Electrolysis

[Pegasus_1]

I recently noticed some pitting in the aluminium of my Stessl plate boat which I believe is being caused from electrolysis. Now all the good books say that any electrical equipment should not be earthed to the boat itself but should run back to a common bus bar and be earthed from there back to the battery. This was done when the boat was new, however there is a little thing called an outboard mounted on the back which is also naturally earthed. Because the motor is bolted to the stern with metal bolts, this then earths the whole boat. I believe the earthing of the motor is causing the problem that I am experiencing with the pitting (electrolysis). Can anyone please provide your comments and or advise. Should I be running a number of sacrificial anodes on the boat itself (apart from those on the motor) to help overcome the problem or is there another solution ???

Thanks


Brett

[finga]

Electrolysis takes the path of least resistance for current and the electrical bonding of your outboard to the negative terminal of the battery is done through the negative battery cable to the outboard and should be the path of least resistance for any current to the outboard.
The same theory should be used for all the accessories.
If any lead is crook or if there's a dodgy connection then not nice things start to happen elsewhere.
Where's the electrolysis happening as lots of other things cause the cancer??
When you say earthed from the busbar back to the battery your talking about a cable and not using the hull aren't you matey??

[Kerry]

Brett, Where is the pitting occurring? Above or below the waterline. What's this pitting look like?

Motor bolted to the stern is of course connected to the hull (at that point) but then the battery negative is connected directly to the motor, not the hull. For a trailer boat there should be no need to be running extra anodes apart from what's on the engine.

Regards, Kerry.

[FNQCairns]

I only understand this in a gross way although when I looked into it a while back the international standard is earthed via the hull, always been a confusing point for me that I would like clarified.
But you are correct the boat is already earthed to the hull via control cables and engine mount bolts so earthing to the hull proper shouldn't make a great difference just need to ensure a battery isolator is fitted and used.

Interested myself to hear from people with a more comprehensive understanding as I am about to wire a boat up and am unsure exactly which thinking to follow for longterm good results.

cheers fnq

[Pegasus_1]

Hi Kerry and Finga
Thanks for the quick replies

Yes the bus bar is earthed directly back to the negative post of the battery by a heavy duty cable (15mm). The pitting is appearing mainly on the inside particularly in the bilge area where a bit of seawater may sit and also where two sheets of metal join/are welded. I noticed on the weekend that a bit of additional water to normal came out of the bungs when removed. On returning home I supported the boat and trailer and filled her up with water to just above the bung level to see if I could find any leaks. None could be detected. The bolts on the motor tightened up a turn or two so I am guessing the additional water may have came in through here???

Brett

[tigermullet]

I only understand this in a gross way although when I looked into it a while back the international standard is earthed via the hull, always been a confusing point for me that I would like clarified.
But you are correct the boat is already earthed to the hull via control cables and engine mount bolts so earthing to the hull proper shouldn't make a great difference just need to ensure a battery isolator is fitted and used.

Interested myself to hear from people with a more comprehensive understanding as I am about to wire a boat up and am unsure exactly which thinking to follow for longterm good results.

cheers fnq

You are correct FNQ in that the hull (metal boats) will have the same earth potential as the engine - it is difficult to avoid - but you do not want a current running through the metal of the boat.

Therefore the power should be taken from the battery to bus bars (which must be isolated from the hull) and then power is led to individual devices. The hull itself should never be used as an earth return.

The cables from the battery to the bus bars should run in conduit and not be allowed to droop into wet areas.

Cables running from the battery to the engine itself should also not be allowed to drop down into wet areas but be supported in such a manner that free movement of the engine is not inhibited.

Over the years I have tried just about every trick known to man to prevent the hull from having the same earth potential as the engine but it is almost impossible to do. Having the same earth potential has not caused any corrosion during the last nine years that the boat has been kept on the water. The boat is equipped with a warning light so that if current is running through the hull it lights up.

The easiest solution is to have a couple of batteries - one for the motor and one for general supply and to make sure that the general supply battery, leads etc., are insulated from contact with any electrically bonded metal parts of the boat.

Good luck with the wiring. It is important to get it right.

[big moose]

Hey Pegasus1,
Agree with everything tigermullet says except that it is absolutely critical for the engine/hull connection to be at the same earth potential or in simpler terms correctly bonded this can be checked with a low reading ohm meter the resistance should be as close to zero as possible then keep all electrical circuits above earth and electrolysis should not be a problem. If you are getting electrolysis check the bonding of the motor to the hull if resistance is greater than about 0.005 milliohms thats your cause
regards bm

[finga]

