SUPERYACHT #11
Winter 2007

Article selected from our quarterly magazine dedicated to the largest and most luxurious boats with information, interviews, technical articles, images and yachting news

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Article by
Marta Pizzarello and Cristiano Battisti

Superyacht comfort
NOISE AND VIBRATIONS

Owners, naval architects and builders are ever more interested in noise and vibrations since onboard comfort became a performance parameter.

On Superyachts the worth of a project is measured in terms of costs, tonnage, range, design and now even in terms of onboard comfort because competition among yards involves the habitability level of the vessel and not just the more traditional aspects such as design, costs and performance.

Fig. 1: the main sources of structureborne noise onboard are the engines, the exhaust system and the in-line shaft. Initially, noise propagates in the hull through the decks and the exhaust system casing, then airborne noise irradiates from the decks to enclosed areas.

Actually, the main characteristic is the frequency composition: in fact, noise is generated by the sum of plenty (at the most, infinite) elementary frequencies that cause the typical and apparently casual trend of the noise.

On the contrary, other sounds such as classical music or the human voice are made up of a very small number of elementary frequencies (from ten to one hundred) that generate a more "pleasant" sound to the ear.

To measure noise intensity, an average value of sound pressure is obtained in the range of audible frequencies (10-20000 Hz). Such value is expressed in Decibels (dB) and its trend is logarithmic: with a 20 dB increase (or reduction) the sound pressure increases (or diminishes) 10 times as much (see table):

The Owner, his guests and the crew perceive noise through the sense of hearing, but this does not mean that it is transmitted only through air. As a matter of fact, depending on the case, noise transmission may be airborne or structureborne.

Fig. 2: by varying the frequency, the areas subject to local resonance may be far from the vibration source, for this reason an accurate preliminary calculation is necessary in order to determine the critical areas onboard before sea trials

This phenomenon typically occurs in the lower decks, where machinery is concentrated and it may be restricted by sealing the noise source so as to hamper airborne transmission (for example, by enclosing machinery in acoustic insulated cabinets).

On the contrary, structureborne noise is more pervasive for it propagates to adjacent compartments through the hull structure. Of course, the farther from the source, the minor the structureborne noise transmitted, but the more rigid the structural element, the better its noise transmission. This explains why structural noise is preferably transmitted vertically rather than horizontally - generally bulkheads are stiffer than decks.

Other means of noise transmission are the pipe connections, the main engine exhaust system, the drive shafts couplings and the resilient joints of main engine supports.

Noise reduction may be obtained either with acoustic insulated materials that are used to hamper noise transmission or with absorptive materials that dissipate part of the noise power as heat.

In the case of structureborne noise, these two solutions are obtained by using floating floors or viscoelastic vibrations damping layers.

In floating floors the floor does not lie directly on the structural deck but is suspended 30-50 mm (1"1/4-2") above it by means of resilient supports. In this way the floor is "isolated" from the vibrations of the underlying deck.

On the contrary, the viscoelastic layer is 2-3 mm (1/16"- 1/8") thick and is placed between the structural deck and the real floor. Thanks to its mechanical characteristics, the viscoelastic layer "absorbs" part of the deck vibrations, dissipating them as heat, without transmitting them to the floor above it.

We examined high frequency and low energy vibrations by treating structureborne noise and we also saw how they propagate in well-defined areas of the hull, preferably along the vertical direction.

On the contrary, low frequency structural vibrations are linked to the cyclical nature of the propelling system - engine-shaft-propeller - and may become unpleasant for onboard comfort only when they enter into resonance with the frequencies of the hull structure.

Fig. 3: the cross-section of a floating floor shows how the floor is structurally separated from the interior bulkheads of the hull structure to hamper transmission of structureborne noise

These frequencies are especially dangerous for onboard comfort (and not only that) because when the cyclic stress occurs at the resonant frequency, the intensity of vibrations cannot be reduced in any way.

We will not explain in detail how to calculate the frequencies of a ship and her different ways of vibration because this has already been done in details by Angelo Sinisi in a previous Superyacht issue (March 2006).

Instead, we will describe the measures that need to be taken during the design phase and the areas on board that are most sensitive to this problem.

Once the natural frequencies of the structure of the ship are defined (today this is possible thanks to the modern structural calculation software), it is necessary to assess which forces acting on the hull have near-to- resonant frequencies.

In general, the first elements to be verified are the shaft and propeller revolutions. The vibrations of the engines on their supports and the pulses of the propellers under the arch piece are two of the most common causes of resonance.

Since, as the proverb goes, prevention is better than cure, then the causes are to be coped with. Engines should be balanced for cruising speed and should be installed on antivibration supports; the number of propeller blades and their shape should be chosen in order to reduce pressure peaks on the arch piece (which in some cases may be likened to "hammer blows").

Only when such steps are not sufficient, local stiffening or additional pillars may be applied to reduce resonance in limited areas of the hull.

However, it is not always easy to act on the structure without interfering with the interior design. For example, the use of ample interior supportless areas to create charming effects, stepped floors or stepped ceilings with a double or triple height, generates flexibility points in the hull which favor the appearance of local resonance even at low frequencies.

From the vibrations point of view, the most delicate areas are those where Superyacht guests linger for long periods, either by sitting or lying for it is in these two positions, rather than standing, that one is greatly disturbed by the vibrations of the underlying deck. Therefore, special care should be taken when designing the dining room, the sitting areas and the cabins. On the contrary, the connecting areas, the galley, the discotheque or the bar lounge and the gym are areas where the vibratory phenomena are less critical.

In brief, when developing the design, it is no more possible to leave aside the study of the acoustic and dynamic behavior of the Superyacht in consideration of the growing increase of the comfort level requested by Owners.

Modern calculation methods and a careful preliminary study enable to reduce to minimum levels the discomfort caused by vibrations and noise, even in complex and articulated structures such as Superyacht hulls where such phenomena are pervasive and cannot be totally eliminated.