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Old 20-04-2021, 07:48   #1
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Basic Electrical for a Boat?

I hate to ask this, but after reading the stickies and lots of youtube videos, I still can't get a grasp on the electrical system.

I want to keep this from being too broad, so I want to ask how volts, watts, and amps relate in a real world situation, not that water and pipe analogy again . I always hear things like "it only draws XX amps!" and "I need a 1200watt inverter for it" and "That won't be enough with a 12v battery". I just made those up, but can someone walk me through it please?

My boat was bought used and seems to have everything wired well as everything keeps charged and works when it needs to, but I fear the day when something goes wrong or I want to add something to the system.

I have a 12v starter bank, 24v house bank (600 amp battery bank, AGM, marine gel), 750w of solar, wind generator, 2kW portable gen. I look to, in the future, add a small marine washer, washdown pump, and converting the stove to electric but I have no idea how to even shop for these electrical-wise.

Please help!
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Old 20-04-2021, 08:00   #2
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Re: Basic Electrical for a Boat?

Pretty broad question not easily answered. You are essentially asking us to make the effort to educate you on power systems.



My suggestion:


"Boatowners Mechanical and Electrical Manual" 4th edition, available on Amazon among other sites. Will give you only a basic understanding but sufficient for most boaters.
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Old 20-04-2021, 08:04   #3
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Re: Basic Electrical for a Boat?

The relationship between voltage, resistance, current, and power are defined by Ohm’s Law, which states that: the current through a conductor, between two points, is directly proportional to the voltage across the two points; which is mathematically expressed as:
For Voltage: E = I x R
For Current in Amperes: I = E ų R
For Resistance in Ohms: R = E ų I
For Power in Watts: P = E x I
and their various transpositions, as below.


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Old 20-04-2021, 08:16   #4
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Re: Basic Electrical for a Boat?

As a newbie here and one that has spent the last year + self educating myself on marine electrical I would say this....it takes considerable time and effort to be able to ask a focused question about marine electrical systems. I’ve had good resources in that my dock neighbor is a master marine electrician and is willing to help me...but I’ve found that I need to do my homework before I can ask good questions.
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Old 20-04-2021, 08:26   #5
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Re: Basic Electrical for a Boat?

Quote:
Originally Posted by GordMay View Post
The relationship between voltage, resistance, current, and power are defined by Ohmís Law, which states that: the current through a conductor, between two points, is directly proportional to the voltage across the two points; which is mathematically expressed as:
For Voltage: E=IxR
For Current or Amperes: I=E/R
For Resistance: R=E/I
For Power P=ExI
and their various transpositions, as below.
I saw that in the stickied post, but I don't see how, say, a 1000w coffee maker fits into it. If I have (for the sake of easy math) 500w of solar, 500amp battery bank, how would I know what else I can add to it? I see my (home) coffee maker says 1000w, how would I know how many 1000w appliances I could support with the system?
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Old 20-04-2021, 08:28   #6
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Re: Basic Electrical for a Boat?

Quote:
Originally Posted by Randy Bogardus View Post
As a newbie here and one that has spent the last year + self educating myself on marine electrical I would say this....it takes considerable time and effort to be able to ask a focused question about marine electrical systems. Iíve had good resources in that my dock neighbor is a master marine electrician and is willing to help me...but Iíve found that I need to do my homework before I can ask good questions.
I've been slowly studying up on it as well (among many other things). I think you're right, I'll keep at it, and I just ordered the book suggested in the third comment!
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Old 20-04-2021, 08:40   #7
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Re: Basic Electrical for a Boat?

Another point on learning electrical systems....I drew an “as built” electrical plan for my boats electrical system that really helped me understand my boat. It also helped when I had questions to ask.
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Old 20-04-2021, 08:47   #8
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Re: Basic Electrical for a Boat?

The following book has been invaluable to me over the past 20+ years of fiddling around with boat electrical systems - improving, fixing, modifying etc etc.

Sailboat Electrics Simplified. Don Casey. Available on Amazon and many marine stores.

The book is now a little dated but the basic advice remains sound because as noted above, the physics involved in electricity has not changed, only the way we apply it has.

Good luck.
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Old 20-04-2021, 08:53   #9
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Re: Basic Electrical for a Boat?

Volts X Amps = watts

I explained this to my wife this way. A 1800 watt hair dryer at 120 volts is 15 amps.

Therefore, for simplistic sakes. If house bank is 12 volts with inverter, it would require 150 amps at 12 volts to power her hair dryer at 120 volts.

I know there are some loss with resistance due to wire size and inverter but..start small.
Once you work with that information for a while, then we can start with resistance to current flow. Using the phrase, Stick a PIE in your EAR. P=IE, E=AR
P=power=watts
I or A=current=amps
E=volts
R=resistance=ohms
BTW she either didn't understand or care! Hope this helps. It seems you may be more motivated than she.
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Old 20-04-2021, 08:55   #10
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Re: Basic Electrical for a Boat?

Quote:
Originally Posted by crybllrd View Post
I hate to ask this, but after reading the stickies and lots of youtube videos, I still can't get a grasp on the electrical system.

I want to keep this from being too broad, so I want to ask how volts, watts, and amps relate in a real world situation, not that water and pipe analogy again . I always hear things like "it only draws XX amps!" and "I need a 1200watt inverter for it" and "That won't be enough with a 12v battery". I just made those up, but can someone walk me through it please?

