On SI the ground wire (hooked to ships hull) was snipped off at the shore power receptacle. I decided to reconnect it to see what effect it would have on the Capac. Reading went from .92 to .63. The reading jumped back up when it was cut.

ps-found this in the 'sticky'...

I too am in fresh water and have a consistant reading of .65 to .70. This is true when at the dock, connected to shore power or when out in a cove with no-one else around (and without generator). I've checked the prop shaft to hull voltage and see about .25 on the port engine and zero on the starboard engine. The new anodes we installed before re-launching this spring are burning up. I think I may have to replace them annually based on the amount of activity. We have started using a drop over the side guppy anode while at the dock. Should the meter readings change with the guppy connected?

DiverDennis:

Before your wiring was fixed, did you experience the same low readings whether connected to shore power or not? I would really like to get in the good range!!!!!

If you have not done so already, calibrate the CAPAC with a digital multimeter.
Hook a meter in parallel with the leads feeding the CAPAC and read the voltage. Use the little plastic adjust screw on the face of the CAPAC to set the needle as close to the reading of the digital meter.

If the readings are still low you may have a wire that draws significant current too close the CAPAC wires. I know when I run the engine blowers the CAPAC takes a dive. I think the wires are so close to each other there is an induction issue, but not really sure about that.

Test the CAPAC with the shore cord unplugged, like you did out in the lake, and also test it with the battery switches turned off.
You may also want to test the CAPAC reading with all of the battery negative cables disconnected.

I've also found as the summer progresses and the Lake gets warmer the reading goes down.

Also be suspicious of your dock neighbors. I had a guy 2 docks down from me with a melted shore cord plug. It was dumping voltage on the ground, making my galvanic isolator buzz.

-Karl

That makes sense. I'll have to trace the wire.

-Karl

Sounds like you have to work back. My boat is a in Lake Barkley and I also boat in KY lake.

After three years (three full summers) my readings have gone down a little.

.65-.72. I replaced anodes at last haul out and readings were up to .95 for the past 2 summers.

I have growth on bottom and am losing continuity between anodes and cathode so I added mag guppy this summer.

With guppy redings jumped up to 1.05 and are steady.

Now it sounds like you have either an isolation problem (since guppy is not changing readings) or your silver silver Chloride cathode has been damaged or painted over. also sometimes the single wire which is going to the cathode is disconnected broked or just damaged.

When I haul in a week for paint I will clean anodes and assuming they are still in good shape my hull readings should come up to normal without the guppy.

one test is take a jumper wire and have helper push activate your meter while you intentionaly ground the shaft to the hull or the engne the block. If the meter deflects downward they your shafts are properly isolated. If not they are not isolated and they will lower your readings and you will be unprotected.

You can check with an ohm meter also. Check for continuity between shaft coupler. There should not be continuity. There will be a very small amount o ohms but not enough to activiate a tone.

Also clean your anodes and see if the readingts change. Readings underway are not reliable and it can take several hours for readings to come up while at anchor.

Through process of elimination you should find the ground fault.

dave

ALL ground wires?

Your 12 volt ground grounds to the stringers or the bulkheads.
110 volt NEVER grounds to the hull! Your shore power, gen, inverters must should run through a gavanic isolator or a isolation transformer. If DC current leaks through on your incomming 110 system it will cuase electrolysis.

Test your system with both your pos and neg cables off your batteries and disconnect your shore power.


dave

Roger,

Battery negative must go to the engine block (best to a stud on the starter) so that the starter will work. The engine block, in turn, should be connected to the hull but a large wire is not needed. This is how most cars and trucks are wired.

If you have a good ground block (mine is 6 3/8" bronze studs welded to a piece of 1/2" copper) then you can run the battery negative to one of the studs, and another heavy wire to the block. Fastjeff also does it this way. This setup is easier to work on at the cost of putting the starting current through a couple of extra connections.

Either way, the engine block is at the same potential as the hull. The block and hull are insulated from the shaft and prop by a nonconductive connection between shaft and transmission, and by the rubber and fiber bearing between the strut and shaft.

My 32' had things grounded to the hull all over the boat -- the trim tab pump, the head, and so on. I recommend replacing these grounds with wires of the same size as the power feed wires, leading back to the ground plate.

Marinettes came from the factory with the negativ12 volt DC grounded to the hull. THis does NOT include the 120 volts AC ground, unless it goes through an isolation transformer first.

I doubt if you could isolate every 12 volt DC item from the hull.

Jeff

Mike:

I have BOTH. Without the mag, in fresh water, I struggled to get above 0.70 volts. No problem now.

Jeff

12V DC hull grounding is in accordance with acceptable marine practice. It is NOT the source of your problem.

Now grounding of disimular metals (SS shafts, props etc. will deflected you readings down. GALVANIC corrosion occurs when two metals are submerged in water. (Not pure distilled water is a poor conductor and there would be little or no raction between metals. ex: bronze/SS/Alluminum. Now is salt, or fresh lake water with sediment etc. conductivity increases and so the flow of electrons from less noble to the more noble metal.

It is easy to confuse electrolosis with galvanic. Stray DC voltage will cuase damage much more quickly than galvanic.
Stray AC while EXTREMELY DANGEROUS has minimum effect on corrosion.

Your CAPAC actually is reading Negative voltage. and ideal reading of .95 is actually -.95 of a volt.

If you have water in your bilge pay attention to what is in the bilge water. A missing tool in a partially flooded bilge will have an effect on your readings and can also cause a hot spot for corrosion.

dave