I am doing some rewiring on my boat and want to make sure I have the best possible ground. I assume the major grounding point is the engine, but wondered about also running a ground wire from the engine mount [which is probably insulated from the engine. Is there anywhere else I should consider?
Best Ground Point
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The fundamental rule to avoid stray current corrosion is to have one single ground. Although it is often difficult, particularly on older boats, to maintain this practice, having multiple paths to ground sets up the likelihood of having different potential at multiple grounds which is often to cause of corrosion.
Having said that, it is a common practice where multiple grounds are evident to interconnect them to preclude corrosion and this works if done properly. So, in answer to your question, if you can't have a single ground, making sure that multiple ones are connected with the proper size wire and the connections are of "no" resistance will suffice.
To state the obvious, having one point is usually easier, safer and more easily maintained and it is typically on the engine block
Having said that, it is a common practice where multiple grounds are evident to interconnect them to preclude corrosion and this works if done properly. So, in answer to your question, if you can't have a single ground, making sure that multiple ones are connected with the proper size wire and the connections are of "no" resistance will suffice.
To state the obvious, having one point is usually easier, safer and more easily maintained and it is typically on the engine block
Ross, To compensate for that, I understand that you can put a ground strapping to "bridge over" the shaft saver coupling. That regains grounding to shaft & prop, and thus to sea water.
On my H34 all metal parts of the boat below water are grounded to the engine block and to the negative terminal of the battery bank. Also make absolutely sure that the shore power ground and the negative or boat ground are not tied together in any way.
" Also make absolutely sure that the shore power ground and the negative or boat ground are not tied together in any way. "
Dumb question because I'm not an electric guy: I don't understand this. The engine is the DC ground and it grounds to water via the shaft - right? So what else is there for the AC (Shore Power) to ground to? How can you isolate the AC ground from the DC ground?
Dumb question because I'm not an electric guy: I don't understand this. The engine is the DC ground and it grounds to water via the shaft - right? So what else is there for the AC (Shore Power) to ground to? How can you isolate the AC ground from the DC ground?
If all goes well, the AC will never ground through the same ground as the DC. It is wired that way so you don't shock yourself if something goes wrong with the wiring IN a device, such as the hot touching a metal case on an appliance.
Now I am confused. My galvanic isolator calls for the ac ground [not neutral] to go the same place as all other ground wires. I also think that the a/c system has its own ground buss near the distribution panel, but I am not sure where that is actually grounded.
Neutral wire and positive wire is where the AC goes through. You can measure current flow through your positive wire or your neutral. The ground is if something goes wrong.
I think.
I think.
Yes it probably does. Some believe that is the only ground you need on a boat. I am inclined to think that way also. I guess it depends on how much swimming you do.
It is recomended that you test the ground in your marina becasue, well other than your life depending on it, marina grounds are often reported to be poorly grounded.
[/quote]So what else is there for the AC (Shore Power) to ground to? How can you isolate the AC ground from the DC ground?[/quote]
This has become very confusing. First, is it really possible to have 2 separate grounds on a boat? It would seem that ultimately anything that really operated as a ground, would manage to connect to everything else that is actually grounded.
Another post makes the point that all grounds should be intentionally interconnected to avoid corrosion. This is challenged by a post that suggests one would not want ac and dc grounded in the same place. This last point may be based on a failure to distinguish an ac "neutral" wire from its "ground" wire, which, as Hermit pointed out is only there as a failsafe and did not even exist until, what, the late 50s.
If I were to act on what has been written thus far, I would probably use multiple interconnected grounds just to make certain I always had a good one. My experience from the car world is that some engine based grounds can deteriorate over time. I would think that marine grounds are even more prone to that.
Another mistake we often make in grounding is forgetting that the ground is usually the post, not the engine surface which may be painted or prone to surface rust. This means that a thin nut should be the first thing on the ground screw, followed by the wire, then a flat washer, lock washer and nut.
This has become very confusing. First, is it really possible to have 2 separate grounds on a boat? It would seem that ultimately anything that really operated as a ground, would manage to connect to everything else that is actually grounded.
Another post makes the point that all grounds should be intentionally interconnected to avoid corrosion. This is challenged by a post that suggests one would not want ac and dc grounded in the same place. This last point may be based on a failure to distinguish an ac "neutral" wire from its "ground" wire, which, as Hermit pointed out is only there as a failsafe and did not even exist until, what, the late 50s.
