Voyaging gensetsDec 16, 2013
A Lugger 6-kW installed in a Beneteau 411 with access via a deck hatch.
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As voyaging sailboats grow in size and complexity, having a reliable power supply becomes more important. Although much can be done with DC power alone, it is often necessary to have a good AC power supply as well. When plugged into shore power this is not a problem, but as soon as we venture out away from civilization, AC power becomes a bit more complicated.
Limited AC power can be produced from batteries with the use of an inverter to convert DC to AC. This, however, is usually not capable of providing large loads for extended periods. With increased and ever-growing power demands, an adequate AC generator is often needed to supply all of today’s needs.
The most common type of AC generator is one powered by a small diesel engine. These generators can provide clean stable power for all the AC systems on board while reducing main engine run time for battery charging.
A Fischer Panda 4200 Eco with an integral sound enclosure.
With increased power demands, generator manufacturers are finding ways to get more power in smaller and lighter packages. All diesel powered generators consist of two basic components. The engine or power source and the generator unit itself, sometimes called the back end or alternator. Both parts of the system have benefited from recent technological improvements. Before we can fully understand these improvements it helps to understand the basics of generator design and construction.
Most generators have the generator unit attached directly to the back end of the drive engine. This is a simple reliable set up, but may not always be the perfect solution. Because the speed the generator must turn is fixed by the number of windings it has, direct drive has advantages and disadvantages. In order to create the 60 cycles required for U.S. power supplies (50 cycles in most other counties) the engine must maintain a steady rpm. If the generator end is a two pole type, then engine speed will need to be 3,600 rpm, if it is a four pole type the engine speed is set at 1,800 rpm.
At first this would seem fairly straight forward, but there are some distinct advantages and disadvantages to one speed over the other. Lower speed motors will be quieter, use less fuel, and have less vibration. The drawback is that with lower rpm the engine is running below peak output. This results in requiring a larger heavier engine for a given amount of AC power produced. Slower engines will also have a harder time handling surge or startup loads.
Peak power ratings
Faster engines can be smaller and thus lighter as they operate at peak power ratings, but because of the higher speed will produce more noise, heat and vibration. Faster running motors can handle start-up loads better as well due to higher inertia and the fact they are operating at near full horsepower. It is generally thought slower running motors last longer as there is less stress and wear on the components. Others however feel it is best to run diesels at rated speeds and power to prevent cylinder glazing and carbon build up. The jury may still be out on this point, but it is clear a slower running motor will be quieter, produce less vibration, and use less fuel.
Larger vessels are not as affected by weight and tend to have more room to accommodate a heavier, larger generator package. With these vessels it makes sense to take advantage of the slower running units with their larger motors. It is on smaller vessels (less than 60 feet) that the size and weight become more important. Some manufacturers such as Next Generation Power and Phasor use a belt drive to reach a compromise in their smaller units. By utilizing different pulley sizes these units can run at 2,800 rpm. This gives the best power rating while keeping weight, size, and vibration to a minimum. The drawback is you have greater maintenance with belt drives. These units, however, use high quality drive belts designed for high loads which improves belt life and reliability.
Most generator manufacturers use small tractor or industrial motors to couple with their own generator units. This keeps costs down and helps when it comes to getting repair parts in far-off locations. For small units in the 3 to 4-kW range, the Farymann single cylinder diesel is popular as it is compact and reliable. Kubota is also a popular engine for the small units, particularly the belt-drive generators. As units get a bit larger manufacturers will often choose a small Kubota or Yanmar tractor motor. Although there may not be a lot new with the motor side of generators, one improvement is the change from belt drive to direct drive of the raw water pump on many new units. This simplifies maintenance and makes for a more reliable water supply. Additional engine improvements come in meeting the new EPA Tier III emission standards.