Building a Quiet Vessel without a Navy Budget

The term "Quiet Vessel" evokes images of stealthy nuclear submarines such as the Seawolf or the new Virginia Class. You may even think of the super-secret Sea Shadow, a radar evading surface vessel designed and built by the makers of the stealth fighter and stealth bomber, the Lockheed "Skunk-Works". However, you would probably not relate the term, "Quiet Vessel", to a new pair of Station Pilot-Boats built for the San Francisco Bar Pilots, a Voith-Schiedner Tractor Tug or a new Ferry for the Alaska Marine Highway System. Whereas the Seawolf and Sea Shadow were designed to be quiet underwater, the Pilotboat, tugboat and ferry were simply designed to be quiet for their occupants. It is possible many owners and operators believe that to build a quiet commercial vessel you need a big Navy budget. This is not the case! Many vessels can incorporate noise-reducing elements for little additional engineering or construction costs. In fact, treating noise problems after a vessel is completed can be at least ten times more expensive. Dealing with these issues during design & construction can add only a few more steps. Here's how to go about building a quiet vessel. What are the appropriate noise goals? As a vessel owner, the first step to acquiring a quiet vessel is to know, "What is quiet?" and how to specify it. Terms like "zero noise", "no appreciable noise" or even "below hazardous noise levels" would not constitute well-defined noise limits. Two good documents that provide guidance for shipboard noise are: U.S. Coast Guard's Navigation and Vessel Inspection Circular, NVIC 12-82, "Recommendations On Control of Excessive Noise", dated June 2, 1982 and the International Maritime Organization's (IMO) Resolution A.486(XII), "Code on Noise Levels on Board Ships", dated November 19, 1981. ABS and DNV have also recently issued shipboard noise requirements for larger vessels. Sound or noise can be measured and reported in many ways. The most widely used metric for shipboard noise is the overall A-weighted sound pressure level in dB(A). The "A-weighting" is used because it accounts for the way the human ear responds to sound. Other weightings such as B, C or linear (un-weighted) can also be used. Unless you know what these terms mean, avoid their use and stick with dB(A). The one and only way to specify shipboard noise requirements is on a compartment-type basis (i.e. staterooms, machinery rooms, etc.). Chapter 4 of the IMO Resolution A.486 provides maximum acceptable sound pressure levels for typical shipboard compartments. Table 1 lists these requirements. IMO allows for very high sound levels in machinery spaces. The maximum level allowed in non-continuously manned machinery spaces is 110 dB(A) and hearing protection must be worn at all times in these compartments. Most accommodation or mission spaces (such as state rooms or pilothouses) have sound limits from 60 to 65 dB(A). This is a comfortable sound level for a ship that would not overburden it with acoustic treatments. What are the Best Noise Control Treatments? Once the noise requirements are put on paper, the next step is to determine what if any noise control would be required to achieve these requirements. To accomplish this, a first draft of vessel's general arrangements must be available. The task of developing a list of noise control treatments can be as simple or as complicated as you desire. Use of three stock noise control treatments as discussed below may be adequate for non noise-critical vessels such as small harbor tugs or short-run passenger ferries. For noise-critical vessels, it is usually necessary to have an acoustical analysis performed. This should be done by an experienced "Marine Acoustical Engineer". They can compute what the sound levels will be from vessel drawings and a machinery list. This would determine what if any noise control treatments are then needed to achieve your requirements. Before introducing any noise control treatments consider the location of the vessel's compartments as a first approach to noise reduction. Compartment location relative to the vessel's machinery is the most important factor that contributes to the room's sound level. It is foolish to think that a State Room two decks above the Engine Room will be as quiet as one adjacent to it. The least expensive noise control is to locate noise sensitive compartments away from machinery. Another good practice is to locate non-noise critical compartments such as storerooms or laundry rooms between the machinery and noise critical spaces. Using air locks between machinery spaces and accommodations areas reduces the loud roar when crew move in and out of each space. The three most useful noise control treatments should be included on all new vessels. They are: machinery vibration isolation, acoustic absorption and acoustic insulation. It is important for any vessel operator to understand that noise transmission through to the accommodation areas of most vessels is mostly due to vibration transmission. Known as the "structureborne noise path", noise is generated by the transmission of vibratory energy from the feet of the propulsion engines and diesel generators, down the machinery foundation, to the hull and then to any structure in the ship. The bulkheads, decks and deckheads in any compartment then radiate this vibratory energy as sound. Thus, one of the best treatments for shipboard noise control is actually vibration isolation. Vibration isolation of a diesel generator, air compressor or hydraulic power unit is simple during the vessel design stage. Isolation of the propulsion machinery is not as simple. It requires an understanding of the engine's movement due to static and dynamic deflection and vessel motion. A flexible torsional coupling is usually required along with the marine grade elastomeric vibration isolators and some specialized engineering. However, for noise-critical vessels, isolation of the propulsion machinery is a great start to a quiet ship. The second important noise control treatment is acoustic absorption. Shipboard compartments are usually very hard or acoustically reflective. Acoustical absorption cuts down on sound waves bouncing around in a room. It should be located in the overhead of every compartment in both machinery and accommodation spaces. Acoustical absorption in the overhead of a machinery room would take the form of 2 inch thick fiberglass or mineral wool with a protective facing (Mylar is good). The material is usually provided in rigidized boards that should be pinned to the deckhead of all machinery rooms. In many cases, the acoustical absorption can be one in the same material as the thermal or structural fire protection insulation. For the material to work for both applications, the fibrous material must not be covered with any impervious sheathing, like sheet metal. Acoustical absorption should be incorporated into the overheads of all accommodation and mission spaces. The most common form is a dropped ceiling with perforated sheet metal panels that has mineral wool within the top of the panel. These are commercially available from joiner material vendors. The third important treatment is usually the first one used when trying to solve a noise problem, acoustical insulation. It consists of a high-density fiberglass or mineral wool just like the acoustical absorption. Unlike absorption, which is used to absorb sounds bouncing around in a room, insulation blocks the sound from leaving one compartment and traveling to the next. Whereas absorption needs to be exposed in the subject compartment, the acoustical insulation can be located between structural and joiner bulkheads. If higher noise reduction is necessary, a layer of lead sheet is located between two layers of fiberglass or mineral wool. In general, acoustic insulation should be located on the bulkheads or deckheads between quiet accommodation spaces and machinery rooms. In the next edition of MarineNews (February 4, 2002), Part II of this two part series will discuss how to put all these treatments together in a vessel specification. Special situations such as noise from HVAC Systems, Bow Thrusters and hydraulic systems will also be discussed.