Negative Hull Bonding

Colleges
This subject has reared its ugly head again and I would like opinions on which is the correct method.
We have an Aluminium boat, trailered. it is powered by an inboard diesel.
This is so we all talk about the same vessel.
We all Know that each and every electrical component we need to run twin wires both positive and negative to make the component work and to keep it isolated from the hull. We all know if using more than two batteries, the negatives from both or even a third are connected through a common bus bar , or neutral bonding bar and then wired to each component with its correct sized wire.
We all should know that we should put a bonding wire from motor to hull and theoretically have a bonding wire from hull to a sacrificial anode of appropriate size to hull or have a good clean connection through hull.
As i have read and being lead to believe that we should also have a bond from the negative Bus or Neutral bond to hull, in my case a lead from starter motor negative to hull, is the correct method of wiring?
I have just being informed from a distributor of sacrificial anodes that the bonding on the negative to the hull in an aluminium trailer boat is not the correct thing, and he negative should be isolated as well as the positive.
What are your Thoughts.
Katoh

 

Your source is correct, galvanic action decreases if there is no current carrying connection between engine and aluminum hull.

But with complete isolation there will be a lot of electrical "noise" on the 12V leads which may cause poor performance of electronics. My advice is to connect the negative bus to the hull with an RC combination (1000 ohms and 0.5 uF parallel). The resistor prevents static charges, the capacitor makes a low resistance path for RF voltages.

 

CDK

I am trying to picture how you would wire it? from the Negative bus joined to hull is only one wire how would you put both of those in parallel?
Could you post a Picture or wiring diagram.
I thought it was impossible to isolate the negative to the hull, just simply through motor groundings, it would be impossible to isolate the the motor.
Katoh

 

Wires of capacitor and resistor tied together, one side to the hull, the other to the engine.

To isolate the engine from the hull it has to be mounted on rubber feet, have plastic fuel hoses, an isolated heat exchanger, plastic through hulls etc. All rather difficult to achieve.
Another option would be to provide isolation in the form of a non-conductive shaft coupler. That would also prevent galvanic action between the shop/prop and the aluminum.

But I didn't say you should do it that way, just that your source was correct....

 

Thanks CDK
I see exactly how its set-up now.
Well the big hassle is were running the motor through a leg, so we have an I/O, there is so many ways a bond can transfer to the hull it, it would be virtually impossible to isolate ever point, even steering and accelerator cables act as a bond.
In theory couldn't you just disconnect the battery when not in use, or would that be just too simple?
I suppose the question is now are we damaging our hulls with the negative bond to hull for the limited time we are on the water?
Katoh

 

With an I/O there already is an electrical path, but little chance of corrosion because the amount of exposed noble metal is very small compared to the aluminum action as a sacrificial electrode.
What you do inside the boat has no effect on galvanic corrosion, so it doesn't matter if you disconnect the battery or not. The steel from the prop shaft and trim cylinders forms a battery with the aluminum parts around it. Early Mercruiser legs got holes in the lower gearcase after just a few months in seawater. Then they connected all the parts with straps and washers (the bonding kit) and solved the problem completely.

In your case there may be a small amount of corrosion of the I/O leg when the zinc is not replaced in time. The hull itself is not at risk.

 

I think its far more important to get a good bonding wire between all these parts on the boat like the motor to hull, leg to motor, leg to hull anything basically that's got that potential to carry that negative, if that makes sense.
I am toying with the idea from the starter negative lead, to run the same size cable to the hull connected via a zinc connection, that way if things start going wrong that's the first place to be corroded.
Cheers
Katoh

 

katoh,

Saltwater application?

Do you have an ohm meter?

Disconnect the engine battery positive and measure resistance between the various engine/driveline/battery ground component connections and the hull and see what you get. Create a list and record the various resistances and locations and then post your results.

Even a small leakage path is unwanted for a system which is supposed to be floating.

For the moment I will not discuss exact solutions to engine as a noise source for RF like CDK talked about.

Before you put in his RF ground, we would like to look at ground noise with an oscilloscope (after we have confirmed isolation) to get a feel for noise for radio interference. The RF grounding design would then have to have a low impedance relative to the RF bandwidth of interest.

By designing a ground connection from the inboard to the hull as part of the design approach, we do not have to deal with isolation or how this "isolation" may break down over time.

