Electric drive conversion info needed

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Electric drive conversion info needed

Post by Transferer »

[originally from: Stephen Bondelid, USA, 17 Oct 2006]

Has anyone installed an electric drive to their Dragonfly?
The more I search on the internet about electric motors on boats, the more I find. For example, Lagoon 420's now come with dual electric drive.

Have any Dragonfly's been converted, and if so, what are your experiences?
Bo Wetzel
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Post by Bo Wetzel »

The biggest problem with electric drives (for boats or vehicles) is the storage of power (some useful calculations later which are also handy for your boat's service battery).

The most commonly used form of storing electricity are batteries or more precise lead-acid batteries. We know all these type of batteries from our cars where they're used as starter batteries. In boats they're are used for starting the engine and running all other consumers like lights, radio etc.

There are other types of batteries, but they're much more expensive. Hence you will find lead-acid type batteries (or a variation thereof) in every milk-float or electric forklift truck. These type of batteries are heavy, very heavy. A battery for a forklift truck weighs, if my memory serves me right, well over a tonne (1000kg/2200lbs). Put one of those in your DF920 and it will think for sure.

20 years ago, I constructed a solar powered vehicle. I used it for three years to drive from my farm to my office in town, a 7 mile (12km) hilly trip. It worked fine but I needed to replace the special traction type batteries regularly after less than a year. Why, I hear you ask, is my car starter battery is still going strong after 5 years?

A few home truths about batteries, their capacity and life cycle.

Capacity:

You will have noticed that you battery capacity is quoted in Ah (AmpereHours).

Say, you have a battery with 100Ah. You might think it means you can draw 10 A (Ampere) for 10 h (hours). Well, you can, if you want to discharge you battery completely and shorten its life considerably!
To be on the save side (and to get a few years use out of your battery) you can discharge you battery comfortably to 50%. Ergo that leaves you with useful 50Ah. For example 5 hours at 10A or 50 hours at 1A.
What does that mean in practical terms? Let's assume you want to run your 12V fridge which is rated at 35Watt (35W).
It will draw a current of about 3A (35W/12V=2.9A).
Luckily it does not need to run continually but at normal room temperature only maybe 25% of the time.

To find out the battery capacity needed to run your fridge for 24 hours our calculation looks like this:
3A x 25% x 24h = 3A x 0.25 x 24 = 18Ah

Say, you have a 140Ah battery (weighing some 35kg or 75lbs - this will be important later) which is fully charged. You could run your fridge for 4 days (and nights) assuming you discharge you battery to a safe 50%. (140Ah x 50% = 70Ah, 70AH/18Ah = 3.9, say 4 days).
But of course, you will have used also some lights, the radio and maybe other things electric. So, your battery has been also emptied by these electricity consumers. Sorry, your beer might be getting warm on the 4th day of your journey.

So remember: Real Battery Capacity is much lower than what has been stated on your battery and Battery Life depends on it's discharge. If you always discharge a battery completely, it will reduce its life expectancy to maybe 50% or less.
You can always use the 50% which rest in an emergency but don't make a habit out of it or you'll be buying lots of new batteries.

Life Cycle:
Normal might be 150-300 charges/discharges. If you discharge your battery completely this might drop to as little as under 100 (depending on your battery's physical construction). If you discharge your battery very little each time before recharging it again you can get several hundred cycles even out of a lead-acid battery.

Now, what has this to do with electric drives?
Lets look at electric motor consumption.
To get the equivalent drive moment out of an electric motor as out of your 10hp outboard engine you will need approx. a 4kw motor (give or take a bit). Lets stay with our 140Ah battery. How many minutes could you run the motor before the battery has been discharged?

As before, Current(A)=Power(W)/Volts(V).
4kW=4000W, 4000W/12V=333A (you will need mighty big cables to connect your motor to your battery but nobody would do this in real life).
So, we have a safe capacity of 70AH from our 140Ah battery.
Hours(h)=Battery Capacity(Ah)/Current(A): 140Ah/333A=0.42h or 25 minutes.

