Unintentional Ground Returns to Hull?

[Champlain]

We all know that, unlike cars, boats should have an isolated ground wire back to a single point, generally the battery(ies) negative terminal.

But lots of electronics and electrical items have their negative attached to a metal chassis, which guarantees contact with a metal hull, and an unintended ground return through the hull itself, even if a separate ground wire exists. It can also be expected that a ground-return via the hull will be of much lower resistance than any practical size ground wire.

This can happen through metal mounting brackets, or even flush-mounted electronics in a metal case, and in the case of things like VHF, via the antenna mount, since the coax shield is generally attached to both the radio case and the hull at either or both ends.

Does anybody attempt to isolate equipment from hull contact, like using rubber spacers, nylon shoulder-washers, plastic panels, etc? I would think without the equipment "floating" there would be a huge potential for stray-current corrosion, compared to a hull of you-know-what (hint: ends in "glass").

Thoughts?

[kmorin]

Champlain , I'm not completely convinced, but I'm not an EE either, that the chassis BOND is the DC equivalent of the DC Neg. in the typical electronic device with an RF shield chassis? I'd say if the DC plus and neg were reliably wired to a DC buss PLUS and MINUS, and that to the battery(s) then there would be no stray current to the hull? I do expect to see a single point Negative Only Pole at one point bonded to the metal structure with 'no resistance' in this condition.

I'm not really sure that your idea is completely accurate that the DC return to a (relatively) high resistance alloy like Al would be remotely equal to the extremely low resistance of Cu? I think a single #12 copper wire is very much less resistance to current flow compared to a cadmium plated chassis mounted on a few screws in terms of bonding connections' resistance? So I think the two wire system would eliminate a potential that would run through the hull to DC neg?

I'm not enough of an EE to make this an argument, well, but my understanding is the metal chassis of electronics is mainly an RF shield not a DC Neg. or DC return, so the 'chassis ground' idea comes in as a safety feature if the power supply fails and that provides a safety-only type of path to 'ground' or zero potential.

I see two separate and different discussions in your post; A) the DC electronic device's proper installation, mounting and wiring, and B) a powered device like motors, lights, and other higher powered equipment mounting, wiring and installation?

not sure I'm talking apples-to-your-apples due to the terms we're both using, but this is one discussion we've had before - I think? Maybe the Search function would help explore others' contributions and views on this subject in the past?

Cheers,
Kevin Morin
Kenai, AK

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[Champlain]

Many (most?) marine items I've tested show zero ohms between the usual black ground wire, and the metal chassis...virtually all audio headunits and amps do, and most VHFs, unless in a plastic case, but even then, mounting bracket screws usually do contact internal threads at ground potential, rather than isolated.

Also, I would think the shear mass of a hull would be a lower resistance ground return path than a 12 ga. copper wire...when house wiring with aluminum was briefly popular, the rule was just to go one gauge heavier
with aluminum, like a #12 wire instead of a #14 wire for a 15 amp. branch. The equivalent gauge of wire a hull represents, vs. a copper wire, is hard to even guess at: the resistance is related to circular-mils of the wire, and with a hull, we're talking circular inches rather than mils.

I do not own a metal-hull boat and never have, but am giving it some thought, and do all my own electrical work, and want to abide by "best practices"...

My gut feeling is to electrically isolate every 12vdc item to insure the only ground path is via the ground wire. I have most of Calder's books, and I'm sure he addresses the issue, but mine are in storage boxes at a moving company right now.

[ an aside: my "wet dream boat" would be a 25' aluminum RIB pilothouse, but after checking prices, see that they are priced for government clients, and even in a 25' version, exceed the price of many homes in my area, even deleting the $5,000/each suspension seats, and machine gun mounts...a new Hewescraft is even stretching my budget...]

[MacGyver]

You can't compare a car wiring system to a boat they are two different animals.

A boat wired per ABYC or C.G. has four separate grounds, going to a single point, battery negative, AC ground, bonding ground, and lighting ground if needed. For more information on grounding systems look up star grounds, ground loops and ABYC grounding systems.

My take on the subject is a chassis that is isolated from negative power wire should go straight to the bonding bus and the negative power wire go to the negative bus. As long as there is no current flowing
from the chassis (The bonding conductor are normally non-current carrying circuit.)

If you don't want to do that, and you also don't want to isolated it from the hull. Measure the hull potential with everything turn on full power. If there is a difference between in the hull potential with the chassis contacting the hull you have a problem. If it bad enough to cause corrosion it will depend on other factors.

[Chtucker]

The only two items that I see this on are my radar and my VHF radios. I do know for a fact that they are chassis grounded. I did not chassis ground any of my navigation equipment (Furuno has the provision on most the system components.
There is only ONE bond to my hull. The DC system and AC system are all bonded to the same post.

I can't say for sure with scientific evidence, but I believe the wire provides a lower resistance/shortest path to "ground" The outboards have a negative cable that goes essentially directly to the battery. Their metal structure is also "ground"

I think if it was an issue, ABYC would be pretty clear about the need to isolate everything (almost impossible)

[Champlain]

Also, keep in mind that when there are multiple paths to ground from the same item, the lower resistance ground doesn't handle 100% of the ground, since you have created a parallel circuit, so the ground-return will be shared. It's not like a lightning strike seeking only the single best path, but will divide according to Ohm's Law.

It would be interesting to check the actual resistance of the hull, say from the helm area, to your common ground point vs. the resistance of the ground wire to that same common point. I can't do the test, not having a metal hull boat, but
I predict that once you're through the oxide layer, the resistance through the hull will be virtually zero ohms. You will need long, heavy probe wires, unless your meter has a zero adjustment with the probes shorted, of course. My old analog meter does, but my DVM does not. If yours lacks the zero adjustment, you could measure the resistance with the extended probe leads shorted and subtract that from the final value.

