As I wrote previously, we are going to make our lovely Blu Emu into a parallel hybrid. The key though is how we want to be a hybrid. A hybrid sailing vessel can plan on using their electrical propulsion only when entering or leaving a harbour or mooring, essentially only a small part of the time. Sailing Uma is one of the preeminent YouTube examples of this. They don’t need much power and crucially they don’t need it for very long.
Blu Emu though is a powered vessel only: no sail. So the benefit of electrification is to get as much as possible for as long as possible. Some powered vessels go the Uma route, like the smaller Greenline hybrids. Given the expense of adding electric propulsion, we want to use it as much as possible, so our goal is:
For short trips, upto 50nm, and where time permits:
For coastal multi-day trips, and where time permits:
For long journeys, like crossing seas/oceans:
Whenever time is crucial, because of weather, sea state or anything else, we will use the diesels!
use electric propulsion only (small motors, large battery bank).
use a combination of diesel-electric (small-medium motors, large battery, fast recharge)
watch the weather and use either diesel alone, or a combination diesel-electric as needed to get to the destination safely.
We think our boating will be 70% of the short trips, 20% of the second, and less than 10% of the third. But instead of going entirely electric, we want to give us the option of the longer journeys by keeping our lovely old, functional and functioning, diesels.
To go hybrid for our uses, we will need to:
- Add solar, so we get some “free” energy and don’t have to use a diesel engine for everything
- Add big alternators, so we get storable electric power from the diesel engines while we have to use them
- Don’t use a generator! A big enough generator is another diesel engine in the boat, and the noise, cost, maintenance of such engines is something we are trying to steer away from. Instead of a generator, we will use large alternators off the main engines which, as our main propulsion, will be on much of the time we travel (unlike a sailing vessel) while at anchor the solar array is more than sufficient for non-moving needs.
- Store power
- Use the power
Add a large lithium battery bank (why lithium is explained but is really for another blog post!)
Add a parallel electric motor to each diesel, and electrify as much as possible. This includes getting rid of potentially dangerous gas-powered cooking.
- Make it all work
- Check whether it does work!
Redo the entire electrical system, buy all the stuff and install it.
Essentially the idea is to get as much solar as possible. Given we are a power boat, we don’t have the limitations many sail boats have wherein their mast and sails shade the solar panels reducing output considerably. On the other hand, more panels equals more space that can’t be used for anything else, more cost and more weight up high, increasing the moment of the boat (movement side to side) and windage, both of which should be as low as possible. For us, the benefits of large amounts of solar are such that the cost and space are accepted.
The weight issue goes hand-in-hand with the “as much solar as possible” end goal. Flexible panels are extremely light, but aren’t as efficient so the same area delivers less solar power. So we have decided that the weight penalty of the heavier panels, like are used on houses, is worth it as we have such a limited space (compared to a house: compared to other boats, we have heaps!).
One other point about solar panels is that their directionality can impact their performance: if you can keep them cool, and pointed at the sun, then the power they provide is maximised. Unlike a house though, a boat moves around while travelling, at anchor or moored. So careful angling of the panels, or having them turn continuously to face the sun, is very difficult and would require solar tracking mechanisms electronics which aren’t suited to the marine environment and are heavy – and incidentally would have to move a lot as the boat moves a lot on a mooring or anchor!
Blu Emu would just be able to have 9.6sqm on the roof above the internal steering position. Once we have designed the flybridge we may have upto another 20sqm. A further sliding extension over the front 9sqm could provide another 9sqm or so.
The panels we are evaluating are probably the 20.4% efficient, LG Neon 2. They are a good mix of efficiency and cost and are 204W/sqm. Sunpower Maxeon 3 are higher efficiency at 22.6% and 226W/sqm, but are a lot more expensive and harder to get. Panasonic HIT are possible too at 203W/sqm. The LG panels though meet our sizing well to pretty much perfectly use up the 9.6sqm space. 9.6sqm would just accommodate 6 panels, making 2.1kW. The sliding extension would add another 4-6panels, or 1.4-2.1kW making a total (excluding flybridge) or 3.5-4.2kW. I’ve made the calculations since on an average of 3.8kW.
