There seems to be a lot of interest in our plans to power our catamaran with an electric propulsion system. And of course there are a lot of critics with proven points. If Lagoon had to replace all their hybrid systems in their charter fleet because of never ending problems why should we do better. Lagoon, probably the biggest catamaran shipyard, if they can´t do who else? Well, that was back in 2006, I don´t know the system of the Lagoon hybrids but I know that diesel-electric propulsion works. What is different to 2006? I think the key is the advance in battery technology, namely the Litium Iron Batteries. And of course the improvement in efficiency of all components, Generator, Inverter, Motor, shaft and prop makes the whole system work better than 9 years ago. So far in theory. We made our decision, for a practical review you´ll have to wait 😉
First of all, what are our needs. With twin diesels I would have decided for 2 X 15-20 hp, e.g. the Sole mini 17 or any other 2 cylinder diesel engine (Volvo, Nanni, Lombardini, Yanmar etc.) But what does that mean for electric motors. Why can we choose them smaller in terms of power and how small can they be? Basically, we are able to turn a prop of same size with a less powerful electric motor because of immediate and evenly distributed torque in the whole range of rpm. Diesel motors supply torque very unevenly, especially at low rpms there is a lack of it. That makes it hard to match the prop in terms of diameter and pitch perfectly to the motor. To compensate that yacht diesels are usually a number to big for the prop (and the yacht). That makes them speed up fast but the load at higher rpms is actually to low. In addition there is loss due to friction of a saildrive or a gearbox. Because of the evenly distributed torque it is easier to match a prop to an electric motor. Greenstar uses a slow rotating disk motor without the need for a downshift. The motor axis is connected straight to the shaft. A low friction bearing is used for the shaft. All in all the system should be a lot more efficient than a diesel Motor. Is the chosen power enough? For now I have to rely on the calculations Greenstar did for our boat. The Greenstar 20D System nominally has only one third of the desired diesel power but if you look at the facts mentioned above it should be enough. It drives two 14×8 Flexofold props. Both motors weigh 28 kg (without shaft and props).
So we got the motors, what electrical power do we need? The Motors run on 24 V DC which is nice, because I don´t have to change the voltage for the boats DC grid. At full throttle each motor draws about 150 Amps, makes a maximum of 300 Amps which need to be delivered at least for a short period of time. Half throttle, which is supposed to be cruising speed (flat water, no wind, 6 knots, the Arrow with low resistance hulls maybe a little faster) draws about 50 Amps each Motor. We want to be autonomous, so we need a source that delivers the apx. 100 Amps for cruising speed continuously, even when the battery is getting low. At first sight the Whisperpower M-GV 4 Piccolo marine system we chose seems complicated but actually it is pretty smart.
First of all there is the Motor/Generator. A 306 ccm one cylinder 5.8 hp diesel motor is turning a permanent magnet alternator at 2800 – 3600 rpm. The Motor/Alternator is fully capsulated (quiet 65 dBA at 1 m distance), fuel consumption is 0.8 – 1.2 l / hour, weight is 69 kg, it has electric start (need for an additional 12 V 55 Ah lead battery, 18 kg) and remote control. It has a unique exhaust system where fumes and cooling water are separated with the outlets for fumes above and for the cooling water below the waterline to reduce exhaust noises. All in all it should be a very quiet system. The alternator produces electricity with 94 % efficiency (using the torque of the motor ideally) but in voltage and frequency not suitable for our needs (up to 400 V at 500 Hz).
To change that the WP PMG module (7 kg) is connected next in line to get pure sine wave 230 V / 20.5 Amp AC (3.5 kw continuous/4.0 kw max/8.0 kw for 5 sec. max power 95 % efficiency). It also has a 12 V/7 Amp output to recharge the starter battery.
Next in line is the WPC 24/3500-90 sinewave inverter/charger (28 kg). The numbers say it charges at 24 V/90 Amp max and inverts from 24 VDC to 230 VAC with 3.5 kw max. The efficiency is 94 %. You can not tell where is in or out. On the one side you can connect a) the Generators 230 VAC and b) 230 VAC shore power, on the other side you connect the 24 V battery bank. The basic idea is that the WPC delivers the power where it is needed automatically. It is fully programmable with loads of parameters and does for example
- start the Generator automatically when the battery drops below a certain programmed voltage (manual start possible as well)
- charges the battery with up to 90 Amp according to programmed parameters
- automatically inverts from 24 VDC to 230 VAC when shore power or generator power or both together are insufficient for whatever you need it for.
- simply delivers 24 VAC when it is needed here or 230 VAC when it is needed there, provided the battery is not low.
