Steel boat ground

[hpeer]

I have a DC only boat. NO AC. Steel hull.

I have a Volvo MD7A diesel. 12 start batt, and 12 house bank. They are charged through a Balmar Duo Charge.

Long story short, engine has been reinstalled with new mounts, shaft isolator, and control panel.

All works well, except my new bottom paint is blistering and zincs seem to have taken a lot of wear in the short 2 months she was in the water. So I am questioning everythinng.

With all wiring disconnected there is about 35 ohms from the Hull to the engine

My DC ground wiring, through the panel, (excluding bilge pumps, etc.) has about 0.4 ohms to the hull with meter zeroed properly.

My sense is that rather than chase down all ground connections I should simply bond the engine to the hull.

It sounds like there is no consensus but also no strong objections either way? But important to pick one way or the other?

Secondly, does this condition sound conducive to starting the paint to blister? Some blisters had a bit of rust in them, ameron 302 zinc primer.

Thanks.

I should add that before removing and reinstalling the engine I had very little wear on zincs. When I removed the engine I found the engine mounts to be "interesting" to say the least. They were hung below the rails and protruded through, it is very possible, perhaps likely the engine was previously grounded through this screwball mount system. I lowered the rails and now have a traditional mount system, and isolated.

As I said, questioning everything. Don't much like having to grind off a bunch of newly applied bottom paint, yet again!

[Andina Marie]

I can see no advantages to be gained by not grounding the engine directly to the steel hull. Unless you have an isolated starter motor and alternator (very rare) the hull is essentially grounded through your DC panel ground anyhow. By grounding the engine there is less possibility of feeding DC stray voltages from your panel through the prop shaft to underwater metal. In fact it is also common practice to add a grounding brush to the prop shaft since it can be isolated from the engine by a flexible coupling and isolated from the hull by rubber cutless bearings.

[hpeer]

Thanks for the reply.

I never questioned the ground connection before, but I am absolutely sure I never removed a ground connection when I removed the engine. The prop has never had a ground brush either.

Come to think of it, I don't recall any ground connection on our big boat either, but I have never looked. No ground on the shaft either, but the zincs were near perfect on that boat when we hauled it.

[noelex 77]

There are two philosophies regarding the grounding of metal boats.

1. Isolate the ground from the hull
2. Ground the hull at one point, and one point only

The low resistance you are recording between the DC panel and the hull suggests that it is deliberately, or accidentally ground to the hull. If it is the former you need to find this point and attach all grounds to this point. If it is the latter you need to find this point and repair it.

Originally Posted by Andina Marie

I can see no advantages to be gained by not grounding the engine directly to the steel hull. Unless you have an isolated starter motor and alternator (very rare) the hull is essentially grounded through your DC panel ground anyhow. By grounding the engine there is less possibility of feeding DC stray voltages from your panel through the prop shaft to underwater metal. In fact it is also common practice to add a grounding brush to the prop shaft since it can be isolated from the engine by a flexible coupling and isolated from the hull by rubber cutless bearings.

It is not difficult to isolate an engine. Many marine alternators are isolated as standard. Isolated starter motors are not common as you point out. The simple solution is to ground the engine for the few seconds while starting. This can be done easily and automatically (if you want).

The lack of isolation during starting is only very brief and is of little consequence. The rest of the time the engine is isolated.

[hpeer]

The "ground at one point and one point only" is something I am wondering about. Elsewhere I have read otherwise, which got me thinking.

[Andina Marie]

Originally Posted by hpeer

The "ground at one point and one point only" is something I am wondering about. Elsewhere I have read otherwise, which got me thinking.

Agree, it is a misguided myth. The theory it that with only one ground point there can be no current flowing through the hull from point A to B etc.. The basis for avoiding that was current flowing through the steel will produce voltage gradients and those voltage differences will cause electrolytic corrosion.

But unless your boat is made from tin foil, point to point voltage differences for normal currents are less than 0.0001 volts and even for the negligible time starter motor currents would flow, it is below 0.001 volts beyond an inch from the grounding point.

Perhaps the myth carries over from the recommendation that in non-metal boats the bonding system should be grounded at only one point?

[hpeer]

There seems to be much myth and theory surrounding grounding in general. Some years ago I was put in an akward position by my employer to seal some drawings for some security control centers. I had not supervised the design and I disapproved of the grounding scheme. Or more correctly, the lack of any coherent approach. Although I am an electrical PE, I don't have tons of experience in this area.

Long story short I had a hell of an argument to change the design, especially since I eventuall found out the wiring had already been run. Lots of push back.

Various engineers cited this or that code or standard for the correct method of grounding and the need for a "halo" (or not) and what that was supposed to do. After much ballyhooing no consensus was ever achieved.

It was eventually resolved by picking one particular standard and using that uniformly.