If I were to act on what has been written thus far, I would probably use multiple interconnected grounds just to make certain I always had a good one. My experience from the car world is that some engine based grounds can deteriorate over time. I would think that marine grounds are even more prone to that.
Another mistake we often make in grounding is forgetting that the ground is usually the post, not the engine surface which may be painted or prone to surface rust. This means that a thin nut should be the first thing on the ground screw, followed by the wire, then a flat washer, lock washer and nut.
Cars and boats are very different. You kinda gotta forget most of car wiring.
If you choose to run your grounds into the water it is important to only have one point at which they enter the water. If you have more than one yourun the risk of having them at different voltages creating a battery in the water with your metal components.
There are different schools of thought on boat wiring. So you will get different opinions. You haven't even mentioned lightning protection yet. Just kidding, forget that for now.
All my through hulls are isolated. My prop is not connected to my grounding system. It is isolated by the coupler. I am with the "do not connect anything" crowd.
I also choose to only run my AC ground to the marina box not into the water. I do use my engine as a DC ground because it is set up that way with the starter and alternator.
If you choose to run your grounds into the water it is important to only have one point at which they enter the water. If you have more than one yourun the risk of having them at different voltages creating a battery in the water with your metal components.
There are different schools of thought on boat wiring. So you will get different opinions. You haven't even mentioned lightning protection yet. Just kidding, forget that for now.
All my through hulls are isolated. My prop is not connected to my grounding system. It is isolated by the coupler. I am with the "do not connect anything" crowd.
I also choose to only run my AC ground to the marina box not into the water. I do use my engine as a DC ground because it is set up that way with the starter and alternator.
To further expound on the through hull battery theory; if you have two through hulls connected 5' apart and a stray current is running through the water from the marina, the one closer to the voltage source will be at a higher potential. And since they are connected inside the boat, that is by definition a battery. One will be eaten away.
That said there are people on here that have had their through hulls wired together for 30 years and have never had a problem. Go figure.
It seems to me wiring everything together is a good idea if your are not in a marina.
As a further measure or prevention, I have flange adapters on my through hulls and I coated them outside and in with epoxy. So if the ball valves get eaten away, I can just unscrew them and replace them in the water.
That said there are people on here that have had their through hulls wired together for 30 years and have never had a problem. Go figure.
It seems to me wiring everything together is a good idea if your are not in a marina.
As a further measure or prevention, I have flange adapters on my through hulls and I coated them outside and in with epoxy. So if the ball valves get eaten away, I can just unscrew them and replace them in the water.
Ross,
The best set of guidelines we have is currently the ABYC standards. Here's what they have to say about flexible couplings in ABYC P-06:
ABYC P-06 said:
Is the basis for that for lightning protection? or just for the AC ground?
I have also read not to route the lightning protection through the engine because it could ruin the bearings somehow.
This is for grounds DC and AC (ground only). Most lightning systems are separate or go directly to a keel bolt or sufficiently sized ground plate because the engines are not usually directly below a mast and horizontal runs of wire in a lightning system are advised against as are sharp bends in the wire..
ABYC TE-4 said:
Lots of misinformation in this thread. I suggest reading about electrical grounding in a respected book such as Nigel Calders books or David Smeads "Living on 12 Volts With Ample Power" or check with people like Don Casey, Chuck Husick or Peter Kennedy that have good Web sites.
AYBC, UL Marine Supplements and NFPA all recommend connecting the AC ground (green) wire to the boats negative system on board the boat. You should read and find out why.
To keep you, your crew and others that may touch the rigging on your boat from a dock or be swimming nearby safe, please become knowledgable about the electrical systems on your boat if it is wired for shore power.
The best way to connect the shaft to the bonding and ground system is by using a shaft brush. These are avaliable at West and others. Bridging a flexable coupling relies on continuity through a gear box full of oil. This will give unequal electrical potential between underwater parts and promote electrolysis.
As to grounding to the engine block or engine mount, remember that the negative side of the alternator will be through the engine block in most cases. Its best to have a separate ground post and a wire of approprate size from the alternator and the starter to the negative post. Connections to the engine are susceptable to vibration and corrosion that can break the wire crimps.
AYBC, UL Marine Supplements and NFPA all recommend connecting the AC ground (green) wire to the boats negative system on board the boat. You should read and find out why.
To keep you, your crew and others that may touch the rigging on your boat from a dock or be swimming nearby safe, please become knowledgable about the electrical systems on your boat if it is wired for shore power.
