Rattle and Hum
A quiet boat stops noise in the engine room—before it gets to you and your crew.
“Live” mounts use special rubber to stop much of the engine vibration from getting to the stringer.
Most boaters, except perhaps those with certain brands of go-fast boats, prefer the sound on their vessels to come from the entertainment system, not the engines. In fact, low ambient sound levels are a principal criterion prospective boaters cite when shopping for a boat. That’s why builders invest so much in eliminating or minimizing sound reaching an owner’s ears.
But it’s not only buyers who are concerned with sound. Plenty of boat owners would love to have a quieter boat, and will spend freely in the quest to accomplish this. The good news is that you can make your boat quieter; the bad news is that it’s often a complicated and expensive task, and improvements are typically small. If you think you can just install some pricey acoustical insulation on the walls of your engine room and your saloon will assume the sonic qualities of a medieval cathedral, you’re in for an unpleasant surprise.
But if you’re serious (and realistic) about creating a quieter boat, you need to understand some basics. One is that sound is inextricably tied to vibration. If something vibrates within the frequency of human hearing—generally 20 to 20,000 hertz—most of us will hear it. The second is that for sound to be transmitted, it must pass through a medium, which could be air, a solid, or a liquid like water. Some media are good at transmission, some not so much.
The main source of sound in boats is, of course, the engine room. Lots of things vibrate here, but most notable are obviously the engines, both mains and gensets. To isolate their vibration, these engines sit on “live” mounts that let them move without making whatever they sit on—and ultimately the rest of the boat—also move. In the case of mains, this is no mean feat, because engine mounts must also be rigid enough to absorb the thrust created by propellers.
Engine manufacturers and boatbuilders have done a pretty good job of fashioning engine mounts that do both. Unless you have a particularly old or cheap boat, it’s going to be hard to realize much improvement by switching engine mounts. Better to look elsewhere.
A good next stop are compressors and pumps. Remember, anything that vibrates can produce sound. When pumps and compressors are running, they will vibrate the structure on which they sit unless their movement is insulated. Once again, live-mounting is the solution. The most common and cheapest way is to just bolt the vibrating device to some pieces of flexible material—freshwater hose is a favorite—and then bolt the hose to the structure.
The kind of hose is critical. If not sufficiently flexible, it won’t damp the vibration; if too flexible, the device will shake even worse, exacerbating the problem. If you decide to use hose expect some trial and error until you get it right. But better to spend a little more to purchase purpose-built rubber mounts. In my experience the results almost always justify the added expense.
Anything that shakes sends vibrations through the air. As engineroom air vibrates, it tries to vibrate whatever it touches, including engine room-mounted fuel tanks. Think of these as giant kettledrums: Thump them with your fist, and you’ll see what I mean. Simply wrapping the tanks in foil-covered foam will go a long way toward reducing the kettledrum effect. (The foil keeps the underlying foam from deteriorating and flaking off.) Do the job with actual acoustical insulation, and the improvement will be even more pronounced.
But the fastest, cheapest, and best way to keep sound in the engine room is to seal it tight. If air can’t escape, neither can vibration. Obviously engines need outside air for combustion, but you should seal all other passages leading from the engine room—even to an unoccupied lazarette. This includes hatches, doors, and passages through which piping and wiring pass. Abundant and judicious use of gaskets and caulking will almost always make your boat noticeably quieter.
This is not to say that insulation is not important; it prevents vibrating engine-room air from shaking the surrounding structure. The most effective insulation combines a porous material like foam to absorb and damp vibration and a dense solid like lead to impose a physical barrier against passage. Applying insulation to the walls and overhead of your engine room will inhibit the escape of sound, but the degree of improvement will be directly proportional to the percentage of surface area covered. If you install insulation panels only in areas where you have easy access, vibrating air will still contact the uncovered ones, and the overall improvement will not be as dramatic.
Indeed, if you’re looking for a truly dramatic improvement in your boat’s sound levels, your best approach is probably to turn the job over to a professional—your trusted service advisor should have some ideas. But if you can be satisfied with incremental improvements, you can accomplish a lot by just sealing up your engine room.
This article originally appeared in the October 2013 issue of Power & Motoryacht magazine.