The £1.9 million project will support the future of the pier by preparing it to accommodate larger, more powerful vessels than the original pier was built to withstand.
Shearwater Marine will mobilise at the harbour during the week commencing 20 May and the first dive will be on Monday 27 May.
Shearwater will carry out work overnight to minimise disruption to ferry operations and day-to-day travel for Bute residents and visitors should be mainly unaffected.
However, to allow the contractor access to the site, the berth will not be available overnight. This will mean one of the Bute ferries will berth overnight in Gourock.
Work should take nine months to complete, and this is dependent on the weather and supply of materials.
Stabilisation works at the harbour include:
• Constructing a toe (retaining) wall at the base of the original wall to provide stability to the main harbour structure. This will extend 143m from the eastern edge of the harbour wall to the overnight ferry berth:
• The toe wall will prevent any potential sliding or further loss of fine materials from the sea bed behind the original wall: and
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• Repair to the surface of the harbour where there has been visible settlement.
This new work builds on previous investment at the harbour to minimise scour and so mitigate damage caused by vessels over the years. Scour is when the seabed is dug out by vessels moving the water and creating channels against the harbour wall. We fitted a wavescreen which prevents vessels moving the water and creating scour and so protects the harbour wall. Since the wavescreen was fitted, we have seen a notable reduction of settlement on the top of the harbour wall.
Councillor Ross Moreland, Chair of Argyll and Bute Harbour Board, said: "It's welcome news that work is about to start in Rothesay Harbour. The pier has played an important part in the island's history and will continue to play a role in its future, through supporting the economy, transport and tourism.
"With that in mind, we have arranged to halt the work during key events such as Bute Fest and the Bute Highland Games. This will enable CalMac to return to its full ferry service."
Regular updates on the work's progress will be available on the council website, and through community, and harbour user groups. We thank everyone for their patience as works progress.
Pauline Blackshaw, Director of Strategy & Change for CalMac said: "We welcome the investment being made by Argyll and Bute Council to upgrade the infrastructure at Rothesay to secure the long-term sustainability of a reliable ferry service.
"We appreciate the efforts made by the Council to carry out these works overnight but recognise this will still lead to some disruption for customers as we must make minor adjustments to the timetable to accommodate the works.
"As we can only accommodate one of our two vessels at Rothesay overnight, the other will berth nightly at Gourock. Consequently, we must cancel the second daily sailing from Rothesay and the second last sailing from Wemyss Bay.
"We fully appreciate the inconvenience this may cause our community and have made every effort to reduce disruption. We are committed to maintaining a full service on key dates such as Bute Fest in July, and the Bute Highland Games in August. We'll be in regular contact with the Council and the local community whilst the works are ongoing."
