Diesel Digest: by Capt. Jeff Werner
The phrase “old school” is heard so often now in the yachting industry that older captains and engineers sometimes feel out of sync with their millennial crew members. Usually this level of discomfort revolves around the subtleties of using apps and smartphones, as well as the nuances of social media.
Older crew tend to fall back into their comfort zone with traditional systems aboard yachts, such as when supplying power to the myriad alternating current (AC) appliances, equipment and entertainment systems that make today’s boating experiences rather luxurious. Just crank up the diesel generator and let it run constantly to supply endless AC to feed the whims of owners and charter guests. However, these conventional diesel generators have also become old school in recent years. Generators using new technology still operate with diesel fuel, but they are decidedly different in their approach to alternating current generation.
This workhorse aboard yachts is a very straightforward system. Couple a diesel engine, operated at a fixed speed, directly to an AC alternator and generate household current. The fixed speed is necessary to produce alternating current at the proper frequency, measured in hertz (Hz), for the electric appliance it is designed to run. In the U.S. that frequency is 60 Hz; in Europe, 50 Hz. A generator motor must turn at a constant 1800 rpm for the alternator to produce 60 Hz AC, and at 1500 rpm to make 50 Hz AC. If the rpm varies, so does the frequency, and the appliance will not operate properly. Traditional diesel generators use a device called a governor to maintain the engine speed at the required rpm regardless of the load. Whether powering a small 1000-watt hair dryer or three power-hungry air conditioners, the generator runs at a fixed speed.
Using advanced alternator design employing compact permanent magnets, an inverter generator initially produces AC in the same manner as a conventional generator. However, rather than feed the AC power directly to the appliance that needs it, it is first converted into direct current (DC). This DC power is then inverted back to AC using sophisticated electronic processing and is then ready for distribution around the vessel. Since the resultant AC is produced by an inverter, the desired frequency of 60 Hz or 50 Hz is formed independent of the generator engine’s operating speed. That means that the generator can run at a variable speed: lower rpm when load level is smaller and higher rpm as the load level increases.
Inverter generators are smaller and lighter in weight; conventional generators have a larger footprint because they are oversized to meet the initial high demand for power caused when electric motors start. And the bigger the generator set, the heavier it is.
Fuel efficiency is another positive aspect of inverter generators. Since they vary in rpm based on load, they burn less fuel than a conventional generator running at steady rpms.
Conventional generators operate at a noise level between 65 and 75 decibels, which is as loud as busy street traffic and close to that of a lawn mower. With quieter engines, special mufflers and sound dampening technology, inverter generators operate in the range of 50 to 60 decibels, which is the background sound level of most offices and normal conversation. That translates to even quieter operation aboard yachts, once soundproof housings are added.
The AC power from inverter generators is also very clean – that is, it produces pure sine waves rivaling that of shore-based utility companies. Clean power is needed today by more and more products: computers, smartphones, TVs, game consoles, printers, galley appliances, power tools, and yacht communication, navigation and monitoring systems.
In the marine industry, the leader in design, development and implementation of inverter generators is WhisperPower, a Dutch company that was spun off from Mastervolt 10 years ago. Interestingly enough, WhisperPower has dubbed this next generation of power systems in its marketing as “new school.”
Capt. Jeff Werner is a 25-year veteran of the yachting industry as a captain and as a certified instructor for the RYA, MCA, USCG and US Sailing. He also owns Diesel Doctor (MyDieselDoctor.com). Comments on this column are welcome below.