Lingering Light

Boat Wiring Guys,

I put in new VHF radio into my sailboat. The installation seemed to be successful except...

Now when I turn off the marine electrical panel switch for the VHF, the green light from the breaker goes out but not right away, slowly turning off in about half a second.

Is there any problem with this? Did I screw something up?



It is not a problem.

The light in the switch gets power when the output side of the switch has power. The residual light is caused by a capacitive charge that is in the VHF radio. This capacitor in the radio helps with noise and voltage spikes but will give the appearance of voltage on the power wire which makes the light stay on.

Hope this helps

Turn of the Screw


I have a 2000 Four Winns.

It has dual trumpet horns installed, one of which was not working when I bought the boat in 2005. The second horn failed last year.

My voltmeter shows 12+ volts when I press the horn button but when I hook up a new horn, nothing happens. If I direct connect the horn to the battery, it sounds. I’m confused because the voltage is there.



Hi Stephen,

This is a simple fix.

Carefully remove the sticker on the rear of the trumpet (it probably says AFI). Turn the exposed screw slowly until the horn sounds.

Hope this helps,


Ground Charge?


I need your help with a marine electrical shore power question.

My boat is plugged into dockside power and there is a 100 millivolt charge on the green ground wire. I have checked with the marina and they have checked their system which is in perfect conformity to the rules.

There is no indication of this on the shore power plug but it appears when you put a meter to the boat which is in the water and seems to make a ground through the water.

What is the reason for this?



Hi Ray,

How are you measuring the 100 mV?

Is it on the shore power plug before you connect it to your boat or is it on your boat when you compare the green to the white?



It was measured on the boat.



The 100 mV difference is simple to explain.

When current passes down neutral wire there is a slight voltage drop. Voltage = Resistance x current. Since there is no current passing down the ground wire, there is no voltage drop on it.

Your dock box is tied in with several other dock boxes with the same neutral and ground wires. If every boat was disconnected from the system, you would not see the 100 mV difference. At times, it is possible to see voltages higher than 100 mV.

Hope this helps,


GFI Trips Common?

Hi Kevin,

I trailer my boat but when it’s in the driveway – or at a marina with household GFI 3 prong plugs – I will use an 30 amp to 15 amp adapter to plug shore power into home to charge batteries, etc. (Not the A/C).

My problem is when I do that and plug it into any GFI outlet, it trips the GFI protector. It doesn’t when I plug into non-GFI outlet. And it works fine at marinas with normal 30 amp shore power outlet.

I understand the GFI detects small inequalities of the inflow current vs. the outflow current. Is there something wrong with the boat wiring like a short cicuit? Or is this common with boats?


Hi Steve,

It is common for boats.

To help reduce marine electrical system noise, some older AC components have a connection between the AC neutral and the AC ground internally. This worked great for noise, but will trip most GFCI outlets.

The culprit is either your battery charger or the AC/DC converter for a refrigerator. You can either replace the components on the boat and hope you solve the problem or just plug it into a non-GFCI outlet.

Hope this helps,


Anode Analysis


I’m burning zincs faster than I think I should.

A local shipwright came to help and measured 4.5 volts AC at the boat’s battery terminals when the charger was turned on (zero when charger off) using a good digital multimeter. (DC voltage was normal – about 14 with charger on, 12.8 or so with charger off.)

He concluded that the inverter/charger is shot and I need a new one. Is this correct or is there likely to be a different wiring problem.

Boat is a 30 foot fiberglass sailboat. The inverter/charger is a 10 to 12 year old Freedom 10 (old black box type) with a Heart Interface R2000 remote.

Your advice would be much appreciated.


Hi Malcolm,

Zincs are designed to burn. They are the sacrificial portion of the galvanic cell. If your zincs are protecting more than your boat, or if there is a problem on your boat, then will corrode away faster than normal.

Galvanic/stray current corrosion is DC. The fact that there is 4.5 V AC on your batteries may be a sign that the charger is bad, but has nothing to do with the zinc corrosion. The correct way to test the system is to use a silver/silver-chloride probe to test the corrosion protection of your boat.

Hope this helps,