The best way to connect the shaft to the bonding and ground system is by using a shaft brush. These are avaliable at West and others. Bridging a flexable coupling relies on continuity through a gear box full of oil. This will give unequal electrical potential between underwater parts and promote electrolysis.
As to grounding to the engine block or engine mount, remember that the negative side of the alternator will be through the engine block in most cases. Its best to have a separate ground post and a wire of approprate size from the alternator and the starter to the negative post. Connections to the engine are susceptable to vibration and corrosion that can break the wire crimps.
An example of why the AC power green should be connected to the boats ground..
Ill use this example again but last summer, someone posted about a kid swimming in a marina who got electrocuted by an outboard motor which had AC on it. What possibly happened is that the boat owner with the outboard ran AC wires and DC wires very close together and had no AC to DC ground connection. This boat probably had too large of load on the AC wires and they got very hot melting the insulation. Or, maybe is was vibration? The hot wire on the AC melted though some insulation and came in contact with the 12 volt DC line so now the 12 volt DC line had AC imposed on it - which put AC + 12 volt on the DC ground which was attached to the outboard. So now.. a very dangerous outboard sitting in the water with 110 VAC on it.
If the boat owner had the AC earth ground connected to the boat DC ground, as soon as the AC hot wire made contact with the boat 12 volt wire, big current would have flowed from the AC hot - through the 12 volt battery - to the green earth ground and the breaker fuse would have blown.
But.. its not a good idea to connect the AC earth ground directly to the boat ground - you need to use one of the diode galvanic ground isolators to connect these two grounds.
(GFCI would have also blown for this case)
Will you ever really need the grounds to be connected.. probably not (and I and my kids wont be swimming in a marina). But.. the rules need to make things idiot proof and that’s the case here.
Also, the reason for single point ground.. If you have a ground wire that has current flowing in it, there will be some resistance in the wire so different points along this wire will be at different voltages. The differences will be small - but the wire still has some resistance so there will be voltage differences. If you tap grounds along this wire and connect these tapped grounds to underwater metal structures, you now have a small voltage between the underwater metal structures - and electrolysis issues..
However, if you have a single point ground, the grounding wires to underwater structures have no current flowing in them - and all go to the same ground point - so all the grounding structures are at the same potential.
Ill use this example again but last summer, someone posted about a kid swimming in a marina who got electrocuted by an outboard motor which had AC on it. What possibly happened is that the boat owner with the outboard ran AC wires and DC wires very close together and had no AC to DC ground connection. This boat probably had too large of load on the AC wires and they got very hot melting the insulation. Or, maybe is was vibration? The hot wire on the AC melted though some insulation and came in contact with the 12 volt DC line so now the 12 volt DC line had AC imposed on it - which put AC + 12 volt on the DC ground which was attached to the outboard. So now.. a very dangerous outboard sitting in the water with 110 VAC on it.
If the boat owner had the AC earth ground connected to the boat DC ground, as soon as the AC hot wire made contact with the boat 12 volt wire, big current would have flowed from the AC hot - through the 12 volt battery - to the green earth ground and the breaker fuse would have blown.
But.. its not a good idea to connect the AC earth ground directly to the boat ground - you need to use one of the diode galvanic ground isolators to connect these two grounds.
(GFCI would have also blown for this case)
Will you ever really need the grounds to be connected.. probably not (and I and my kids wont be swimming in a marina). But.. the rules need to make things idiot proof and that’s the case here.
Also, the reason for single point ground.. If you have a ground wire that has current flowing in it, there will be some resistance in the wire so different points along this wire will be at different voltages. The differences will be small - but the wire still has some resistance so there will be voltage differences. If you tap grounds along this wire and connect these tapped grounds to underwater metal structures, you now have a small voltage between the underwater metal structures - and electrolysis issues..
However, if you have a single point ground, the grounding wires to underwater structures have no current flowing in them - and all go to the same ground point - so all the grounding structures are at the same potential.
There should ideally be a separate ground buss and then only the one large negative ground wire should go to the engine or strap brush. I have yet to see a 2, 1, 1/0, 2/0 or larger proper battery lug fail from engine vibration but it's a boat and anything can happen.
Also isolated ground alternators or starters are rather rare. If you can get one that's good but many factory alts and starters are not set up for isolated ground so a clean ground point to the engine must be maintained without an isolated or insulated ground.
Unfortunately most boats do not have dedicated ground plates and engines are used. It is good practice to check the resistance between the ground point on the engine and the prop shaft aft of the coupling. If you have anything but perfect continuity a ground strap or brush on the shaft is a good idea if you have the room.
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