In early September, Tallink Grupp’s passenger ferry ’Baltic Queen’, operating on the Tallinn/Stockholm route, underwent a planned technical maintenance period in Naantali, Finland.
In addition to work on the vessel’s hull and interior, various technological and environmentally friendly renewals and upgrades were also carried out.
One of the major technical upgrades undertaken was the replacement of the vessel’s propeller blades with new and more innovatively shaped blades, which increase the efficiency of the ship’s propulsion system in accordance with the ship’s operational needs.
This enables the reduction in the vessel’s fuel consumption and also cuts emissions. The test results also demonstrated that it was possible to reduce the underwater noise generated by the ship.
Whereas the ship’s decreased fuel consumption already showed positive initial results, this still required more detailed analysis, but the company already had the results, which showed the reduction in underwater noise.
To measure underwater noise reduction, as a result of changing the vessel’s propeller blades, Tallink partnered with Tallinn University of Technology (TalTech), who measured the initial underwater noise level produced by the former propeller blades prior to the vessel’s docking from 30th August to 4th September, 2023, and again, after the docking and installation of the new propeller blades, from 15th to 25th September, 2023.
The measurements showed that 40 Hz decidecade source level with the new propeller blades, moving at a speed of 17.6 knots, was by 16 ± 5 dB lower and that a source level in decidecades from 20 Hz to 20,000 Hz with the new propeller blades, moving at the speed of 17.6 knots, is lower by 8± 5 dB.
This means that the acoustic pressure radiated into the water at low frequencies is now at least three times, and at all frequencies combined, at least twice as low as with the old propeller blades.
According to Tallink Grupp’s Head of Ship Management and Chief Captain, Tarvi-Carlos Tuulik, the results were most gratifying and encouraged the company to continue to look for and pilot environmentally friendly solutions.
”Every new solution, which helps us as a company to reduce the negative impact on the natural environment around us is most positive indeed and achieving good results inspire us to find more and more such solutions. Solutions that have a positive effect in multiple ways, such as ‘Baltic Queen’s’ new propeller blades, are particularly pleasing as they reduce fuel consumption, emissions and underwater noise at the same time,” he said.
Commenting on the co-operation on measuring the underwater noise, Aleksander Klauson, Professor of Structural and Fluid Mechanics at TalTech’s Institute of Civil Engineering and Architecture, said: “Ships are a major source of continuous underwater noise at sea. Reducing ship noise is an important environmental issue, as underwater noise disturbs marine mammals and fish and can lead to habitat degradation and declines in marine species populations.
“The European Union deals with the issue of human-induced marine underwater noise through The Marine Strategy Framework Directive where one of the characteristics of good environmental status of the sea is that the continuous underwater noise is at a level that does not harm the marine environment. Several international organisations, such as the IMO, are also working hard to make shipping as environmentally friendly as possible,” he said.
According to Klauson, the results again proved that ship propellers were the main source of underwater noise, due to the fact that when the blades rotate, a phenomenon such as cavitation occurs, ie, the formation of steam bubbles in the water near the rotating propeller blade.
“The measurements demonstrated that by optimising the ship’s propeller, it is possible to mitigate cavitation and significantly reduce the underwater noise caused by the passenger ship,” he explained.
The new propeller blades were designed and manufactured by Kongsberg Maritime Sweden, especially for this type of ship, considering the speed ranges required for her operation.
Fuel and emissions savings from the new blades are expected to be at least 14%, the company said.
If the collected measuring results confirm these savings and the pilot project on the ‘Baltic Queen’ is considered successful, Tallink will extend the use of these propeller blades to other ships across the company’s fleet, the company said.