How to Use Voltage Generated by Your Outboard to Power Your Boat’s Systems
Take Charge
Multiple outboards produce plenty of 12-volt amps. Use them to power your ship’s systems and you could potentially lose the genset.
I love outboard motors. You can hang a bunch of them off the transom for a hellacious herd of horses without taking up space on board. They require no separate cooling system, no through-hulls, no exhaust elbows to worry about, no mufflers—all you
really need is a fuel line and filter. And, unlike outboards of a generation ago, today’s motors have robust recharging systems to meet the 12-volt needs of modern boats. Or almost, anyway: As outboard boats grow in complexity, they need more juice, often more than a single or even twin engines can produce to keep up with all the appliances. But install three, four or even five outboards, and you’ve got enough electrons to replace the genset and rely on battery-backed 12-volt power.
Extra amps for battery charging hasn’t always been a characteristic of outboard motors. Back when outboard boats were pretty simple affairs, there wasn’t much need to generate amperage beyond what the motor needed to fire off the spark plugs. There were no amp-gulping electronics suites and appliances like we have today—usually just running lights, a bilge pump or two and maybe a radio. As long as there was enough extra juice to recharge the starting battery, life was good. Heck, if the motor had a pull-cord, you didn’t even need a battery—power for the plugs came from a magneto under the flywheel. Years ago, I had a 13-foot Boston Whaler with a 40-hp Evinrude. The engine started with a cord, and the only battery on board was a dry cell I carried to power the running lights when necessary, which was almost never. It rusted out every season.
The heart of the Fathom-e Power System is a bank of four batteries.
A magneto generates electricity by the interaction of a magnetic field moving in relation to a wire; you probably remember that from high school physics. Basically, it consists of a wire-wrapper stator surrounded by an array of magnets that rotate with the flywheel. The rotation causes the relative motion that generates the power. A magneto takes power away from the propeller, so manufacturers rarely used more magneto than necessary to run the engine and provide a minimum of recharging current, especially in smaller motors. As horsepower increased and electronic ignition systems arrived, outboard recharging systems grew in capacity. Some manufacturers still use a modern version of the magneto (now they call it a stator) while others are switching to alternators in their larger motors.
No doubt, our electrically erudite readers will remind me that a magneto is itself an alternator. That is true. The rotation of the magnetic field changes the polarity of the generated current with each rotation, thereby creating alternating current. Excess current that’s going to recharge the battery runs through a rectifier/regulator that both converts it to DC and regulates the voltage. Otherwise, at faster engine rpms, the voltage would be too high.
This leads into another downside for stator recharging: The output is directly related to engine speed, and at low revs a stator often won’t generate enough power to offset the battery drain on a modern outboard boat with the bells and whistles today’s boaters demand. Belt-driven alternators spin faster than the engine and produce rated output at lower engine rpm than a stator, but not at idle. So, folks who spend a lot of time at low speeds—fishermen, I’m looking at you—often find their single or twin engines can’t keep up with the demands of radar, multiple nav screens, bait wells, 12-volt refrigeration, ear-busting stereo systems and so forth. Add air conditioning for the console and a gyrostabilizer, whether 12-volt or 120-volt powered through an inverter, and things soon turn upside-down, amperage-wise. That’s why many builders of outboard boats these days equip them with gensets. And now you’ve got the associated maintenance issues of an inboard engine and, sometimes, another fuel system, too: In the interests of safety and longevity, builders often install diesel gensets with dedicated fuel tanks.
Bolt On Another Engine
Rather than deal with a generator, some boatbuilders and boat buyers are using modern technology to keep the batteries charged without an annoying genset. Even if an outboard produces only 30 or 35 recharging amps, with three or four of them on the transom, combined with high-capacity batteries, these modern systems are producing all the juice that’s needed. And the best of the lot feature lithium batteries that can be deeply discharged thousands of times and recharged at high amp rates. They’re smaller, lighter and longer-lived than lead-acid batteries. They’re also many times more expensive, but that cost is offset by the added lifespan. Add a couple of macho charger/inverters for dockside use, and most folks won’t miss the genset.
When did this new technology get its start? At CES 2020, the engineers at Power Products LLC, now Advanced Systems Group (ASG), and Sea Ray introduced the system described above. They called it the Fathom-e Power System and installed it aboard a Sea Ray SLX-R400e powered with triple Mercury outboards. Sea Ray, Mercury and ASG are all Brunswick companies; according to a Brunswick spokesperson, “ASG is the world’s leading supplier of products and integrated systems to the marine industry.” ASG’s brands include Mastervolt, Blue Sea Systems, CZone, Marinco, ProMariner and others—in short, it’s a one-stop shop for marine electrical systems.