I was able assure a code compliant design. Beyond that, I'm not so sure.

God only knows if the corrections were ever made. Eventually the prime was terminated and suites ensued. Thankfully nothing I had a hand in.

[noelex 77]

Originally Posted by Andina Marie

But unless your boat is made from tin foil, point to point voltage differences for normal currents are less than 0.0001 volts and even for the negligible time starter motor currents would flow, it is below 0.001 volts beyond an inch from the grounding point.

I think everyone is familiar with the voltage drop that occurs in a wire. This is ideally about 3% (ie 1.5% for both the positive and negative sides of the circuit). Even if we ignore the voltage drop in switches, connections etc, the ground point of equipment will be 0.2v different to the battery negative and this is in well designed circuit with appropriately sized wiring.

So if we ignore the metal hull for the moment, the ground point of equipment that is on will be at least 0.2v different to the battery negative, or to a circuit that is switched off. The switch only disconnects the positive side.

Of course we cannot ignore the metal hull. It is a bloody big conductor and will conduct current to even out the 0.2v difference.

This is problem we are trying to avoid by grounding all circuits at one point.

[dannc]

Kasten has quite a few articles from Metal Boat Building about corrosion and bonding that he combined in one PDF file, http://www.kastenmarine.com/_pdf/mbqCref.pdf.

Could stray voltage from other boats or from shoreside be a problem?

Later,
Dan

[dannc]

From Kasten again, Corrosion Prevention. Skipping a few paragraphs to Bonding (bolding and underlining are mine),

Bonding

Bonding is the practice of tying all of the underwater metals together with wires or bonding strips. It is done in order to 'theoretically' bring all of the underwater metals to the same potential, and aim that collective potential at a single large zinc. It is also done in order that no single metal object will have a different potential than surrounding metal objects for the sake of shock prevention.

However for maximum corrosion protection, metal boats will ideally NOT be bonded. This of course is contrary to the advice of the ABYC. Keep in mind though that the ABYC rules represent the consensus of the US Marine Manufacturers Association, and are therefore primarily aimed at satisfying the requirements aboard GRP vessels, about which the MMA is most familiar. Naturally, aboard a GRP boat the boat's structure is electrically inert and not subject to degradation by corrosion, therefore aboard a GRP boat there is no reason to recommend against bonding - except perhaps the fact that bonding all underwater metals using a copper conductor invites the possibility of stray current corrosion of those underwater metals due to the possible potential differential in the water from one end of the boat to the other.

Little by little though, the ABYC is learning more about the requirements aboard metal and wooden vessels, and recommendations for aluminum and steel boats have begun to appear in the ABYC guidelines. Even so, the corrosion vs shock hazard conundrum aboard metal boats is not 'solved' since the solutions are not as simple as they might at first seem. For an introduction to some of the issues with regard to bonding, please see our "Corrosion, Zincs & Bonding" booklet.

However, before you read that booklet and possibly take on the design of an onboard electrical system, please thoroughly study the materials at the Electroshock Drowning web site..!! The following is a brief summary of the issues involved.

The sections after the Bonding section might be of interest.

Later,
Dan

[hpeer]

Originally Posted by dannc

Kasten has quite a few articles from Metal Boat Building about corrosion and bonding that he combined in one PDF file, http://www.kastenmarine.com/_pdf/mbqCref.pdf.

Could stray voltage from other boats or from shoreside be a problem?

Later,
Dan

I just read through this stuff once again, I'm not sure he really gets to the nub of this particular question. For example he doesn't seem to mention isolating the engine.

To your question about other boats and shoreside..we have two steel boats ( yeah, nutzoid, I know) and they were tied bow to stern on the dock, except for a 3 week cruise we took in the boat in question. The zincs are near perfect on the other boat after 2 YEARS.

That is one of the bits that makes me consider an engine related electrical issue. We ran the engine for maybe 50 hours on our little cruise.

The engine, battery, and alternator grounds were all connected, the the hull was not connected.

[hpeer]

Originally Posted by noelex 77

I think everyone is familiar with the voltage drop that occurs in a wire. This is ideally about 3% (ie 1.5% for both the positive and negative sides of the circuit). Even if we ignore the voltage drop in switches, connections etc, the ground point of equipment will be 0.2v different to the battery negative and this is in well designed circuit with appropriately sized wiring.

So if we ignore the metal hull for the moment, the ground point of equipment that is on will be at least 0.2v different to the battery negative, or to a circuit that is switched off. The switch only disconnects the positive side.

Of course we cannot ignore the metal hull. It is a bloody big conductor and will conduct current to even out the 0.2v difference.

This is problem we are trying to avoid by grounding all circuits at one point.

There are bits I could quibble with but I don't think they are relevant.

