The demand for electrical energy on boats is constantly increasing. Not long ago most boats less than 45 to 50 feet in length satisfied their electrical energy needs from a battery bank of modest capacity. Today, those same vessels consume substantial amounts of electrical energy, including significant amounts of 120-volt, 60-Hz AC power.
Even limited-duration use of relatively minor power consumers can require 1,500 to 2,000 watt-hours (1.5 to 2.0 kWh) of AC power per day. A single, one-ton, 12,000-btu, air conditioner can add 11,000-watt-hours (11 kWh) to a vessel’s daily power appetite. While the 2-kWh-per-day demand may be met with use of an inverter, supported by a 340-amp-hour battery bank, powering the 11-kWh-per-day air conditioner load from an inverter would require a 2,50- amp-hour battery bank. AC power loads of this magnitude require an engine-driven source of electrical energy.
Regardless of any other attributes, a successful generator installation must be quiet. Generators operate when the vessel is at anchor, or tied to a dock where shore power is not available. The availability of electrical power will be of little value if genset noise makes its use unacceptable to those on board. The severity of this problem increases with a decrease in boat length. Noise levels that may be tolerated when underway will be unacceptable when at anchor. The genset should make no more noise than the vessel’s air conditioner.
In addition to noise, a number of other installation considerations must be addressed. Gensets require significant amounts of fresh, preferably cool air to support the combustion of fuel. Genset diesel engines, like propulsion engines, are about 34% efficient in converting the heat value of their fuel into mechanical energy (thus, about two-thirds of the energy in the fuel consumed by the gensets engine is converted into heat energy). Although virtually all gensets designed for belowdeck installation use water-cooled engines, they radiate a significant portion of the waste energy into their environment. In addition, in most gensets, the heat created by the passage of electrical current through the windings of the alternators is removed by a flow of air, forced through their windings by fans built into the rotors. This heated air adds to the heat energy delivered to the space surrounding the genset. Since air conditioner operation is one of the most common motives for installing a genset, the addition of heat to the accommodation space below should be avoided. As noted in the table, a few gensets use water-cooled alternators, thereby avoiding the necessity for the discharge of hot alternator cooling air and somewhat reducing the genset’s noise output.
Keeping it cool
As with any other internal-combustion engine, the genset engine’s maximum power output varies inversely with the temperature of its environment. Development of full rated power requires that the genset be supplied with an adequate volume of air at the lowest practical temperature. Fuel temperature should not exceed about 100° F. A constant supply of filtered cooling water, preferably not warmer than about 90° F, is also required. Any combination of excessive temperatures will limit maximum power output. The small gensets most often installed on sailboats are often required to operate at virtually full output power, making them especially sensitive to any condition that limits driving engine performance.
The exhaust fumes and noise from a genset can create problems for both those on board and for nearby boats. The exhaust outlet must be located to minimize the flow of fumes into the cockpit or into the accommodation through an open port or hatch. It is likely that a genset will be powering an air conditioner when the crew of the boat is asleep. Although diesel engines produce relatively little carbon monoxide, the installation of a quality, marine-rated carbon monoxide detector is advisable. The noise created by the mixed flow of water and exhaust gases can be annoying. If desired, it can be minimized by installing an exhaust separator that discharges the fumes above the waterline while disposing of the cooling water through an outlet below the water.
Choosing a genset for a boat, particularly a sailboat less than 50 feet in length, can be particularly challenging. Many of the 1,800-rpm, 5- to 8-kW sets may be too large to fit within the available space, especially when equipped with a noise-containing sound shield. A number of compact gensets, many of which operate at 3,600 rpm, are available for use in restricted spaces. Some of these machines power their two-pole, 3,600-rpm alternators with cog belts, allowing the driving engine to turn at a somewhat slower speed. Concerns about diesel engine operation at a constant 3,600 rpm are largely unfounded. The claims that 3,600-rpm sets are noisier than those that operate at 1,800 rpm are generally untrue. The amount of noise energy produced is directly related to the amount of power being produced. The sound heard in the boat’s accommodation will depend on the total sound energy and its frequency distribution. High-frequency noise can be easier to eliminate than sound energy at lower frequencies.
To produce AC current of the correct frequency, the genset’s alternator must rotate at a particular and constant speed, determined by the number of poles in the alternator. A two-pole alternator must rotate at 3,600 rpm to produce 60-Hz current (3,600 rpm = 30 revolutions per second; with two poles per revolution, 60 Hz current is produced). Four-pole alternators must turn at 1,800 rpm. Alternators for large generators are built with more poles, allowing slower rotation speeds. A six-pole alternator rotates at 600 rpm when producing 60-Hz current. The number of poles in the alternator does not affect the quality of the AC power produced. The voltage produced by the genset’s alternator may be internally controlled within the alternator (self regulated) or through the use of an external control circuit.
