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Back to basics. Here is a simple, theoretical schematic. I've never had formal training in electricity beyond high school physics, and I don't want to presume anything.

1. The engine block is in the negative side of the DC system. What creates "boat ground potential" in that system -- the connection to the negative posts of the batteries? What is the relevance of the fact that the propshaft is in the ocean and electrically continuous with the engine block -- is that important in the nature of DC ground, or does "boat ground potential" in that circuit just mean return to the negative battery pole?

2. What happens if the boater unhooks black cable E from the starter mounting bolt in order to disable the engine circuit but leaves house circuit intact and switched on? Is the house circuit still "grounded" because cable F completes the house battery circuit to the house battery negative post?

3. ABYC E-11 states "11.5.4.7.4 If the negative side of the DC system is to be connected to ground, the connection shall be made only from the engine negative terminal, or its bus, to the DC grounding bus. This connection shall be used only as a means of maintaining the negative side of the circuit at ground potential and is not to carry current under normal operating conditions." What does "not to carry current" mean? Does not the black wire in a standard two-wire DC system carry current, and so the engine block also carries current for devices that ground through their metallic attachment to the block?
 

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Marine applications are one of the few instances where "negative" and "ground" are not synonymous.

"Negative" deals with the current carried from a battery, genny, etc whereas "ground" refers to the current generated by dissimilar metals within an electrolyte (seawater).
Right away, you can see that the grounding system's purpose is to prevent the boat from becoming a battery by making sure the "battery's" electrodes (underwater metal components) are shorted out and allowed to dissipate through the sacrificial anodes (zincs)

The ABYC bit you quoted specifically forbids using any of this metal as part of the grounding(bonding) system. Essentially, you are electrically connecting all the underwater metal components. In your example, if the battery negative were disconnected, then obviously the engine wouldn't start. Ignoring that for the moment, the zincs would begin to corrode much faster. After being eaten away, more expensive metal begins to disappear.

In building aluminum power boats, we would bond all ac components and all significant hunks of metal together with 4 ga wire (green). Otherwise, the practical results are gremlins like hot water tank and heat exchanger zincs that don't last the season.

A marina shorepower hookup can make this even worse because now you can have two adjacent boats acting as individual electrodes.

This is all completely independent of the DC power system.
 

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I'm considering not bonding my tru-hull fitting on my FG boat...I hope to hear more pros and cons on this thread.
 

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I'd prefer for this thread not to go down the bonding rabbit hole, if that's OK. My question is simpler than that.

To better focus my question, is there anything wrong with sailing along using the house panel with the engine taken out of the circuit by disconnecting cable E at the battery in that theoretical schematic? (Other than the fact that none of the engine circuit devices will be usable, of course, and for safety cable A ought to be disabled too, using the battery switch.)

[My understanding: There would be nothing wrong with that practice, if the boat's wiring is installed and maintained to code.]
 

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I am specifically chasing clearer understanding of "ground", "grounded" and "negative" in the DC circuits, not "grounding" or "bonding", which are not part of the current-carrying wires normally supplying devices in a DC system with current. I left AC green grounding, lightning and DC bonding wires out of that schematic to avoid all the distraction they create. Hopefully, we can leave bonding out of this thread as much as possible, and just focus on DC negative and the role of the engine block. My amateur understanding is shown in square brackets and open to correction.

E-11 gives these definitions:

Ground: The potential of the earth's surface ... established by a conduction connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.

[My understanding: in a self-contained DC system in a fibreglass boat, "ground potential" is determined by the negative posts of the batteries. But, I wonder what role the propellor shaft's connection to the ocean plays in defining a boat's ground potential, and whether it is important.]

DC Grounded Conductor: A current carrying conductor connected to the side of the power source that is intentionally maintained at boat ground potential.

[My understanding: any DC device's black cable or wire that is ultimately connected to the negative terminals of the batteries is the negative wire, and can be considered "ground" = "boat's ground potential". These normally carry current, unlike bonding wires that some boaters might connect between a metallic throughull fitting and the boat's DC wiring.]