Someone want to give it a try? I'd actually like to be proven wrong.

[Goingfishing]

All out boards and inboard engines are grounded to the alloy hull. The block of the outboard is negative this is why it is very important to matain the anodes on an outboard motor.(have a look at the starter motor it has a small negative loop to the block also look at the rectifiers they are grounded to the block). It is then electrically grounded to the hull via the mounting bolts holding the motor to the hull and in the case of inboards they are connected via gearboxes, prop shafts etc.

What is IMPORTANT is both the negative and the positive cables are clean and have a good connection. The electrons will always flow via the path of least resistance. when you have dirty or bad connections and they make a circuit it is when the problem begins, in the case of the starter motor the circuit is created between the positive cable and the negative cable the electrons (amps) flow through the cables to form a circuit if the negative has a bad connection and it can get a path of negative return with less resistance via the hull through another grounded item in the boat the electrons will flow through this circuit and use the hull as part of the circuit.

This is why when wiring a alloy boat it is very important to plan and Calculate all the circuit loads first, this allows you to use the correct gauge cable required. Use insulated postive and negative bus bars behind dashes and in areas where there is multiple acessories to connect.

One dangerous example I came across recently was a stuck open throttle cable coming back into the harbour. When the catapiller engine was serviced the negative strap was disconnected to access the oil filters and not reconnected, so it used the throttle cable as a negative return melting it and causing it to jam in the open position.

One last important issue to never to over look is circuit protection, fuses or circuit breakers. These must be installed with the he correct amperage to the cable size. I can not tell you how many times I have seen 60amp breakers installed on 20amp rated cable you might as well not even waste you money and install one, the cable will melt before the breaker will trip. I know of at least 7 cases of boat fires where this was the primary ignition source.
If you have never experienced an onboard fire at sea I can tell you it is not fun, we had a oil line on a turbo split pumping hot oil onto an exhaust within seconds it became a life and death situation, it was lucky we were able to remotely activate the fire suppression system and save the boat and ourselves.

[Champlain]

The electrons will always flow via the path of least resistance.

We have an obvious difference of opinion here.

With multiple paths to a common ground, the ground return current will be divided between multiple return paths, inversely-proportionally to their resistance. To complicate the situation, if several items share a common return path to ground via a metallic hull, all such paths will be combined.

I see no good results, unless all items are isolated from the hull, which can be very challenging to do, requiring plastic or rubber insulation, and mounting bolts using plastic shoulder-washers to keep mounting screws/bolts from making hull contact, along with a plastic washer under the securing nut.
This assumes thru-bolted items, rather than screws going into tapped holes into parts of the hull, impossible to electrically isolate, unless using nylon screws, which generally lack both shear and tensile strength.

The idea of seeking the path of least resistance is simply wrong in DC circuits...the current will be divided, and be a potential of stray-current corrosion, and/or accelerated need for sacrificial anode replacements. The motor internal anodes are often neglected...out of sight, out of mind.

I would recommend a silver chloride electrode test, a brief introduction can be found here, but will not make you an instant expert:
http://assets.fluke.com/appnotes/electr ... 269b_u.pdf

The issue is particularly difficult to solve with metallic hull boats, with any item which has a metallic chassis internally connected to the negative ground wire, including most VHFs, stereos, and radars.

My opinion: all such items must be isolated from an aluminum or steel hull, with the only possible ground return being via the "black wire", and not the hull.

[Chtucker]

The electrons will always flow via the path of least resistance.

We have an obvious difference of opinion here.

With multiple paths to a common ground, the ground return current will be divided between multiple return paths, inversely-proportionally to their resistance. To complicate the situation, if several items share a common return path to ground via a metallic hull, all such paths will be combined.

I see no good results, unless all items are isolated from the hull, which can be very challenging to do, requiring plastic or rubber insulation, and mounting bolts using plastic shoulder-washers to keep mounting screws/bolts from making hull contact, along with a plastic washer under the securing nut.
This assumes thru-bolted items, rather than screws going into tapped holes into parts of the hull, impossible to electrically isolate, unless using nylon screws, which generally lack both shear and tensile strength.

The idea of seeking the path of least resistance is simply wrong in DC circuits...the current will be divided, and be a potential of stray-current corrosion, and/or accelerated need for sacrificial anode replacements. The motor internal anodes are often neglected...out of sight, out of mind.

I would recommend a silver chloride electrode test, a brief introduction can be found here, but will not make you an instant expert:
http://assets.fluke.com/appnotes/electr ... 269b_u.pdf

The issue is particularly difficult to solve with metallic hull boats, with any item which has a metallic chassis internally connected to the negative ground wire, including most VHFs, stereos, and radars.

My opinion: all such items must be isolated from an aluminum or steel hull, with the only possible ground return being via the "black wire", and not the hull.

You are wasting your time. It has been proven that you don't need to. If you follow AYBC you won't have any trouble. There are plenty of other areas to worry about, like poulstice, stray current, and dissimilar metal corrosion. If you don't INTENTIONALLY use your hull as your return, you will be fine. Run your DC grounds to the batteries. Thousands of aluminum boats have radar, VHF radios, FM radios etc that are not isolated from the hull. If it was an issue, Furuno who provides the electronics for thousands of USCG boats would be pretty specific about it.

You will have more issues from your neighbors boat in the marina than your worries about minor chassis/antenna connections to the hull