If you have a large AC electrical load and spend time away from dockside shorepower, you’ll probably need to look beyond the capabilities of DC-to-AC inverters and start investigating gen-sets. Even if a properly sized inverter is capable of handling your load, the charging system needed to replace battery drain may well be impractical.

A good example of an AC load too large for most inverter installations is air conditioning. The instantaneous AC current draw from air conditioning ranges from fairly modest to high, depending on the BTU rating of the unit. But it’s not the current draw that creates the problem for inverters, which can comfortably handle large loads such as microwave ovens, toasters, and coffee makers. The problem lies in the length of time and the time of day air conditioning is normally operating. Air conditioning runs for long periods of time and is used primarily at night, when charging sources have a tough time replenishing the battery drain. Generally speaking, large AC loads that run for extended periods of time require an engine-driven AC power source, the most common option being a diesel- or gasoline-fueled gen-set.

Gen-sets use two primary pieces of equipment, a rotary AC generator and a matched diesel- or gas-fueled engine. For the sake of efficiency, a gen-set’s engine is properly sized to do the job required, usually with little or no capacity left over. You can increase the overall system efficiency even further, however, by using an inverter along with the gen-set.Use the gen-set to power large or constant AC loads, then use an inverter to power smaller or intermittent AC loads. This reduce wear and tear on the gen-set, fuel consumption, and the amount of time you are subjected to engine noise.

Choosing a Gen-Set

Equipment weight and size are important considerations for multihulls, especially those with light payload capacity, and cost is a universal concern for cruising boaters. Gen-sets can be big, heavy and expensive to purchase and to install, but don’t let that deter you from considering one. There are compact units on the market that are reasonably-priced and relatively lightweight, ideal for multihulls, and installation can be simplified if you know what you’re doing. Once you’ve accepted that a gen-set is right for your situation, take some time to research the various units on the market.

An ideal gen-set would be lightweight, smooth, quiet, fuel-efficient, powerful, low cost, reliable, and fit in a spare locker.Since it’s difficult satisfying all of these requirements in a single gen-set, you’ll need to establish an order of importance for your situation. If high power and low cost are your priorities, you may end up looking at models that are heavy, bulky and only moderately quiet. If silent operation, light weight and compact size are the top requirements on your list, you’ll undoubtedly have to pay a premium price in terms of $/kW. It’s best to determine what characteristics are most important for your situation before you shop around.

Fuel and Engine Type

The first thing to determine is fuel type. It’s logical to choose the fuel that your auxiliary engine runs on. If you are choosing between the two, diesels are the marine engine of choice as they tend to last longer and be more fuel efficient, yet they are typically more costly to purchase and repair. As with an auxiliary, proper venting and exhaust is crucial for a gasoline engine used below decks; if you are considering a gas-fueled model make certain it is approved to operate in a gasoline atmosphere.

Gen-set engines vary in how many cylinders they have. Many of the small, high-speed models use Farymann or Yanmar single-cylinder engines, which are surprisingly balanced and relatively quiet. Yanmar, Kubota and Mitsubishi 2- and 3-cylinder engines are also popular for modest sized gen-sets. In general, three and six cylinder engines tend to be the most quiet and have the least vibration.

RPM Rating

Industry standard gen-sets such as those from Kilo-Pak, Kohler, Northern Lights and Onan are made to run at 1800 rpm (for 60HZ output; 1500 rpm for 50HZ output). Large gen-sets (over 20 kW) using six pole sets in the generator run at 1200 rpm (60HZ) or 1000 rpm (50HZ), while the compact gen-sets from Entec West, Fischer Panda, HFL and Mase are designed for high-speed operation at 3600 rpm (60HZ) or 3000 rpm (50 HZ). Relative newcomers to the field are the compact models from Next Generation Power, which operate at a mid-range speed of 2800 rpm. The 1800 rpm proponents claim lower noise and longer engine life, while the manufacturers of high-speed models claim they’ve designed their units accordingly and can produce more electrical power for a given weight and volume by using high-speed diesels. Next Generation claims their mid-speed models are as quiet as the 1800 rpm units, without the need for sound enclosures as with the high-speed units. While it’s true that gen-set noise varies widely, I feel all engines on board need to be soundproofed if you’re going to enjoy your time on the water. Most of the high-speed models use water cooling in both the engine and the electrical generator itself, allowing the gen-set to be completely enclosed in a well-insulated, soundproof box.

