Is this a ground leak?

[ccriders]

I had been out of town for a couple of weeks so I go down to the boat to check it out and I found a residue somewhat like that often found around batter terminals all around the lag bolt that attaches the ground wire to the mast base. I looked around past threads here and found a DC ground leak test procedure posted by sailingdog, so I did those tests and here is what I found:
With the battery selector switch on "1" and the positive cable removed from No. 1 battery, I measured 1.090 volts, 0.908 M ohms, and 0.32 amps.
With the battery selector switch on "2" and the positive cable removed from No.2 battery, I measured 1.105 volts, 0.910 M ohms, and 0.33 amps.
With the battery selector swich on "both" and both positive cables removed I measured 1.085 & 1.080 volts, 0.910 & 0.910 M ohms and 0.31 & 0.31 volts.

I then measured between the ground wire and the mast base and found 200mv +/-25 mv and 19.6 microampss (the miliamp scale read 0.0000).

My boat is a 1976 Pearson 28. It has a keel stepped mast that sits on a cast aluminum base that has a butyl (I assume butyl as it is a very tough black rubbery material) between it and the floor. The ground wire is lag bolted (3/8 x 2 1/2" stainless) through the mast base into the floor. When I removed the lag bot I noticed discoloration and some fouling in the upper half of the bolt. The bottom half was very clean. The ground cable ring connector was completely encrusted with the white residue. When I replaced the bolt I noticed that it would not snug down and left the ground cable loose.

The electrical equipment on my boat is very simple consisting of a VHF radio, knotmeter, depth sounder, navigation and cabin lights and the wiring for an Atomic Four.

There is an AC system that feeds the battery charger and two outlets, but I was unpluged from shore power. All switches were off and the only thing hardwired to the battery is the charger, which was unplugged.

I concluded that I have a DC ground leak and that the white residue formed because of a loose connection.
Does this sound right? And how do I address these problems?
Thanks for listening
John

[sailingdog]

CCRiders—

The white powder is probably aluminum oxide, a corrosion by product from the mast base corroding. Where does the ground cable from the mast step go to, and why is the lag bolt holding it down so long??? What does it screw into? If the ground cable lag bolt screws into a wooden block that is saturated with salt water, that could be causing some problems. Also, is your bilge normally dry or wet??

If you've got .3 amps or so—it is probably that something is still on while you're testing. Double check to make sure everything that is hardwired to the battery has been disconnected.

[ccriders]

My bilge is almost always dry and its been totally dry for the last six months. I've owned the boat for 2 &1/2years and in that time have fixed leaking windows some stanchion bases and where the chain plates penetrate the deck. Probing around found no problems with the deck core so the leaking must hae been minimal with the PO. However there was significan corrosion on the mast base and I took 5/8 of an inch off of it when I replaced the standing rigging.
The lag bolt is what I found when I bought the boat so either it was Pearson or the previous owner that decided this was the right size.
The ground wire is connected to the grounding system that ties together all the through hulls, electronics, engine, ground terminal block, chain plates, etc.
I assume that the sole upon which sits the mast base is laminated wood. Based on some pics I saw of a P26 keel stub, there is probably a plywood structure that the floors sit on and then everything covered with fiberglass.
I'll go probe it with a pic and see if I can figure out what is there. I believe that the lag bolt was intended to hold the mast base in place (the two forward bolts are much smaller diameter)and was a convenient place to put the ground wire and repairing it so the lug is snug should not be a problem.

How do you determine where the DC Negative shorts over into the boat's grounding system? That's the question as until I can solve that, the DC negative will bleed into the ground and go to sea water via all the things it bonds together.

I appreciate your help.
John