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Should You Have a Sound Shield on Your Generator?


It’s rare to see a generator without a sound shield, but leaving it open could mean easier maintenance.

Does Out of Sight Mean Out of Mind?

Your generator’s hush box could be causing a maintenance issue.

For most of us, life afloat without a generator would be a lot less fun. Sure, we could run electrical off a battery bank or a combination battery bank and inverter. But batteries—even AGMs and gel cells—have limited capacities, especially when you turn on air conditioning or anything with a heating element. When batteries tap out, you must either fire up the mains or go searching for shore power. A generator’s electrical output is limited only by the amount of fuel available, which means many more comfortable hours unplugged.

The modern generator is a paragon of unobtrusive reliability, but it wasn’t always thus. When I first began working on boats in the ’80s, generators—especially gasoline ones—were balky and temperamental. The engines often wouldn’t start, requiring a trip to the engine room just when everyone was primed for some relaxation. The electrical components, notoriously vulnerable to the marine environment, weren’t much better.

Today’s generators are so reliable that we hardly give them a thought. You need only stroll over to the conveniently located remote control panel and flip one or two switches. It’s so simple that some boaters aren’t even sure where their generator is, or what it looks like. Such unfamiliarity often leads to problems.

Generator maintenance is simple and straightforward because a marine generator is an uncharacteristically simple device on a vessel that’s typically awash with complexity. A marinized gasoline or diesel engine is directly coupled to an alternator or generator, which when turned, produces electricity. (An alternator produces electricity when its magnetic field spins inside the stator or windings; in a generator, the windings spin inside a fixed magnetic field.)

This simplicity is a bit illusory in that regardless of load, the engine must turn the alternator/generator at a specific speed to produce electricity of the frequency demanded by the vessel’s electrical appliances. In North America, our alternating current is 60 cycles (or 60 Hertz); Europe’s is 50 cycles. The number of cycles or frequency is directly related to how fast the generator/alternator turns, which is why the same generator sold in North America can be adapted for Europe by simply slowing down the engine rpm.

In the old days, correct engine rpm was maintained by a mechanical governor that reacted slowly to changes in load, which is why the lights often dimmed when you turned on the air conditioning. This minor inconvenience became a potential calamity once devices such as computers and LED/LCD TVs, which are highly sensitive to frequency variations, came aboard. Fortunately, the problem that advanced electronics created they also solved: Today, electronic governing ensures delivery of the proper frequency at all speeds and loads.

Besides unreliability, older generators suffered from excessive noise and vibration to the point that cabin comfort was compromised—especially at night. Much of this was corrected through improvements in engine and mount design, and with the introduction of the acoustical enclosure—more commonly known as a hush box or sound shield, but with it came a potential maintenance problem.

Like the generator, the sound shield is simple. You craft a box, pad it with sound-absorbent material, and place it over the engine and generator. The problem is that the engine needs air for combustion, and the alternator/generator needs it for cooling, and when you make a hole for air to get in, you also make a way for sound to get out. That said, generator enclosures today employ internal baffles that absorb much of the sound before it escapes.

One potential problem with sound shields is that you must remove them, or at least part of them, to perform maintenance. This is by no means difficult, but it can be bothersome enough to impede the less scrupulous from necessary tasks such as fluid checks. Proper generator maintenance is often already an issue because many boaters schedule it at the same time as the checkup on their mains—despite the fact that a lot of generators rack up two to three times as many engine hours. 

This leads us to the question of whether all boats really need acoustical enclosures. Generators are usually situated in the engine room, which is typically loaded with enough acoustical insulation to contain the roar of a pair of big diesels. Shouldn’t this insulation be sufficient to also snub the comparatively puny mumble of one or two generators?

And consider this: Besides adding a potential impediment to giving the generators the attention they need, a hush box also prevents you from seeing the unit’s workings, along with warning signs such as oil leakage and discoloration.

I’m not aware of any testing that has quantified the effectiveness of adding a hush box in a well insulated ER, but I can’t help but wonder just what difference it may make to human ears ensconced in a saloon or stateroom. I’ll bet not much.

This article originally appeared in the August 2016 issue of Power & Motoryacht magazine.