Batteries and power system????

[Justme]

Yes the higher the volts the lower the amps for the same amount of effective work.

 

Thats why some go for a 24 or even 48v domestic battery bank set up. The cable can be lots smaller / cheaper as cable size is down to the amps it carries not the watts.

 

You also have to allow for charging losses. You will get people say they allow from about 10% to about 30%.

 

I tend to go high to build in more safety margin.

Edited February 6, 2010 by Justme

[fabius]

A diesel engine needs to be pushed reasonably hard for maximum efficiency and economy. On the inland waterways where most of the water is still or slow moving, you would probably find that 10hp would push it along quite happily and at amazing low fuel consumption. However, as your boat will have a lot of mass, I'm not suggesting you go to 10hp, it will need more power for emergency stopping or pushing through a faster flowing current on occasions. There are formulae for working it all out based on displacement, waterline length etc, but it needs someone with more knowledge than me to help with that. As most cruising will be done at very low revs, perhaps 1000-1200 rpm, 75hp would be a waste of money, power and efficiency IMHO. If you are only going to be on faster waters occasionally, even 45hp might well be sufficient, as it would be very happy to be pushed hard if needed. I would certainly try to get someone to do the maths for you.

 

I understand where you are coming from with a salty water background, as there is a need to push through fast tidal flows, large waves etc. I think the norm is for 50% above the expected required maximum power, as a safety margin. I was reading an interesting article a few moths ago about a seagoing boat with a hybrid system. The boat was about 45ft, with a 10hp equivalent electric drive which gave a reasonable cruising speed under calm conditions and for harbour work. It was in tandem with a 75hp diesel, which was used when conditions needed it or a higher speed was required. It also had a high output alternator for charging the batteries.

 

Roger

The smaller of the donkeys it'll be then. I'll get onto the builder monday.

You know when i think about it i already knew how efficient displacement craft were and it makes sense. The "stopping" bit however i had not thought of and your right about the mass so I wont want to go too small. I have the equation for the hull around here somewhere, i think it's the same one that we use to calculate handicaps in sailing races.

 

As for hybrids well!! Sounds lovely but light bulbs a proving a challenge at the moment.

It does seem like the logical way forward though with battery technology improving and getting cheaper. it would be uber efficient all right.

 

Thanks for the help.

[steelaway]

So if i'm factoring in 240v items does that mean that the amp's would be 20 times(ish) less than if they were 12 v.

 

i.e........... a 12.8v item at 6amps = 76.8 watts

 

the exact same same item 240v would be 240v x 0.32amps =76,8watts. ???? (Plus whatever power is lost in the inverter)

 

 

THE MUD IS CLEARING...Here was me beginning to think that you somehow got something for nothing using 12v... Bloody Newton and his stupid laws!

 

 

Hi

 

As said earlier W=AxV therefore A=W/V

So a toaster at 1000w is A=1000/230 = 4.3 Amps

But as the power is coming from the battery's (via the invertor)@ 12V

The calculation is A = 1000/12 = 83.3Amps !!!! - (plus a little in invertor losses)

I think you are right is considering a smaller engine, by the time you have a prop matched to that amount of HP if would become a bugger to maneuver on the canals.

 

Alex

[fabius]

Hi

 

As said earlier W=AxV therefore A=W/V

So a toaster at 1000w is A=1000/230 = 4.3 Amps

But as the power is coming from the battery's (via the invertor)@ 12V

The calculation is A = 1000/12 = 83.3Amps !!!! - (plus a little in invertor losses)

I think you are right is considering a smaller engine, by the time you have a prop matched to that amount of HP if would become a bugger to maneuver on the canals.

 

Alex

So the electric shower is out then....

presumably that 86amps is only in the cabeling between the batteries and the invertor right? After that it's sent round the boat in 230v at 4.3 amps. Is that ampage a problem for the batteries or the wiring, i'm not planning on having an electric toaster but just so i know what the restrictions are. I'll have 230v sockets as most of my stuff for now is at that voltage. Is it normal for the ampage to get that high or do you just have to avoid high powered items all together.

[steelaway]

So the electric shower is out then....

presumably that 86amps is only in the cabeling between the batteries and the invertor right? After that it's sent round the boat in 230v at 4.3 amps. Is that ampage a problem for the batteries or the wiring, i'm not planning on having an electric toaster but just so i know what the restrictions are. I'll have 230v sockets as most of my stuff for now is at that voltage. Is it normal for the ampage to get that high or do you just have to avoid high powered items all together.

 

 

Hi

 

You find that you will inspect every electrical item you put on the boat for its wattage.

You might have to fight with the wife/partner about an hair dryer

A friend maintains that his wife has the worlds most expensive hair dryer - he had to buy a 2Kw Honda genny to power it!!!!

I had to have a toaster as my wife insists on toast for her breakfast and the grill is crap - I've found it best to do as I'm told

The battery cables to the invertor are always huge, mine has 300A fuse.

You will have to find out how much current ever item on the boat is and wire accordingly.

Pumps/fridge/TV/dvd/toilet/lights/headlight/horn/CH/ etc.

There will be reference to these some where on the furum.

 

Alex

[ChrisPy]

there are several extremely good books on marine electrical systems, a good investment if you are learning from scratch. Try a search on Amazon.

[nb Innisfree]

Ref large battery banks, above 75-80% SoC a bank will reduce charge due to resistance so a large bank above 80% SoC will charge just as quickly as a small one. It's only below 80% SoC (bulk charge) that alternator/charger size is relevant which means in practice a large bank takes only slightly longer to recharge than a small one especially when batts are getting on in their life.

[Lady Muck]

You might have to fight with the wife/partner about an hair dryer

 

Or tell them to adapt - I grew my fringe out when we got the boat, it was the noly reason i ever used a hair dreyer because it would stick out at right angles if I didn't dry it. I waved goodbye to dry land and hairstyles....