The typical daily ability to collect power from solar panels is 4-5 times their power depending on where on the globe you are and winter/summer. Averaging to 4.5 for simplicity, 4.5×3.8=17kW of solar input per good sunny day. There is a loss in conversion from higher voltage at the panels to the battery, but using MPPTs and series connections for higher voltages and earlier in the day power should minimise this. We have allowed an 8% loss in calculations.
Apart from the sun, we do need other means to create electricity if we want to use it for propulsion. Typically, alternators are used in most motor vehicles to make sure the starting battery is charged. However, since we are a motor vessel our diesel engines are going to be on a lot, and instead of using a third diesel engine in the form of a generator we will use large alternators on the main engines.
Our main engines are Perkins M135 naturally aspirated. They are 600kg big lumps of metal. Perkins states that the PTO (power takeoff) at the non-prop shaft end can handle 6kW/8hp being taken. This will slightly increase fuel usage (10-15% at our guess), and would certainly be a problem if we ran the engine anywhere near the maximum power. Luckily, we are already purposely downrated from 2600rpm to a maximum 2400rpm, or 132hp maximum down to 100hp per engine. So there is plenty of room to take off 8hp.
While in theory 6kW/8hp can be “taken off”, an alternator only works at a certain level of efficiency. Average ones are 50-60% efficient only. But there are some that are more like 80-85% efficient. This turns the 6kW/8hp into an actual 5.1kW/6.8hp to go into the batteries (with a little loss in transmission).
Most alternators are small: 12V units for cars and trucks, from 50amp to 100amp (or 0.6-1.2kW). These can be driven from V-belts between the engine and the alternator. Large-frame alternators though may be 180amp or more (2.1kW), and most then need serpentine belts to take off the power successfully, and are usually externally regulated. Some boats even run twin alternators, one on either side of the back of the engine so that the radial loads – the sideways pull on the PTO shaft – is offset by an equal pull from the other side for another alternator.
As the battery will be 48V to cater for the largest use, propulsion using a 48V motor, the alternator has to be 48V as well or a step-up transformer (with loss!) is needed. We have found an Australian company, Rapid-Power, who make alternators for the mining industry and have 48V, 100amp (4.8kW) 85% efficient units. We will likely ask them to make us a 48V, 125amp (6kW) unit (their webpage show a 28V 200amp 5.6kW unit, the RP5, which is close).
Importantly, alternators can be irreparably damaged if they suddenly lose their load, such as the battery “saying” it is full, or misuse of the 1-2-both switch (Marine HowTo has some excellent information on why not to use a 1-2-both switch). So the external regulator usually will turn off (aka regulate) power from the alternator to suit the needs of the battery. The older regulators just used voltage to decide what do: they were programmed for lead-acid batteries and their particular needs. But there are now new ideas and products to cater for lithium batteries and even combination of chemistries. Options we have looked at for a regulator include: a Wakespeed WS500, fairly basic but known Balmar, and something with much more “smarts”: a digital CANbus controller. The particular controller, or regulator, is important as it can limit the amount of power that goes to the battery, making charging longer or even impossible especially for new large lithium batteries with 0.5C charge possibilities. So matching the alternator, battery and regulator/controller is very important!
If (and that’s still in question) a separate lead-acid starter battery is needed, there is also the option of a Victron Argo FET 200A Charging Splitter with ignition switched activation to ensure the different battery chemistries charge properly. We would probably use the Argo FET anyway to protect the alternator from sudden shutdown.
Power is stored in a battery. Interestingly, you can have a battery made out of many things – a spring is a type of battery! The Baghdad Battery was made in 200BC or so and made from pottery and copper. In our case, as a catamaran we need light weight, lots of power storage so we can run on electricity for a long time, the ability use the power stored when needed, make sure it is safe on a vessel that’s moving about sometimes violently, and is cheap. Hmmm, that’s hard!