I take you point that if there is a voltage drop between the intentional ground and the unintentional ground a current will flow. Agreed. Do I care? That's what I'm trying to sus out. I'll come back to this point.

First I think I've convienced myself I want to bond the engine to the hull. I think that is my bigger question. My reasoning, if it is sound, goes like this.....if I use a floating approach then I need to maintain it, I need to test it routinely. If I don't then a failure can cause some unintended problem of unknown consequence, perhaps significant. If I intentionally tie them then I think the failure modes and subsequent consequences are less.

[Pelagic]

My steel hull is over 30 years old and after 10 years of ownership and sandblasting exterior hull down to bare steel in 2010...to remove too many layers of bottom paint... I was happy to have found no problems.

Everything from Neg buss bar, main engine/gen/thruster/windlass has cables of appropriate size going to a common ground point frame in the engine room.

My zincs are now 5 years old and after hammering on last dive..are at 75%

Is there a possibly when replacing engine, the ground point was inadvertently painted over?

[hpeer]

Coming back to the remote unintentional ground and the question "do I care". I'm going to try to work this out as I write.

Let us accept for the moment the hypothetical 0.2VDC imposed on the inside of the hull.

If the hull had NO connection to the outside water, the hull, prop, etc. were perfectly encased in epoxy then it would not matter. Because the hull would be (electrically) floating, there is no reference between the inside and outside potentials.

But it is not floating, it is referenced to the outside via the zincs, prop, and shaft. But it would be Positive to the water, thus protected.

Yet, I fail to see how we can get that 0.2VDC potential as the zincs are connected to the hull, the hull to battery ground, and hull to the fault ground. So the voltage, as Marie points out, will be effectively zero. But even at that a wee bit above zero, which is good(ish?)

What you do get is current. The hull will act as a parallel conductor to the intended return wire. The hull being a very low resistance will carry most of the current. Again, do I care? What are the negative consequences?

Kasten makes the point that if your boat is between two electrodes communicating current then you could make a "cage" to direct the flow around your boat and not through your boat. This only makes sense if the epoxy paint system is a poor insulator and allows current through it.

Elsewhere I read a paint expert say that no epoxy keeps water out, it just slows it a whole lot. Maybe that is so? Then Kastens comments about a cage would make sense.

It also would make sense that zincs only work for up to 50 feet. They create a ZERO voltage point in the water, then there is an increasing voltage gradient out from the zinc, increasing through the water, faster in fresh than salt. So then, if the interior hull is all at one potential common to the zinc, you get an increasing voltage potential between the water and the hull the further from the zinc, with the hull being negative to the water outside. This is BAD!

But it still does not mean I care about the ground fault!

......

So, providing the above is correct, not sure, here is what I'm gonna do and why.

1. I'm going to ground the engine to the hull, and put on a prop brush.
2. I'm gonna try to find that damn ground, just cause not to is sloppy.
3. I'm gonna have a beer, cause if I get another failure it won't be a big deal, a bonded hull is pretty fail safe without huge down sides.

So what went wrong before? Why the rapidly dissapearing zincs?

Prior to the reinstallation the engine was unintentionally, but firmly, grounded through the wacko engine mounts. I removed that, unknowingly. The alternator was tied to the battery. But not to the hull. My hull ground was somewhere else, in the fault, possibly through some small gage wire. Electrons leave the alternator, go through the load via the small gage wire, then back to the alternator. But the small gage wire is connected to the hull at the fault. This puts a Negative voltage on the zincs relative to the surrounding water. BAD, VERY BAD!

May not explain the paint blistering, but does seem to explain the zincs for sure.

Does this fly!

[hpeer]

Originally Posted by Pelagic

My steel hull is over 30 years old and after 10 years of ownership and sandblasting exterior hull down to bare steel in 2010...to remove too many layers of bottom paint... I was happy to have found no problems.

Everything from Neg buss bar, main engine/gen/thruster/windlass has cables of appropriate size going to a common ground point frame in the engine room.

My zincs are now 5 years old and after hammering on last dive..are at 75%

Is there a possibly when replacing engine, the ground point was inadvertently painted over?

Pelagic, when you say to a common ground frame, I presume you mean a frame that is also connected to the hull?

Assuming that, The answer is not painted over but, yes, removed.


As far as I can tell ther never was an intentional ground connection from the engine or alternator or buss bar to the hull. I SUSPECT there was a firm ground through some wacko engine mounts. (pretty sure really the more I think about it) When I reinstalled, with normal mounts, I removed the tie to the hull, without realizing it.

Although he is a fine craftsman and a person I admire greatly, I know the PO had some strange ideas about electricity. So this would not surprise me at all. I know for a fact that there is no intentional ground connection anywhere near that engine.

I should have noted that myself in the reinstall. (gotta stop doing that)