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.