How Important is this Ground Thingy ?

[Jetexas]

I had a new prop installed this weekend, and the diver found that of the two fresh zincs I put on Jan. 1, 2012, one had already disappeared and the other was dissolved enough that it was about to fall off. Obviously, I've got an electrolysis problem.

I knew something was grounding out my fuel gauge because it reads "full" all the time. I have no idea if the two problem are related or not.

Long story short, I spent Sunday tracing wires. I was under the impression that everything should ground to the engine and that the engine was grounded through the propshaft into the water. Then I traced a ground wire from my bellhousing back to this thing under the seat.

It looks like at one time, this bolted one of the batteries and had all the ground lines leading back into it. Should I worry about replacing this thing and running all my grounds to it or am I ok with just being grounded to the motor? If I do replace this thing, should I remove the grounds connected to the motor and just run them to this or should I leave them connected to the motor and to this?

Thanks for any info.

[senormechanico]

It looks like a slab zinc backing plate/connection to me.
If that's it, I'd ground everything to the motor and then run one decent sized wire (#8) to the plate

[Ike]

Agreed. The engine block should be the ground, the only ground. If you have zincs they should be bonded to the engine block.

[sailorboy1]

What! You guys don't seriously believe that your boat's ground is the engine and not the battery negative do you?

Yes you need to run the grounds back to the battery. The engine ground also needs to go to the battery negative.

And yes to the OP if all your ground is though the engine and into the water you will go though zincs. This is until the zincs are gone, and then you will probably go though props.

[Ike]

Yes we seriously believe it.

Every electrical system standard for boats says to ground the battery and the system to the engine block.

From ABYC: E-11 Electrical Systems

11.4.5 DC grounded conductor

A current carrying conductor connected to the side of the power source that is intentionally maintained at boat ground potential.

11.4.6 DC grounding conductor

A normally non-current carrying conductor used to connect metallic non-current carrying parts of direct current devices to the engine negative terminal, or its bus.

11.4.8 Engine negative terminal

The point on the engine at which the negative battery cable is connected.

11.4.12 Ground

The potential of the earth's surface. The boat's ground is established by a conducting connection (intentional or accidental) with the earth, including any conductive part of the wetted surface of a hull.

ISO 10133 Extra Low Voltage Installations

3.1
equipotential bonding conductor
normally non-current-carrying conductor used to put various exposed conductive parts of direct current
electrical devices and extraneous conductive parts at a substantially equal potential
3.2
engine negative terminal
terminal on the engine to which the negative battery cable is connected

3.3
craft's ground
ground that is established by a conducting connection (intended or accidental) with the common ground (potential of the earth's surface), including any conductive part of the wetted surface of the hull in permanent contact with the water

3.4
main grounding
earthing point
main point that provides connection for conductors to the craft's ground (for example a unique point of the metallic part of the craft for metal craft, a specific a ground plate, etc), protective conductor and conductors to
conductive parts of the craft (for example mast, metallic tanks, etc)

I could cite a dozen more references.

The reason is to provide a point that is effectively zero voltage potential in relation to the earth. We in the US call it ground. Most of the rest of the world calls it earthing. Ground is there to provide a zero potential point that all voltage measurements can be taken from, and a path back to ground in the case of a short or ground fault. On a boat the only effective means of connecting to ground is some large body of metal that is in contact with the water. On most boats that is the engine. On metal boats or boats with a large ground plate, they can be used but that is not common practice. Common practice is to use the large metal mass of the engine block.

[Jetexas]

Just to be clear, my negative battery terminals are grounded to the engine, so technically everything is currently grounded to the engine and negative battery terminals.

[Stu Jackson]

The West Advisor: Marine Grounding Systems

[Cheechako]

Are you sure that isnt an SSB ground device or a lightning ground? Are you in a marina with shore power attached? Odd s are that is more likely your electrolysis issue.... Or as one poster said... is that for a big plate zinc outside the hull? Maybe the boat has had this problem a long time if that's the case....

[Jetexas]

Lighting ground comes down out of the mast and straight to a keel bolt. It's not attached to the DC circuit in any way.

I have no idea what's on the outside of the hull where this plate is. The boat had been left rotting and flooded for several years before I bought it. No idea what problems it had before, but it hadn't had shore power connected or working batteries in it for quite some time. That's probably what saved the previous prop from completely disintegrating even though it had sat without new zincs or a bottom cleaning for at least a couple years. However, the prop, which was already pink and questionable when I had it checked two years ago had a chunk break off when the diver was cleaning it in January. That's why I was putting a new one on over the weekend.

I've definitely got to stay on top of the zinc situation and track down the problem as I want this new prop to last.

The zinc in my heat exchanger is almost two years old and still looks brand new. Of course, it's in the fresh water system.

[Cheechako]

You should get a look at the outside. Could be a bronze SSB ground plate... some people attach any and everything they can to the SSB ground, including the engine block. Or as mentioned it could be another large zinc.... and might need replacing... Maybve if you see a diver cleaning inthe water ask him to look.... How cold is the water in Houston this time of year?

[hlev00]

Originally Posted by Jetexas

Lighting ground comes down out of the mast and straight to a keel bolt. It's not attached to the DC circuit in any way.
...

I thought that was true on our boat too until someone commented that they had discovered that their VHF antenna/coax ground was connected to the mast due to the manufacturer's standard metal mounting bracket. The antenna ground is connected to the coax shielding which in turn is connected at the other end to the VHF DC ground. This leaves me with two dissimilar metals in the water all connected by the ground wiring and coax shielding. In my case an iron keel and the prop/zinc. While our mast is down this winter we are going to fix that at the mounting bracket. Others have suggested using a DC block device somewhere along the coax route. Unfortunately, the boat had already been pulled and mast unstepped when I came across the post in another forum so I couldn't do any verification.

I felt we went through the zinc a lot faster than we should have since buying the boat in August 2011. Maybe the changes will make a difference, maybe not. In any case, you might want to verify that your keel really is electrically isolated from the ship ground if you haven't already.

[Jetexas]

I just had my bottom cleaned, and the diver didn't mention finding anything over on that side, but I'll give him a call. The water is probably 50-60f right now. We've managed to make it the entire season without a freeze.

Update: Just called the diver. He said there were no studs coming out of the bottom.

[smj]

I may be wrong here but I would worry more about corrosion of thru hulls than my prop.

[Jetexas]

Originally Posted by smj

I may be wrong here but I would worry more about corrosion of thru hulls than my prop.

The through hulls aren't metal.

[capn_billl]

I would check all wiring, any stray DC current will burn through zincs. Any wire left in the bilge, (cracked insulation, etc...), will also. A bad guage can leak current to ground also. The Neg on the battery must be connected to ground to insure all bonded objects, (like the engine), are negative compared to the zincs. If positive they become the sacrifical anode.