Doesn't sound too bad hey?
Sorry, but there is another problems with batteries. The higher the current you draw from your battery the lower the real capacity!!!!!

A 140Ah battery has only this capacity if you draw a steady current of 1.4A for 100 hours. It's what they call the c100 capacity.
The c10 capacity might only be 120Ah (12A for 10 hours).
Now if you draw a mighty 333A you will certainly look at less than 100Ah you're getting out of your 140Ah battery.
So 50% of 100Ah=50Ah, 50Ah/333A=0.15h or 9 minutes which is a lot less than the 25 minutes we have calculated earlier.

Now lets assume you want to be able to run your motor for 4 hours.
333A x 4h = 1332Ah.
How many of our 140Ah batteries do we need for this? 1332Ah/50Ah=26.6; say 26.
Do you remember from earlier they weight of one was 35kg (75lbs). The total weight of batteries would be 910kg (nearly 2000lbs).
Does that remind you of something I said earlier? A forklift truck battery in your 920?

Of course, in realty one would use not a 12V motor but for example a 48V motor in a marine environment. Therefore reducing the maximum current to a more realistic 85A (4000W/48v). The batteries certainly like this better than the original 333A. However, this changes little on the principal problem. To store electrical power you need a lot of weight.

Multihulls are fast because they're light. Or in other words: per kg of weight you have a lot more sail area than a monohull. It's like having a lightweight car with a big engine. It goes fast. Now add a lot of weight (your batteries) and the whole lot slows down.

But certainly, I think it might be a good option for a monohull to replace the useless lead weight in the keel with batteries. Remember, you read it here first!

The day we can have a battery with 1000Ah in a 50kg package will be the day everybody will have electric propulsion in his boat.
I've been waiting for more than thirty years now to see real improvements in battery storage. One new, very expensive and in my opinion at the moment not viable option are Li-Ion (lithium-ion) batteries. But guess, why you only get 6 months guarantee with one of those batteries?

Anyway, I hope the above explained a few things.

In order to use electric propulsion on a (big) boat you will need also some backup for generating (large amounts) of electricity. For example a diesel generator with an output of 3-6kw (and a fuel tank for this beast). You will end up with a very heavy boat.

However, if you only want to get in and out of harbour under motor it becomes a more feasible proposal even if it will be still heavier package than your 50kg 10hp outboarder.

One further thing: Battery capacity drops in lower temperatures. Above calculation are made at the assumption of a normal (20 degree Centigrade) temperature. Below freezing you might have only 50% of your original capacity.

And don't forget to recharge your batteries immediately after use, they will be grateful for this.

P.S. if you want to know why your car battery last for years (if you don't leave the lights on overnight) just do the following little calculation.
To start a car you turn the starter motor for approx. 3 seconds.
The starter motor current is anything between 200A and 500A depending on the size of your car engine.
Lets take 300A: 300A x 3 sec= 900Asec
to convert this to our familiar Ah divide this by 3600 (there are 3600 seconds per hour):
900Asec/3600 = 0.25Ah.
Nothing at all for your car battery which might have a capacity of 70 or 100Ah as long as the battery has been designed for this type of current. by starting your engine you're discharging your battery only very little. Your car battery is fully charged again after a few minutes drive.
Vimsa
Posts: 15
Joined: Sun 07 Aug 11, 18:46
Your Country: Sweden

Post by Vimsa »

Bo: What you forget is that not everyone is using the full 10 hp of the motor all the time.
I for example often use a low speed setting, that is around 0.5 - 1 hp to drive my DF 800 on and out from the marina with my 5 hp motor.
If I were to install a 55 lbs trolling motor (roughly 1 hp), and use it 30 minutes every day (probably more like 10 minutes just going into and out from the marina), I would use around 10-20 Ah @ 12V.
With a full charge I should be able to run more than an hour full speed. Other people have mentioned 2-3 knots in speed with this configuration and heavier keel boats.

