Generators Done Right

ATDMarch11-01.jpgSteve D'Antonio (all)

New generations of smaller onboard generators can be fit into increasingly tight spaces, but correct installation of these diminutive units is as important as it ever was for their hulking predecessors.

The change is noticeable. Just 15 or 20 years ago, generator-equipped boats under 40′ (12.2m) were a rarity. Today, gensets are available in diminutive sizes, making installation possible in locations that previously would have been unthinkable for any gear, much less an internal-combustion engine.

Unlike other segments of the marine-accessory market where consolidation has narrowed competition, an increase in the number of generator manufacturers has resulted in a crop of compact, lightweight, reliable, and relatively affordable models. But none of these new options diminish the importance of proper installation.

As a systems consultant I routinely encounter generator installations with defects from mildly annoying lack of sound insulation, to fuel and exhaust leaks.

Among the most frequent problems are seawater- and exhaust-plumbing defects. Many gensets are installed at or below the boat’s maximum heeled waterline (for powerboats this is the waterline when heeling at an angle of 7°; for sail craft the waterline is denoted by the waterplane of the hull when heeled to the toerail. And remember that waterline is dynamic even for powerboats, so take heeling into account before completing the installation design). Nearly every generator manufacturer provides explicit instructions for installing the exhaust and seawater systems. Those guidelines ensure that seawater will not migrate into the generator’s exhaust manifold and cylinders. One genset manufacturer aptly entitles its instructions for this procedure “Don’t drown me!”

The prudent boat builder or mechanic tasked with installing a new or replacement generator will follow the manufacturer’s instructions without deviation. (Don’t simply assume a previous installation was correct.) In nearly every case where a generator installation exhaust or seawater system does not meet the manufacturer’s guidelines, it’s easy to tell where the technician went wrong. With the availability of most manuals online in pdf format, the information is readily available not only to the installer but also to the owner.

Where exhaust systems are concerned: follow the instructions as well as the following tips.

Most generator and engine manufacturers call for a specific minimum “drop” in the exhaust, often 12″ (305mm), from the exhaust elbow or spray ring to the muffler, with a continuous downhill slope of ½″ per foot (42mm/m). Probably the most common violations I encounter are wet-exhaust hoses that form a lazy-U shape, dropping from the injected elbow to the bottom of the generator sound enclosure, exiting the enclosure, and then traveling upward to a waterlift muffler. Depending upon the generator’s orientation, water trapped in this section of hose can—when the vessel rolls, heels, or pitches—sluice up and into the exhaust manifold, and then to the open exhaust valves, and finally into the cylinders.

When I challenge builders on this subject, I often hear the refrains, “That’s the way we’ve always done it,” and, “It’s never been a problem before.”

I have two responses. First, I’ve seen generators and engines in service for years suffer from just this type of exhaust-system-design failure. Why it took seven or 10 years to occur is anyone’s guess; nothing about the system appeared to have been changed, although apparently something about the way it was used caused the flooding.

Second, if, as an installer, you knowingly violate this or any other generator installation standard, you assume responsibility for any failure that results from the design deviation.


This installation on a crude plywood shelf looks precarious even for dockside use.

Another common fault involves mounting the unit. Generators are comparatively heavy and tall, a combination that exerts significant leverage on the fasteners that secure the base trestle to the boat. While the boat is in a seaway, the forces can be significant. Yet in many installations I encounter generators lag-bolted or even secured by self-tapping screws (in one case sheetrock screws) to a stringer or shelf.

Generators require secure mounting, ideally with through-bolts and/or angle brackets, much like those on propulsion-engine installations; or threaded metallic inserts can be laminated into the stringers that generators rest on and into which mounting machine screws can be secured.

Generator electrical connections are also fraught with potential for failure. Most generator sound enclosures are made of sheet metal, and penetrations in these panels often have sharp edges. I’m surprised by the minimal or nonexistent chafe protection and strain relief where wires penetrate housings.


Sharp edges of this metal enclosure threaten to chafe through insulation on penetrating electrical cables.