Fortunately, battery technology is changing very fast. Lithium iron phosphate (LiFEPO4) is our chosen battery technology and meets all the needs. It is much lighter weight the old AGM or other chemistries, it has vast storage for its size and weight, some examples can be charged at 0.5C or even 1C, and discharged at 1C. But it has some safety issues, and some thermal runaways and explosions have been experienced for some installations and particular chemistries. Marine Howto has some good points.
With the chemistry choice made (and watching for the many changes happening in the field!), the next choice is then whether to work with a respected manufacturer (like Victron) and use their batteries, or DIY. The cost difference is substantial! We will keep watching, but the cost savings of DIY mean we are leaning that way: making up our own battery from purchased cells and a BMS (battery management system).
Right now (early 2021), the 280ah cells from EVE and Lishan are looking good (DIYsolar has some excellent information and people who are doing similar things for boats and RVs; Marine Howto has great information on how to make your own battery cables). These 280ah 3.2V cells x 16 to make up 48V, would make a battery bank of 13.4kW. For Lithium, discharge to 80% is generally ok meaning a usable 10.8kW of storage. These cells have a 2000+ cycle lifetime (so 80% or 90% of their initial size), or about 7 years of usage where every day is discharged and recharged once. Discussions with some gurus seem to indicate that regular charge-discharge in an electric boat sense (perhaps hourly even switching between electric (using power) and diesel (generating power)) may not constitute an entire cycle.
Use the Power
The two uses of the power is electric propulsion, and things working on the boat (cooking, anchor winch, fans, recharging phone and computer, etc.). These uses are split into the voltage each needs, and then whether they are always-on or “switched”. Some items are 12V, some are 48V, and some are 240V.
- 12V DC is common in cars and smaller boats. There are a lot of 12V products. 12V won’t kill you, but also is typically low powered. If a 12V thing needs a lot of power, it needs a lot of amps which means heavy, expensive cables and connectors and so more 12V products don’t use too much power.
- 48V DC seems to be the sweet spot between “high enough for small amperage and good enough power” and “not quite high enough to kill you if something goes wrong” which seems to cut in at about 50V. Anything over 50V needs a qualified installer and extra care. There are a variety of 48V products available now, including anchor winches (which take a lot of power), and electric motors for propulsion.
- 240V AC is the Australian standard for home electrical. This covers various goods we may use aboard like air-conditioning, fridge/freezer, microwave, induction stove top, etc. These typically need big amounts of power, even if used for shortish periods of time.
Propulsion is the main reason though that we want to go down this path – if we were doing it only for the purpose of using induction cooktops and other high-draw electrical items then our choices may be different. But for propulsion, the stakes are higher.
There are many options for electric propulsion: electric outboards, electric motors connected to straight propeller shafts, electric motors connected to sail drives, and then serial hybrid or parallel hybrid. We have thought about electric outboards, which would entail no internal changes to the boat but are severely limited in their prop size. At the moment, a parallel hybrid is preferred, a bit like Beta marine is marketing.
The motor is planned to be 48V. This limits our continuous power quite a lot to less than 16kW because of the cabling size (amps) and runs required: 333amps is enormous! Fortunately, we have had a quote from Oceanvolt which gives a general indication of the power we will need for particular speeds:
This graph immediately demonstrates the limitations of electric power for a semi-displacement (and displacement) hull shape: the power required to go fast rapidly goes exponential. While some hydrofoils and specialty boats may be able to go faster than 9-10kn using electric only, most vessels won’t be able to – and certainly not using 48V!
So we are limiting our electric usage to between 4 and 7 knots. A cut down version of the graph is easier to understand:
This shows the amps (at 48V) required for 3,4,5,6 and 7kn. So 5kW of power would draw 100amps from the battery to go about 4.5kn. We think we will be happy with 5kn, as the place where the curve starts getting worse quicker. At 5kn, draw from the battery is about 7.7kW, 160amps.
