Currently we are rethinking our choice of propulsion. Jeff Schionnings original idea was rather simple: one central 30 hp outboard would keep the boat light and the cost down. In the beginning I liked the idea a lot but there are downsides: a lack of maneuverability with the central motor and the rudders far away from the flow, petrol instead of diesel, noise, you need a lifting device for the heavy motor … all in all not perfect. Of course we could have just gone for twin diesels but recently I stumbled upon an interesting alternative.

By accident I found the website of electric propulsion specialist greenstar marine of Sweden. They offer a system that seems ideal for our catamaran. Greenstar20D1Unlike other providers it consist not only of motor and controller but is more or less plug and play with charger, shore power connection, power recuperation and even comes with a matching folding propeller and a specially designed shaft. The main question was how to generate and store the electric energy.  And how much would be needed?

The greenstar 20D system has two 24 V DC motors with continuous power of 3.6 kW each. The recommendation for yachts is to have 1 to 1.5 kW power per ton ship weight. The weight of our Arrow 1200 is calculated with 5000 kg. Of course we want to keep the boat light but if we assume at least another 1000 kg for equipment the 7.2 kW continuous power is definitely on the low side. With this setup motoring into strong wind against current an waves for hours is not possible. Maneuvering in and out of a berth and accessing anchorages should be just fine. So how much energy storage do we need? And what if we want to motor away from calm areas. How can we extend our range?

If we assume that 7.2 kW is full throttle we need a current of 7200 kW / 24 V = 300 Amp. Wow, that is quite a lot. And definitely to much to draw out of a lead/acid battery, even the `good ones´. The max discharge rate of a good AGM battery should not exceed 1/2 C (capacity), so for 300 Amp you need a 600 Ah battery bank weighing approximately 300 kg – rubbish! The only alternative is Lithium Ion batteries which are able to provide up to 3 C of current. Theoretically a battery of 100 Ah would be enough but for how long? Wit a maximum DOD (depth of discharge) of 70% for LiIon batteries we would have 15 min of full throttle – hardly enough to get out of the berth, so 300 Ah at 24 V should be the minimum.

Straight out of the box you can buy for example the Mastervolt MLI Ultra 24/5000. It is a 360 Ah Lithium Ferrit batteryMLI 245000 with intergrated BMS (battery management system). The Lithium Ferrit (LiFePO4) batteries have not as much capacity in relation to their weight as the Lithium Ion or Lithium Polymer types but are a lot safer when short circuited. The battery weighs 58 kg which is related to the AGMs pretty good. But – this battery costs 5100,- EUR, quite a lot for 45 min of motoring. The alternative is configuring a system by yourself. This is of course a lot more complex but manageable.

For our 24 V 300Ah we need 8 single LiFePO4 cells and a balancing/BMS system. The best known cells are probably the Winston (Thundersky) LiFeYPo4 cells. You can get them from 40 Ah up to 1000 Ah. Each cell has a nominal voltage of 3.2 V which is kept even at high discharge rates. The operational Voltage range is 3.8 – 2.8 V which equals a DOD of 80%. The producer claims a cycle life of 5000 charge/discharge cycles (Mastervolt – 2000). Constant discharge current should be <= 3C with an optimum for charging and discharging at 150 A.

Winston 300 Ah LiFeYPo4 cell

Winston 300 Ah LiFeYPo4 cell

There are various balancing/BMS systems on the market. Basically the BMS is needed to level the voltages of the individual cells and protect them from low or high voltage. I might do a separate post on that topic some time. A system mentioned above should be around 4000,- EUR with the advantage of higher cycle life. Still, one couldn’t´t do only with the battery – we need a range extender in form of a Diesel Generator.

To keep the advantages of the system – small and light – I chose a generator supplying 150 Amp constant current at 24 V. That would

  • charge the battery within 2 hours
  • give me 1.5 hours at full throttle (150 Amp from the battery, 150 Amp from the Generator) when battery is full
  • let me cruise at half throttle until I run out of Diesel when battery is down
  • let me do without propane gas using electric cooking instead

The smallest and lightest Generator I found is the Whisper Power M-GV 4 Piccolo Marine. It is a very small and light (70 kg) 3.5 kW permanent power AC Generator.