The heart of the Fathom-e Power System is a single bank of four 400-Ah (Ampere hours) Mastervolt batteries with integral BMS (battery management systems; lithium batteries need more attention than lead-acids) and battery monitoring. At 1600 Ah total and 80 percent depth of discharge, there are 1280 amps on tap from the fully charged batteries, which should be good for 3,500 full recharge cycles. (Discharging lead-acid batteries, even deep-cycle batteries, by more than 50 percent will shorten their life, which isn’t as long as lithiums to start with.) Dockside, two 3000-watt CombiMaster inverter/chargers charge the batteries from depleted to topped-off in about 4.5 hours.
Underway, the Sea Ray’s triple Mercury Racing 450R outboards, each with a 115-amp alternator, feed the lithiums. But not all 115 amps go to the batteries—like all modern outboards, the 450Rs are power-thirsty, too, with lots of computers, controllers and components that tap power from the alternator’s output. But even if only two-thirds of the output goes to recharging, that’s still 230 maximum recharging amps. That’s enough juice to cover the drain on the 12-volt system and have plenty left over for battery charging.
Does It Work?
Sounds good in theory, but does the Fathom-e Power System produce as predicted? Now, almost two years after its introduction, the system’s installed on many boats. In 2021, Yellowfin Yachts of Sarasota, Florida, launched a flybridge version of their 54 Offshore center console powered by four Mercury Verado 600-hp V-12 outboards, with a Fathom-e system engineered by Joe Himmelspach and his crew at ASG. The boat is owned Jim Hill, an experienced boater who cruises from his home port in Puerto Rico.
“The quad Verados put out 4.8 kW of charging current from their alternators, and they run virtually silent,” said Hill. “The huge batteries recharge very fast when running the motors.” The batteries will run the cabin A/C (the Yellowfin 54 has a full-headroom cabin with a queen berth, galley, lounge seating and a head below the console), all chiller plates, three refrigerators and lights from afternoon until morning when anchoring overnight. “We can island-hop and recharge between stops. I can also save on slip fees since I don’t have to pay for shore power if I’m spending only one night in the marina. I no longer need a diesel generator with its noise, vibrations, pain-in-the-butt refueling diesel and gas, and so forth.” Finally, he said, there’s no genset running constantly to add to the boat’s carbon footprint. “I realize it’s hard to say I’m an eco conservationist with 2,400 hp on the back of a 54-foot Yellowfin … but this new Yellowfin really runs super clean.”
A Change in Attitude
Not every lithium-based power system is a Fathom-e; some folks engineer their own. Capt. Vinnie LaSorsa is one of them: He’s the full-time captain aboard singer/songwriter Jimmy Buffett’s customized Freeman 42 cat, Last Mango. Buffett likes to live-bait fish, drifting and at anchor, and doesn’t like noise while he’s doing it. LaSorsa couldn’t find a place to locate the genset that wouldn’t be offensive to the boss, so he took the 12-volt route.
After hours of study and much consultation with experts, LaSorsa built a system that uses two banks of three 140-Ah lithium batteries, for 840 total. They’re recharged underway by Last Mango’s quad Yamaha 300 V-6 outboards; at idle, each Yamaha produces 14 to 16 recharging amps, and 34 to 36 amps at cruising speed, said LaSorsa. Two Victron Quattro inverter/chargers provide a fast charge at the dock. (Last Mango is primarily used for day fishing, so is plugged-in most nights.) After 1,100 hours on the engines and 2,500 hours on the boat in total, the system has worked great. “I have put these batteries through hell,” LaSorsa said. “We have had literally no issues at all.”
LaSorsa determined his power needs very simply: He added up the amps required for each appliance and rounded up. He also increased the draw by 10 percent for every load running through the inverter to compensate for inefficiency. The biggest single load is a 16,000 Btu A/C. Even in the Bahamas, with the A/C compressor running all the time, one bank of batteries will last for three or four hours without the engines running; up north, it’s a lot longer. When the voltage drops to 11.6, LaSorsa switches over to the other bank. Power management is crucial, he said. “You have to pay attention.”
LaSorsa said that when he was engineering the system, lots of people said it wouldn’t work. “‘You’ll need a generator,’ they told me. But we haven’t had dead batteries yet. I’d do it again in a second.”
