Earth Leak to Steel Hull

[noelex 77]

It is easy to measure the voltage difference. Set your multimeter to volts and put one end on the negative battery terminal and the other end on say the negative thermal of the bilge pump where the wires have been joined. With the bilge pump running the *voltage difference will not be 1/1000 of a volt, but typically about 0.3 v. If the wiring or contacts are poor the voltage difference can be much higher.

If these voltages sound to low to do any damage remember that if we disconect *the zinc anode from the steel hull the *voltage difference between the zinc and steel *in seawater is *about 0.4 v. It is this voltage difference and the current it produces that causes the corrosion *of the zinc.*

It *would be cheaper and easier to use the hull as a negative return. *Metal boats do not connect negative wires around the hull like cars do for very good reasons. This system needs to be maintained .

[Vyndance]

Lightning ground should be assumed in a steel hull. The mast, rigging or super structure is in contact with the hull. Your pump returning to it's frame connected to the hull is a problem. When you are working on these boats do you follow anyone's established practices?

[Andina Marie]

Typically pump frames are bolted to the hull, I would make no attempt to insulate it from the hull. In fact is it also indirectly connected to the hull by the salt water in the inlet hose so to minimize electrolysis on the impeller housing, grounding the frame is advantageous so any leakage current does not appear as a positive voltage through the raw water inlet.

All my electrical connections are marine grade copper wire to the nearest 12 volt distribution point. Any leakage current will be conducted to the hull through the motor frame but this is INSIDE the vessel. Even if the whole pump return path was via the hull it would make no difference to the external electrolytic voltage and have no effect on sacrificial anode life.

The problems reported were active deterioration of the anodes. This has nothing to do with internal negative wiring or bonding. Readers are confusing the problems of a fiberglass or wooden boat with that of a steel hull. On non metallic boats, stray voltages on underwater items can have electrolytic consequences but on a steel hull everything is at the same voltage so the internal negative wiring has no impact and there is no advantage to isolating it from the hull or providing a breaker in the negative battery lead.

[noelex 77]

Originally Posted by Andina

Typically pump frames are bolted to the hull, I would make no attempt to insulate it from the hull. In fact is it also indirectly connected to the hull by the salt water in the inlet hose so to minimize electrolysis on the impeller housing, grounding the frame is advantageous so any leakage current does not appear as a positive voltage through the raw water inlet.

All my electrical connections are marine grade copper wire to the nearest 12 volt distribution point. Any leakage current will be conducted to the hull through the motor frame but this is INSIDE the vessel. Even if the whole pump return path was via the hull it would make no difference to the external electrolytic voltage and have no effect on sacrificial anode life.

The problems reported were active deterioration of the anodes. This has nothing to do with internal negative wiring or bonding. Readers are confusing the problems of a fiberglass or wooden boat with that of a steel hull. On non metallic boats, stray voltages on underwater items can have electrolytic consequences but on a steel hull everything is at the same voltage so the internal negative wiring has no impact and there is no advantage to isolating it from the hull or providing a breaker in the negative battery lead.

If I understand correctly you think when a wire is making contact with the hull on the inside the current is only conducted by the hull on the inside.
In simple terms this is not true the whole of the conductor ( in this case the steel hull - will conduct the current). If there is a minor defect anywhere in the coating of the hul, or fitting that has electrical conductivity, then this is in contact with seawater, which is an electrolyte.


(There are some skin conduction effects that occur with higher frequencies that do not have much bearing on DC currents, but the conduction is still on all surfaces. )

[Vyndance]

From E-11
11.5.2.2 DC Grounding Systems and Bonding - A metallic hull, or the bonding and DC grounding systems, shall not be used as a return conductor.

[Andina Marie]

No the current is conducted by the bulk of the steel.
Resistance in the hull is metal resistivity * length / cross-sectional area.
Steel varies in resistance depending on alloy from 10 to 100 * 10^-8 Ohms per square,
I will use an average value of 50 * 10^-8.
Resistance is going to be highest in a circle at the connection point before the cross sectional area gets large
For a distance of only 1cm from the grounding point, on our boat the cross sectional area calculates to
2* 3.14 * 0.6 = 4sq cm (Diameter * PI * thickness)
So the electrical resistance over the first 1cm is 50*10^-8 * 10 / 4 = 125 * 10^-8 ohms or 1.25 Micro ohms
If the negative cable was disconnected and all 10 amps from the pump went through the hull the differential surface voltage 1cm from the connection point is 12.5 Micro Volts. Resistance will go down as the radius increases so this is the highest differential voltage location and 12.5 micro volts is not going to be any source of electrolysis.

Electrolysis is caused by current flowing in the water. For current to flow from point A to point B there has to be a difference in voltage. If the voltage is zero no current will flow. The voltage on the outside of the hull from either stray or even currents drawn by equipment is only a few thousandths of a volt spread out over many feet. The incremental voltage along the surface is only 12 micro-volts at the worst location There is no way this is going to cause any electrolysis.

For current to flow to or from the hull you need another electrode to complete the circuit that is at a different voltage than the hull. Just one piece of metal (the hull) in the water doesn't make a battery until you connect it to a different piece of metal in the same water. The wiring inside the hull cannot provide a circuit for electrolysis since it is not in the water, or even the same water if there is water inside the hull.

A shore power connection, however, does complete the circuit. The ground lead is connected to the hull on one end and all the shore power ground at the other end which will be at a different voltage. That is why you need a Galvanic Isolator in the shore power lead to break that circuit and leave the hull isolated.

[noelex 77]

I think we will have to agree to disagree on this one.
Hopefully we will hear from the OP and we can get back to solving his problems.

[Nemo55]

Disconnect the alternator/s and run your tests again. (Could be an iffy set of diodes) also,,when considering grounds or relay for power,,,always use central buss distribution.
In other words,,use single source grounds originating at a single Buss bar. (totally cleans everything up) and when running (Pos+) for length run suitable size wire and terminate for distribution at Buss bars like i said.
Follow these simple configurations and your headaches will be lessened.
P.S. get back to us on the Power siphon problem after temporarily disconnecting the Alts.

[goboatingnow]

It's worth pointing out that simply connecting a very high impedence digital multimeter between the hull and 12v is not really indicative of anything

I would ( and many experts also ) favour completely isolated hulls for metal boats.

Ps most through hull fittings that are not steel are isolated from a steel hull. They arnt connected to it.

Dave