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Replacing Your Generator

Power Planning

Replacing the generator on a Grand Banks 42 promises to return peace of mind (and quiet) to her operator.


You’re sitting in your cockpit watching the final rays of a spectacular sunset disappear over the horizon. As the warmth of the day dissipates, you retreat into your saloon and flip the switch to fire up the generator. It begins cranking but fails to start. It’s said that you never appreciate a good thing until it’s gone; this is especially true when your generator fails and your saloon starts getting chilly while the temperature of the food in your refrigerator begins to climb. 

Four Signs that You Might Need a New Generator

  1. If your generator is burning a lot of oil like an old car or if you find yourself needing to replace major components, you might want to consider the value of a new model. 
  2. A generator’s lifespan is measured in hours of operation but it’s the quality—not quantity—of those hours that really matters. A trawler generator with 15,000 hours of running under constant, high load will usually outlast a generator with 800 hours of under-loaded operation at a marina. If your generator falls into the latter category, you might want to consider a new model before heading off for an extended cruise. 
  3. If you’re adding new electronics, a new, more powerful air-conditioning unit, or a gyrostabilizer to your list of ancillaries, such items require a sustained supply of power that your old generator may not be able to provide. 
  4. What’s that?!? If your circle of cruising friends has dwindled because the loud rumbling of your generator disrupts an otherwise quiet anchorage, a new and quieter model might be in your future.

With upcoming plans to cruise our Grand Banks 42 project boat, Arawak (, from St. Thomas up the East Coast, the editors began questioning the reliability of her old 8-kilowatt Westerbeke generator, which like the rest of the vessel, had fallen on tough times. So we reached out to our partners at Northern Lights who are working with us to find a replacement, a task that typically requires plenty of careful planning and preparation. 

The first question that arose on our journey towards electrical emancipation was: What size model do we need? According to Colin Puckett, vice president of sales and marketing at Northern Lights, that is the first question boaters should ask in the genset-replacement process, and the one they most often get wrong. 

“A lot of people make a big mistake when going into a project like this by oversizing their generators,” says Puckett. “They take the electrical demands of all their equipment onboard and then round up to the next size. They’re operating under the assumption that all your switches will be flipped on at all times, which isn’t a realistic way to operate a boat. You’re never going to be running with your AC, oven, and washing machine on.”

The problem with a regularly under-loaded generator is that a fixed-speed diesel engine, like a Northern Lights generator, is deprived of a full-combustion cycle (meaning the fuel being dumped into the cylinders is not fully burned) causing excess soot, carbon buildup, wet stacking, and smoke, which severely reduces genset lifespan and may eventually cause a total shutdown. 

So, what is the best way for you to determine the best generator for your boat? Well, according to Northern Lights, you don’t. You need a trained technician to sit down with you and evaluate your load requirements. This is especially important on a boat like Arawak that is having many of its electronics upgraded and systems replaced. But there’s the rub. According to Puckett, a common misconception boaters have is that because they’re adding electronics and upgrading systems they’re going to need a larger-model generator. 

Three Tips on Buying a Used Boat With an Old Generator

  1. Ask the seller if he has spare parts aboard; if he doesn’t that is a warning sign that the generator may not have been well maintained. 
  2. Inspect rubber hoses, gaskets, and belts. If any are cracked or split this is another warning sign the system wasn’t properly cared for.
  3. Ask for a log. “Asking for maintenance records is like asking for a CARFAX,” says Puckett. “If the system was maintained and winterized properly there should be a record of that.”

He goes on to explain that today’s helm electronics are more energy efficient than their predecessors and that an upgraded vessel may mean lower power requirements, a smaller generator, and added space belowdecks. 

The biggest factor in determining the ideal generator is the size of the air-conditioning unit it must supply. “If you were to walk into my booth at a trade show and say ‘I have a 42-foot Grand Banks, what size generator do I need?’ I’d respond with, ‘Tell me about your air conditioning.’ A Caribbean boat will have a different unit [with a larger compressor] than one in the Pacific Northwest.”

After power consumption is calculated, the next factor that goes into choosing the best model is the footprint of your previous generator. Replacing a physically larger model with a smaller, more efficient one could cut down on installation time. But to determine exactly how long it will take for a replacement, technicians need to physically crawl around your engine room, examine where your water and exhaust hookups are and sometimes get creative with the types of mounts employed. That last factor brings us to a major benefit of the modern generator: rubber mounts that reduce the noise and vibration of your system. (Perhaps your friends might raft up with you again after all!)

If you’re handy and have worked on engines in the past, you may be thinking, “I could probably replace the generator myself and save a few bucks.” You’d better think again, explains Puckett. “You’re generating electricity, which is feeding the lifeblood of your boat, like your communications systems, your air conditioning, nav systems, and these are sensitive electronics that people have grown to become dependent on,” he says. “Don’t get too cavalier and try to tackle a project like this alone. We’re talking electricity so a unique skill set is needed.” 

Northern Lights,  800-762-0165;
Cummins Onan, 812-377-5000;
Kohler, 800-544-2444;
MASE, 754-703-5153;

Another benefit of having a company like Northern Lights—with more than 300 dealers worldwide—tackle our installation is the warranty they offer. On new models, they cover everything for the first year, parts for year two, and then major components for the following three years. 

Replacing a generator is a serious decision. Bring in the professionals to properly size your new genset, keep up with its routine maintenance, and enjoy knowing that you’re going to be cruising in comfort for a long time.

This article originally appeared in the April 2015 issue of Power & Motoryacht magazine.