goes into dry dock for an overhaul.
Parts of MV Clansman's propulsion system have had to be sent to Denmark to be repaired.
CalMac managing director Robbie Drummond said: ‘We are facing a highly regrettable situation that we need to manage as best we can with the vessels we have.
"Our first priority is to continue to provide lifeline service to island communities.
"The overrun of our annual vessel dry dock schedule into the summer season is regrettable and we realise this is going to impact adversely on some communities, which we apologise for.
"However, under the circumstances, the timetable arrangements we have now put in place for the period of disruption are the best options we have available.
"Through vessel redeployment and timetable amendments we are confident that we can satisfy all demand for services, including the extra traffic that will be generated by the Islay Whisky Festival and the upcoming World War I commemorations.
"Although we realise that communities such as Mallaig, Campbeltown and Armadale will be disadvantaged for a short period, we need to deploy resources where they are needed most."
Mr Drummond hpreviously warned that CalMac's "ageing" fleet would be "stretched" this summer.
Half of its 32 ferries are beyond their 25-year working life expectancy, and the risk of mechanical breakdowns was "significant".
On the Arran route from next Thursday, its second vessel will also help cover the Islay route, which has also been reduced to a single vessel.
The Armadale route will be operated by MV Loch Fyne until Tuesday, then by MV Loch Fyne and MV Loch Bhrusda.
Ms Forbes said the Mallaig to Armadale route had been "plagued with cancellations and local businesses are reporting a substantial drop in visitors compared to the same period last year".She said the 125 cancellations in the first 25 days of the summer timetable was “simply not acceptable”.
CalMac said they comprised just under 15 per cent of sailings.“This is having a seriously adverse impact on the local economy, with one popular tourist attraction in the south of Skye reporting a 25 per cent drop in visitor numbers and a 50 per cent coach cancellation rate compared to the same period in 2017."Ms Forbes said the MV Loch Fyne could not operate at low tide, which had reduced sailings.
She said: “We have been here before in the summer of 2016, and it is deeply unfortunate that we’re here again – in perhaps an even worse situation.
“Notwithstanding the significant impact already, this has got to be fixed before the height of the peak tourist season.”
A Scottish Government spokesperson said: “While these issues are an operational matter for CalMac, we understand the frustration of the communities which rely on their ferry services.
“CalMac is seeking to manage the situation by redeploying vessels within the fleet to ensure lifeline connections are maintained to all the communities it serves on the Clyde and Hebrides network.
"The frequency of service for some routes may, however, be reduced from normal during the period of disruption.
“Our recently published vessel replacement and deployment plan shows how existing, planned and prospective vessels could be deployed across the network to deliver the commitments set out in the Scottish Ferries Plan, and better address forecast demand."