My boat was bought used and seems to have everything wired well as everything keeps charged and works when it needs to, but I fear the day when something goes wrong or I want to add something to the system.

I have a 12v starter bank, 24v house bank (600 amp battery bank, AGM, marine gel), 750w of solar, wind generator, 2kW portable gen. I look to, in the future, add a small marine washer, washdown pump, and converting the stove to electric but I have no idea how to even shop for these electrical-wise.

Please help!
Adding an electric stove and a washing machine .... Your generator could be t running 24hrs on a given day. It's not just about what you can use. It's about what you can recharge.
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Old 20-04-2021, 08:56   #11
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Re: Basic Electrical for a Boat?

Wth Ohm's law, just don't confuse dependant and independant variables, or cause and effect. You can change the voltage and the current will change, but you can't just arbitrarily change the current. Voltage pushes current, not the other way around.
And for God's sake, don't confuse amps (rate) and amp hours (quantity)

Another good book is The 12 Volt Bible for Boats.
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Old 20-04-2021, 09:01   #12
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Re: Basic Electrical for a Boat?

Well, the water in a pipe analogy is the best way I know of to explain it. But here it goes: Ohm's Law says Volts times Amps equals Watts. Watts is a measure of the work a device is doing, actually the power needed to make it do that work. A particular device would use the same number of Watts no matter what voltage it is operating at. The number of Amps required to deliver that number of Watts would vary according to the number of Volts that are being supplied. For example the household type refrigerator in my trawler draws one amp at 120 Volts AC when plugged into shore power. That's 120 Watts, 1A x 120V = 120W, or 120W / 120V = 1A. But if I'm unplugged and powering it with my inverter, the inverter is running at 12 Volts DC from my batteries. The draw on the batteries is now 10 Amps plus whatever loss there is in the inverter itself, call that 1 Amp. 120W / 12V = 10A + 1A loss = 11A total draw. So when running on the batteries the fridge draws 11A when actually running. Now a fridge doesn't run continuously, it only runs when it's internal temperature is above the thermostat's set point. If it runs half of the time, it would draw 132 Amp Hours per day or 11A x 24 hours = 264 amp hours / 2 = 132 amp hours. (The actual run time varies with room temperature, how full the fridge is, how well insulated, and how often you open the door.) These calculations can be made on all devices on the boat, and are the basis on deciding how big a battery bank, generator, and solar array are required. You audit all the devices on the boat and add up all the Amp Hours likely to be used in a day to decide how much battery and solar you need. Add up how many Amps (not Amp Hours) are likely to be consumed at once to determine generator and inverter size.
The equipment listed in the original post seem to be pretty adequate, but you need to do the math to be sure. Everything should have it's power draw documented somewhere, and household stuff has to have it labelled on the device. As for adding equipment, Amps or Watts used by the added device is one of the most important factors. How often will that device run and do you have enough capacity to run it? Example if I were to replace my fridge, I would go with a 12V version as they are much more efficient, but at about 3x the purchase price.
As for switching to an electric stove, I wouldn't do it unless you plan to be plugged in most of the time. Making or removing heat (stove or air conditioning) uses massive amounts of power and even a robust 12V system will have trouble doing it. Even the 2kW generator will have trouble, may not run the oven and a stove top burner at the same time. A friend of mine tried this unsuccessfully and is considering adding a second 2kW generator in parallel just to to run the stove. He's not going to be popular in a quiet anchorage with two of those things running. Conduction stove tops use less power, but you don't get an oven with them. I took the electric stove out of my trawler and went to propane. Not as safe but much more energy efficient and just like the gas stove at home we're used to.
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Old 20-04-2021, 09:01   #13
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Re: Basic Electrical for a Boat?

Amps dictate the size of the wire and the size of the breakers and or fuses. A wire rated at 10 amps needs a 10 amp fuse or breaker to protect the wire.

Amps tell you how much power your devices are drawing.

a device using 12 volt is going to draw twice as many amps as the same exact device running on 24 volts.

To get amps from Watts you just take the Watts and divide it by the voltage.

to get the Watts from amps you just take the amps and multiply it by the voltage.

A 1200 w inverter running on a 12 volt system would be pulling 100 amps at its maximum continuous. So in this situation you would have a wire or cable rated for over 100 amps with a fuse of 100 amps.

Whenever you branch off and go smaller you need a smaller fuse or breaker to protect the smaller circuit and so on and so forth.

That was my shot at simplifying electrical explanation. I hope that helps.
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Old 20-04-2021, 09:05   #14
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Re: Basic Electrical for a Boat?

I don't know if it have in english but take a look, nice regards.
https://youtu.be/1fdEgkVaNdY
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Old 20-04-2021, 09:08   #15
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Re: Basic Electrical for a Boat?

I took a "Marine Electrical" course with the local US Power Squadron (Now called "America's Boating Club").

I am an engineer by trade, and I still learned a lot. Course was not expensive, especially if you become a "member" of the boating association.

I took the course as my newly purchased 30ft sailboat came with most of the electrical system ripped out, and I needed to know how to install the new wiring.

You can take the course with an online instructor, or just buy the book and do it at your own pace by yourself.

Course: https://americasboatingclub.org/cour...trical-systems

cheers
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