Shock between boat and dock!!
 

Today while winterizing the boat I happened to be holding on to the propeller and grabbed on to the dock step. To my surprise I got a mild shock!! Getting out my DVM I found there is a 70 VDC potential between the boat and the dock (stair, bolts holding the dock together, any metal even bolts that only go into the wood pilings). If I disconnect the shore power it goes to under 2 volts.

The boat is on the lift and completely out of the water. The only thing connected to the shore power circuit are the battery charger and the cabin dehumidifier.

Testing between the circuits on the boat there is a 12 VDC and 70 VAC potential between the shore power neutral and ground and the boat battery negative terminals... even after I completly disconnected the battery charger from both batteries.

I'm very confused. If there is no circuit between the boat electrical system and the shore power (after I disconnected the battery charger), why am I seeing such a big electrical potential? I'm sure the current is small as it takes a couple seconds on the DVM to build the AC potential from zero to 70 VAC.

-Greg

Re: Shock between boat and dock!!
 

Sounds like there is a short between the green leg for shore power and your negative side of the 12v system. Many builders do this through the bonding system or it could be an accidental rub through of the AC wiring.

Re: Shock between boat and dock!!
 

I don't think the two crcuits are connected anywhere. Should they be?

-Greg

Re: Shock between boat and dock!!
 

There are some reasons to do it, usually if they bond all the metal components to help prevent corrosion. Throw the ohmmeter between the green leg and the neg side of the 12V system and see what it reads. You may also find the white and green legs of the AC system connected together at the breaker panel, I have no idea why they do that but it will trip a GFI every time if they do.

Re: Shock between boat and dock!!
 

The 70 VAC/12 VDC reading was from the AC green/ground to the DC negative. It is the same if I have the battery chargers connected or if I unwire them from both the battery + and - terminals.

-Greg

Re: Shock between boat and dock!!
 

Do you have a 12v/110 fridge on the boat?

Re: Shock between boat and dock!!
 

The only 110V on the boat are the battery charger and the cabin outlet (it is a GFCI outlet), which I plug the dehumidifier into during the winter.

-Greg

Re: Shock between boat and dock!!
 

Okay, I have heard of moisture in the pilings conducting current and causing galvanic corrosion of the lift bolts so you likely have that going on (my lift guy tells everyone to unplug the lift when not in use) but I'm not sure how it is effecting the boat. Are any of the metal fittings on the boat touching metal on the lift?? Either way you probably want to unplug the lift when not using it since you seem to have a current leak that will corrode the lift components.

Re: Shock between boat and dock!!
 

I looked at a couple marine electric sites (including Marinco) and they all say to connect the AC ground to the boat ground, so I guess I'll try putting a lead between them tomorrow and see what it does.

As for the lift, there is no metal of the boat contacting the lift, just the fliberglass of the hull setting on the bunk carpets. The lift was wet as I had just had the boat down in the water to run the motors so winterize them (outboards are a lot easier than I/O's). There is no potential showing between the dock outlet ground terminal and the dock bolts/steps etc, so they all seem to be at the same ground potntial. The only lift corrosion I've seen is from paint peeling on the one lift motor mounting plate, which seems to be caused by the cover bracket scraping away an edge and letting water get under the paint. The lift's were completely underwater during hurricane Isabel. The lift motors both died and were replaced right after the storm.

- Greg

Re: Shock between boat and dock!!
 

Disconnect your shore power at the breaker source and hire an electrician. This could be a deadly scenario. I recently read a case where a young boy was swimming near the dock and died due to drowning from electrical shock that was caused by a accidental rub through of the AC wiring to the DC negative wire. The voltage was coming into the water from the outdrive. This may or may not be the problem. Hire a professional to determine the source of voltage. As a marine surveyor I can assure you that I would NEVER get in the water around boat docks that have shore power.