I have been advised to install a six-horsepower Westerbeke diesel to run a 150- amp, 12-volt alternator since I don’t have space for a generator on board. What is your opinion about these units?
Tom Wood responds:
I may be missing part of the goal here. If you really want a generator, that means that what you’re trying to produce is 120-volt alternating current (or 220 if you’re in Europe) to drive household-type appliances. But by installing a little diesel to drive a 12-volt alternator, you’ll not be attaining that goal unless you also put in an inverter.
If you are installing an inverter to "invert" the 12-volt power produced into the desired 120 volts, you’ve come to a highly opinionated and backward source for advice. I have nothing against Westerbeke engines, but I hate with a passion little diesels driving an alternator, regardless of the brand. They are expensive, take up a lot of space, need another fuel line and fuel filter, a saltwater intake, an exhaust, and they add a lot of weight to the boat. They usually get mounted in some out of the way corner where they are hard to maintain and don’t get enough airflow, and as a result, they don’t run right half the time.
There are better ways to achieve your ultimate goal. First, why not just mount the 150-amp alternator right on the main engine. While this is initially the lowest-cost option, I understand that the main auxiliary will require more maintenance and die an earlier death from the many hours of unloaded running. But I’m not sure that it would cost any more than having the separate little diesel in the long run. And if power is an issue, you can opt to mount two 200-amp alternators on the main auxiliary.
A second way to produce 12-volt power to feed the inverter is to beef up the battery banks and use solar or wind-generated power to replenish those banks when sun and wind are available. These environmentally clean power sources are reliable, cheap, and require almost no maintenance.
If your boat's transmission will allow a freewheeling propeller, you can add a high-output alternator belted off the main propulsion shaft and gain your additional power while under sail. Even without a turning prop, a towed or water-driven 12-volt generator can supply a lot of power at a very modest price.
A fourth option is an AC generator belt-driven by the main propulsion diesel. There are several companies that build these in sizes up to nearly 10-megawatts. You would have the same problem with running the big diesel at light loads merely to get AC power, but the space and weight requirements are far less than a dedicated little diesel. And with this approach you’d get both 12-volt DC and 120-volt AC power whenever the engine was running.
The last option is the most drastic. Have a garage sale and get rid of all the 120-volt appliances and go out to dinner with the proceeds. Put all the money you saved by not installing a generator, alternator, filters, inverters, and exhaust systems into a good investment and use the income to go sailing to a quiet, remote anchorage where there are no engines and generators running. Good luck.
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