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Old 24-11-2007, 15:02   #1
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Small Boat Electrical Q's? 12v

I am currently working on my '83 Mac 25's wiring system. I have never really understood what I was looking at till I purchased The 12 Volt Bible for Boats by Miner Brotherton revised by Ed Sherman (second edition), which really helped me understand the electrics on my small boat.

The running lights have never worked since I have had the boat and currently dont still after fiddling with them today. My FM/CD player was hooked up straight to the battery so I rewired it to my control panel(this was the first thing that I have done correctly in my vast trials with electricity).

My wiring system (I think) is not normal. The + and - coming off of the battery run directly to an approximatly 4x12 peice of thick plexiglass with 2 narrow copper sheets running parrell down it lengthwise (one is for + and one for -). This has about 4 seperate + and - studs on it for various items. From one of them is wire leading to my control panel. Also off of it are wires leading to my bilge switch and bilge, and my VHF radio. I dont think this is a good thing but I am not sure. Do my bilge and radio need to be ran to my control panel? If anyhting what is so bad about my switch and fusesless plexiglass control panel?

The main panel works and I wired my FM/CD to it today and it works well. I have a total of four ummmm... switches? to wire different applications too. BUT there is only a place for ONE ground????? I dont get it, shouldnt it have 4 different places to ground things?????? A freind and home electrician told me to splice wires together and ground all the applications on the one ground. Is this correct?

On my control panel, the ground is a copper strip with a ground wire attached that runs to the - point on my battery. I guess that being so then everything that I ground (on the panel and the plexiglass panel) is then grounded to my battery. Is this alright? Should I make a ground that is on my swing keel winch or something?

Do I need a master battery switch for only one battery? My assumption is that the master switch is only there to eliminate anything that may use the battery even when it is not turned on. For example, a VCR still draws a little power when switched off for the clock. Is this needed?


I have a good freind who is an home electrician by trade and good at what he does but I am not sure what he knows about boats. (he is the one that wired up my FM/CD to the battery) I knew that wasnt good but it worked at the time.

Sorry its long but I worte down these questions today as I was working. Thanks for all replies!!!!!!
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Old 24-11-2007, 16:24   #2
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Home and marine electronics are fundamentally different things and the wiring process is also quite different. You should have only one common ground on a boat. The reason is more complex than you probably care but having multiple grounds leads to problems. You should absolutely have a master battery switch for no other reason than safety.
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Old 24-11-2007, 17:07   #3
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Hmmm ok, when you say one common ground, would the battery be considered one common ground? Or, should it be elsewhere? Why should there be only one grounding spot and not as many diferent place that I can find?
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Old 24-11-2007, 17:58   #4
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Actually, your "system" is correct as far as earth etc are concerned.
The easiest way to think of wiring is just like plumbing water. Your battery is the water tank. You want a tap(switch) at the in case you have a leak down the line and want to turn it all off. In the case of electricity, it would be a fault that could cause wiring to melt. So the Tap or switch in our case, wants to be a circuit breaker or fuse. It needs to be large enough to cope with all demands of the equipment it is supplying, but small enough that the fuse or breaker goes before the main supply cable melts. I tell people, fuses are there to protect the cable, not the equipment. The equipment will have their own fuses. A breaker gives you the ability to manually turn the power on/off if you want a main switch to shut everything down. The distribution panel needs to be replaced. You need a panel with individual switches and fuses to protect each smaller branch of the circuit. Thus is you have a short in a light, it only affects that circuit and not the entire system. An earth and a positive wire needs to run to each device you are powering. The earth of that battery needs to run to the control panel and a seperate earth from that panel to each device you are powering. The Battery earth Terminates to the Engine block if you have an inboard engine. There should be no other earth wire terminating to any other part of the block, other than that one connection.
I hope that wasn't to complicated and makes sense.
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Old 24-11-2007, 19:39   #5
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I understand everything you stated, not complicated at all but a good explanation.

"The Battery earth Terminates to the Engine block if you have an inboard engine."

I dont have an inboard but an outbaord motor. So does this mean that I will have to settle with my earth being only to the battery?
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Old 24-11-2007, 19:44   #6
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It isn't critical with an outboard which location you choose for the single ground point so either the neg buss or battery grd post will do
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Old 24-11-2007, 23:06   #7
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And then?

So what could I expect if I did not earth my house bank to the engine block?
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Old 25-11-2007, 08:08   #8
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So what could I expect if I did not earth my house bank to the engine block?
Not much as the engine won't start
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Old 25-11-2007, 18:05   #9
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House bank...

I did specify house bank...
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Old 25-11-2007, 18:46   #10
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Go to the Blue Sea web site. They have some easy to understand wiring diagrams there for outboard boats.
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Old 27-11-2007, 18:14   #11
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Wow, yes the easiest way to think of electricity is a water tank just like Alan said. Do you need to understand OHM's law and Kerschoff's law and kuloms and such? I think not, you need to understand a circuit. That is you have a source of power(battery) a conductor(wire) and a load or resistance(lightbulb or radio, etc.) The circuit is complete when you have these things and the conductor connected positive to negative. I think a better book for you would be Boatowners Illusstrated handbook of Wiring by Charlie Wing. I read through Miner's book and he was still talking about single strand wire which is a no-no today. All wires today are multiple strand copper and for boats should be tinned copper. There are so many standards out there as far as the ABYC is concerned and they are meant to make boating safer. I think Charlie Wing will help you a lot as it goes over basic electrical thoery and ABYC standards. it should give you the confidence you need to tackle a decent sized project. I could go on for hours, but I hate looong threads. If you have anymore specific questions please feel free to email me. You will need to remember this as I will never forget it..."All amps go home". It may not seem like much now but wait till you get into stray current and galvonic corrosion. I'm here if you need me! Good luck!

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Old 27-11-2007, 18:17   #12
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Oh let me not forget the *gawdfather* of electricity when it comes to boats, Nigel Caulder. Any of his books are good info and up to date.

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Old 27-11-2007, 18:32   #13
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Ok i suck at this, ditch the wierd set up as soon as possible. I think you'll get much better results from a newer system, I think it will save you a lot of trouble (also money)down the road and it will be a LOT easier to understand. I can't say enough on this.
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Old 27-11-2007, 18:44   #14
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Oh, if you don't have fuses, you are a pretty good fire hazard which might put your boat with Davey Jones and his locker. Please get some fuses or a panel with circuit breakers, i'll sleep better. You have them in your house on dry land, don't you think you would want them 20 miles off shore?
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