Mercury’s 48-volt alternator and Fathom e-Power, state-of-the-art boat power?

Ben Stein

Ben Stein

Publisher of, passionate marine electronics enthusiast, 100-ton USCG master.

8 Responses

  1. Colin A says:

    It actually burns a bit more fuel than I thought it would in gen mode. But still given the cost of adding a generator and maintaining it seems like this is a better deal.

    • Ben Stein Ben Stein says:

      I had the same thought. I was guessing it was going to burn less, but then when I looked at the overall numbers the difference in burn seemed less consequential.

  2. Sven says:

    It’s pretty cool. With outboard boats it’s been a weird decision of what to do: gas outboards and gas generator (gas generators aren’t as efficient as the Onan in your example). gas outboards and a diesel generator (and maybe diesel heater) with separate gas+diesel, or diesel outboards (unproven) and diesel generator.

    This should free up a not-insignificant amount of space, also means no generator thru-hull and discharge hull, and if these are as quiet as Verado V8s, then they are quieter than any generator we’ve run. The space and weight can make up for the tankage, and we would need to buy a lot of fuel to make up for a 10kW generator (twins).

    The last puzzle piece for the northwest is heat – particularly going up to Alaska – ideally some sort of DC heat pump that doesn’t involve tons of thru-hulls on a 40′ boat – but I have a feeling it really just means a diesel tank + diesel hydronic heater with 40 degree water temps not favoring the heat pumps.

  3. Bruce Schwab says:

    Too bad they couldn’t fit 2 (or even 3x) of the 48V alternators on those huge engines. The fuel/kWh ratio would be vastly improved in generator mode, for the same reason that AC generators are more efficient when more highly loaded.

    They’ve tried to improve the efficiency with lean burn, etc. However even 5kW of output is miniscule relative to the size of the engine. Needs more load to improve the efficiency…

  4. Ryan Daniel says:

    Looks like Mercury has cracked the “code”. Very impressive engineering integration from engine alternator through to charge management controller, engine controller etc. This is modern boating with big LFP house battery banks. Great job mercury engineers on commercializing this advance in tech from the Stone Age 12v systems still marketed by all the other major brands.

    Clearly well thought out and executed…leaves all the other outboard (and onboard) engine vendors in the 20th century!

  5. Grant Jenkins says:

    Hey Ben, thanks for the detailed update on Fathom. Clearly a lot of thought and engineering went into this. I still think it only makes sense for for a limited size/style of boat, and a limited mission, which you pointed out in the article.
    However, you also stated this:
    ” With a generator, the engine is running any time you need 120-volt (or 240-volt) power available. The e-Power system, by drawing power from a large battery bank, only runs the engine(s) when the batteries are depleted and require charging. Only running the engine(s) when the batteries need charging also means the batteries will accept large charge currents.”
    Most cruisers in the PNW with generators operate in exactly this same way – they have a large house bank (often LiFePO), a large inverter, and can anchor in silence for many hours before needing to run the genset for recharging . I suppose the big variable is heating/cooling – if you need to run AC-powered heat or A/C full time to stay comfortable, then that becomes limiting. But otherwise the fast-idling Fathom outboard ends up being used pretty much exactly like a generator. The biggest advantage I see is eliminating the space/weight/maintenance of the generator, which is not insignificant, and the other efficiencies of 48V vs. 12V. And yes, the automated digital switching and monitoring is impressive.
    Anyhow, until it becomes available in a retrofit or re-power scenario, I doubt if we’ll be seeing much of it soon . I just got back from the Seattle Boat show and the least expensive Brunswick day cruiser starts at about a half million bucks. That’s a big investment to experience 48V charging!

    • Ben Stein Ben Stein says:


      I think we’re very much on the same page except that our respective climates drive a significant difference. Being in southern Florida, air conditioning is a major factor. I also think Brunswick’s boats with the Fathom system onboard are targeting a warm weather climate where A/C dependency is the norm.

      As for the cost of the system, to my eye that’s not the only trouble, the current crop of boats with the system installed aren’t a style of boats I find compelling. In fact, as I was writing this I found myself thinking about the potential for something like a Ranger Tug fit with the system. Pair that Ranger Tug with Bruce’s suggestion of multiple alternators on a single motor and you could be talking about cruising energy management perfection!

      -Ben S.

  6. Colin A says:

    I haven’t seen it mentioned but the basic concept is similar to the Integral system that was talked about a lot 3-4 years ago. Using the main engine as a power source when it had extra power available thru a 48V alternator. I seem to recall early installations of those had issues, but it would seem there will likely be more of these systems in the future.
    Last year I was playing with building a small cruising tug for myself, and I started to draw up a shaft driven alternator with a belt system to the prop shaft allowing forward and reverse without a transmission (similar to how Rob white coupled a Kubota to the prop shaft on his Resuce Minor). At some point I wondered if it might make more sense on a small displacement cruising boat to just make a 48V generator and use an electric drive. This wouldn’t be about propulsion efficiency but instead about using unused HP for house loads. It also allows for layout flexibility.

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