The maximum we could imagine, for a short time as it would deplete the battery very quickly, is probably 7kn, or 25kW of power. However, this could be delivered through two motors, one in each hull. So each motor would draw 12.5kW or about 250amps from the battery pack. This wouldn’t be good, or even possible, with a single battery pack at nearly 2C discharge, so a separate second battery pack would be needed. The motor itself could be rated larger, continuously, than 12.5kW as running electric motors at their maximum for long periods of time can effect the cooling and efficiency. Something like a 20kW motor would probably be fine, one in each hull with a second battery pack.
Now, although 5kn only draws 7kW from the battery pack, there is loss in efficiency: between the battery and the motor, in the motor, and from the motor to the prop shaft itself. It is likely to be around 85% (at lower speed) to 90% efficient (5kn+). So while 7.7kW is drawn from the battery, only 6.9kW makes it to the prop shaft which is where a form of comparison to a power of a diesel engine can be made – at the prop shaft. Note that the propeller itself is anything from 55-65% efficient, but since for a parallel hybrid the same propeller is used for both diesel and electric, I haven’t bothered taking that into account!
So after all that – if you’re still with me! – we want an electric motor of about 10kW continuous for each engine. This would give us a total of 20kW of propulsion making 6.2kn and draw about 22kW from the battery(s) at 225amp per pack. It would also allow us to run on one side of the boat only at 5kn, freewheeling the other propeller.
Make it all work
A key part of the change that needs to occur is to redo the electrical system. Blu Emu is over 20 years old and the electrical system has clearly been put together over a period of time without much care – there are wires hanging in the engine room that are open at one end, and no labelling!
The first thing is to have a plan! The following is our current (Feb 2021) plan.
The key parts of the plan, from the diagram, are:
- Inputs are on the left:
- Solar – as much as possible at 48V, coming through MPPTs, and connected to the +ve and -ve bus bar with fuses. 2-3kW is easy, and upto 9kW is possible on Blu Emu. As a powercat, there should be little problem with shading and therefore panels can be in series to increase the voltage which, for MPPTs, should make them generate power earlier in the day.
- Alternator per engine – upto 6kW each at 48V, coming through regulator (Wakespeed, Balmar or CANbus digital). Our Perkins 135hp diesels allow upto 6kW/8hp power takeoff
- Shore power – 240V AC coming through an isolating transformer as we are an aluminium boat, and directly into the charger
- A 48V DC diesel generator is something we don’t want: another expensive diesel, but should not be needed.
- Each electric motor can also function as an electrical generator while the diesel is running. It isn’t clear this is a good idea while the diesel is also running large alternators, but may just happen unless we can work in a clutch on the electric motors.
- Storage is in the middle
- A 48V battery bank, LiFEPO4. As large as practical and cost effective given the rapid change in battery technology. We may start with a group of 16x 280Ah cells from China, which would give a bank of 13.4kwh bank with 10.7kwh usable (at 80% discharge).
- Shown is also a starter battery, 12V. We don’t think that this will be needed though and does complicate the system quite a bit because of the need for charging.
- For simplicity, there is only one battery bank. It is 48V as this is the largest voltage that is commonly seen as electrically safe (the EU apparently has much more stringent rules for >50V as it can kill). There are also many products for 48V, including anchor winches. Finally 48V is a bit of a sweet spot for electric motors – there are many available at this voltage and it provides a decent continuous output potential without enormous amperages making wiring hard/impossible.
- Voltage change is in light grey
- The charger-inverter needs to convert from 240V to 48V to allow shore power to charge the batteries. It is difficult to find a better option than a Victron Multiplus II (the 48V version of course).
- The 12V systems need conversion from 48V to 12V (which could be a buck boost, like the Scotty from Safiery, or the named Victron Orion).
- Green shows systems that consume power
- 12V always on – direct from the battery, stepped down, these are for systems that should not be off the standard 12V panel
- 12V – all the usual 12V systems on a boat, caravan, etc.
- 12V NMEA – this is only for the NMEA2000 systems on the boat, separate to the other 12V to keep those requirements separate
- 48V – any systems that are 48V, the primary is the electric motors themselves. This could include other major systems too such as anchor winch (perhaps in the future) and dinghy battery charger.
