More Power to Ya!

Today’s gensets are smaller, quieter and more efficient than ever. Here’s what you need to know to choose the right one for your boat.       


by Frank Lanier
Chesapeake Bay Magazine (May, 2004)

            It’s often vilified as noisy, smelly, maintenance-intensive or decadent, or all of the above. It’s the generator, or “genset”—scorned by purists and naturalists everywhere. But let’s face it, it’s quite a convenient thing for those who can look past its shortcomings. And true optimists will point out that today’s gensets are quieter and more compact than ever, making them a practical alternative for boats even in the 30- to 40-foot range. Considering the demands of power hungry electronics and creature comforts like air conditioning (and the ever-popular blender), many boaters these days see generators as a necessity, not a luxury.

            Installing a genset is a major decision, not least because it will probably be your most expensive piece of equipment after the engine. You’ll want to examine the choices carefully to make sure you select the one that best fits your boat’s needs. Still, you should buy the best quality unit your budget allows. Depending on your boat and its power needs, generators can log up to three times as many hours as your engine. A “good deal” isn’t a good deal at all if you end up with a poorly built, underpowered product. A generator that’s too small will be constantly laboring to meet demands, leading to poor performance and a shorter life. Conversely, one that’s too powerful will lead to “underloading” problems such as carbon buildup in the engine, incomplete fuel combustion and overall inefficient operation.

            That’s why before you start shopping you need to figure out your power needs. Any generator dealer will be happy to conduct a load survey or analysis of your vessel to determine what size generator you’ll need (how much wattage, that is), but it’s also fairly easy to do it yourself. The first thing you’ll need to do is record the wattage requirements for each piece of electrical gear onboard; in most cases these are listed right on the machine, on a manufacturer’s data plate. If they only show amps, you can convert to watts by simply multiplying amps by volts (amps times volts equals watts).

            While electrical devices such as toasters, lamps and coffee makers present a constant “resistive” energy load and are easy to calculate, “reactive” loads (saws, drills and electric motors) are tougher to nail down, since their power requirements vary during operation (that’s why you’ll see the lights dim when you start a large motor, but return to normal once the motor is running). Starting power for an electric motor can be up to three times the required running power. “Loaded” power—the extra juice required when a saw, for instance, starts cutting into wood—needs to be factored in as well.

            Some motors’ equipment data tags include starting power information, so check there first. If you don’t find it, you can calculate it by multiplying its running power by three (five to be even more conservative) and use that as its required load. Doing the same thing with the running power of tools and other equipment covers loaded power requirements for them as well. For example, a water pump with 300 watts of running power would have a calculated start power of 900 watts (if multiplying by three), while an electric drill with a run power of 500 would have a calculated load power of 1,500 watts. 

            When calculating your boat’s power needs, consider ordinary power management techniques too. You’ll rarely run every tool and appliance simultaneously, and something as simple as first turning on the largest electrical motor in your system, then bringing the others on line one at a time, lets you use running power rather than starting power in your calculations. Even shutting down the air conditioning while using a power saw will let a smaller generator manage a larger overall load. 

            Once your list is complete, it’s simply a matter of adding the maximum amount of loads that will most likely be pulling simultaneously. Then consider doubling it to cover any future power needs. Once you come up with your final number, select a genset with sufficient capability, plus enough reserve power to ensure it normally operates within its optimum range. Don’t base your choice on a generator’s “maximum” wattage—because gensets shouldn’t operate at maximum power for more than 30 minutes or so. Instead, look at the “rated” power, which is normally around 90 percent of maximum power and an output level the genset can maintain for long periods without ill effect. Then choose a unit that will normally operate at about 50 percent of its rated power based on your vessel’s expected loads. For instance, if you need 2,500 watts of power, you’ll want a genset with a rated power of about 5,500 watts.

The Basics

            Simply put, a generator converts the mechanical energy of its engine into electrical energy. The basic setup consists of an engine, alternator, AC switch gear (which switches the electric load between generator and shore power), cooling and exhaust equipment and some type of sound shield to quiet them down.

            Most recreational boaters will find that single-phase, 15-kilowatt gensets producing straight 120 volts or a combination of 120/240 volts will meet most of their onboard requirements. Single-phase 120-volt works just fine for powering most household type appliances such as microwaves and coffee makers, while the dual output (120/240) units give you the option of powering larger 240-volt equipment too, like a dive compressor or a heating system. (Though three-phase gensets can be easier to start and less pricey than single-phase gensets, they’re really overkill on midsize recreational boats, instead better suited for much larger vessels with big heating and air-conditioning systems, for instance, or specialized equipment.)

            Generators are designed to operate at certain engine speeds, typically between 1200 to 3600 rpm. In general, lower engine speed means quieter operation and longer engine life, while units with higher engine speeds tend to be physically smaller and less expensive. Gensets in the 1800-rpm range typically offer the best balance of weight, efficiency, engine life and noise level, making them an attractive choice for small to midsize vessels.

            Air-cooled gensets require large amounts of fresh air, often more than can be realistically supplied in the close confines of a tight engine room, and they’re also noisy. Liquid or water-cooled units are quieter and for the most part are considered a better choice for marine applications. Liquid-cooled technologies range from closed systems (using a heat exchanger or keel cooler) to open (raw water) systems.

            Sail or powerboats can accommodate gensets using heat exchangers. Raw water from outside the vessel is pumped through a heat exchanger located on the unit itself, where it cools the coolant that in turn cools the generator. Keel-cooler units, on the other hand, are typically found on powerboats. They use a grid of tubing mounted outside the vessel’s hull through which coolant is pumped, cooled by the outside water, and returned to the generator’s cooling system. The genset’s cooling grid should be separate from the vessel’s engine for maximum cooling efficiency. Raw water-cooled units simply pump water from outside the hull through the generator for cooling. If you’re boating in fresh water, raw-water units fare okay, but if you’re in brackish or salt water (as in the Bay) you may experience the same problems as raw saltwater-cooled engines—increased corrosion and decreased service life.

            Once you’ve determined what capacity genset you need, next figure out how much room you have for the installation. Gensets should be as small and light as possible. If you can, obtain dimensional drawings to help plan the installation and make sure it’s possible given your boat’s space restrictions and layout. And don’t forget that you need to include not only the genset’s weight and size, but also all the required accessories, such as exhaust systems and sound shielding enclosures—some of which can double the overall size and add significantly to its weight.

            While you’re figuring out the space issues, don’t forget about maintenance. Routine maintenance is critical to a genset’s longevity, so you need to put the unit someplace you can get to it without major acrobatics. Many generators now have all service points located on one side, which saves space by letting you install the generator next to a bulkhead without losing accessibility—a feature that may also open options (particularly in the close confines of many sailboats) that would be unable to accommodate traditional gensets.

You’ll also want a separate starting battery. You’d feel pretty silly if your engine batteries were dead but you couldn’t charge them because you didn’t have enough juice to start the genset.

            Finally, choose a genset that uses the same fuel as your engine, so you don’t have to carry two types of fuel on your boat. This will also protect your investment to some extent—nothing depresses the resale value of a diesel-powered vessel like a gasoline generator.

As with all alternative energy systems onboard, gensets require some compromises. They’re not cheap and they aren’t necessarily easy, either. But when properly selected and installed, they can bring a whole new dimension to your life afloat.