Sacrificial Anodes are good way of providing a warning of galvanic or stray current corrosion as the result of something beyond your control. Either to do with shore power, or a device on-board aging and producing leakage current.

So the Anodes provide us with a way to understand degradation over time.

CDK

You seem to have a lot of electrical background where did you get your training?

In looking at bonding, it is possible to have a galvanic corrosion circuit completed by stagnant salt water in the bilge. So this is a consideration unless the bilge is always dry. This is an often overlooked conduction path which is inside the boat.

Mark

 

Hi Mark
Thanks for the input. My boat has no wiring at the moment, it is just a bare hull. I have totally stripped it and am rebuilding it from scratch. All I'm doing is getting an idea how and where to run my main cables.
99% of my wiring I have worked out, I have read the books, taken advice and asked the questions. The question that come up lately was about bonding the negative to the hull.
I think I will use a good boding wire from motor block to hull an another from transom shield to hull and possibly from leg (somewhere within the engine compartment) to hull as well. If negative is bonded to the motor, the bonding wire from motor to hull should be ample. There would be no need to run a bonding wire from battery to hull or from the negative bus to the hull.
Cheers
Katoh

 

Make a formal wiring diagram even if it is a sketch on a large piece of paper. If you have more than two wires on the boat put wire numbers that are waterproof on each end of each wire. Then if you have a problem or want to change something down the line finding it will be a lot easier.

 

I've had none, at least not officially. It started back in 1948 when I built a working radio at the age of 5, using a potato and a needle as a detector. You may call it autism: anything about electricity I understand and remember, many 1000's of datasheets stored in my head, but I cannot remember my kid's birthdays.

 

The boat's electrical system has NO effect on galvanic corrosion (unless you are sitting in the water and tied to shore power) because galvanic corrosion requires a "circuit path" and that normally requires two different metals in contact with the water which are ALSO tied together electrically. That normally happens between a metal hull and drive train components that are in the water (or thu-hull fittings, etc.) Whether the negative of your electrical system is tied to the hull at some point is irrelevant to the corrosion issue.

In any metal hulled boat, we do NOT want to be passing electrical current through the hull - current for instruments, starter, etc. - as the hull is not made to be an electrical wire but, for safety's sake, it is a good idea to have ONE connection, usually from the engine to the hull. If that one connection is a single conductor, there is the possibility that a short from a positive wire to the hull could cause a spark but that shouldn't be an issue in a proper electrical system. Having an impedance (resistor) between the negative of the electrical system and the hull eliminates that possibility but can constitute a safety issue where shore power is used.

I have been in the electrical/electronics field for nearly 50 years, 30 as a designer, and the debate of solid ground v.s. impedance ground has been around THAT long but solid ground (bonding) usually wins out on the safety issue.

 

Thanks to all for their advise.
Just to throw two other equations into the the field of play.
Does this all depend on the the position and placement of the batteries on the boat, I ask as I plan to house my batteries some 5m (15') or so away from the motor in the dryness of the cabin, towards the bow. So If my only bonding contact is at the motor or off the starter motor, this will be a long run to negative side of the the battery, will this be acceptable?
The second part, we will have a removable auxiliary motor fitted to the transom, this cannot be permanently fixed due to storage length, but will this motor need to be bonded as well, some how?the motor is pull start and does not need any battery power to it, but it has provision for external 12v output.
Katoh

 

Katoh,

We would like to minimize DC cable length between high current devices and the batteries, like battery chargers and inverters.

ABYC has the most comprehensive requirements for pleasure boat electrical installations.

DianneB,

I cannot speak for CDK, but the suggestion he made regarding engine grounding was for a proposal that the engine electrical system is floating relative to the hull, and that it is possible that engine control could generate RF emissions that may interfere with the radio. So CDK's proposal was for an RFish type ground to deal with this. However, CDK also states that a floating propulsion engine system is very difficult to achieve.

What is interesting is what the Marine engine manufacturers recommend for connecting engine ground to a metal hull ground. For example I have a Volvo installation guide which requires the Engine Ground (0 V) and the hull (Earth) remain unconnected. Although they have no recommendation on how this should be achieved and do not specifically address stray current corrosion.

Do you have 30 years experience as a Marine electrical designer?

Because if you do there are some higher level topics I would like to discuss.

Thanks,

Mark

 

Good on ya CDK. You make a good case for Wood or F/glass boats if it's gonna have complex electrical system.