Running a Boat Generator at the Right Load: Switching Sequence, Balancing and Avoiding Wet Stacking

Walk through the boat and list what uses the most AC power. Typical big loads: air conditioning units, the water heater, the electric stove or cooktop, the battery charger when batteries are low, a watermaker, a washer/dryer. Typical small loads: lights, TV, phone chargers, a coffee maker for a few minutes, the fridge once it is cold. Air conditioning compressors also draw a short surge each time they start, much higher than their running current.

Before you change from shore power to generator (or back), switch off the large AC branch breakers: air conditioning, water heater, battery charger, stove. Power & Motoryacht's experts give the same advice: don't start the generator under load, and bring things back on one at a time once it is running.

If you are plugged in at the dock and want to run the generator, turn off the shore power main breaker on the boat first (and the pedestal breaker if you are unplugging). Northern Lights warns that a worn shore/generator selector switch can let the generator connect to shore power, with expensive and dangerous results. Never try to "help" a switch that feels loose or sticks: call an electrician.

Start the generator from its panel with the generator's main AC output breaker off. Confirm cooling water is coming out of the exhaust. Let it warm up for about a minute (Northern Lights) before adding load. Turn on the exhaust blower if your boat has one and keep nearby hatches and ports closed so exhaust doesn't drift inside.

Turn the source selector to GEN (or close the generator breaker on interlocked panels). Watch the AC voltmeter: in the US it should read close to 120 volts per leg, and if your panel shows frequency, close to 60 Hz. If the voltage is far off or jumps around, switch the selector off, shut down and call a technician.

Turn on the largest loads one by one, giving each air conditioning unit time to start and settle (about 30 seconds to a minute) before the next. This spreads the starting surges so the generator isn't hit with them all at once. Watch the voltmeter dip and recover with each one. A voltage that sags a lot and stays low, black smoke at the exhaust or the generator laboring means you've asked for too much. A useful habit from a BoatUS surveyor: write down the voltage with no load and again once a big load such as an air conditioner has settled. The rpm may dip for a moment, but voltage and frequency should come right back; a gap of more than about 5% between the two readings is worth having checked. Frequency matters as much as voltage, since power that is off-frequency can damage sensitive electronics. If your panel has no hertz meter, a technician can check it with a meter, or you can add a panel-mounted one.

On boats with two legs, check the amp meter for each leg if your panel has one. Try to keep the load roughly even, for example one air conditioner on each side, and the water heater on the leg with less on it. Your panel labels show which breakers sit on which leg. An unbalanced generator can trip one leg's breaker even though the total load is well within its rating. If your labels are missing or confusing, ask a marine electrician to label them.

Look at the total amps and compare with the 40% and 75% figures you worked out. If the generator is barely loaded for hours (say, just lights and a TV overnight), you are building carbon. Options:

• Run small loads on the inverter and batteries instead, then run the generator for a few hours with real work to do: charging batteries, heating water, cooling the boat, running the watermaker.
• Batch the big jobs into the generator run: hot water, battery charging and cooling at the same time.
• Shut it down when the big loads are done instead of letting it idle along.

If your boat has an inverter/charger, it's the right tool for small, quiet loads like overnight lights, electronics and a cold fridge. When the generator runs, the charger refills the batteries, which also adds useful load. Never connect the inverter output to the same circuit as shore or generator power unless it's a factory-designed inverter/charger with an automatic transfer switch (see our inverter safety guide).

Turn off the big AC loads, then switch the generator's output breaker or selector off. Let the generator run with no load for about a minute, or the 5 to 10 minute cool-down your manual gives (Westerbeke stresses this for turbocharged units). Then stop it. Northern Lights says this habit protects the voltage regulator and diodes. Switch back to shore power only after the generator is off the line.

A generator that sits suffers. Westerbeke's monthly routine for idle generators: do the daily checks, warm up 5 to 10 minutes, run about an hour at 70 to 85% of rated output, then 5 to 10 minutes at little or no load before shutdown. At the dock, the water heater and air conditioning are usually enough load. Check your manual for your brand's version.

Black, oily soot at the exhaust outlet, a wet sooty transom, more smoke at startup, or a generator that seems to lose power are warning signs. Write down hours, load readings and anything unusual in BoatMend after each long run. If you see these signs, a few hours at a solid load may help, but carbon in the exhaust elbow often has to be cleaned out by a technician (see our generator maintenance guide).