DC Grounding Conductor: A normally non-current carrying conductor used to connect metallic noncurrent carrying parts of direct current devices to the engine negative terminal ...

[My understanding: if a wire is connected from a DC device's casing to the boat's DC negative wiring, the purpose of that wire is to carry current only if there is a short in the device that would otherwise make the device's casing electrically hot. This seems equivalent to the third green or bare wire in an AC system, but I don't recall ever seeing a third, normally non-current conducting wire from DC device.]

[Calder (page 231) emphasizes that the term "grounded" is reserved for the current-carrying wires that normally supply current to DC devices, while "grounding" is reserved for wires that only carry current in unusual situations, such as stray current (bonding), or lightning.]

Polarized system DC: A system in which the grounded (negative) and ungrounded (positive) conductors are connected in the same relation to terminals or leads on devices in the circuit.

[My understanding: "Grounded" in this sense means that the black wire intended to complete the current-carrying DC circuit for a device is continuous with the battery negative terminals. So, it makes sense to me that "boat ground potential" (phrase used in E-11) might not be the same as earth (ocean) ground if the electrical connection is only to the negative posts of the batteries to create a local "ground" within the boat, and not to the ocean.]

[My understanding: The engine block in that schematic is a giant black wire, carrying current for devices that attach to the engine block and use the metal attachment as their current-completing path to the boat's ground (battery negative terminals). 1. That's why it is so important to keep the main DC ground cable connection pristine from engine block to battery negative post. 2. This analogy explains why I think it is OK to disconnect the engine circuit, but safely carry on using the house circuit.]

The propellor shaft connects the engine block to the ocean. That's not like in a car, where the engine block normally has no connection to earth. I am still fuzzy about a couple of things:

1. Whether the DC current from a device like an oil pressure sender or alternator that grounds through its connection to the engine block could return through the prop shaft to the ocean instead of back to the battery, and whether this is important. Calder (p 121) points out that current actually flows from negative to positive.

2. Whether the propellor shaft connection therefore puts the boat's ground potential to earth, over-riding the potential at the battery negative terminals.

2a. Whether point "2" is important.
 

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The ABYC bit you quoted specifically forbids using any of this metal as part of the grounding (bonding) system.
Ah yes, I see now what they were getting at. I was distracted by the "if" in "If the negative side of the DC system is to be connected to ground...".
 

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Back to basics. Here is a simple, theoretical schematic. I've never had formal training in electricity beyond high school physics, and I don't want to presume anything.

1. The engine block is in the negative side of the DC system. What creates "boat ground potential" in that system -- the connection to the negative posts of the batteries? What is the relevance of the fact that the propshaft is in the ocean and electrically continuous with the engine block -- is that important in the nature of DC ground, or does "boat ground potential" in that circuit just mean return to the negative battery pole?

2. What happens if the boater unhooks black cable E from the starter mounting bolt in order to disable the engine circuit but leaves house circuit intact and switched on? Is the house circuit still "grounded" because cable F completes the house battery circuit to the house battery negative post?

3. ABYC E-11 states "11.5.4.7.4 If the negative side of the DC system is to be connected to ground, the connection shall be made only from the engine negative terminal, or its bus, to the DC grounding bus. This connection shall be used only as a means of maintaining the negative side of the circuit at ground potential and is not to carry current under normal operating conditions." What does "not to carry current" mean? Does not the black wire in a standard two-wire DC system carry current, and so the engine block also carries current for devices that ground through their metallic attachment to the block?
Floatsome, I just found this thread, and I'm trying to understand what it is exactly that you are asking. It sounds like what you are asking is if the negative potential in a circuit needs to be connected to earth ground, is that what you want to know ?

