Alex Cross also said its annual maintenance programme was so tight there was only time for two weeks’ work on each of the 34 vessels, so not everything could be checked every year.
But Britain’s biggest domestic ferry operator has no spare vessels, so when one goes out of action, a stand-in has to be redeployed from another route, which can have knock-on service reductions across the network.
That has forced CalMac to balance the sometimes competing demands of islanders trying to get to schools and hospitals, time-critical commercial traffic such as fish and whisky, and tourists bringing vital income to island businesses.
More than one third of the Scottish Government-owned firm’s ships are beyond their 30-year design life, which has left them more likely to break down.
Their average age is 24 years when it should be 15.
Cross said: “There's always a risk of a random thing going wrong, just like with an older car.”
However, increased traffic fuelled by the Scottish Government significantly cutting west coast ferry fares since 2008 under its “road equivalent tariff” (RET) has also led to more services being added and the ferries having to work harder.
Cross said: “Some of them operate 16 hours a day, seven days a week, 350 days a year.
"With the pressure on the fleet and the amount of time the ferries are working, we do not have time to do deep maintenance.
"But with up to 500 timetabled runs a day, the fact the fleet availability is 96-98 per cent is good.”
As Brian Fulton, head of business support for Caledonian Maritime Assets Limited (Cmal), the Scottish Government firm which owns CalMac’s ferries, put it: “The irony is that the ferry services have never been so good for the islands, quantity-wise, but if something breaks down, more people now feel the problem – and RET has just increased the numbers.”
Cross said as an example, the 33-year-old Lord of the Isles, which normally operates between Mallaig and Lochboisdale on South Uist, “is getting to the point where it is at an increasing risk of failure.
“We are still driving it hard, while RET is using up our assets with less opportunity to maintain them.”
Lord of the Isles was taken out of service for eight days in May and moved to a yard in Greenock for repairs to its “drencher” system, which sprays water onto the car deck in the event of a vehicle fire.
Corrosion in its pipework was flaking off and causing a blockage, but the problem was only spotted at the end of its annual overhaul.
The Maritime and Coastguard Agency had given the vessel temporary approval to continue operating because there was not time for the necessary remedial work, which involved flushing chemicals through the system.
Cross said: “The ferry is beyond its design life so we get that sort of thing.
"Older vessels are more likely to fail.
"Anything could go, because there is only so much we can check every year [in annual maintenance], so we try to do over a five-year period.
Cmal said it expected to order a replacement for Lord of the Isles in 2024.
Other large CalMac vessels to have broken down this year include the 29-year-old Caledonian Isles, which suffered an engine failure that left Arran without its main ferry for 12 days in April, triggering a major fall in visitor numbers at some of the island’s biggest attractions.
The ferry is among CalMac’s 11 major vessels, with the fleet also including three medium-size ferries and 18 small ones, with the latest of which, Frisa, from Norway, entering service on the Oban-Mull route two weeks ago.
However, the ships’ average age has increased because their replacement rate has slowed down over the last few decades.
Cmal chief executive Kevin Hobbs said: “There was quite a lot of activity back in the 1980s and 90s, and from that point onwards, it’s pretty much tailed off.
"It’s not a great story.”
He said the current Scottish Government funding of £580 million for new west coast ferries and port upgrades between 2021-22 and 2025-26 “goes some way to addressing what we need to do, but it’s not enough to do everything.”
He said the “overall requirement” was about £1.25 billion.
Hobbs said: "There is more money required in the next five years, which we hope will come along.”
He said the current funding included for two new ferries for Islay, worth £130m, being built in Turkey, seven smaller electric ferries, costing around £17-20m each, including charging infrastructure, and three for the Gourock-Dunoon/Kilcreggan routes, costing around £6m each.
These new vessels are in addition to the two separately-funded but massively late vessels being built by Ferguson Marine in Port Glasgow for the main Arran route and Skye-Harris-North Uist, which are due to be completed next year at an estimated cost of £240m – two-and-a half times over budget.
Hobbs said acceleration of the current five-year programme to be able to order two more Islay type vessels would be “very useful...a real boost”.
He said that would leave just two more large ferries to be ordered in the next ten years.
However, Hobbs stressed: “We are not criticising any government – this isn’t party political.
"Any government has got a limited amount of capital spend, and a lot of that has gone on other major projects which are very worthy”, such as the Queensferry Crossing and the Queen Elizabeth University Hospital in Glasgow.
But he said the current £580m funding “is a recognition that we do need to start accelerating the replacement programme.”
In our print version of Scotland on Sunday on June 26 (page 1), we inaccurately state in an introduction to this story (printed on page 5) that an increased number of sailings meant the CalMac fleet "could not undergo annual checks".
We acknowledge this inaccuracy and apologise for any confusion.
As stated correctly, in the full story, all vessels are checked annually, but the maintenance programme is currently tight, "so not everything could be checked every year".