This can be dangerous because the positive DC starter cable that travels from the remotely mounted battery, through the enclosure to the starter, nearly always lacks over-current protection. (Note: this is compliant with American Boat & Yacht Council guidelines.) If the insulation chafes against the sound shield, a short circuit is a certainty, and a fire very likely. The same holds true for AC cabling; however, add electrocution to the list of risks.


A tidy installation with an anti-siphon loop. Most loop manufacturers state the fitting’s minimum installed height above the waterline.

Finally, seawater supplies to generators are another area where manufacturer instructions are often disregarded or only partially observed. Beginning with the intake, generators should not be equipped with scoop strainers (hull strainers are covered in detail in Professional BoatBuilder No. 127), as this arrangement might force water into a generator that is not running when a boat is under way. Ideally, and where practicable, strainers installed inside the boat should be mounted below the waterline to reduce the likelihood of air locks. Hose for seawater supply, from seacock to the generator and in anti-siphon valves, should be specifically rated for the application, preferably designated J2006R Raw Water and Wet Exhaust Hose. An even more conservative approach will include wire reinforcement, making the hose virtually crush- and kink-proof as well as resistant to flattening when exposed to high vacuum (during an intake clog, for instance).

If the generator is installed at or below the maximum heeled waterline, you need to install an anti-siphon loop between the generator’s seawater pump and the injected exhaust elbow (typically between the heat exchanger and the injected elbow). In far too many cases, I find such valves installed just a scant few inches above the waterline, making their effectiveness questionable. I also see anti-siphon valves installed on the engineroom overhead, giving the appearance of elevation; when questioned, installers often say, “That’s as high as it will go.” Many anti-siphon loop manufacturers clearly state the minimum installed height above a waterline for effective and reliable operation. Without the elevation, the weight of the water in the hose is not enough to open the vent.

If the valve is mounted too low, the anti-siphon loop is actually inoperable, and the siphon is prevented merely by the generator’s seawater pump impeller—until that impeller becomes worn or damaged. It might take years for circumstances of the defect to allow siphoning that floods the genset.

If the engineroom overhead is too low for proper anti-siphon elevation, it might be necessary to mount the loop in cabinetry or behind a settee in the cabin. Ensure that there’s easy access for inspection and service (anti-siphon valves should be inspected regularly and serviced/cleaned at least annually).


If the anti-siphon loop can’t be mounted high enough, the valve may be located higher in the boat and plumbed to the loop apex with a small-diameter hose.

Another alternative is to do away with the valve mechanism, or mount it remotely atop the inverted U-shaped anti-siphon loop, and simply plumb a small hose to its apex. The hose can be routed to a higher location and directly overboard, into an overboard drain, or to the valve mechanism. But be cautious. Small-diameter hoses are easily crushed; or they might even collapse under the seemingly light pressure of a wire tie, especially over time and when exposed to heat. I’ve seen at least one engine completely ruined when the remote anti-siphon-valve plumbing made of clear-PVC hose running adjacent to the wet-exhaust hose, collapsed and flooded the engine. In another case, a newly installed generator flooded when the remote anti-siphon hose, led to the bilge, ingested bilgewater upon its first shutdown.


Small-diameter PVC hose can be easily crushed by a wire tie. When remotely plumbing an anti-siphon valve for a generator (or anything else), employ crush-resistant hose such as fuel line.

When remotely plumbing an anti-siphon valve for a generator (or anything else):

  • Employ crush-resistant hose such as fuel line.
  • Keep runs as short as possible.
  • Ensure that the terminus of the vent hose can never become clogged or exposed to standing water.
  • Observe manufacturer’s height requirements.

Even experienced boat builders, mechanics, and generator installers will benefit from periodically rereading installation manuals. Guidelines change, memories fade, and it’s possible that a familiar practice that has always seemed to work turns out to be in clear violation of the manufacturer’s installation instructions.

About the Author: For many years a full-service yard manager, Steve now works with boat builders and owners and others in the industry as “Steve D’Antonio Marine Consulting Inc.” He is a contributing editor of Professional BoatBuilder, and awaits publication (by McGraw-Hill/International Marine) of his book on marine systems.