The answer to that question is, not really, you can float a ground. Airplanes do this all the time, they are not grounded to earth. Solar panel systems sometimes do this as well though I don't think that is to code. You can hook a solar panel up to a battery with two wires and the panel will charge the battery, but it is normal to connect the negative side of the circuit to an earth ground. What they mean by it not carrying any current is that the earth ground connection just insures everything is at the same potential, but that connection really doesn't carry current to do that, it's more like touching metal and getting a static spark, once it is discharged, there is no measurable flow of current after that.

If you disconnect your electrical system from earth ground it will still work, it is called a "floating ground".
 

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I am specifically chasing clearer understanding of "ground", "grounded" and "negative" in the DC circuits, not "grounding" or "bonding", which are not part of the current-carrying wires normally supplying devices in a DC system with current. I left AC green grounding, lightning and DC bonding wires out of that schematic to avoid all the distraction they create. Hopefully, we can leave bonding out of this thread as much as possible, and just focus on DC negative and the role of the engine block. My amateur understanding is shown in square brackets and open to correction.
I will take a shot at this.

E-11 gives these definitions:

Ground: The potential of the earth's surface ... established by a conduction connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.

[My understanding: in a self-contained DC system in a fibreglass boat, "ground potential" is determined by the negative posts of the batteries. But, I wonder what role the propellor shaft's connection to the ocean plays in defining a boat's ground potential, and whether it is important.]
What you call self-contained is a floating ground system, a system that has a ground that is at an unknown potential. Connecting that ground to an actual earth ground will put earth and the negative side of your circuit at the same potential. Your prop, since it touches water, is at earth potential. If there is an electrical connection between the ocean and the negative side of your circuit, your circuit is grounded to earth and is no longer floating.

DC Grounded Conductor: A current carrying conductor connected to the side of the power source that is intentionally maintained at boat ground potential.

[My understanding: any DC device's black cable or wire that is ultimately connected to the negative terminals of the batteries is the negative wire, and can be considered "ground" = "boat's ground potential". These normally carry current, unlike bonding wires that some boaters might connect between a metallic throughull fitting and the boat's DC wiring.]
Ground wires in a circuit carry current, in fact the negative side of your battery is the source of electrons and they flow backwards from the negative wires through the devices into the positive side of the battery, the "holes" left by the moving electrons flow from positive to negative (reverse direction).

DC Grounding Conductor: A normally non-current carrying conductor used to connect metallic noncurrent carrying parts of direct current devices to the engine negative terminal ...

[My understanding: if a wire is connected from a DC device's casing to the boat's DC negative wiring, the purpose of that wire is to carry current only if there is a short in the device that would otherwise make the device's casing electrically hot. This seems equivalent to the third green or bare wire in an AC system, but I don't recall ever seeing a third, normally non-current conducting wire from DC device.]

[Calder (page 231) emphasizes that the term "grounded" is reserved for the current-carrying wires that normally supply current to DC devices, while "grounding" is reserved for wires that only carry current in unusual situations, such as stray current (bonding), or lightning.]
The only purpose of this conductor is to bring the negative side of your circuit to earth ground, or to bring the negative side of two floating grounds to the same ground potential. If you had two batteries, for example, that were not connected together in any way, and independent circuits connected to each battery's terminals, you would have two separate floating grounds and they could be at different potentials. Connecting the negative terminal of the two batteries together with a DC grounding connector would bring both floating grounds to the same potential, the result being one floating ground. Connecting them together and then to earth ground brings them both to earth ground.

Polarized system DC: A system in which the grounded (negative) and ungrounded (positive) conductors are connected in the same relation to terminals or leads on devices in the circuit.

[My understanding: "Grounded" in this sense means that the black wire intended to complete the current-carrying DC circuit for a device is continuous with the battery negative terminals. So, it makes sense to me that "boat ground potential" (phrase used in E-11) might not be the same as earth (ocean) ground if the electrical connection is only to the negative posts of the batteries to create a local "ground" within the boat, and not to the ocean.]
Right, a floating ground.

