Keeping Your Marine Generator Running Smoothly

Marine generator maintenance has gotten complicated with all the brands and models flying around. As someone who’s nursed gensets back to life more than once, I learned everything there is to know about keeping yours running. Today, I will share it all with you.

Quick Answer: Your marine generator needs regular oil changes, coolant system checks, and load testing to stay reliable. I’d say the single biggest thing you can do is run it under load for at least 30 minutes each month — that alone prevents wet stacking. Beyond that, annual service means swapping the impeller, replacing fuel filters, and checking belts.

Understanding Marine Generators

Generator Sizing

Getting generator sizing right is one of those things that sounds straightforward until you’re anchored out and your AC keeps tripping the breaker. I’ve seen it happen plenty of times. You’ve gotta match your electrical loads to the generator’s output capacity. Go too small, and your air conditioning struggles. Go too big, and you’re wasting fuel while the unit suffers from light-load issues.

Here’s the thing most folks don’t realize — air conditioning is almost always the biggest power hog on board. A 16,000 BTU marine AC unit can pull 15-20 amps just on startup, then settles down to maybe 10-12 amps running. Got multiple AC units? You’re probably looking at something in the 8-12 kW range for a mid-sized cruiser. Do the math before you buy, trust me on that.

Diesel vs Gasoline

If your boat runs diesel propulsion, a diesel generator is the obvious pick — it simplifies your whole fuel setup. No second fuel type, no extra storage headaches. Gasoline generators work fine on smaller boats with gas engines, but you’ve really gotta stay on top of ventilation and safety because of fuel volatility. That’s not something to take lightly.

Diesel generators tend to outlast their gasoline counterparts by a wide margin. We’re talking 10,000+ hours with solid maintenance. Yeah, the upfront cost stings a bit more, but the fuel efficiency and longevity make up for it over time. That’s what makes diesel gensets endearing to us boaters — they just keep running when you treat them right.

Regular Maintenance Requirements

Oil and Filter Changes

Probably should have led with this section, honestly. Oil changes are the foundation of everything else. You want to change your generator oil every 100-150 hours, or annually — whichever hits first. Use marine-grade oil that matches what your manufacturer specifies. For most diesel generators, that’s 15W-40. And don’t skip the oil filter; swap it every time you change oil.

Before every single use, check your oil level. I know it seems tedious, but low oil can destroy an engine fast. Sure, most modern generators have a low-oil shutdown, but do you really want to bet your weekend on a safety system kicking in? I don’t. Prevention beats protection every time.

Raw Water Cooling System

Marine generators typically rely on raw water cooling — seawater gets pulled through a strainer, runs through an impeller pump, passes through a heat exchanger, and then gets discharged overboard. It’s a proven system, but it absolutely requires your attention if you don’t want overheating surprises when you’re miles from the marina.

Get in the habit of eyeballing the raw water strainer before each use. Debris builds up and chokes flow faster than you’d think. The impeller needs inspection every 200 hours or once a year, and if it’s showing any wear or damage, replace it right away. I always keep a spare impeller on board — and honestly, so should you. It’s cheap insurance against being stuck without power.

Fuel System Care

Fuel problems are behind most generator headaches I’ve dealt with over the years. Replace your fuel filters every 200-300 hours and you’ll dodge the majority of running issues. If your boat sits for long stretches, a fuel polishing system is worth every penny.

In warm climates especially, treat your diesel with biocide to keep algae from taking over your fuel tank. That diesel bug is no joke. And drain water from your fuel separators on a regular basis — water in diesel wrecks injectors and leads to lousy combustion. It’s one of those small tasks that saves big money down the road.

Operating Best Practices

Load Management

Here’s a number to remember: keep your generator running at 40-70% of its rated capacity. That sweet spot gives you the best combination of efficiency and lifespan. Too little load and you get wet stacking on diesel units (more on that in a minute). Too much load and you’re wearing things out faster while burning through fuel.

When you fire up the generator, don’t just flip everything on at once. Let the unit warm up for a bit, then start adding loads — air conditioning first since it’s the heaviest draw, then work your way through. Dumping a huge electrical demand on a cold generator stresses the system and can trip breakers. Nobody wants to be resetting breakers in the dark below decks, right?

Exercise Running

Even when your boat’s sitting at the dock and you haven’t gone out in weeks, you still need to run that generator under load for at least 30 minutes every month. I can’t stress this enough. Exercise running keeps seals from drying out, lubricates internal components, and stops fuel from going stagnant in the system.

But here’s the catch — running the generator with barely any load is actually worse than not running it at all. You need to put real electrical demand on it during these exercise sessions. Turn on the AC, the water heater, whatever you’ve got. Give it something to work against.

Wet Stacking Prevention

So what’s wet stacking? When a diesel generator runs at light loads, exhaust temperatures stay too low to burn fuel completely. That unburned fuel builds up in the exhaust system, and you’ll notice it as black, sooty smoke. Over time it causes real damage if you don’t address it. The fix is simple: maintain adequate loads during normal operation, and periodically run the generator hard to burn out any accumulation. Think of it like clearing your throat — sometimes the engine just needs a good workout.

Troubleshooting Common Problems

Starting Issues

Won’t start? Nine times out of ten it’s a fuel delivery issue. Start by checking your fuel filters, then bleed any air from the fuel lines, and make sure fuel is actually flowing from the tank. Could also be the battery — check voltage and make sure your terminals are clean and tight. I’ve seen corroded connections fool people into thinking they had a major problem when it was just a loose cable.

Overheating

If your generator’s running hot, look at the raw water cooling system first. Check that strainer, verify the seawater intake isn’t blocked, and pull the impeller for inspection. And don’t laugh, but closed raw water seacocks are a surprisingly common culprit after maintenance. Someone closes it to do work, finishes up, and forgets to reopen it. It happens to the best of us.

Output Problems

Seeing voltage swings, frequency wobble, or weak output? That’s usually pointing at the generator itself or the voltage regulator, and honestly, this is where you want a professional to step in. Modern generators have built-in diagnostic systems, so check your owner’s manual for error codes before calling the tech — it’ll save you both time and money if you can describe exactly what the unit is reporting.

Safety Considerations

Carbon Monoxide

This one’s deadly serious, no pun intended. Generator exhaust contains carbon monoxide, and it can kill you. Never, ever run a generator while accommodation spaces are open to the exhaust discharge area. Get carbon monoxide detectors installed in every living space on the boat and test them regularly.

Pay attention to where your exhaust discharges, too. Keep it pointed away from the cockpit and swim platform. There have been fatal poisoning incidents from people swimming near operating exhaust outlets. When the generator’s running, nobody goes in the water. Period.

Fuel Handling

Shut the generator down before you fuel up — no exceptions. Let a hot engine cool before you start adding fuel. If you spill any, clean it up immediately, and make sure the engine space has good ventilation before you hit that start button again. These aren’t suggestions; they’re rules that keep your boat from becoming a headline.

Annual Service Checklist

Once a year, you want to go through the full service routine: oil and filter change, impeller replacement, fuel filter swap, belt inspection, raw water hose check, exhaust system inspection, tightening all electrical connections, and a load bank test to confirm you’re getting full power output. It sounds like a lot, but it goes pretty quickly once you’ve done it a few times.

I’d recommend having a qualified marine tech handle the annual service, at least for the first couple of years until you’re confident doing it yourself. They’ll catch developing problems you might miss, and you’ll have proper records for warranty and resale purposes. Good documentation has saved more than a few boat deals from falling apart at the survey.