This system alone should be able to power our Arrow 1200 up to almost cruising speed (90 Amp output). The Generator is then loaded with 2.3 kW. There is still 1.2 kW power left to install another 30-40 Amp with an additional charger if needed. But of course we need a buffer, a battery suitable for our system. It must a) be able to deliver up to 300 Amp DC at 24 V and b) be able to supply power to go full throttle for some time, let´s say at least half an hour. When you see the figures it is pretty clear, that even deep cycle AGM lead acid batteries are not suitable. To go 30 min at full throttle you need 150 Ah. Considered not to discharge a lead battery more than 50 % you need a battery bank 300 Ah/24 V – which weighs 180 kg – nonwithstanding that a current of 300 Amp will destroy the battery within a few cycles. The answer is Lithium, in our case Lithium Iron because of the minimal fire hazard. Unlike Lithium Ion (Tesla) the Lithium Iron batteries contain to little Lithium for a severe fire or explosion. Nonetheless you have to be careful and never ever short circuit these batteries. The nominal voltage of the batteries is 13.6 VDC, not dropping even at high discharge rates. Greenstar sell their own range of Lithium Iron batteries which are basically relabeled Super B´s.
Now what is important about Lithium Iron batteries. Unlike lead acid batteries they do not need to be fully charged and discharged every time but it is crucial to never overcharge or discharge them. Once deep discharged and they are dead! This is valid for the single cell of the battery, e.g. a nominal 12 VDC Super B is made of 4 single cells and their voltage must not get out of a range of 2.5 V to 3.65 V. Lithium Iron batteries out of the box like the Super B´s have an integrated BMS (battery management system) which monitors these parameters, balances the individual cells and cuts off either the charging current or the discharging load to prevent the battery from damage. The parameters that influence the cycle life (No of cycles of discharging/charging) are a) the DOD, depth of discharge in percent and b) the charging/discharging current. To get as much as possible out of your expensive batteries the DOD should not exceed 80 % of the nominal capacity. The charging current should not exceed 1C (means 1 x capacity, e.g. 100 Ah battery, max charging current 100 Amp) and discharging current should not go further than 3 C. Less is always better. The maximum size for a Super B LiFePO4 battery is 12 V/160 Ah. We need at least 2 of them connected in line for 24 VDC. Charging them with 90 Amps equals 0.56 C which is OK. Discharging them with 300 Amps (Full throttle, no Generator running) equals 1.87 C- not perfect but acceptable. 80 % DOD are roughly 130 Ah. For that we get 25 min full throttle only on battery or 75 min at cruising speed. These numbers increase when the generator is running. All in all it is not perfect! The biggest advantage is that I buy from one dealer/distributor which is of course better in case of a failure and guarantee. Still, for the same amount of money I can buy an individually configured battery system with BMS made of Winston LiFeYPO4 single cells with 400 Ah capacity.
The advantages are obvious. Lower charging/discharging rates, lower DOD, resulting in higher cycle life. The range without the help of the generator is extended (about 3 times as long). In addition they work at a slightly higher voltage. The weight: 2 Super B 12V160Ah 60 kg, individual bank of 8 Winston LYP400 with BMS 110 kg. If I reduce to 300 Ah it is 80 kg.
We want to keep our catamaran light which is crucial for a good sailing performance. If we sum up all the weights (regardless the shafts and props since they are the same) we have for diesels
- 2 diesel engines with gearbox and motor mount – 220 kg
- 2 55 Ah starter batteries – 36 kg
- cabling, wiring, exhaust etc. 20 kg
- about 280 kg in total
- 2 electric motors with motor mount – 35 kg
- Generator M-GV4 pcc. with PMG – 76 kg
- 55 Ah starter battery – 18 kg
- WPC Charger/Inverter – 28 kg
- cabling, wiring, exhaust 15 kg
- roundbout 180 kg
The weight for the battery is about the same, e.g. 300 Ah Winston indiv. batt. bank 80 kg, 300 Ah 12 V AGM 90 kg. The diesel tank remains the same but we might not always fill it up to the top with the small consumption of the generator. If electric cooking works we spare the weight and the hazard of a gas installation. A total saving of about 100 kg should be possible.
The initial costs for the installation are higher than with twin diesels but the follow up costs should be a lot lower. We can produce electricity ourselves by wind, sun and – recuperation. From 5 knots boatspeed upwards the motors can produce electrical energy. With this it even might be possible not to run the generator at all.
Again, so far in theory. We are happy with our decision but only time will tell if it works out … The delivery of the parts start beginning of May, it´s going to be an interesting summer 😉