I have always had low Capac readings and made a few changes this winter that have helped. I had new grounding busses installed in the engine room and on the flybridge to ground all electricals. They are connected to each other. I noticed that the yard also grounded the engines to the busses. I've read Doug's well written post on engine/hull grounding and understand that engine is the only item that should be grounded to the hull; although I also read that the gas tank should be hull grounded (mine doesn't seem to be and I have two wire gage sending units). Is there a problem not grounding the engine to the hull with satisfactory Capac readings? My Capac readings are 0.9 to 1.1, depending on the day I check. The engines seem to be isolated from the hull by wooden blocks under the motor mounts. New non-conducting cutlass bearings and rudder show no continuity and the shafts are isolated. I have the correct alumaloy anodes. Should I return the engine to the hull grounding post or leave well enough alone?
Jim

Thanks for the reply and good description. Neither my engines nor any other electronics, unless there is something I don't know about, are grounded to the hull. They have all been grounded to the grounding busses added by my yard (one in the engine room and one on the flybridge). The busses are grounded directly to my batteries. My engines were previously grounded to the two factory grounding points in the engine room, along with most other electronics, just as you describe. The engines and everything else are operating properly and the Capac is calibrated and reading within tolerances. I was just wondering, if every thing is working, is there any reason to be concerned about my present set up.

This discussion keeps surfacing.

On our Big YOU DO NOT BOND METAL THRU-HULL FITTINGS TO THE HULL! All dismimular underwater fittings need to be isolated from the hull. Examples, Shafts, Rudders, water intakes etc.

Part if the confusion comes from electrical bonding used on fiberglass and wood vessels. Now this bonding which often went to a dineplate (a rectangular mass of compressed metal shot attacted to hull under the water line.) grunding plane. This was used for several reasons none of which was corrosion protection. The bonding is used to provide a lightning ground path. (Very important on sailboats) Anodes are still used on these vessels to protect underwater metal fittings.. outdrives, shafts, props rudders etc.

Our electrical grounding is from the 12 DC supply directly to the hull. (in our case the forward engine room balkhead. One ground for each engine. All DC current passes through the hull since the battery is in the ground loop. The DC current should NOT pass thru your underwater fittings.. again shafts, rudders, metal thru-hulls.

What actually happens is when there is DC grounding thru the shafts etc. that is a positive potential which will deflect your voltage down. (Your CAPAC reading of .95 is actually -.95. Now that is really MV but is commonly referred to as volts. That is way a cheap volt meter will not show the voltage.

Trying to keep it as simple since that is me anyway.. You want the electrons to flow FROM THE ANODE to your hull and Cathode. (Cathode is the CAPAC reference and it does not provide protection unless you have an impressed corriosn system. ) This current flow is what your sacrifical anode is suppose to do and it will desolve. If things get too part out of wach.. then the electrons are pulled from your hull, rudders, props, etc and that is a bad thing.

Now to confuse it a little more... your galvanic isolator prevents DC voltage from gong through you AC ground wire.

Hope this helps a little.

safe boating,
dave

Sounds like you have a Negative Bus ( a good thing ) not a ground bus.
I don't believe in using the hull as a conductor, which is what happens when you have the battery neg. in parallel with the ground.
Just think that when you run the starter motor you have 400- 500 amps flowing. --Ed

My boat surveyor informed me otherwise as well as the DNR inspection officer. Who am I to argue.

I have isolated my thru hulls but then I bonded them to the hull thru the green wires I was told to use..

On our aluminum boats ALL thru hull fittings and shafts, rudders, etc must be isolated and never ever bonded to the hull. Sounds like the DNR And your surveyor are plastic boaters.

I am sorry but the surveyor and the DNR inspecting officer are not correct.

Where the confusion lies for many is there are Three (3) grounding systems on boats!

1. DC grounds we all know what that is. Electrical connections ie the negative terminal of you DC banks services as the completed 12 volt DC cirucit. Buss bars are added to make grounding electrical components easier to connect to the DC negative.

2. AC (Green Wire safety grounds) This is the wire that often confuses inspectors. The 115 vac must have a Green saftey ground wire. This comes in through the shorpower to your galvanic isolator. It is a safety ground.

AC Neutral)(white) this is your return AC wire which is also referred to as the common. It will be attached to the green saftey ground wire at the main junction box on shore to an earth ground. Sub boxes the common is normally isolated so your GFIC will function properly.

3. Equipontental Grid. This is used in many installation where stray current is distributed over a wide area. The most common use is swimming pools. All metal is bonded with a solid copper wire to a ground grid. (ladders, pumps, diving boards etc.) Any stray current that may enter the ground static or electromagnet like high tension lines or leaking current from a neighbor etc, will be dispated safely through the entire grid. (grid is normally the re-bar in the concrete around a pool)

Ok confused.. we all are.. the difference is the Equipontental NEVER commes in contact with an AC safety equipment ground or a neutral.

Now Example the Alluminum fuel tank must be bonded to the alluminum hull. Any static charge from a nozzel etc will prevent a spark. The current will be discharged through the entire hull safely.

Bonding systems are very common on sailboats to provide the "Cone of Protection" from lightning.

Bonding would work on ours except.. brass, bronze etc are higher on the noble scale thann alluminum therefore the electrons will flow from our alluminum hulls to protect the bronze, brass, stainless etc. fittings. That is why you never bond dissimular metals on an alluminum boat. On a fiberglass boat for example a proper bonding system ties all the fitting together to form the equipontental grid which then is connected to a Dyna plate to spread the discharge over a large area.

I probably confused some.. heck I confuse myself!

Safe boating,
dave

References would be appreciated. I HAD to ground fuel tanks and all metalic thruhulls (raw water intake, exhaust flapper valves) or I would not pass the charter boat inspection. I since I had them isolated from grounding to the hull at the mountings I had to run green wire from each to the hull grounding tab. The CAPAC electrode was not required to be grounded or else it would be useless.