Genset selection and installation, a firsthand account

Jan/Feb 2004

The time had come to replace the Mase 8.0 genset that had served well for more than 12 years. As in the past when the Mase 8.0 was selected, the priorities were to find a unit that would fit in the available space, ensure that it would be at least as quiet as the Mase had been, and select a unit that would produce an adequate amount of electrical energy. The priority order reflects prior experience. Regardless of what else a genset may be able to do, if it is not quiet, it does not belong on the boat. Location of the genset�s components — especially replaceable filters and any zincs used in the cooling system — was also given due attention with regard for the often limited access that would exist once it was installed.

Sailboats present notoriously difficult installation challenges for almost anything, especially a diesel genset. On the Irwin 46, the genset must fit in a compartment below the sole of the saloon that measures 31 inches wide and provides a maximum usable length for the genset of 33 inches. The boat was originally equipped with an Onan MDJE, 7.5-kw machine. The installation provided no room for a sound shield, and as a result, the genset created a noise level that made it impractical to use it when anyone was in the saloon. The noise level in the forward and aft staterooms was barely acceptable. The totally enclosed Mase was much better; in fact the noise level it created in the saloon was actually one decibel less than the sound of the 1-ton A/C unit. Our objective was a new genset that would be at least as quiet as the unit being replaced.

Comparing the specifications for all of the gensets that would fit in the available space rapidly narrowed the possible choices. We were particularly intrigued by the low noise and superior voltage and frequency stability specifications claimed for Onan�s new e� QD diesel genset product line. The 5.0 MDKAU was the largest unit that would fit in the available space. A careful check of the boat�s actual power demand showed that with relatively minor load management — for example, not powering the 1,500-watt electric heating element in the domestic water heater when all three A/C units were operating — would make a 5-kw genset an acceptable choice. The noise-level specification of 71 dBa at a distance of 1 meter was particularly interesting. Most genset noise specifications are given for a measurement distance of 7 meters. Although 71 dBa at 1 meter is the mathematical equivalent of 54 dBa at 7 meters, we were interested by Onan�s decision to publish the close-in noise measurement. Discussions with their engineers disclosed that they had identified the genset engine�s combustion air intake as a major source of noise and had taken specific steps to minimize it. As anyone who has removed the air intake silencer from a boat�s diesel propulsion engine can attest, the sound created by the pulsing flow of air entering the engine creates a first-class din. Onan designed a computer model of the air-intake system and used the model to specify a series of Helmholtz resonators that, combined with the air-intake plumbing, would greatly diminish the noise. They succeeded.

Gensets tend to be quite conventional in the way they control voltage and frequency and in the way operating parameters such as coolant temperature, oil pressure and electrical load are monitored. The Onan e-QD gensets use a microprocessor to control engine speed and the alternator�s excitation current. We found the result to be as advertised, precisely 120-volt 60-Hz power with very rapid response to and recovery from the large momentary loads imposed by A/C system compressors.

Automatic monitoring and protective automatic shutdown is particularly important for gensets. Unlike propulsion engines that are usually attended when in operation, a genset must run unattended for long periods of time. Although monitoring of coolant temperature, oil pressure and alternator temperature is standard on most gensets, few provide a usable history of the sequence of events that caused an automatic shutdown. Onan�s monitoring system resembles a modern automobile in its ability to store a series of fault codes for later use in diagnosing the cause of a problem.

To provide 60-Hz power, a genset�s alternator must rotate at a precise speed — 1,800 rpm for a four-pole alternator or 3,600 rpm for a two-pole alternator. A small diesel engine running at 1,800 rpm produces only a small percentage of its power potential. Running a diesel continually at 3,600 rpm can be less than optimum when it is only lightly loaded. Onan had addressed this problem as they have in some of their RV line of gensets by using a drive belt to couple a two-pole alternator to a 2,900-rpm driving engine. Some may question the use of a belt for this purpose; however, our experience with the multiple drive belt system used in some small helicopters allayed any concerns we might have had. In addition, Onan warrants the belt for the life of the genset.

Out With the Old — In With the New

With fuel supply and return lines, exhaust hose, electrical power and control cables disconnected and the cooling-water supply hoses to the ventilated loop disconnected the 350-lb Mase genset was raised from its well using a four-part tackle secured to the boat�s main boom and moved aft into the cockpit. The lifting tackle was repositioned farther aft on the boom so that the unit could swing outboard and be placed on the dock. The mounting area was cleaned and a resin-coated three-quarter-inch marine plywood mounting board installed. The Onan genset was prepped for the cooling system ventilated loop and placed in position using the lifting tackle.

The genset was securely bolted down and fuel, cooling water and exhaust hoses connected. The electrical power wires were reconnected and the remote-control panel installed beneath the chart table. The next task was to assay the temperature around the genset when it was operating at maximum load in Florida�s hot summer climate. The temperature probe of a digital remote-reading thermometer was placed adjacent to the alternator cooling air exhaust of the genset�s sound shield. A bit of experimentation showed that a four-inch flexible air duct connected to an existing 250-cfm exhaust blower would keep the maximum temperature at the genset below 112 degrees F even on a hot Florida day, warm but not excessively so.

The new genset installation is very successful. The sound level from the genset, installed in its �well� with the saloon floor open, measured about 76 dBa due to reflections from the sides of the mounting space. With the floorboard in place, the noise level in the saloon from the fully loaded genset is a moderate hum at 60 to 61 dBa and is about 3 dBa less than the noise created by the saloon A/C unit. The genset is inaudible in both the forward and aft cabins when their A/C units are in use. The 5-kw power capability of the genset is more than adequate in spite of the fact that two of the three A/C units are 23 years old and consume significantly more energy than more modern rotary compressor units.

The bottom line is that if we had it to do all over again, we would proceed exactly as we have just done it.

By Ocean Navigator