Technical Team Leader Pekka Peltonen from Meyer Turu explains the purpose of these tests.
The purpose of the current vibration test is to check the vibration levels of general passenger areas and cabin areas, and compare these to the values based on calculations. The test aims at ensuring that the vibration values fall below the maximum allowed to the ship. The goal is to guarantee passenger comfort when the ship is in operation.
The measurements are being done now, after launching, in the early stage of interior works. When something needs to be fixed, it is easier to do at this stage.
The design of hull structures is co-operative work. The architects wish for capacious spaces and we in turn take care of sufficient strength and low vibration, by supporting the structures with columns, for example. Of course, the functionality of spaces is also important; on car decks we cannot place columns in ideal spots, usually we cannot add any columns there at all. The aim of co-operation is to find solutions that suit everyone, says Peltonen, revealing the challenges associated with building Megastar.
The hull of Megastar is challenging in terms of vibrational design. There are many car decks and other capacious spaces. Ensuring low or no vibration requires careful designing.
The vibration of the main engines is isolated by flexibly fastening them to the ship hull, and the design of propellers takes account — in addition to energy efficiency — the impulse forces and frequencies directed towards the ship.
The preliminary design of the placement and amount of ship hull constructions is based on experience. After this the vibrational behaviour of the ship is estimated by different calculations, using computer modelling. Based on these models the construction is changed or strengthened, if necessary, so that the compliant low vibration levels are achieved.
Next, the ship and the calculations are checked with tests and the final test is done on the ship’s test voyage. On the test voyage, the vibrations in general areas and cabins are measured while the ship is on the move.
During the testing, 32 measuring sensors are attached to Megastar, and in course of the test the sensors are moved to 350 different measuring points.
On the lowest car deck of Megastar, above the propellers, two devices the size of a large chest of drawers are installed, which in 20 minute sequences produce vibration similar to the rotating propellers.
These devices simulate the vibration created by the propellers at different speeds. The devices have computer controlled eccentric motors which are rotated at frequencies from 6 to 16 hertz. 16 hertz is already more than the blade frequency of the propellers’ rotational speed.
From technology to practice
The machines are started, our feet sense a little vibration. It feels as if the ship moved. All in all the vibration is minimal, even though 13 hertz is now in use, which corresponds to full speed. Based on sole feeling we are in for a comfortable ride.
For measurement purposes the ship is divided into two areas: front and back. While measuring is taking place in one area, in the other area the sensors are moved to a new location. The results are continuously monitored in the command centre, set up in the future storage room of the shop. Megastar is a large ferry, over 200 metres in length. Despite the size of the ship the measurements are done meticulously. Weighing 80 kilograms I was not allowed to walk in the measurement area. My walking would have disturbed the sensitive measuring devices. I felt like a bull in a china shop, even though I was surrounded by enormous amounts of steel.