Whisperpower Piccolo Genverter

Whisperpower Piccolo Genverter

It has to be run in combination with the 24V 150 Amp DC power cube, transforming the AC power to 24 V DC with an efficiency of 94 %. The decision is not made yet. I still have to do some research because it might be better to install a DC Generator with more power in combination with more powerful motors to have a real diesel-electric prop system.

Some web links, tbc

Marine electric propulsion

Distributor of Winston LiFeYPo4 cells (Europe)

BMS Systems

Marine generators

7 thoughts on “Propulsion

  1. Nick

    Hallo Chris,
    the question i have is: why? why, go the diesel-electric way instead of just the good ol’ diesel? diesel-electric is much less efficient compared to a diesel engine. and the advantage of the diesel-elecric-setup (immediate torque) doesn’t seem to be a real point in a sportsboat. also, spareparts for a diesel engine can be found all over the world. with an electric-engine it might look a little different.
    anyways, great project and i love to see the progress u make 🙂
    best regards

    1. Christian Post author

      Hey Nick,

      that is what we are asking ourselves 😉
      The twin diesel setup is well proven, reliable, as a saildrive easy to install … but I do not quite agree on the efficiency. As I know from my last boat a propeller hardly ever matches the motor. It is ether to big and the motor does not achieve the maximum rpm or it is to small and does not produce the power you need. The easiest way to avoid this is to oversize the motor what I don´t want to do because of the weight. In a Diesel-Electric setup the diesel generator runs at ideal rpm and torque, the motors provide the same torque throughout the the whole rpm range. So far in theory! We are undecided yet but what I like is the idea of motoring quietly into a calm and secluded anchorage and to occupy myself with a matter that is new to me. Anyway, thank you for your interest in our project. In January we have the BOOT exhibition here in Dusseldorf where we can talk to the various manufacturers – and maybe we end up with two nice small diesel engines 🙂


  2. Tom


    I never was a fan of the single outboard setup. After questions to Schionning they also came up with drawings for retractable dual outboards in the hulls.( as other schionning designs have too) I’ve talked to an F-boat builder (who built a Schionning cosmos earlier) about this. He was very satisfied with the performance of two 9,9 Yamaha high thrust outboards. downsides are the resell value this is lower if there are no inboard diesels and small alternators. I like the clean bottom of the boat if the engines are up.
    Not building the Arrow yet but I keep a close eye on anything conserning this. I would love to use electric engines but right now they are pretty expensive. That said, the developments in electric propulsion are going fast so maybe its wise to wait as long as possible before choosing a propulsion system.

    With regards,

  3. Robert B

    Reliability would be really good if you could install a double 2.8 kW gen set instead of a single 3.5 kW if you can afford the extra weight. This way you would have reliability due to the spare gen set and you could run full throttle almost all the time with both gen set running. I would install the batteries and the gen set in the center of each hull for good weight distribution. 400 ah on each side would be good.

    I have been following this technology for many years. Ten years ago I would have passed but today, I would seriously consider it. Specially if you can get ride of propane and run off an electric stove. Having two small gen set would be ideal to only run diesel based on the load you need. ie 2.8 kw or 5.6 kw.

    I think I would go for it.

    good luck


    I am at present setting up to have an Arrow 12 built for me in South Africa.I also considered hybrid electric propulsion (Fischer Panda DC genset/Solbian solar(about 4sqm)/Li battery/Ocean Volt drives). but rejected on ground of too heavy/too expensive/not reliable enough in circumstances such as a foul weather lee shore or low battery + difficult mooring in confined spaces-considering windage/and if dispensing with propane what does one do for hot water?/
    I am going for 2x Lombardini diesel 20hp drives/Solian pannels(4sqm)Watt&Sea water generator/EFOY fuel cell backup/propane for cooking and heating/alternators only for use while motoring anyway)therefore saving the engines from working with too little load and only then for propulsion for which they are designed, and of course fuel savings.
    I consider the single petrol outboard as dangerous and not suited for cruising boats.
    Thanks for an inspiring blog.

    1. Christian Post author

      Hey Anthony,

      I am still making up my mind. In two weeks we have the “boot” fair here in Dusseldorf. Lots of exhibitors, both for electric and diesel propulsion will be there and probably will make up my mind then.

      Anyway, thanks for comment and good luck with your project!


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