My 45 W solar panel charges 15 Ah on average every day, so if I just add some more solar panels that just might work with the rest of the electronics.
Of course I would like to have a 86 lbs motor in case of strong winds (in which case I would sail anyway). However, then I would need to add one more battery to be able to get to 24 V.
Also there are batteries designed for deep discharge which discharges down to 20% without damage.
Bo Wetzel
Site Admin
Posts: 166
Joined: Thu 01 Jan 70, 1:00
Your Country: France, Provence

Post by Bo Wetzel »

Hello Vimsa,

I think you should borrow a trawling motor and try it on your DF800 going in and out of the marina. IMHO if there is NO wind you will be ok but when you have some wind this becomes another story. I remember being unable to enter a marina and manoeuvre with my 920 and a standard 8hp motor on full throttle. I quickly changed the motor to a 10hp low speed high thrust one!
So, think about the day you have a lot of wind in your sailing area and you want to get back in. Will your tolling motor (55 or 86 lbs) cope? In my experience NO because you have a lot of windage on a trimaran.
Unless you can sail back into your marina it would be an unacceptable risk in my opinion.
Vimsa
Posts: 15
Joined: Sun 07 Aug 11, 18:46
Your Country: Sweden

Post by Vimsa »

Yeah, wind is my biggest concern. My goal is to do some tests with this next season. Fortunately, 55 lbs trolling motors have come down a lot in price so it will not ruin me to perform tests.
It's pretty easy to find shelter behind islands everywhere here, and my marina is pretty good sheltered from all directions too. Navigating in strong winds and shallow waters with rudder and fin brought up may pose a problem. Maybe I should install a bow thruster as well :-)
JP
Posts: 8
Joined: Sun 01 May 11, 8:22
Your Country: Switzerland

Electric motor

Post by JP »

I have been tinkering on that subject. I manage to fit a 24V 500W electric motor on an outboard shaft. I did test that configuration with two 12V 27Ah batteries and got a 2.5 kn speed (no wind). We are back to yes it would work for say 10 min. getting out of harbor and 10 min. getting back in. And that only if wind is light...
I still have my 6hp Honda installed on the boat wich provide up to 5-6 kn if needed.
DF 800 on Lake Geneva
Steve B.
Posts: 235
Joined: Thu 02 Nov 06, 1:58
Your Country: USA, Whidbey Island WA

Electric Drive Batteries

Post by Steve B. »

I haven't converted my DF1000 to electric drive (yet), but I have installed a single bank of 200 aH Lithium Phosphate cells and couldn't be happier with their performance as well as weight savings.
They weigh HALF of L/A batteries and can be discharged 3000 times to 80%.
They will accept up to 3C charging current, i.e. a 200 aH bank will accept up to 600 amps charge or discharge without damage.
They charge with almost no taper up to full charge, and the discharge voltage curve is almost flat.
My Spectra desalinator makes more water than before because the normal battery voltage is around 13.4V.
WE LOVE THEM !!!

More here:

http://www.cruisersforum.com/forums/f14 ... 65069.html

Lead Acid? Never again !
Vimsa
Posts: 15
Joined: Sun 07 Aug 11, 18:46
Your Country: Sweden

Re: Electric motor

Post by Vimsa »

JP wrote:I have been tinkering on that subject. I manage to fit a 24V 500W electric motor on an outboard shaft. I did test that configuration with two 12V 27Ah batteries and got a 2.5 kn speed (no wind). We are back to yes it would work for say 10 min. getting out of harbor and 10 min. getting back in. And that only if wind is light...
I still have my 6hp Honda installed on the boat wich provide up to 5-6 kn if needed.
I hybrid would be best, where you used the electric engine for going in and out of the marina in calm conditions and start up the petrol engine when there is need for more power.
The electric engine could then double as starter engine for the petrol engine.. anyone wanna construct? :-)
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