Can solar energy power a catamaran?

What I really would like is a way of converting engine revs to power and then getting a resistance curve for power/speed. Then it would be easy to get real world data from boats and put these in to a spreadsheet. Then you could refine the model based on real world data - basic science really.

 

And another thought - that 3.78 kW comes out pretty much to 5HP. Anyone interested in solar could just try putting around a local waterway with the power limited to 5HP. Do the experiment. I have tried it for brief times but if anyone was serious it would be good to work out the minimum required power first and work backwards from there.

 

The question for a hybrid setup is: How long do you need the engine if you really want to sail? How long takes it to motor into or out of a harbour and at which speed?
As Richard pointed out, the propulsion of a ship in dead calm conditions doesn't require much energy, but do you have to plan for a possibly fatal situation where you have to fight with your ship against wind, waves and current?
It's surely a question of seamanship to avoid such severe situations, but sometimes the good outcome of a situation relies on luck and maybe a strong engine in the right moment. I don't know, just an armchair sailor... .

 

I have probably written about it before but I have friends who did the electric route on their 40ft mono. It was great for daysailing in and out of the marina but terrible for coastal sailing. They basically couldn't motor sail or motor for any time ofhshore. There was a time they had light sloppy headwinds off Jervis Bay and they spent the day tacking back and forth against the East Coast Current. They couldn't get too close to the cliffs (and into less current) as there was even less wind and the batteries were not being topped up enough by the 5KVa genset. A bugger of a day considering that with their old motor they could have powered by into the slop at 6 knots in 2 hours.

Guess what? They have ditched the electric drive and taken a large financial hit. Now they have a nice 50HP Yanmar. The electric drive is in the shed or Ebay now.

Reality can be a pain as well as being very expensive.

 

Well, a mono has much weight to accelerate compared to a multi and if you have the time to wait for a favourable weather window, most of the described circumstances can possibly be avoided.
However, Torqeedo for instance states some impressive performance data on their advertising (and therefore this has to be taken with a grain of salt), so there is definitely the possibility to provide the power to escape hairy situations but the question remains for what period of time this power has to be supplied with energy.

 

It doesnt quite work like that... its not a simple kw-hp conversion like the math suggests... for example, i have a scuba compressor on my boat, the same compressor (bauer PE100) with electric motor uses a 2.2kw electric motor. The same PE100 compressor packaged with a gas engine is a 4.2kW (6hp) petrol engine. Probably a torque/rpm difference between petrol and electric motors but you always see electric boat drives using rather small kW motors to drive rather large boats...

In any event, its not the motors that are the problem but rather energy storage and energy harvest from the sun. Forget using lead batteries, the only viable battery on the market today with a high enough energy density is lithium. The best of which being the lithium polymer type although these can be dangerous... never the less if one wanted an electric drive boat, its pretty feasable to build a 40ft lightweight catamaran with 10kw solar array and a large amount of lithium polymer or lifepo4 or limn2o4 or licoo2 or liNMC etc battery for energy storage and use. For example;

At todays prices in australia, 10kW solar array will cost you $8000AUD and is 64m^2. Weight is approx 800kg.
17.1kwh lipo battery weighs 105kg and costs $9200AUD. lifepo4 = ~200kg $10000AUD.
If you use good quality inverters your upfor;
$3600 for the AC coupled solar inverters, 50kg and,
$12000 for 15kw continuous output inverter/chargers, 116kg, fully programmable.
Id probably look at custom electric motors, i could rewind the stators to suit my purpose but choose large diameter frames with big bearings from standard 3phase induction types... These are pretty cheap these days, the last 2.2kw AC motor i bought was only $220AUD...
Variable speed drives for each motor. Also cheap these days, id allow $1000AUD per drive to handle 5kw continuous output. Fed from simple pots 0-5VDC output on the throttle levers.
The final drive would probably be a simple direct flex plate coupling from the prop shaft... also very cheap...

I know these prices are accurate as ive recently finished installing an off-grid solar system for a farm using the same inverters and a 15.1kwh lifepo4 house battery - This was $10k but much heavier than Lipo, very safe tho.