Regardless of the type of voltage regulation used, the stability of the AC energy delivered by most gensets, (±3% to 5%) will be satisfactory for the loads being powered. Potentially damaging low voltage can occur when the genset is very heavily loaded, especially when powering inductive loads such as air conditioner compressor motors or the large AC motors that power some reverse-osmosis watermakers. Most modern electronic air conditioner controls incorporate voltage monitors that will automatically shut the system down, preventing damage, when a low-voltage condition exists for more than a few seconds.
Genset choice usually begins with an analysis of the electrical load that must be powered. Figure 1 lists a number of typical 120-volt appliances and their power requirements. When deciding on the power capacity of a genset, consider that it is usually unnecessary and can be quite expensive to provide sufficient power to operate all loads simultaneously. Comfort will not be sacrificed by having to turn off an air conditioner unit for the relatively brief time when the microwave, toaster, or hair dryer is in operation.
The power output of a genset is specified at a particular power factor. Power factor is a means for defining the effect of the electrical characteristics of a powered device on the alternator’s ability to deliver power. Resistive loads, such as toasters or hair dryers, have a power factor of 1.0 and pose the least problem for the genset. Inductive loads, such as AC motors, that must create magnetic fields in order to operate usually have power factors of approximately 0.95-0.97. Low-power-factor loads consume reactive power, which has the effect of reducing the ability of a genset to deliver its full rated power output. Power-factor problems are most likely to exist when large motors, such as those powering high-output reverse-osmosis watermakers, are in use. Powering such inductive loads can be made more tolerable through the addition of power-factor correcting capacitors connected at the load.
The power required to start air conditioning system compressors and some watermakers can be as much as three times the operating power demand. Some air conditioner system controls can automatically sequence the starting of individual units to prevent the simultaneous starting of compressors.
When deciding on the power rating for a genset, it is advisable to include an additional allowance of 5% to 10% to account for the decrease in genset output that will likely occur when the unit is highly loaded on a day when the air and water temperature and humidity are high. Each of these adverse environmental conditions reduce the power output of a diesel engine.
Selection of a genset can begin once the required power output is determined. Assembling a matrix, which includes rated output power, all dimensions (with sound shield), weight, and manufacturer’s stated noise level when operating at full power output, will prove useful. Take careful note of the manufacturer’s noise specification. Many genset noise measurements are stated at a distance of seven meters. Measurements are often made at this distance to eliminate the distorting effects that accompany measurements made at shorter distances such as one meter; however, at a distance of seven meters, you may find yourself on an adjacent boator in the water.
A number of installation requirements must be observed regardless of which genset you choose. All gensets will require both a source of air for combustion and sufficient ventilation to prevent unacceptably high compartment temperatures. The installation should include an AC-powered exhaust blower, operated whenever the genset is running. Provide sufficient room for service access, including provision for removal of parts of the sound shield.
Gensets are heavy and must be very securely fastened to the structure of the hull. Most gensets are fitted with flexible, vibration-absorbing mounts. If space does not permit use of the manufacturer’s sound shield, be sure to allow sufficient room around the unit to prevent unintended contact with adjacent structure. When space is at a premium, a flexible, blanket-type sound shield may be used to quiet the unit.
The genset must be provided with a supply of clean fuel. Tapping into the propulsion engine’s fuel supply is a poor practice. Separate fuel supply and return lines are required. A primary fuel filter/water separator and an electric priming pump should be a part of the installation. Electrical connections to the vessel’s shore power system must be made in conformance with UL procedures, using an approved multi-pole power switch. There must be no possibility of interconnection between the on-board genset and the shore power supply.
Supplying a constant flow of sea water for genset cooling can present problems in many boating areas. Unlike propulsion engines, which are in operation primarily when the vessel is underway, the genset will often be used when anchored or when at a dock where shore power is not available. Strainer fouling from sea grass, weeds, jellyfish, or, occasionally, mud or sand can become a significant problem. The supply of cooling water for the air conditioning system is often a companion problem. Starving the genset of cooling water will trigger an automatic shutdown. Starving the air conditioner cooling pump will shut down the air conditioner system and, if prolonged, can cause cooling pump failure. Installing a self-cleaning seawater strainer, like the GROCO Hydromatic, can eliminate troublesome strainer fouling prob-lems. Functioning as a sea chest, the Hydromatic can simultaneously supply filtered water to the genset, the air conditioner system, and other consumers.
