Engine mounts are components often neglected in scientific papers but very precious for onboard comfort. In this respect, they perform the very important function of absorbing and mitigating the vibrations of operating components that they are connected to. As soon as engines are started up, both at rest and when sailing, they produce annoying airborne and structural vibrations and noise. Air and structure borne noise depend on the intrinsic dynamic characteristics of both engine and engine supporting system. In recent years, a new double-stage mounting system, called "raft mounting", has been developed in order to drastically reduce the structure borne noise transmission of superyacht medium speed diesel engines. In this paper, an investigation on the performances of a double stage mounting system installed on a superyacht is carried out.
Comfort on board luxury superyachts at present is the object of great attention by the most important yacht builders. At the same time Classification Societies, taking advantage of the experience gathered in the field of merchant and passenger ships, adjusted existing rules to meet yacht requirements as well. This activity resulted in a wide set of noise and vibration limit values proposed in different areas and situations (Boote et al. 2013). Low vibration and noise levels on a yacht are becoming the new key factors to attract potential customers. Besides passenger comfort, engine mounts are fundamental to preserve the hull structures to all these kind of damages caused by vibrations. The engine is connected to structure foundations through supports, connecting rods, mounts of different shape and layout, all designed to dampen and minimize as much as possible annoying vibrations (Biot et al. 2015). As a matter of fact, if the engine mounts are not properly designed, vibrations generated by main engines and generator sets are perceptible everywhere on board, significantly lowering the quality of onboard life and introducing also fatigue loads which could create non negligible structural damages (Pais et al. 2017). Most part of experimental data gathered in recent years show that, in the case of steel and aluminum light alloy vessels, the noise transmission through structures is usually more important than airborne transmission. The structure-borne sound influences the acoustic comfort at long distance from the sound source because of the low damping values of steel and aluminum light alloy. On the contrary, air-borne sound has significant relevance only in the proximity of the source. For the reduction of the structure borne noise transmission, an improvement of the mounting systems can be attempted by changing from a conventional single stage design to a double stage mounting system (also called "raft mounting"). Raft mounting is a quite new solution by which main engines are resiliently mounted on an intermediate mass, called sub-frame. This allows to significantly reducing the noise and vibration level. This sub-frame, in its turn, is again resiliently mounted to the ship structure, usually the thick keelson flanges. The scantling variables of the system are represented by the mass of the raft and choice of upper and lower stage mounts. The sub-frame can be quite large (up to the 50% of the controlled mass), and results in a taller foundation. Particular attention should be devoted to the raft installation in existing engine rooms because of the consequent weight increase and with regard to hull girder strength and engine room height.
