Boats DC electrical off battery's.
Next to hull, DC floats your boat.

Greg Moore Marine electrician electronics tech and electrical engineer. Links to resources for do it yourself marine electricians. 

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Float your boat now before its to late
DC Electrical Safe float Special
:  Fall Winter 2012/13 
(up to 40 ft and 10 miles from 92106) 
1. Check, tighten put new terminals every bad connection on boat
2. Make checklist of bugs found
3. Check all electronics and calibrations
4. Cut out and discard obvious unused wire
5. Check VHF and HAM radio VSWR and output power
6. Check epirb
7. Check and service battery's and post
8. Check tank level gauges
9. Check bilge pumps and alarms
10. Provide email report of all discrepancy's found

DC 12 volt , 24 VDC, 32 VDC, 144 VDC system theory 
Batterys hold a charge rated at 20 amps for how ever many minutes in amp hours. 200 amp hrs supplys 20 amps for 200 minutes. So you can see batterys dont hold much of a charge so they always need to be replenshed and ideally have an alternate charging source greater than all DC loads combined. 

Battery types matter most if your batterys could ever end up submerged or your sleeping over a big bank while hdrogen and sulfric acid gases escape into your cabin. In those conditions technologys other than flooded open cap or maintenance free batterys will be needed and that drives up price. The best batterys going are lithium because they can take extremely fast and very high current recharge rates and hold more reserve. Lithiom batterys cost ten times what the battery in your car cost for same size.  Use AGM for sleeping on top of gases and use GEL if they need to be stored down low  where flodding may occur. A gel can be underwater and deeply discharged for extended  periods and fully recover. The battry like your car uses once it goes flat you may as well throw it away. 

Biggest fault cause in a DC system is low voltage and loose connections. Tighten up every connection in boat and I mean everyone and you will eliminate many problems on the Dc side. Second biggest fault is wiring for low voltage being way to small or not have enoug strands that are tinned because someone is trying to save money. 

Biggest fault boat owner make. iring non  qualified workers. Every mechanic and handyman or Capt out there says they do electrical. They dont and cant. Navy lectricians and household electricians are about te worst until they get trained. They enough to charge you and thats about it.  if you hire someone to do electrical or electronics and they didnt go to long expensive scools and spend many years after learning, your really adding a great risk to your boating safety and pleasurre. Every seasoned cruiser will tell you its electricity tat keeps them floating and your electrical guy needs to know more than every other trade out there. A simple question like how many watts are in one horsepower the most basic priciple is 746 watts to one horsepower. Most guys on docks runnign wires couldnt even answer it correctly. Thats a rip off even if its free labor. 

Surveyors dont understand enough about electrical to protect you. Most boats arent built with good enough systems at factory because the price would be to high. When you see very expensive boats they always have better electrical systems.