Warship Technology: January 2017
2016 saw two full-scale ice trials undertaken in the Bay of Bothnia between Oulu and Kemi, where ice conditions are extremely challenging in cold winters. The was to perform ice trials for the Finnish Border Guard’s offshore patrol vessel Turva. The second was to test a newly developed bronze propeller in heavy ice.
The bronze propeller for ice has been developed by Aker Arctic in cooperation with Tevo Oy and the Technical Research Centre of Finland, VTT Oy. Although not as strong a material as stainless steel, bronze has some benefits compared to a stainless steel propeller. These include better corrosion resistance; it is easier to manufacture and maintain, and, not to mention cheaper. Bronze is a widely used material in propellers intended for open water and Tevo Oy wanted to find out if it would be feasible to use it in ice-going vessels. Tevo manufactures NiAl bronze customised and fixed-pitch propellers at its facility in Turenki. It also reconditions and repairs propellers.
The bronze propeller was tested on the Finnish Navy’s multipurpose vessel Louhi, which was designed primarily as a spill response vessel. The vessel is owned by the Finnish Environment Institute (SYKE), but manned by the Finnish Navy. Louhi was built by Uki Workboat Ltd in 2011 and is a multipurpose response vessel designed to operate in all Baltic Sea ice conditions. The ship is fitted with mechanical oil spill response outfit, including equipment designed specifically for collecting oil in ice. It can also be used for emergency towing, firefighting and other rescue operations. Working on behalf of the Finnish Navy, it is also designed to be able to lay subsea cable, act as a platform for diving operations and lay mines in the event of a conflict. Louhi is also fitted with a suite of equipment to measure environmental conditions at sea. The spill response vessel is equipped to operate in the Baltic all year round and has a maximum speed of 15knots. In 50cm (20in) thick ice the vessel is capable of 7.5knots. The ship has a bollard pull of 60tonnes. It is fitted with four Wärtsilä 9L20 generating sets and is normally fitted with a pair of Rolls-Royce azimuthing thrusters, each of 2,700kW that have stainless steel propellers, but for the test one propeller was removed and replaced with a bronze propeller. The tests were performed in ahead and astern direction in 60cm and 85cm thick level ice, 6m thick ice ridges, and ice channels.
“The extreme ice conditions for the tests were excellent,” said Kari Laukia, head of ship design and engineering at Aker Arctic. “We wanted to test the propellers in as heavy conditions as possible and succeeded in finding sufficiently different conditions. In addition to strength measurements, we also took underwater videos in order to see how the ice and propeller interaction took place. After the testing, Louhi returned to drydock and the test propeller was remounted and sent to Tevo for inspection.”
Aker Arctic said that, based on the trials, it has concluded that bronze propellers can be suitable for vessels in 1A Super ice conditions and believes that the conditions during testing provided reliable results and input for the design of propellers for 1A and 1A Super ice class vessels.