|Now Hear This|
in the art of hushing up boats.
By Tim Clark — September 2001
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 J & 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.
This article originally appeared in the June 2003 issue of Power & Motoryacht magazine.