- 240V No Break – for 240V systems that may be needed even if shore power is not available.
- 240V switched – for 240V systems that can only be used if shore power is available. This is a function of the Victron Multiplus II that we will use as a combination charger-inverter.
- The underlying colours show the voltage of the different areas: light green for 12V, light yellow for 48V and light blue for 240V.
- A few further comments:
- RCDs (residual current device) are not shown
- We haven’t worked through sizing for wires, fuses or switches
- Some of the systems have question marks (?) – we aren’t sure whether we’ll have them, or if we do what voltage will be used. For example, the windlass is 12V currently but 48V versions are available; we have a 12V watermaker now, but 240V are available.
- ACR is an automatic charge relay, and isn’t shown. These are used to combine two batteries so when charging they appear as one battery, yet when using them they are separate. As we hope not to install the starting battery for the diesel engines, we hope not to need the ACR.
Check it works
Who knows! At this stage, we only have plans. But it isn’t any use if the plan is not returned to afterwards and it can be evaluated and lessons learnt, so this is a placeholder for the return time.
What I have done is a spreadsheet to play with options of amount of solar, battery size, speed desired and other options. The output is a table of when diesel is needed and when electric propulsion is to be used, over the entire 24hr day. So a certain solar input, alternator input, speed under electricity, speed under diesel, battery size, will show that electric propulsion is possible for 12 out of 24 hours, and total distance is 158nm at 1.8lpnm (for example). If people are interested (hint – leave a comment, or send me email!), I’ll clean it up and make it available on the blog.
Random links to others
A Norwegian tourism company who built their own hybrid electric power cats. 800kW of batteries, 10 hour run time, 1.5m propellers! The Brim Explorer. BUT – you have to be careful with batteries, especially lithium!
Good information sources:
- SV Lynx – hybrid. Detailed long article on why hybrid, scenarios, and their own uptake
- SV Entropy. Good long article of the system design of a LiFePO4 battery system.
- Fischer Panda Easybox hybrid. Manufacturer of generators, but a good overview of hybrids with easy to understand diagrams.
- SV Pleione’s story. Good article (PDF) on an electric conversion with lots of pictures, speed facts and learnings.
- Maine sail has good information on planning and especially design diagrams of electrical systems in general, such as here, and the site marinehowto.com
- Plus all the others referenced already in the text above!
Please let us know what you think, and maybe whether you are thinking of the same!
5 thoughts on “Electrification big picture”
Love your boat. I’ve been looking for something similar to do in retirement which is at least 5 years away. Wish Prometa made more instnaces of your boat.
I think your eclectic plan will work. It does allow good use of the existing engines with some added complexity and weight.
Regarding solar, one possibility is to have sliding panels on the sides. Your beam is 4.8m and many panels are usually 1x2m in size . You could have a strip 1x2m panels going down the middle of the boat from front to back (these would be mounted with the 2m length from side to side). Then you have two panels on each side of this strip that slide in up to 1m underneath the middle strip. So when docked you may need to slide them in (total length from side to side would be 4m) but when sailing they slide out and you have a total of 6m across of solar from side to side. You could mount over 40 panels this way from front to back which would give you a lot of power for day trips as well as quickly charging a battery bank.
Now 40 panels is a lot and could make the boat dark or under too much shade but you can use bifacial panels to allow the light through and get a boost in panel output from boat and ocean surface reflections — https://news-media.energysage.com/wp-content/uploads/2018/10/Screenshot-2017-11-15-17.52.26.png. LG has nice bifacial panels in the neon line. There are also flexible ones that weigh just 12.5kg per panel (http://www.jolywood.cn/upload/202004/26/202004260933598965.pdf) (so making it light and strong can be done. A lot of these panels are close to 400w on the front and the bifacial boost should be 10-20% so you are probably looking at 15-20kw if you cover your whole boat.
How do you mount it and keep it light? Aluminum would work well but carbon fiber is lighter ( https://dragonplate.com/carbon-fiber-truss has some nice truss systems).
