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.

Re: Shock between boat and dock!!
 

I kind of agree with Steve, it seems like you have some current leakage and it's only gonna get worse. What the lift guy told me about the galvanic corrosion is that the bolts corrode inside the pilings when moisture wicks up the pilings it is like the bolt being immersed in water. The current then travels through the lift when it's plugged in, through the bolt, and trashes the bolts, eventually the lift falls off the piling. They now install all their lift motors with short pigtails and plugs so you can unplug the lift right at the dock to prevent this. If you want go over to cbpba.com and ask about an electrician over there, I think there are a few in the club that could help.

Re: Shock between boat and dock!!
 

First thing I checked was between the electic supply lines, both the lift 220 circuits and the 110 dock outlets, to the dock fittings, stair and bolts to see if there might be a short somewhere in that wiring, and there is no voltage between the AC grounds and anywhere on the dock or the lift.

Re: Shock between boat and dock!!
 

Here's a couple of good links on Corrosion, but there's a lot of info there on electrical issues as well (since that's what causes corrosion), and how to properly wire your boat/dock.

http://www.boatus.com/boattech/MarineCorrosion.htm

http://www.boatus.com/seaworthy/galvanic/default.asp

Re: Shock between boat and dock!!
 

After several hours with the DVM (and several more re-wiring) I finally tracked down the problem and corrected it. The short answer (no puns) is that the shore power line passed though an under-deck conduit along with several DC leads and the conduit had water setting inside in a low spot (which I've blown out with compressed air as best possible to try to dry it). Even though there were no breaks in the insulation, the resistance between the AC and DC circuits when wet (even though all the wire is boat/wet use rated) was only about 50,000 ohms.

I pulled out all the shore power wiring and re-ran it separate from the DC lines in adels inside the hull sides instead of in the underdeck conduit (I had to move the inlet recepticle, so I will have to patch the old mounting hole in the center console sometime).

I did confirm that, once the voltage leak was eliminated, the shore power ground is tied to the boat DC ground through the battery charger. Once re-wired the AC hot shows 110 VAC to the boat ground, and the AC neutral and ground both show about 3-4 VAC.

The thing that surprised me most is that a coil of wire laying on the deck, connected to nothing and pulled out of all the conduits, still showes a 4-5 VDC potential to the boat ground. The resistance between the wire just laying there and the boat tests out about 500,000 OHMs which is the resistance of me holding the leads.

Anway, after spending a lot of time proving that the boat sitting on the lift in mid air is not quite at the same potential as a true ground I was able to get everything re-wired and behaving as it should.

Thanks to everyone for the suggestions and lthe links.

-Greg