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. Unlike 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 battery 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.
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.
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)