This could allow you to get rid of the diesel engines to free up a lot of weight for batteries and panels and just use the panels to charge the bank as well as the generator for backup or low speed cruising if there isn’t any sun. It would also be simpler than a diesel engine and generator setup. Maintenance should be easier too.
The biggest issue is ocean crossings and that can be done with solar if there are many days of sun and you have a large battery bank (or fuel for the generator). Solarwave 46 did some long crossings and Silent Yachts 64 crossed the Atlantic using solar and generator power.
Do you plant to keep the bridgedeck intact or are you thinking of chopping some of it off to cross European canal bridges?
Thanks so much for your comment.
Yes, a few people have said they would have loved to see more of this style of boat. Prometa did do a larger sister ship, but that just makes two!
I think that Fountaine Pajot ramped up their line at the same time, starting smaller and using glass rather than aluminium, and that hurt Meta/ProMeta. Aluminium has always been a bit of a niche boatbuilding material I suppose.
Your thoughts on solar panels match ours! Having them being able to be wider (and protect from the sun in the tropics) while on anchor or in a marina means the overall boat can be wider. I am in the middle of planning a new hardtop which I am thinking of building out of bifacial panels, rather than a hardtop on which panels sit. It should make the weight lower too as it will be very high. Interestingly, I found a PhD recently showing bifacial of 1.5years ago was only 7% more efficient than single face, with another 2-3% efficiency simply because the bifacial use a better quality product. So 10% better in all. The Neon’s are my preferred panels too at the moment (although the Panasonic HIT look ok).
The flexible seem to be of much lower efficiency though – maybe 16% instead of 20-21%, but they are definitely lighter. Apparently they don’t last as long though – but I suspect panels in 5-8 years will be so much better than today’s that it’s a no-brainer to update early than 20 years!
Yes, carbon fibre would be best. I’d be looking at building my own from resin infusion for weight, but I will look at dragonplate too – thank you.
Our engines are 625kg each (including gearbox), so saving over 1300kg (including various other items needed by them) would be fantastic. I can’t quite see us wanting to limit to <7.5kn though, which is pretty much the max for any reasonable time use of electric motoring. Using the diesels – which are only 135hp each – we can get to 12-13kn. It is on my mind that should one of the diesels go bad, to take both out and replace with one electric and one larger, newer diesel. We'd still save lots of weight, and lessen complexity a lot!
I have a rather complex excel to show that, using 3kW solar and about 14kW lithium, we can motor at 5kn electrically for 12 hours a day, and then at 8.5kn using diesel for 12 hours of the day. So half the time could be on electric and the total average speed is still near 7kn/170nm days and at much better diesel fuel consumption. The more the solar, and battery, the more electric running can occur. So crossing oceans is definitely possible as a hybrid.
The French bridges are 3.5m high, which would require us to cut down our top steering to the coaming, while also being full of fuel and water and possibly a little more ballast. We are narrow enough, so only height is needed. But covid means we won't be doing the European passages any time soon, so we've changed our direction – which is part of an upcoming blog…
Please keep in touch!
Robert and Eraine
Looking forward to your next blog post. I’m also curious about your spreadsheet calcs for powering at 5kn. If your boat is well balanced and hull is fairly efficient then 10-20kw should be able to achieve 5kn. Motor and propeller efficiency can affect things significantly.
There are electric motors that can replace the equivalent amount of power your diesels put out: https://www.diyelectriccar.com/threads/uqm-powerphase-135-brushless-pm-motor-spm195-114-3.160506/
This looks like the same motor that Silent Yachts uses in one of their 55 series solarcat: https://boatingnz.co.nz/boat-reviews/silent-yacht-55/
You can find this motor and controller on ebay for about $2900 USD. Not cheap (prices have been coming down though), but a proven solution that runs fairly efficiently at low rpm and power and with a big enough battery pack can provide quite a bit of speed when needed. I recently watched a youtube video of a sailing catamaran family that put in Tesla Model S battery modules into their catamaran: https://www.youtube.com/watch?v=IEZ0AKeHJP0 These seem like the best for density and cost (55lsb for 5.3kwh) — you can buy used packs on ebay for around $13k usd for 75kwh with a total module weight of around 800lbs: The only drawback is that these are lithium ion rather than the safer lithium iron phosphate. But Tesla packs are safe and have proven themselves in their cars — fires have only occurred during accidents when the pack is punctured and I’ve never heard of a fire in their powerwall or powerpack products which use the same batteries. If you got rid of both diesels and replaced with motors and battery pack you could probably have about 200kwh+ and be about the same weight as both diesels — not including the fuel weight needed for the diesels.