Power Rating and Efficiency

Power ratings for gen-sets begin in the 2.5 to 3 kW range, and continue up to 20 kW or more. The most common gen-sets for pleasure craft are in the 4 to 12 kW range. Gen-sets achieve their rated power at a given engine speed necessary to produce the required AC frequency; engine speed remains fairly constant, regardless of electrical load. Even though the actual load on the engine is related to how much electricity is being used, running a gen-set to satisfy a small AC load is inherently inefficient. Some gen-set manufacturers (notably Balmar) have embraced what is known as VST (variable speed technology). Gen-sets with VST adjust their speed according to electrical demand, while maintaining the correct AC frequency and waveform. These models cost more, so you’ll have to decide if you can afford the extra efficiency.


Cooling & Exhaust

Most marine gen-sets are water cooled with two separate cooling circuits. One circuit pumps seawater from outside the boat, up through a heat exchanger, and back overboard. The second circuit pumps freshwater from the heat exchanger, through the engine block and exhaust manifold, and back to the heat exchanger. Many of the high-speed gen-sets such as FISCHER PANDA incorporate an additional cooling loop in the generator itself, which increases efficiency and allows the unit to be completely enclosed in a soundshield without worry of overheating.

Some gen-sets installed in monohull sailboats are “keel cooled”. In this type of system a cooling pump moves fresh water through a cooling grid on the bottom of the boat. You’ll still need a seawater loop, however, if you want to have a wet exhaust.

Engines on pleasure vessels usually have a wet exhaust to dampen the exhaust noise. Dry exhaust systems are more appropriate for workboats. There are devices now on the market that further reduce exhaust noise by separating the water and exhaust air before they exit the boat, eliminating that tedious splashing sound of wet exhaust outlets just above waterline.

Sound Proofing

All gen-sets make noise, but some manufacturers have gone to great pains to make their units run as quietly as possible. Compare decibel ratings of the various units on the market. Noise on a multihull is usually a big concern, so choose a model with a good sound shield, or have a soundshield made (hard case or soft-sided) to help insulate gen-set noise from the rest of the boat.

Mounting Location & Method

Gen-sets ca n be mounted in any dry, accessible space on board where the sounds and smells can be isolated and the plumbing and wiring can be installed safely and efficiently. Access is important, since the engine will need to be maintained and serviced regularly. The mounting surface must be adequately reinforced. Check with your dealer for his recommendations on platform material selection and thickness.

Any marine engine should be mounted on a well-reinforced platform using rubber isolation mounts. Some gen-set suppliers offer “hydrolastic” mounts that provide the ultimate in vibration resistance. My advice is to spend the few extra dollars on good quality sound and vibration isolation. It will make your time on board much more enjoyable.

Plumbing & Exhaust

For wet exhaust systems, plumb raw seawater through a strainer and into the engine’s heat exchanger, then into the exhaust piping; exactly where depends on how the gen-set sits in the boat. If the point where the cooling water is injected into the exhaust is less than a foot above the waterline, you’ll need a vented loop (syphon break) to prevent seawater from siphoning back through the raw water pump after the engine is shut down. Never use a scoop type water intake on the outside of your hull, since this can inadvertently force water past the raw water pump, into the muffler, then on into the exhaust manifold and into the engine cylinders.

Make sure to plumb a loop between the muffler and the exhaust outlet. This loop should be at least one foot above the loaded waterline. An alternative to this type of arrangement is to use a device which separates exhaust and water before they exit the boat, eliminating splashing or pulsating water flow at the exhaust outlet.

Gen-Set & Inverter-Charger Combination

Gen-sets can provide high performance charging whenever they are running if you have an inverter-charger on board. Most inverter-chargers switch into the charging mode anytime a source of AC is detected, whether gen-set power or shorepower. Make sure the gen-set you choose is sized to meet the demand of the inverter’s charging circuit. Some small gen-sets just can’t provide enough power to operate a large inverter’s charger efficiently. If you own the inverter and are shopping for a gen-set, make sure the gen-set supplier knows which inverter you’ll be using.

Some inverters have an adjustment that allows you to control the charge rate. With this arrangement you can fine tune the charger’s output to match the gen-set output.

Options & Accessories

You’ll need to have a good diesel fuel filter/water separator installed in your fuel line, and a separate battery for starting your gen-set. Gen-sets are designed to maintain starting battery charge. Additional safety and monitoring equipment includes start/stop panels, engine-hour meters, AC & DC electrical meters, and ship/shore switches. Automatic controls can be installed to shut down the engine on high exhaust temperature, engine overspeed, low water level and low oil level.

Partial List of Manufacturers

Fischer Panda
Next Generation
Phasor Marine


About the Author

Kevin Jeffrey is a long-time multihull sailor, independent energy consultant, author and book publisher.  He is the author of Independent Energy Guide, a valuable resource for cruising mutihull sailors, and is the publisher of Adventuring With Children by Nan Jeffrey and the first three editions of the Sailor’s Multihull Guide.