Noise control CB 90 : A Study of the Transporter Room in a Noise Perspective

The main focus of this thesis has been the noise level inside combat boat 90, CB 90H acommon boat in the Swedish navy. The stakeholder of this project has been Dockstavarvet,the developer of the boat. Almost all parts of the project have been carried out at thecompany headquarters in Docksta, except some testing which occurred at a laboratory atLuleå University of Technology. The development of these aluminium boats are goingtowards having higher speed with more powerful engines and at the same time tougherrequirements for the noise level. This thesis aimed to evaluate the transporter room wherethe marines are stationed while transport. The current noise level is above 85 dB(A) whichexceeds the recommended noise level for hearing protection which is 80 dB(A). In recentyears the fire regulations have increased, to meet these stricter regulations Dockstavarvetwanted to see an improvement without using anything that could be classified asflammable. This limited the project to focus on changing the aluminium structure andusing the fire approved viscous compound Noxudol, which is used for noise control today.To evaluate the situation two research questions was necessary to answer”where does thenoise origin and how does it propagate?” To answer these questions a large study ofpervious noise control assessments was conducted and a search for similar projects.Through the research it was determined the CB 90H has two sources contributing to thehigh noise level, water streaming against the hull and the engines, two water jets. Each ofthese sources contribute with a high noise level, the hulls contact with water gives thehighest dB(A) value while the engines contribute with a low frequency noise and thehighest dB value. In this thesis the goal have been to lower the dB(A) level which for themost parts are the requirement in a new development of a vessel. Therefore the main focusof this thesis has been to reduce the noise from contact with water and secondly the enginenoise. A literature study was conducted to understand how these noises could be dampedand if there is any suitable methods. The research showed increasing the stiffness of themain girders would attune the vibrations from the engines. To reduce noises which occurfrom the contact with water, weight needed to be added for parts of the bottom hull plates.After this point the engine noise was unprioritized and the focus was put on detaildevelopment for the radiated noise from hull plates. Due to practical reasons, testing on aCB 90H in water wasn’t possible. Therefore a test rig was built; the test rig was a section ofthe lower hull structure in the transporter room. The test contained three differentconfigurations, bolting plates on the bottom hull plates, adding a layer Noxudol betweenthe plates and finally spraying the test rig with Noxudol. The test showed an expected effectwith the added plates but also showed that the viscous compound had no effect betweenthe plates and small effect when it was sprayed. The effect from adding the plates werenotable and will improve the situation but with a cost of some additional weight. Therecommendations for Dockstavarvet is to further examine the possibility to increase girderstiffness in engine room and search for an more effective way to reduce the radiating noisefrom hull plates.Validerat; 20150910 (global_studentproject_submitter