[My understanding: The engine block in that schematic is a giant black wire, carrying current for devices that attach to the engine block and use the metal attachment as their current-completing path to the boat's ground (battery negative terminals). 1. That's why it is so important to keep the main DC ground cable connection pristine from engine block to battery negative post. 2. This analogy explains why I think it is OK to disconnect the engine circuit, but safely carry on using the house circuit.]
Yes, but will your alternator still charge, and will your starter work ? If your starter is grounded to the engine block then the engine block has to be connected to the negative terminal of the starting battery to have a circuit, for example. Same for your alternator, it has to be a complete circuit.

The propellor shaft connects the engine block to the ocean. That's not like in a car, where the engine block normally has no connection to earth. I am still fuzzy about a couple of things:
Some vehicles such as RV's occasionally drag along little wires that brush against the road to bring them to earth ground.

1. Whether the DC current from a device like an oil pressure sender or alternator that grounds through its connection to the engine block could return through the prop shaft to the ocean instead of back to the battery, and whether this is important. Calder (p 121) points out that current actually flows from negative to positive.
I'm not sure what you are asking here, it sounds like you are wondering if you can use the positive potential from one circuit and the negative potential from another and the answer to that question is no because you wouldn't actually have a circuit.

Said another way, you can actually do this - if you have a solar panel and a battery and you hook the panel up to the battery with two wires, the panel will charge the battery and so long as you do not connect the negative terminal with earth ground you will have a floating ground. Now you can take ANOTHER battery, connect ITS negative terminal to the POSITIVE terminal of the first battery, and connect another circuit to the SECOND battery and it will all still work. You will have two floating grounds, essentially, the first battery's ground will float at some unknown potential, and the second battery's ground will float at the first battery's positive terminal potential, which would then make the second battery's positive terminal 24 volts (12 x 2) away from the first battery's negative terminal potential (another way to say that is that they are connected in series). If you can understand that, you got it. You can use a ground wire to bring your floating ground to any potential you want just by connecting it to something, same as reaching out to touch something to discharge static when you wear wool clothing.

2. Whether the propellor shaft connection therefore puts the boat's ground potential to earth, over-riding the potential at the battery negative terminals.
Assuming it's electrically connected. Don't think of it as "over-riding", think of it as instantaneously bringing them to the same potential, like a static discharge, and then keeping them connected so that neither one of them wanders off and builds up a static charge again relative to the other. You can test this connection easily enough with an ohm meter - run a wire over the side of the boat into the water, connect that to one lead of your ohm meter, then touch the other lead of your ohm meter to whatever you are interested in, if it's something near zero ohms resistance then there is an electrical connection with earth ground.

2a. Whether point "2" is important.
It's important if you don't want a floating ground. If your ground floats then that means that the negative side of your electrical circuit could at some point have a vastly different potential than earth ground, and in some situations that could lead to things like sparks jumping from your floating ground to earth ground for example. That's why they recommend you ground your car and yourself to earth ground before you pump gasoline at the filling station, because your car's electrical system floats and can become charged relative to the gas station's pump, possibly leading to a static spark that could ignite the fuel.
 

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I'm not sure what you are asking here, it sounds like you are wondering if you can use the positive potential from one circuit and the negative potential from another and the answer to that question is no because you wouldn't actually have a circuit.
I thought of another way to explain this. If you have a solar panel hooked up to a battery with two wires and your ground is floating then you have no idea what the difference in potential is between your ground and earth ground. If you take a 12vdc light and hook it up to your battery, it will light up, because electrons will flow from the negative terminal of your battery through the light and back into the positive terminal of your battery, it is almost as if your battery were a pump moving electrons from one side of the battery to the other through your circuit. Now, if you were to hook the negative lead of your light to EARTH GROUND (instead of your batteries ground) and then you hooked up the positive lead to your batteries positive terminal, electrons are not going to flow from your battery into earth ground - instead what is going to happen is that your batteries potential is going to move relative to earth ground instead of pumping electrons and nothing is going to happen. Of course, if you were to hook the negative terminal of your battery up to earth ground, you've got a circuit, and the light would shine. Said another way, you could take a piece of wire and directly connect the positive terminal of a floating battery to earth ground and all it would do is change the potential of the negative terminal to -12vdc instead of 0vdc.