In the tropics where i live we would see input energy of approx 60kwh per day if its a clear summer day in this system. Annualized average for this area would be 42kwh per day which accounts for weather and the seasons etc... So at best, we could use 10kw per hour for 5hours and be energy neutral. We have an additional say 15kwh usable from the battery so an additional 1.5hours if we start with a full battery. On average tho we would be using more like 10kw per hour and have 3 hours + 1.5hr battery reserve.

5kw per electric motor will push a 40ft cat built from composite glass along at around 8kts assuming a displacement of around 5 tonnes. (The last 40ft cat i had used 20hp diesels per hull on saildrive legs and would do 8kts at about 2000rpm) The electric propulsion system would ideally be very efficient in design tho with very large slow turning props on a direct drive, large diameter electric motor (massive torque)

Of course more battery could be added but the solar harvest becomes the limiting factor if you wish to cruise everyday. More solar cant be added because you dont have the real estate to mount it on. The only way of improving that is to use much more expensive panels which have a higher efficiency per square meter. You can get panels that would be 50% more efficient than the ones i quote above but the price is probably quadruple - i dont know as ive never priced them...

So yes its all feasable, however the same money spent on wind power from sails would give a faster boat and much longer range if considering coastal long distance cruising...

I see solar not a good idea for cruising, but i can see it working for day charters running snorkelling tours or the like for eco conscious clientèle where the distances are modest and the weather is predominantly sunny or the boat has shore power available to charge up overnight in times of poor weather... many operators dont have the patience or skill to deal with sails and rigging for this type of operation and dealign with the public. It would certainly save a bundle on diesel based propulsion over the life of the vessel considering fuel costs and maintenance of the diesel engines and gearboxes... At todays prices, the solar electric equipment can be purchased for approx the same cost as the diesel prime movers of a similar sized slow moving catamaran.

If you consider the diesel powered catamaran still needs batteries and an electrical system aswell for general interior use, then the cost of the solar electrical system is even further offset by it. I can actually forsee that a solar electric boat for a specific purpose could actually have a lower capitol cost AND lower running costs...

 

Here's a first attempt at battery sizing. The available solar power is a sine function of the time of day, p(t) = pi/6 * sin(pi*t/12). The coefficient pi/6 is there just so that the integral of power over an entire day works out to 4 kWh/m^2. The collected energy is e(t) = 2*(1 - cos(pi*t/12)), and if you want to maintain a constant power output energy needs to be stored for some time prior to use so that the combination of battery power plus solar power gives a constant. That constant is again pi/6 ~= 0.52 kW which is the peak of the curve. The idea is that you use up battery power to make up for solar power until you hit the peak when battery power goes to zero, and then start using battery again past the peak. The amount of storage needed for this occurs when 2 * e(t) = pi/6, assuming that you've stored energy from the previous afternoon, and again in the morning, that is, the areas in red shown in the plot. I included a round-trip battery efficiency of 80% which gives t = 2.25 hours. In other words, if you have 12 hours of sunlight, then you can start at 8:15 in the morning and go until 3:45 in the afternoon. This is an absolute lower bound with no margin for other power requirements and 100% discharge which can't be tolerated by real batteries. An upper bound might be that you store an entire day's worth of solar energy, or 4 kWh/m^2, again ignoring depth of discharge limitations.

Lithium-ion batteries have an energy density of ~1 kWh per 7.5 kg, so using the orignal assumptions about 60% covered area, the battery mass can be estimated. For a 12m boat, the lower limit is ~200 kg and the upper limit is ~1200 kg. These numbers should be at least doubled to handle discharge limits. The third plot shows boat speed with these limitations, but doesn't account for external conditions of headwinds, current or sea state.

 

Escape artist - all sounds great in theory, but real world considerations wont make it as clean cut as that. You have weather, wind, current etc. You would have to experiment with the boat over time to determine what a practical size for the battery would be and you would likely end up finding that wind and sails were far more practical for this type of cruising

A few more things - originally you spoke of hull speed. Multihulls with very slender hulls dont really have a hull speed, that is, they dont have a significant hump in the resistance vs speed curve.