The trick is not to try to isolate the motor from the hull but to have all structural parts of the boat equipotentially bonded (ie all parts of the boat at the same potential and therefore no electrons can flow) including all structural parts of the hull, motor etc.
In theory if you use different types of material in the boat it is possible for a current to occur with no electrics connected (galvanic reaction or redox reaction). Even if the different materials are different grades of the basically same material (ie different grades of alloy or stainless). One reason why we don't put galv. bolts in ali boats. Maybe this problem isn't a electrolysis problem as the pitting is only a join in ali
If everything is at the same potential then no current or electrons can run. No flow of electrons no electrolysis or corrosion.
That's the reason or why new outboard have all those little wire trace looking things running everywhere and the simplistic reason why I was able to work on power-lines whilst they were alive and the reason why birds don't get zapped sitting on powerlines (unless they touch 2 wires or a wire and an earth. Then it's bzzzz and fried bird)
Electrical theory explained a bit better here
http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/DC-Current/WaterFlowAnalog.html (http://www.ac.wwu.edu/%7Evawter/PhysicsNet/Topics/DC-Current/WaterFlowAnalog.html)
It's a bit late so I probably muffed the explanation a bit. I'll read again tomorrow and 'adjust' if needed and someone doesn't correct it before.

As Bigmoose says. Keep every part of the part as close as possible to zero resistance to any other part of the boat. The closer to the absolute zero reading you achieve the closer to no problems you get.

[tigermullet]

Thanks to BM and Finga we now have better explanations. Sorry if I side tracked anyone with my impossible dream of isolating the motor from the hull. I know it is not possible in a wet environment but IF it was then a lot of potential problems would disappear. (That was a horrible pun);D

[gofishin]

Good to see we are on the right track now.

Having the (alloy) hull properly earthed, is completely different to using the hull as an earth (-ve) carrying conductor. Bolt a motor on the back of an ally boat and your hull is earthed and there ain’t nothing you can do about it.

The ABYC recommends (maybe stipulates by now) that the battery system is adequately grounded to an alloy hull. This ensures an equi-potential (as discussed previously) and also helps protects the motor electrics etc if there is an accidental short somewhere else in the boat.

However, it is surprising sometimes to find that even where there has been great care taken when wiring an alloy boat, the installation of a simple fuel tank sender may lead to electrolysis. One model of a well know reputable brand swing-arm type sender does not have the –ve pole properly isolated from the mounting plate. If it is installed in an alloy tank, which is fixed to the gussets in an alloy hull, your hull will be carrying a current so long as the tank gauge/sender is powered!

Some alloys react differently in the weld zone, and maybe subject to pitting corrosion in the right environment. If two different alloys are used, even of the same series, ie 5083 for the hull plate & 5051/5252 for the structural members (more easily bent/formed), it can also lead to minor corrosion around the weld area.

Any stagnant/wet area in the bilge would have an abnormally high salt concentration, causing increased corrosion. Since you can never get that last bit of water out through the bungs, shove a couple of rags through the bung holes into the wet area, and let the ends dangle below the boat. These siphon the remaining water out, solving the problem – well nearly. In Qld’s higher humidity, any remaining salt crystals can still cause problems, but at a significantly reduced rate.

Some like to splash a bit of fish oil in the bilge. Not sure how much it helps reduce corrosion, but it sure keeps the missus away!

One very important point; do not use degreasers, truckwash, or any detergent that is alkaline. This kills the ability of the alloy to form an oxidising layer which protects the alloy from corrosion.;D

[FNQCairns]

Thanks Tigermullet for the direct reply,

I have read every thing written here 3 times so far. Think I will print it out and go to bed with it.

Am I right to assume that I should run a second battery -ve cable to the hull from any battery fitted then simply wire it all up (the rest) as normal.

Sorry for sounding like a boofhead:(

cheers fnq

[Kerry]

....Am I right to assume that I should run a second battery -ve cable to the hull from any battery fitted....

NO! ......

[Pegasus_1]

Gofishing

As I read your post am I correct in my assumption that I should have all my electrical equipment in the boat earthed back to a bus bar and then a single cable from the bus bar back to the negative post of the battery. I can then, as a added precaution have the battery earthed to the boat via another cable. This way the electrical equipment will take the path of least resistance (cable from bus bar back to negative post of battery).
If I am incorrect in my assumption can you please point me in the right direction.

Many thanks

Brett/Pegasus_1

[FNQCairns]

Pegasus that is exactly what I was trying to say in my post above (badly I might add) but Kerry says NO. still confused.
Thinking normal wiring then at the end after all is working as it should and only then with care to isolate any stray earths only to battery!, connect earth from each battery direct to hull?