In addition to all of the above, there is yet another kind of ground on your boat, and that's the RF ground used in your radios, and that is something else entirely and has little to do with anything you've already talked about.

With your RF ground your radio is essentially acting as a pump, rapidly moving energy to and from earth ground through the radio and into and out of space through the antenna. If you don't have a good RF ground then the pump doesn't have a source for electrons and nowhere to send them and the radio doesn't work well, and if you don't have a good antenna on the other side then the same thing happens, the radio doesn't work efficiently. So you are trying to give your radio/pump a good antenna that is well connected with the sky and passes energy easily, and you are also trying to give your radio/pump a good connection to the earth so that energy also passes easily to/from the earth. That means you need a ground plane of some kind to bind your radio to the earth, this often means creating a capacitor in a fiberglass boat by basically spreading out lots of foil or metal mesh, etc, along the hull so that it is as close as possible to the water and and has a large area. One little wire or your prop shaft won't do the job, it's too puny, and RF energy won't pass through it well, it's like using a really little pipe on one side of your pump, the radio just won't be able to move energy well enough to do its job effectively. This ground is in addition to your other grounds, that's why your HF radios have two grounds instead of one. It doesn't really matter much for VHF.
 

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...if you were to hook the negative lead of your light to EARTH GROUND (instead of your batteries ground) and then you hooked up the positive lead to your batteries positive terminal, electrons are not going to flow from your battery into earth ground - instead what is going to happen is that your batteries potential is going to move relative to earth ground instead of pumping electrons and nothing is going to happen. Of course, if you were to hook the negative terminal of your battery up to earth ground, you've got a circuit, and the light would shine...
I'm annoyed with myself: still not getting it.

I updated my sketch to show electrical continuity from engine block to the ocean via the propshaft. This makes the boat's ground potential equivalent to earth ground, rather than a floating ground where the boat's potential is not connected to earth. This is the usual situation in boats, unless there is a plastic coupling between the transmission and propshaft that breaks electrical continuity.

I am still not getting this critical point: What stops current that flows through components grounded via the block (alternator, senders) from flowing into the ocean, instead of returning to the the boat's battery? (Recognizing that electrons actually flow from negative to positive, but the question remains.)
 

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I'm annoyed with myself: still not getting it.

I updated my sketch to show electrical continuity from engine block to the ocean via the propshaft. This makes the boat's ground potential equivalent to earth ground, rather than a floating ground where the boat's potential is not connected to earth. This is the usual situation in boats, unless there is a plastic coupling between the transmission and propshaft that breaks electrical continuity.

I am still not getting this critical point: What stops current that flows through components grounded via the block (alternator, senders) from flowing into the ocean, instead of returning to the the boat's battery? (Recognizing that electrons actually flow from negative to positive, but the question remains.)
The short answer is because there is no circuit. :)

The long answer ...

The battery wants to do work (for lack of a better word), when it is charged there exists a situation where the chemical reaction that discharges it can only occur if electrons flow from the negative terminal to the positive terminal - if the negative terminal is not connected in some way to the positive terminal the battery will not discharge, it will just sit there, a chemical soup in a state that will not change until it is allowed to move electrons from the negative terminal to the positive terminal.

Connecting the negative terminal to the ocean does not create a situation where the negative terminal can move electrons to the positive terminal, instead all it does is create a REALLY GIGANTIC negative terminal that STILL wants to be connected to the positive terminal, and since it isn't connected the chemical reaction still won't take place.

Instead of connected the negative to the ocean, you could instead connect the positive to the ocean and leave the negative terminal floating, and again you would have a situation where the electrons could NOT move from the negative terminal to the positive terminal and the chemical reaction could not take place. All you would have done is create a REALLY GIGANTIC positive terminal that STILL wants to be connected to the negative terminal.