All precautions appropriate for the main engine’s exhaust system must be observed for the genset. A properly located water lock, a ventilated loop in exhaust cooling-water injection line, and a vertical loop in the exhaust hose adjacent to the through-hull exhaust fitting must be present.
When evaluating a particular genset be sure to ascertain which accessories are supplied as a part of the basic system. A remote-control harness and control panel will be required in virtually every installation. Most sets are equipped with automatic shutdown systems triggered by low oil pressure or excessive temperature in the freshwater cooling system. The addition of an exhaust temperature alarm/automatic shutdown may be desirable.
On occasion, an interruption in the flow of cooling water may cause the exhaust hose to become dangerously overheated before the water in the cooling jacket is hot enough to trigger the over-temperature shutdown. Some genset remote-control panels provide indicators identifying the cause of a shutdown. Others are mute, requiring troubleshooting to determine the cause before corrective action can be taken. Engine parameter gauge packages are infrequently installed on gensets smaller than about 15 to 20 kW. A running time meter is a necessity for any genset.
All genset installations should include a dedicated starting battery. A marine deep-cycle battery will suffice for this purpose. Check to ensure that the genset recharges its own battery when it is operating. The battery charging current from the genset may be limited. However, the battery is only required for starting, making the actual withdrawal of energy per cycle quite low.
AC output genset alternatives
For applications where the total continuous AC power demand does not exceed about 1,600 watts, it may be desirable to install a diesel engine-driven, DC output alternator powering a DC/AC inverter. For example, the Balmar PC 550 can deliver up to 160 amps of 12-volt DC power to an inverter. Using this approach, short-duration loads can be powered from the vessel’s battery bank, with the diesel engine-powered, high-output alternator supplying power for longer-duration loads. Some of these DC output systems incorporate automatic engine-speed controls, allowing the driving engine to turn only as fast as needed to produce the amount of energy being withdrawn from the inverter. Balmar also offers a number of combination units that incorporate reverse-osmosis watermaker pumps in the engine/alternator package. HFL offers a DC output genset and a companion air conditioner unit that incorporates an integral DC/AC inverter.
The use of gasoline-powered, air-cooled, deck-mounted gensets on boats is normally undesirable, in part because their noise output is unacceptable. Honda has recently introduced two innovative air-cooled, gasoline-fueled gensets that may prove useful on some boats. The smaller unit, the model EU1000i, provides a continuous 900 watts of 60-Hz, 120-volt power, with 1,000 watts available for a short time. The unit is remarkably small, about the size of a 2.5-gallon fuel container, and weighs only 29 pounds. It also provides eight amps of 12-volt DC power for battery charging. It generates its power from a very compact alternator that produces AC power at a frequency and voltage dependent on engine speed. This AC power is rectified and then converted to 120-volt, 60-Hz power by a built-in solid-state inverter. Since the frequency and voltage of the output power is determined by a microprocessor, the driving engine need turn only fast enough to deliver the amount of raw power being consumed. Engine speed automatically adjusts to meet the power load, saving fuel and reducing noise when less-than-full output is needed. The output of two or more of these gensets may be connected in parallel, automatically sharing the load. Two of these units on the deck of a vessel could power a one-ton air conditioner. The larger model EU3000is produces 2,800 watts continuously, with three kW available for intermittent loads. It weighs 135 pounds. These new units may find significant application due to their very low noise level, only 49 to 57 dbA, depending on load. Any objection to the use of gasoline power may be mitigated by the fact that gasoline is often carried on deck to fuel the vessel’s outboard engine.
Some manufacturers do not publish list prices for their products, relying on dealers, who do most of the installations, to quote an installed price. The price you pay is subject to negotiation and will to a significant degree depend on the work required to install the equipment in the vessel. Unlike main engines, which are installed before the deck goes on, gensets are typically installed after the boat is complete. Routing the fuel, exhaust and ventilation lines can be a difficult challenge. The installation must take into account the weight of the genset, which can have a material effect on smaller boats. The task of installing a genset is well within the capabilities of many boat owners, provided they are willing to devote sufficient time to precisely plan the installation and carefully adhere to all relevant safety considerations. Electrical energy is a wonderful servant; however, it must always be respected, especially when dealing with AC voltages of 120 volts or higher.
The benefits of having substantial AC power on board will become evident soon after the genset is operational. The first hot, humid, buggy, windless night spent on board, with the air conditioner eliminating the discomfort, will make all the expense and effort clearly worthwhile. n
Contributing editor Chuck Husick is a sailor, pilot, and Ocean Navigator staff instructor.