Do you know how much your boat speed is affected by additional weight? Have you run any tests?
Hi John, I wrote a lovely long reply to you that the system deleted. Thank you WordPress – not!
In essence, Oceanvolt estimated we should take 8.5kW battery draw to go 5kn. The benefits of quite narrow, long hulls on a catamaran!
Of course they are estimates, but it is difficult to do more without trying some motors out – not easy.
I’ve calculated that electrification hp are certainly not the same as diesel hp, at least based on others comments on line and the estimates I have. The cutover for us is about 8.2 kn: under that, it take less “hp” for electric to get to the same speed. Over that, it takes much more electric than diesel hp to get to speed. Essentially, the power curve for electric starts better but goes nasty quite quickly. For 11kn, it may take 300hp of electric vs 135hp of diesel.
Tesla pack’s don’t seem to be cheap here in Australia. Maybe I can’t help by driving into some, lol.
Most cats and weight conscious. It’s one of the reasons why the original plan of going series hybrid with DC genset may still be good – each of our lovely old NA diesels are 600+kg, and 250+kg of AGM house+starters. Going series hybrid would easily save 1tonne out of our 16-18 tonne displacement.
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I wonder how much of this has to do with the type of motor and propeller rpm needed to reach 8.2kn. Electrics lose torque as the rpm gets higher while the diesel at the same rpm may be hitting its stride. Having a larger or more powerful propeller (different blade amount or design) mated to the low electric motor rpm at its high torque point may allow the most propulsion at a given kw (hp). There are some new propeller designs that are significantly more efficient than conventional ones: https://www.sharrowmarine.com/our-story
It is possible that your boat may have a propeller and engine mated for a higher speed than what would be most efficient for an electric system.
8.5kw for traveling at 5kn is not bad. It should be easy to put up 9kw of solar on your boat so cruising 20-30 miles a day just on solar on a sunny day is entirely doable. Doubling the amount of solar won’t double your speed but can add a knot or two as well as charge your pack. One of the things I struggle with regarding going electric is what amount of emergency power do I need to get out of an unsafe situation (high water current channel or river, outrunning a storm, etc). I have no issues cruising around at 4-6kn with an efficient electric motor, propeller, and sufficient solar plus battery. But if I only have a top speed of 10kn or maybe 15kn but can only run for half an hour like that then that bothers me. How often does one encounter an unsafe situation? Hard to say — if one stays near bays and harbors most of the time and looks at forecasts and avoids storms then it most likely is not an issue. But if one explores a lot or does longer open passages then having more of a safety net is needed. It is always better to have more but it does come at a significant cost (especially with electric).
This place in Australia has Tesla battery packs: https://www.evolutionaustralia.com.au/product-page/tesla-battery-modules-24v-5-3kw. It is a 30-40% premium than what can be had in the USA. Wonder if they offer a bulk discount
Looks like it may be cheaper to buy in the USA and ship it by boat to AUS.
Have you considered getting rid of one of your diesels and replacing it with all electric? The AGMs can be replaced with Tesla packs and you’d run on the electric whenever you have sun and high pack power. The diesel is your backup or additional propulsion when needed. You could even keep the genset to charge the battery pack or power the electric (if the diesel breaks). That definitely adds to the complexity and maintenance but does provide good backups and a safety net. The simplest and lowest maintenance but probably most costly solution would be all electric with a very large battery pack and a small reliable genset for emergency backup.