Stray current detector

Colleagues
I would like to ask the more electrically minded than myself, how would you make or measure stray currents in your electrical wiring.
If I assume that your motor has good bond to hull, it would be just a matter of using a meter from Negative bus to hull, and watching for variations.
What I would like is some sort of meter, light alarm to warn me when there is a stray current present. there is a product available but I just cant justify the price
Cheers
Katoh

 

Katoh,
I would try to modify an RCD for the purpose. I hope that's the right term for the circuit breaker that monitors stray currents in an electrical installation. If not, I blame Google Translator (Swedish: jordfelsbrytare). The RCD checks that there is balance between the currents in the wires. If not, there is stray current and the RCD breaks the circuit.

You probably just want a warning for stray current, and you don't want to break the circuits. I've never looked into an RCD to see if they can be modified for your purpose and if the sensitivity can be adjusted, but it would be interesting. I assume that they are sensitive to DC and not only AC. They are cheap so you don't have much to lose.

Erik

 

Hi Erik
Fantastic, I think you know exactly what I want. Its more of an indicator that I have a stray current in the boat, then you can chase it down before it starts to do any damage.
I will definitely look at the RCD (jordfelsbrytare).
Cheers
Katoh

 

The stray current you are looking for is probably DC current. All the RCDs I have ever met, are AC.
If you disconnected the battery at the negative, and measured with a multimeter from the hull to the terminal, I would imagine you would get a reading.
Preventing stray current in a boat is often difficult that is why they have double pole iscolators. As long as you are charging the batteries sufficiently when the engine is going and iscolate the batteries when you stop you shouldn't need to worry.

 

Hi Poida
I'm not sure about RCD being only AC, I'm still researching that one.

If you disconnect the neg side of the battery, and meter between positive and hull you will get a full reading regardless as the negative is bonded to the motor which is bonded to the boat hull. There is a commercial product out there called a seabis, but for the money is not worth buying for one small vessel.
There must be some simple way of having an indicator on board.
Cheers
Katoh

 

Katoh......What is your end goal? Are you looking at preventing unwanted battery drain while docked or trailered, or are you looking at a way to prevent undue (and unwanted) electrolytic-induced corrosion from stray currents?

Roger

 

The end goal is to be notified of stray currents in the boat, before they start any corrosion issues. Like most things prevention is better than cure, if your alerted to a problem, you can fix it before it starts creating other problems, mainly holes through your hull.
Katoh

 

Katoh- In most cases it's not the stray currents within your own craft that you have to worry about. It's all the unbonded wiring(floating grounds) from the boats and shore power supplies of a marina. Simply properly install zincs on your hull and you should be ok. I speak from the experience of building and owning an alum. hulled motorsailer for 9 years with totally no problems other than replacing my zincs as needed. My private mooring had only a few boats in close vicinity but whenever i visited a marina I dangled a large zinc electrically connected to the hull over the side in addition to my regular zincs. There was a long and detailed post on this subject not so long ago on this forum with more than enough info to answer any questions related to the subject Do a search on CDK's posts, I think it was his thread. Even I who have a well trained background in electrical/electronics learned alot from his posts ---Geo.

 

I have read the post by CDK on zinc's, Very good reading, here is the link for anyone else interested.
http://www.boatdesign.net/forums/boat-design/revelations-about-zinc-electrodes-36162.html
My boat lives on a trailer, and never sees a marina. What absolutely gets right up my nose is I just found another bloody hole in the hull, this is No7.
All these holes were from incorrect wiring and stray currents, mind you the hull did not have its own allocated zinc anode, but has now.
I am just trying to get an alarm system in place, as I dont really want to go through this full rebuild again. I sometimes think it would have been easier building from scratch.
Katoh

 

Katoh

I'm not an expert by any means on this subject but if everything is turned off the only current getting through to the negative has to be what is leaking somewhere? I would assume.

On a trailer ?? Sure your not dropping fishing hooks into the hull.

But ali will corrode, you need to wash it with fresh water to remove any salt deposits.

 

Poida
Even if there was dropped hooks sinkers they cant go really go too far. The hole deck is fully welded tight, no way into the hull.
The problem is not when things are switched off, but when there on. If there is even the smallest of leaks a current will flow or can flow into the hull on its journey back to the battery, once these stray currents are there if your zinc doesn't do its job your hull is the next one to be attacked.
So people can see what I'm up against have a look at the following photos, the squares are 50x50mm or 2"x2" one side is the outside of hull the other inside.
There was 6 holes in total till this afternoon until I found a 7th.
Sorry I should also Add, I bought this bought this boat as rebuild project, I have not had it in water, all damage was caused by previous owner/owners.

 

In all due respect Katoh I think your stray current theory is about as plausable as fairies coming out in the middle of the night and hitting your boat with a hammer and chisel.

There is not a problem passing a current through aluminium. They are in fact used as high voltage wires because of their lightness and ability to span over long distances. (There is a high tensile steel core incidentally)

There has to be a medium between where the hole is and where the current is going too. I assume you have rubber rollers on your trailer which insulates it from the hull.

There is every possibility that the previous owner had it propped up for a considerable period of time using steel props. The have square ends which may explain the shape of the holes.

Looks like it could be difficult welding the patches on the inside, which would be desirable.

 

Poida
First things First
If you pass a current through an aluminium hull in salt water, you will turn it into a battery, the first thing that will dissolve is the alloy, then we go further up the electrical list, that's after the hull has dissolved.
Secondly the previous owner wired the cabin lighting positive to battery negative to hull. Hmm make your assumptions! I have not fully stripped all the wiring yet to see what else awaits me.
Lastly if it was Pixies in the night, If not the 80kg Akitta sitting in the yard to pick them off would have been the owner with 1.25oz of No 9's enough for pixies with a hammer.
Boat was bought as is, this is not from pixies or gnomes or gremlins or either little green men, Those holes in the pics are from stray currents and poor wiring!
I must add at this point the original build by Ken Fisher, yes I will name names is acceptional, and the vessel was built to survey, its what happened afterwards!
Now getting back to my original post, which I do believe would be of interest to other aluminium boat owners, is a device notify them for stray currants.
Katoh

 

I won't admit to being an electrical engineer, nor having played one on television, but.......if you can really attribute the problems to improper marine wiring practices, then you should be able to correct said shortcomings as you rebuild. It sounds as if the previous owner used automotive wiring practices in wiring in a batch of accessories, that probably shouldn't have been on a boat.
If I'm not mistaken, it's improper to use the hull as the battery negative, or common return current path. Bonding it to negative is one thing, having it act as a current-carrying conductor is quite another.

BTW......those pics seem to me to be rather suspicious. In parts 1, 2, 4, & 5, the corroded hole seems to be awfully round. I would think "random" corrosion wouldn't be so "perfect". Might it be possible the P/O had actually drilled those through the hull for mounting something?

Roger

 

Your post seems to equate "awfully round" with "perfect" -- I only know what the second description means, and that is clearly not the case. Explain #3.