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"Now Hear This"
Developments In The Art Of Hushing Up Boats.
By Tim Clark


September 2001 Power & Motoryacht

There are plenty of impressive advances in controlling noise on yachts lately, but since this is our construction issue, one aspect of the endeavor strikes me as especially pertinent: Addressing sound and vibration before and during construction can be as much as 10 times cheaper than lowering levels on a finished boat. What can be done on the drawing board is surprisingly extensive and is now being applied to production boat designs as well.

As you might expect, computers are playing a big role. With complex computer modeling software, noise-control engineers can work hand-in-hand with naval architects and designers to analyze every detail of a yacht's design in order to predict noise problems in every nook and cranny. Using Finite Element Analysis (FEA), engineers at I & A Enterprises in Marblehead, Massachusetts, create a model of the boat that includes the materials and structures of her hull, decks, bulkheads, piping, mechanical systems, and more. This data includes not just the materials, but also their stiffnesses and strengths. After the model is complete, the engineers apply a range of virtual vibrations to the structures to cause them to resonate according to their natural frequencies. (Think of them as the annoying notes that a certain bulkhead or beam, like an unlikely tuning fork, might play.) Once these frequencies are known, they can be compared with the frequencies generated by the primary sources of noise and vibration aboard most vessels-the engines, props, and shaft systems.

The interaction of the various natural frequencies of a boat's structures with certain frequencies of its noise sources can be most disturbing-as when you're at cruising rpm and a bulkhead shivers and whines like a child too long in the pool. When the model indicates the presence of these unfortunate harmonies, engineers can alter the composition or dimensions of the structures within the model to minimize their incidence.

FEA deals primarily with low-frequency vibration traveling through the vessel. Another modeling program, Statistical Energy Analysis (SEA), is used to predict higher-frequency noise levels in individual areas of the boat. According to Sjaak van Cappellen, founder of Silent Line Noise and Vibration Control in Miami, Florida, the SEA model is three-dimensional and includes not only the vessel's structures, but also values for their sound absorption and conductivity. What's more, SEA incorporates the boat's sound sources and the amount of noise coming from them. Once all is in place, the program can calculate the flow of sound energy throughout the boat and generate estimates for levels in particular areas. Also, the model will indicate the primary paths of the greatest noise sources onboard. You can then plug in a variety of systems for reducing sound sources and plans for sound insulation and absorption.

Innovative noise-control strategies such as those that would be modeled into an SEA analysis are becoming more widely applied in boatbuilding, trickling down from megayachts to boats as small as 45 feet LOA. Advances in reducing sound and vibration at their sources include better understanding of propeller design in relation to cavitation and to the transfer of vibration to the reduction gear, as well as the development of exhaust system silencers that are more compact and more effective. But a boat's propulsion system will always produce some level of clatter and vibration, and as the differences between FEA and SEA analyses indicate, noise gets around in different ways on a vessel.

The low-frequency resonance that FEA addresses is primarily caused by vibration traveling through the boat's structures. The higher-frequency noise that SEA examines often travels through the air, so once engineers quiet the sources as best as they can, specific treatments are for the most part divided into two strategies: isolating vibration (structure-bome noise) and containing loudness (airborne noise).

The vibration from main engines can be significantly cut off from the rest of the vessel with the use of flexible mounts. Special engine installations allowing for exceptionally soft mounts-such as the Aquadrive antivibration system, the Centa-flex AGM, and Rubber Design systems, all of which include bearings that keep propeller thrust from the engine-can reduce the transfer of engine vibration to the hull by as much as 95 percent while also diminishing prop-shaft vibration. If you also mount gensets, exhaust systems, and even pumps on isolation mounts, you go a long way toward confining structure-bome noise.

Only now do you turn to sound control methods that most people think of first: insulation and absorption. While isolating vibration impedes the flow of most noise on a boat, it doesn't do a thing to diminish the airborne noise the isolated systems create. Acoustic insulation materials (along with complex baffles on air intakes, highly insulated doors, and airtight gaskets) are used to keep such noise confined to the engine room. Materials range from the conventional and specialized fire-retardant composites that) & A's affiliate Soundown manufactures to lightweight Polydamp Melamine Foam (PMF) often used by Silent Line in as many as 15 layers. There is a broader range of such materials than ever, with increased fire resistance and combined acoustic absorption properties.

They are frequently used not only on engine room bulkheads and ceilings, but also throughout hulls and superstructures, for in some hulls, once sound and vibration from propulsion systems are tamed, noise generated by the hull passing over the water can be significant. When a structure-bome sound source is, to use I & A president loe Smullin's words, "so largely distributed," it's best to treat individual locations such as staterooms and saloons. While one option is to heavily insulate a cabin, another is to "float" it-isolate it on ranks of mounts or elastic strips in a box-in-box construction with sidelinings and bulkheads stabilized with other flexible attachments. While whole floating decks are common on mega-yachts, space and expense considerations make floating cabins rare on smaller boats. However, Silent Line has floated cabins on boats as small as 55 feet LOA, and floating a bulkhead between an engine room and stateroom is not uncommon.

A majority of both Silent Line's and J & A's comprehensive, early-design-stage projects are at the behest of custom and semicustom yards, but they are increasingly consulted by production builders. "Some manufacturers are afraid that these solutions are expensive," says van Cappellen, "but we approach every design revision from a practical point of view. On existing boats we go aboard to take noise and vibration measurements, do some computer modeling as verification, then ask the client, 'What levels do you want, and how much do you want to spend?' Then we tweak the model according to that criteria."

"The standard of onboard quiet is advancing in the'United States," says Smullin. "When production builders come to me with an after-the-fact design, it's not to put out a fire; it's for analysis toward general product improvement."

If trends continue we may turn around one day to find whisper-quiet wheelhous-es as industry-wide as bow thrusters. And the loudest noise source on your boat will probably be your brother-in-law, dressed in a Hawaiian shirt.


 

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