You are imagining that hooking the negative terminal up to the ocean is going to discharge the battery because now it can release all the charged up electrons it's been wanting to let go of, but it isn't like that. The negative terminal DOES have a different amount of electron density than the earth when it is floating, but that is a static charge and not an ongoing potential, so when you hook the floating ground up to the ocean you may get a static discharge, but then the terminal immediately comes to the same ground potential and it STILL wants to move electrons from its negative terminal to its positive terminal, because it still needs electrons to move in order to work its chemical magic. Hooking the negative terminal up to the ocean is fine, but you can't get the chemistry to work in the battery because the battery actually NEEDS the electrons at the other terminal in order for the reaction to take place.

It is important to understand that there is no such thing as a Universal zero volts or a Universal 12vdc, you can only have 12vdc relative to something else. When you connect the batteries negative terminal to the ocean any static charge on the battery dissipates into the ocean (possibly as a spark) and no matter what potential the batteries negative terminal ends up at, the positive terminal will end up approximately 10-13 volts above that potential, because no matter what the batteries negative terminal is at in terms of electron density, the battery still wants to conduct 10-13 volts of electrons from one terminal to the other.

The above is why you can hook the negative terminal to the ocean and pretend it is zero volts and you will then be able to read 12'ish volts at the positive terminal. It's also why you could hook the negative terminal to the positive terminal of ANOTHER BATTERY and you will still get 12'ish volts at the positive terminal, and 24'ish volts from the negative of the first battery to the positive of the second. The above is also why you could hook the ocean up to either (1) the negative terminal of the first battery, or (2) the negative terminal of the second battery and the positive terminal of the first, or (3) the positive terminal of the second, and the batteries won't discharge, all you will do is create a potential to do work relative to the ocean that is different in the 3 cases. In case (1) your voltage potentials will be 0, 12vdc, and 24vdc relative to earth, in case (2) they will be -12vdc, 0, and 12vdc, and in case (3) you would have -24vdc, -12vdc, and 0.

If that doesn't make sense I can try to explain it in another way.
 

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wind-magic - that helps, thanks. I plan to read it again when I'm not so beat from a 14 hour day. I'm beginning to see it.
 

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wind-magic - that helps, thanks. I plan to read it again when I'm not so beat from a 14 hour day. I'm beginning to see it.
Floatsome, I thought of an easier way to explain all of the above - with some simple questions. :)

If you put a penny on the negative terminal of your 12vdc battery and it just sits there, will the battery discharge ?

How about a screwdriver ? If you touch a screwdriver to just the negative terminal and don't touch the positive, is it going to discharge the battery ?

How about a wrench ?

What about a 1lb steel weight ? If you hook a 1lb steel weight up to the negative terminal on a battery and it doesn't touch the positive terminal, will that discharge the battery ?

A piece of rebar ?

Something bigger, if you wire up a bowling ball sized piece of steel to the negative terminal, will the battery discharge ?

What about if you hook up a piece of steel like an old car frame ?

Or something bigger, a backhoe ?

Or a steel boat that is sitting on stands ?

Or a WW2 tank ?

Etc, etc ....

So why would wiring the whole earth up to the negative terminal of the battery be any different than the things above - besides being much bigger, is it really any different ? :) No matter how big the negative terminal is (no matter what you wire it up to) it still has to touch the positive terminal on the battery to discharge the battery.
 

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So why would wiring the whole earth up to the negative terminal of the battery be any different than the things above - besides being much bigger, is it really any different ? :) No matter how big the negative terminal is (no matter what you wire it up to) it still has to touch the positive terminal on the battery to discharge the battery.
A temperature sender hooked up to the positive terminal of the battery completes its circuit through the engine block ground when the engine is running and the circuit to the sender is open. Electrons flow from negative to positive. So, do the electrons flow from the negative terminal of the battery through the black ground cable attached to the block rather than from the ocean and up into the block via the propshaft because the battery has a ready supply of electrons and not the ocean (?) and then flow from the block to the sender and back through the red wire to the positive terminal of the battery?
 