Be careful generalising about batteries. There are many different types of lithium battery and they all have differing energy densities. Like i said, the highest is the lithium polymer but its also the most dangerous and has a shorter lifecycle and other impracticalities which make its implemetation quite foolish. Look to the electric car industry, there are good reasons why they are using the particular lithium chemistries that they are - safe and long lifecycles.

In practical terms, like i said, i cant see it as feasible for coastal cruising, at least not when we have good old sail power technology to compare it to. But ill give you an example of where it could be a commercial bonanza...

Where i live there are many tourist charter operations running. Many of these operators run from the local marina out to the nearby islands which are only a short hop of about 10miles each way. The same scenario occurs in many places up and down the coast here. The boats go out approx 9am in the morning, the tourists stay for a few hours on the island and have lunch, go snorkeling etc, and then head for home around 3pm in the afternoon so they can be back in their hotels, showered and ready for dinner. Each tourist pays an average of $150 for this experience, thousands of them do this everyday in this little part of the world...

The boats they are using for this are old school diesel power. The operators have to maintain the engines, gearboxes, fuel systems and ancillary equipment, fill the fuel tanks and the total running costs for these boats are high. For the short 10mile trips, most operators run slow boats at around 8-10kts as the travel times are still acceptable (approx 1 hour each way) - Long enough to enjoy a martini but not too long so people get bored. Running slow boats keeps their costs down compared to fast ferries which burn alot more fuel per mile.

Now in this above scenario i see real possibilities for a solar project. The boat would be specially designed so that it had more like 80% of teh total area covered in solar. - the sun is harsh in the tropics so most boats here already have covered decks all over anyways...

The battery would be sized so that it could run for the required 1 way trip with zero input from the solar on battery alone with a significant reserve to acount for inclement weather which in total amounts to a 70% depth of discharge including the house loads. The batteries would charge up during the midday hours whilst on mooring for the return trip. On complete rainy days, the return trip would be powered by a diesel genset - which would be required to satisfy commercial marine survey standards here anyway as a redundant system in case of primary system failure. But for most days, the genset would never be started... If poor weather discharges the battery on the home voyage, the battery is charged via shore power overnight ready for the next day.

The system i would employ is based around existing industrial electrical / solar equipment and 240/415VAC output with 3phase 415VAC induction motor prime movers - motor efficiency for these off the shelf approx 93% - perhaps better with custom modification. The solar would be arranged into 5kw strings at around 340VDC per MPPT string to match existing equipment on the market for peak efficiency (sine wave 240VAC rms is 339V peak to peak voltage) and low costing. Therefore all the house loads are met by the same electrical equipment which supplies power to the electric motor prime movers. This means that all the kitchen appliances, bar equipment and refrigeration, fire pumps etc etc is standard household/light commercial or marine 240VAC 50hz type stuff. Most all commercial boats here have 240VAC power installed and i believe survey requirements deem it necessary for most larger boats emergency pumping requirements. So your meeting the standard electrical requirements of the boat at the same time as providing power for the propulsion. This offsets a large portion of the electrical installation costs as all this would have to be installed IN ADDITION TO diesel prime movers. And still no genset running...

So in summary, it wold be a purely solar boat in good weather. Bad weather would dictate that it be defined as a hybrid. In any case, i believe it could be built for a comparable cost to traditional diesel boat (diesel prime movers are very expensive to purchase) with prices of solar equipment now fallen to their current levels. Operators should jump to buy them as their running costs would be lower. Much less maintenance and of course very little fuel...

 

There are guys who have done this. A couple of Bob Oram cats got into the electric motor thing, I saw one in Maryborough but didn't get to talk to the owner. I gather he had liked the electric thing but was in the act of changing back. In fact so did the other Oram that had the electric drives too. They were running them with a big genset.

This from Cruisers forum about the Oram, It now runs twin 20 Tohatsus

When it was wrecked it had a single 60hp outboard centrally mounted. Prior to that it had Torqueedo electric outboards, but they proved unreliable. IIRC 5 failed in 6 months, eventually the money was refunded. And before that it had E-Pod electric drives, which were so bad the owner removed them and threw them in the garbage.

