Electric Motors and Battery Size

[peter.bomberg]

Ok so this is a new by question but assuming a Privilege 51 weighing 15 Tons, it appears that two Thoosa 13000 should do the trick (http://www.asmomarine.com/2005/asmo_...OOSA_13000.pdf) they are 72Volt systems so assuming that I need the batteries wired in banks of 3 (based on 24 volt batteries), to get a 6hr motoring capacity I am assuming I need 50% of max current draw to sustain the speed (assuming dead calm as otherwise I would be sailing) so 6*100=600Amps thus I need to have a minimum of 9 batteries to give me 600Amps, they would weigh about 1,152lbs and to double the range I need to double the battery bank and so on.

Can someone who is better at electronics check this as if it's right then I am net zero for 6hrs of motoring versus the weight of 2 50hp diesel engines (of course a genset and solar panels) will extend the range but then how many hours of motoring should I be planning on is the next question as from my reading there can be a day or more on a long ocean crossing with no wind)

Advice and help needed!

Peter

[LtBrett]

Well, the specs say the motors are rated at 200A (I assume continuous) and 400a peak. I'd approach the problem a little differently. First you have to determine the current draw at cruising rpm. Say it's 100a per motor. I suspect it will be less, but let's use this as an example. You'll need a generator that can deliver 200a to the motors. Factor in losses and you'll probably want the ability to recharge your batteries while cruising and you're looking at perhaps 250a or an 18kw dc generator. Now you can cruise until your fuel runs out. Next, ask yourself how long you'd like to run without starting the genny. If your answer is six hours, you need 200a x 6 hours x 2 = 2400 amp hours. 12 8d batteries will get you roughly 2400 amp hours (6x12v per side). Want to go lighter? 12 4d batteries get you roughly 1800 amp hours, good for 4.5 hours of cruising. I multiplied your amp hours needed by 2 so you only draw you batteries down 50%.

Hope this helps.

Brett

[Talbot]

Just to make the calculations more complex, if there is no wind, thus motoring = no wind resistance. Thus on a diesel boat you would be using a single engine. With electric I would suspect that this would make little difference to power consumption to achieve a specific speed. However, if you were using something like autoprop then this would also change the result.

[Rising Star]

Peter,

Your amp usage will be dependent on a number of factors so running your figures is just a starting point. Get the biggest batteries your boat and wallet will handle.

You may want to check out the Electric boats group at: electricboats : Electric Boats. Almost any question possible has been answered there.

Also take a peek at http://www.electricyacht.com/ . I got my system from them and got great support and service at a good price.

[peter.bomberg]

Thanks Everyone, I guess the first question is how long cruising time do I need on Battery alone? Please give me your thoughts is 6hrs adequate as it look like I can be at net zero weight wise up to 6hrs

[Rising Star]

Peter,
I have 2 banks of 4 100A AGM batteries for my 48v motor. I only use one at a time for motoring leaving 1 in reserve for any emergency full-power situation. I can motor full power for 45 minutes on one bank or putter along at 3.8 knots using 18A/hour. My record use at one time is 17 hours at 18A with a Honda 2000i generator plugged into my 18A battery charger. I also have a 200watt 48v solar panel that can recharge a bank after 2 sunny days on anchor.

My feeling is after all it is a sailboat!