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A temperature sender hooked up to the positive terminal of the battery completes its circuit through the engine block ground when the engine is running and the circuit to the sender is open. Electrons flow from negative to positive. So, do the electrons flow from the negative terminal of the battery through the black ground cable attached to the block rather than from the ocean and up into the block via the propshaft because the battery has a ready supply of electrons and not the ocean (?) and then flow from the block to the sender and back through the red wire to the positive terminal of the battery?
Electrons repel each other. Think of electron density like heat - when you touch something that is very hot to something big and cold, the heat from the hot thing dissipates into the cold thing until both things are the exact same temperature. The hot thing won't get colder than the cold thing, and the cold thing can't get hotter than the hot thing, they meet somewhere in the middle. If the hot thing is very big compared to the cold thing (a stove top and a penny) then the hot thing won't cool off very much, and the cold thing will heat up a lot (hot penny). If the hot thing is very small compared to the cold thing (a penny and an ocean of water) then the hot thing will cool off a lot relative to the cold thing and the cold thing will heat up very little (ocean doesn't change temperature if you drop a hot penny into it, but the penny cools off). Whatever happens though, both things share the heat until they both end up at the same temperature (so long as they are not isolated by some kind of insulator like the atmosphere).

Touching the negative terminal of your battery to something works the same way, the negative terminal (and plates) of your battery may have some kind of static charge - that is to say they may have a higher (or lower) electron density than whatever you touch the terminal to, but as soon as you touch the terminal the electrons will either spark out to decrease their density to be the same as whatever it touched, or spark in to increase their density to be the same as whatever it touched, but in the end they will have the same basic density as whatever it touched. Why ? Because like heat (which is really just vibrating atoms) electrons repel one another and "want" to move away from each other, so they "want" to move into areas where the density of electrons is lower. If you touch the engine block of your engine to the ocean, because the ocean is so vast and your engine block is so small your engine block will essentially spark to whatever electron density the ocean has - they will end up with the exact same field strength, the same ground potential, electrically they will become one.

Now, NO MATTER WHAT THE NEGATIVE TERMINALS GROUND POTENTIAL IS, the positive terminal will be approximately 10-14 volts above it. This is important to understand. The battery wants to do work relative to whatever its negative terminal's potential is NOT relative to some Universal zero volt that is a constant. No matter what the negative terminal's electron density the battery will ALWAYS want to do 10-14 volts of work relative to it, the two terminals will ALWAYS want to conduct so long as the battery has a charge (lead sulfate and sulfuric acid in solution).

So ....

Your question is, when the sender on the motor works, how do electrons from the negative terminal know how to go through the sender and not through wire to the ocean.

The answer is - it doesn't matter. The negative terminal hooked to the ocean is like one big giant ground plane, so it doesn't matter what any individual electron does. If an electron goes through the sender from the engine block, in an instant the entire field (engine block, negative terminal of the battery, earth) loses an electron and an electron moves out of the negative terminal of the battery to compensate for the loss, and an electron moves into the positive terminal of the battery and the chemistry in the battery works. By hooking the battery's negative terminal to the ocean all you have done is hook it to the mother of all engine blocks, electrically the whole earth becomes part of your battery's negative terminal. Said another way (in answer to your queston), it's impossible for a stream of electrons to flow out of the negative terminal of your battery "into the ocean" because they are at the same potential the moment you hook the terminal up to the ocean, so why would electrons "flow" there ? If they wanted to flow into the ocean they would have done it (and did as a static discharge) as soon as you hooked the ocean to the negative terminal of the battery.

Said yet another way ...