Groper, I don't really get the 5KW per motor driving a 40 cat at 8 knots. Is it not more efficient to put the power plant into one motor. It seems from the experience of twin motor set ups that double the power in one donk gives greater speed than half the power in two donks. Would not a single engine be more efficient. Cat sailors usually motor on one motor when going long distances to save fuel, halve the fuel consumption for 1 knot less.

 

No i meant 2x 5kw electric motors... that would easily push my old oram at 8kts i reckon... the 20hp diesels were barely laboured at 2000rpm... full speed was 3200rpm...

And for further comparison - the 2 diesels on saildrives were ~450kg, we also had 100kg of solar panels on the roof, 250kg of lead acid batteries, 100kg of diesel... the entire 10kw solar, 15kwh battery system would only be a fraction heavier. Probably about par if you compared the weight to a sailing cat with mast rigging and sails with diesel saildrives...

Horses for courses tho mate - ive read about the commercial marine electric drive stuff many times also - theyve all got major shortcomings and i dont see many of them hitting the mark... and like i said - i wouldnt be interested in it either for a private coastal cruising boat... no way...

 

A slight adjustment to your 64m2 array.
The panels will be positioned flat on deck or flat on the roof, far from ideally facing the sun.
The array will then be less than the 10 kW on a land installation.

Also, hosing them regularly with freshwater to keep them clean of salt, will have the watermaker running all the time.

 

The solar array is actually 10.4kW... 40x260W. Next month the same branded panels will be 265W... theyve been steadily increasing their output over the years... I live in the tropics, so mounting them flat doesnt make a great difference at our lattitude...

These are very cheap panels BTW - you can get much higher efficiency panels, the only thing is the cost increases per watt output. Ideally a boat with limited space should use highest efficiency panels, but theres no point in considering it if the cost makes such a project un feasable...

If we used sunpower E series panels instead the power goes upto 327W per panel to 13.08kW nominal total... price unknown, but 1 retailer has them at $540AUD per panel which would be $21600. - likely much better from a proper wholesaler.

As another price point i can give from my catalogue - using the LG Neon panels @ 19.2% efficiency, 315W per panel, same area yields 12.6kW nominal, $14490AUD - this is probably the best compromise between cost and output.

 

small version

Hi all,

I know there will be differences by virtue of scale, but here's my 18' version of a solar cruising cat, which pending sun, will run every day for a few hours, best in short bursts. This works well for exploring interesting waterways.

PV Solbianflex 500w
Torqueedo Cruise2R
2 x 55AH Optima Blue spiral lead acid batteries.
Redarc BCDC40 controller (the weak point at present - great reliable bit of kit but won't let the PV array output as much as it could).

https://www.youtube.com/watch?v=DnzLvCr9JGw

Moondog

 

CatrigCat - An even bigger problem than trying to keep the panels clean is that the batteries add a significant amount of weight. I'll just have to chuck the crew overboard.

groper - Do you know what the power/speed curve typically looks like for a cat? I've been assuming that power is proportional to v^3, but that might not be true for catamarans.

There are some interesting examples of solar powered ferries so that concept seems to be catching on:
Ocius has hybrid wind/solar: http://ocius.com.au/solar-sailor-ferries/current-designs/
NavAlt Solar & Electric Boats in India has a fully solar 75 passenger ferry: http://www.dnaindia.com/india/repor...-built-india-s-first-solar-boat-ferry-2167266
Another example in Chichester, England: http://www.shorehambeachforum.com/s...lectric-solarpowered-ferry-for-Shoreham-Beach

 

Yes, I designed and built my own 11m cat which has so far traveled 3000 NM and going very well. I'll post the resistance curve when I get some time later.

The idea of a solar boat has always had merit for particular usages, another one is demonstrated above by moon dog. The problem has always been cost. That problem is no longer a barrier anymore due to the huge reductions in the relative cost of solar thanks to the economies of scale now present in the solar industry and the massive uptake of solar across the world. 1/3 of all homes in australia now have solar. The price per installed watt is at a point where the solar boat industry is going to get a big shot in the arm...