Mounting junction box on my aluminum boat

[frankr]

outbdnut2 wrote:

Since there is already a negative copper battery cable running to the motor, most or all of the starter current will follow the lower resistance copper cable instead of the aluminum boat, and, as has been said, it may not find a path through the aluminum at all. I have mounted the box directly to aluminum on several boats over the years with no problems. In addition, it is mounting aluminum to aluminum, so no problem with galvanic action caused by unlike metals and moisture.
Dave

That was sort of my knee-jerk thinking as I first replied in this topic. But then I got to thinking and realized it is only true—to a point. In your example, the copper cable and aluminum hull are, in effect, resistances in parallel and the current through each resistance depends on the electrical rules for resistances in parallel. That is, the current flow through each resistance is related to the resistance of each path. Suppose the hull and cable both have the same resistance. In that case, the current flow would be divided equally between each path. Unequal resistances would result in more current through the path of lesser resistance and less flow through the path of higher resistance.

But to the point, what would happen if the copper cable develops a loose or corroded connection? Then, more or even ALL the current is going to go via the hull. Taking it even further, suppose the path between the stern brackets and the engine happens to be a thin wire, such as a mercury safety switch. Now a heavy current, such as a starter motor is going to try to pass through that thin wire, overheating and probably burning up the wire.

Taking it even further, suppose the rigger decides to save 50 feet of wire by grounding all the other electrical equipment such as lights, depth finder, bilge pump, radio, etc., through the hull. Can you imagine what a diagnostic nightmare he has just created in event of a malfunction? And we haven’t even gotten to corrosion problems yet.

Bottom line is good practice says not to use the hull as a ground return.

[outbdnut2]

frankr wrote:

outbdnut2 wrote:

Since there is already a negative copper battery cable running to the motor, most or all of the starter current will follow the lower resistance copper cable instead of the aluminum boat, and, as has been said, it may not find a path through the aluminum at all. I have mounted the box directly to aluminum on several boats over the years with no problems. In addition, it is mounting aluminum to aluminum, so no problem with galvanic action caused by unlike metals and moisture.
Dave

That was sort of my knee-jerk thinking as I first replied in this topic. But then I got to thinking and realized it is only true—to a point. In your example, the copper cable and aluminum hull are, in effect, resistances in parallel and the current through each resistance depends on the electrical rules for resistances in parallel. That is, the current flow through each resistance is related to the resistance of each path. Suppose the hull and cable both have the same resistance. In that case, the current flow would be divided equally between each path. Unequal resistances would result in more current through the path of lesser resistance and less flow through the path of higher resistance.

But to the point, what would happen if the copper cable develops a loose or corroded connection? Then, more or even ALL the current is going to go via the hull. Taking it even further, suppose the path between the stern brackets and the engine happens to be a thin wire, such as a mercury safety switch. Now a heavy current, such as a starter motor is going to try to pass through that thin wire, overheating and probably burning up the wire.

Taking it even further, suppose the rigger decides to save 50 feet of wire by grounding all the other electrical equipment such as lights, depth finder, bilge pump, radio, etc., through the hull. Can you imagine what a diagnostic nightmare he has just created in event of a malfunction? And we haven’t even gotten to corrosion problems yet.

Bottom line is good practice says not to use the hull as a ground return.

Good points, Frank!

I think the biggest threat is having a lot of other stuff grounded to the aluminum boat, because there would be items that pass current continuously, like lights and radios.

Current to the starter, choke, and Mercury switch only flows while starting, so while (if all wires and terminations are good) a smaller portion of the current may go through the aluminum, it is very momentary, being only while cranking, so likely not an issue.

I agree that, as you said, a broken or worn wire, as you stated, yes, could cause wires to fry their insulation and maybe the wire too if the current finds a parasitic path back through the hull unless these other small wires are fused, and fuses are usually in the other (positive) wire, not the ground side. That alone is a good reason to insulate the box from the hull.
Dave

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