You could, if you wanted to, hook the negative terminal of your battery up to the ocean and then run a wire over the side of the boat into the ocean and make that wire your entire electrical systems ground wire. Whenever your lights were on electrons would flow out of the ocean through the lights and into the positive terminal of the battery and electrons would move out of the battery's negative terminal into the engine block and out into the ocean to complete the circuit. The entire ocean would then become a single piece of wire in your circuit drawing, a really, really, really big piece of wire, and you would not need another piece of wire hooking the engine block up to the negative wire in your electrical system to "complete the circuit", the ocean would serve the purpose.

When you hook one thing up to another thing electrically they take on the same electron density. That's why you can hang from a 10,000 volt electrical line and not die, and then fall and touch the ground and not die (so long as you survive the impact). When you are hanging from the electrical line your electron density is changing with the wire, you may static discharge to or from it when you first touch it, but then you take on it's same electron density, you become part of the wire you are touching. When you fall to the ground the earth has a different density and you will charge/discharge into/out of it in an instant, but you won't die unless you actually become the wire that completes the circuit between the wire and the earth - leave that trick for the household appliances in people's homes, they are better suited to the task than your body is, you don't want to heat up like a light filament. :D
 

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Discussion Starter #17 (Edited)
If an electron goes through the sender from the engine block, in an instant the entire field (engine block, negative terminal of the battery, earth) loses an electron and an electron moves out of the negative terminal of the battery to compensate for the loss, and an electron moves into the positive terminal of the battery and the chemistry in the battery works.
Eureka. Thanks.

So my simple-minded way of thinking now is this: The positive side of a battery has an electron deficit compared to the ground side. Closing a circuit switch in the positive side of the circuit allows electrons to flow from ground to the battery.
 

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Eureka. Thanks.

So my simple-minded way of thinking now is this: The positive side of a battery has an electron deficit compared to the ground side. Closing a circuit switch in the positive side of the circuit allows electrons to flow from ground to the battery.
Exactly.

If the battery were a capacitor instead of a battery it would discharge the negative terminal's electrons into the positive terminal and the device would instantly reach equilibrium. Since it is a battery, however, hooking the negative terminal to the positive terminal does not "satisfy" the battery, because the battery's positive terminal continues to have a lower electron density than the battery's negative terminal because the positive terminal is "eating electrons" to work its chemical magic - as electrons move from negative to positive to increase its density the chemistry in the battery removes those electrons to work its chemistry re-creating the deficit, the result is a flow of continuous current. The reason the battery doesn't instantly reach equilibrium is because the chemistry can only "eat" so many electrons at a time, so it takes it a while to discharge completely. Once the battery completely discharges it will have the same field strength (electron density) at both terminals and electrons will no longer flow from the negative terminal to the positive terminal and the battery will stop doing work. That's why batteries can only supply such-and-such amount of current, and also why it takes them such-and-such amount of time to recharge in the presence of an even lower electron density at the positive terminal (sucking electrons back out of the positive terminal i.e. recharging) than the negative terminal, the chemistry takes time to work.
 

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Discussion Starter #19 (Edited)
This helps too: Safe circuit design : ELECTRICAL SAFETY.

you can float a ground. Airplanes do this all the time, they are not grounded to earth.
That scene in "Hunt for Red October" comes to mind, where the helicopter is hovering over the submarine and the sailor on the sub gets shocked by a spark jumping from the helicopter to the sub as they try to ground the helicopter prior to accepting the hero on the end of a tether. My understanding now is that the helo's floating ground ended up at a different voltage from earth as it picked up electrons during the flight, but within the helo all the electronics were happily working with voltage determined by their own ground. The voltage potential between the helo's ground and earth ground rapidly corrected itself when helo and sub came together. I presume this is the same shock a person gets when they feel a zap as they step out of a car. Our propshaft connection ensures that our boat's ground potential is always the same as earth.

1. So, do aircraft routinely connect to earth ground when they land, especially on a metal aircraft carrier? Just found this: Helicopter Static Charge? [Archive] - PPRuNe Forums

2. Hence also the rationale for connecting a green wire from the grounding bus bar in the AC system to the main DC ground bus bar, so that stray current from a short in an AC appliance can dissipate safely "to earth"?
 
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