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Is Your Engine Room Insulation Really Working?
by Ron Reisner
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Boat owners may know a lot about the equipment in their engine rooms, but most know little about the material that lines the walls of the engine compartment.
As a marine surveyor, I recently received a question from the new owners of a 40 foot twin diesel boat. They thought the interior was much too noisy under way.They wondered about that 1 inch thick foam with the silvery plastic coating on the engine room walls -- and they wanted to know what could be done to improve the insulation.
Here's what I told them:
Acoustic insulation works by either absorbing sound energy or reflecting it away from living spaces. Some actually do both.
After talking to the boat's new owners, it became clear that the foam insulation on their particular vessel did not have a reflecting element.
Sound engineers call the absorbent material the "decoupler" and the reflective material the "mass element," or mass layer.
The two most common materials used as acoustic absorbers are open-cell urethane foam and fiberglass batting (similar to the fiberglass insulation in your house). If the foam and the fiberglass are both of good quality and designed for acoustic insulation, one will be about as effective as the other at the same thickness. These materials act as sponges, soaking up sound waves that bounce back and forth in the compartment.
It is important to note that the effectiveness of absorbent materials increases with thickness. In an engine room with a wide range of sound frequencies, 1 inch of absorber is good, while 2 inches is closer to the optimum size.
Mass layers are typically made of lead sheeting, ranging from one to two pounds per square foot; or plastic sheeting, which is loaded with lead or some other heavy molecular weight mineral.
Sound engineers tell us that the mass layer must be non-porous (so sound waves can't pass through) and must be limp. Stiff materials, such as plywood and fiberglass, transmit noise via sympathetic vibrations.
So, we want our engine room lined with a decoupler and a mass layer, in a thickness that will block the transfer of noises ranging over a wide frequency spectrum. That can mean a pre-fabricated foam (which has a loaded plastic layer bonded into it) of at least 1.5 inch thickness and at least one pound per square foot. If the engines, the auxiliary generator, the pumps and other gear are big and noisy, the thickness and weight will need to be increased.
Also note National Fire Protection Association (NFPA) standards state that material used in engine spaces must have a "flame spread index" of 75 or less. If it does, the material label will either list this index or say the material meets "NFPA 255" requirements.
The reason the silvery plastic coating was placed over the foam in the first place was to prevent hydrocarbon vapors from accumulating within the material, which would make it more flammable.
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This article first appeared in the August 1, 1996 issue of Sea Magazine. All or parts of the information contained in this article might be outdated. |
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