Hope I am not treading on your toes pegasus, just tell me to go away.

cheers fnq

[PADDLES]

equipotential bonding is the key, you must have all items that you want to protect bonded together electrically so they are all at the same potential and no stray currents can flow from item to item via the electrolyte (sea water).

we deal a little with cathodic protection at work and the basic theory is reasonably simple (i only understand the basic bits).

first understand how a battery works, you have a cathode and an anode dipped in an electrolyte, basically one of these (anode) corrodes to create an electron flow if you place a short across them.

now in the case of a marine structure you have the structure as the cathode, you dip an anode made of something that doesn't corrode real easy like platinised titanium into the sea water (electrolyte) and then inject a dc current into the anode to try and corrode it. this then protects the structure and stuff bonded to it (cathode) from corrosion.

an aluminium boat would be fairly similar but instead of injecting current you are relying on the sacrificial anode corroding in the electrolyte to create a current in the right direction and protect the hull. now if you inject a dc voltage (by a fault or other means) onto an aluminium (or steel) hull then it becomes the anode and the circuit works in reverse and that's bad.

have a look here for a bit better info if you're keen

http://www.cathodicdiecasting.com.au/introduction.htm#SECTION_1:__CORROSION_AND_CATHODI C_PROTECTION

[FNQCairns]

Can anyone explain why the international standard is to earth the hull on 12vdc aluminium boats:(

Even our manufacturers had to lift their game in respect to ABYC standards for export as the ones they sell here would not pass the codes, why did they fit the earth strap to hull! Human lightning stike protection? Fuel flash protection (regulation doesn't need to make good common sense at times)

still a boofhead.

cheers fnq

[PADDLES]

you need everything that needs protecting to be bonded fnq so that currents don't flow between items.

[FNQCairns]

Thanks Paddles, so why would in the electrical sense Kerry say NO? does he mean bonding from the battery to the hull proper is a no no but a yes yes from a common bus bar ot th hull proper.

cheers fnq

[wiz]

Fnq

Mate Make Sure All Your Wiring Is Isolated From Your Ali Hull

Having An Outboard Mounted To The Hull Will Provide The Eth Required

Dont Confuse Yourself By Reading To Much Into The Technical Side As This Is Quite Simple.

Cheers Dan

[gofishin]

Gofishing
...
If I am incorrect in my assumption can you please point me in the right direction.

Many thanks

Brett/Pegasus_1

Brett, yes! Keep ground lead between battery and hull as short as possible. Run +ev and -ve cables to all powered components/bus bars. Ensure that the -ve pole on all powered components has appropriate isolation from the hull.

FNQ, you are confusing 'grounding the hull' with a 'common earth' system as used in vehicles. The two are completely deferent, and the latter is a no-no with alloy boats. International standards have been policing best practice concepts for decades, and they are far more stringent than Australia’s non existing, or lax standards, especially wrt the private sector.

[FNQCairns]

Ok Dan will take yours and others advice, in a retorical sense it is still a mystery why the standards recommend earthing the hull to the battery if it will or has the very real potential to turn it into an aspro as some allude to.

My last tinny lasted many many years without the hull directly incorporated by strap so I know that works OK.

cheers fnq

[FNQCairns]

Hi gofishing no I dont think I am, if the entire elecrical system is in perfect working order in an aluminium boat without grounding to the hull proper and deliberate, then a person adds just one extra wire/cable to that fully functioning system - from the battery negative to the hull as per abyc why does that turn the boat into an aspro or have any effect on corrosion over and above the moment before it was fitted?

cheers fnq

[PADDLES]

i don't know why kerry says 'no' fnq. i can only assume he is saying "no" to setting up a negative earth system like in a car. like gofishn is saying above, the two are different. in a negative earth system you would be using the hull as a return conductor, by using negative wires everywhere and then bonding the hull to earth/negative at one point there is no current flowing in the hull unless there is a short or leakage somewhere in the system. you'd be able to pick this up by doing as someone has suggested earlier and have an led or light on the earth connection to the hull and if it lights up then there's a leak somewhere (ie. the hull has charge on it)

ps. don't rule out the effects of dissimilar metals either. it might not be an electrical issue causing the corrosion but the theory of it all still holds. as an example just recently i have had to replace the aluminium fuel tank in my signature. the reason (we think) was the brass fuel pickup fitting in the tank. salt water left in the bilge wet the carpet over the end of the tank which was touching the fitting and dipped into the water in the bilge, presto instant electrical circuit. you could actually see the line of corrosion lining up exactly where the tank used to sit in a bit of water in the bilge before it dryed after each trip. new tank now has different fitting with a barrier lacquer stuff on the thread and it also doesn't sit in water any more and the carpet doesn't go down the back of the tank and sit in water. you live and learn hey?

[FNQCairns]

OK thanks Paddles so grounding to the hull makes no difference from a corrosion point of view but using the hull as THE ground which is plain silly anyway should be avoided, so in reflection running a strap/cable from battery earth to the hull can only be a good idea due to it acting as an outboard electical problem buffer at least (as mentioned above).
So if abyc do consider it a good idea possibly because of the added electrical protecton provided I might do it if there are no negatives.
Not totally sure I have it, but it sounds sort of straight forward.

cheers fnq

[gofishin]

...why does that turn the boat into an aspro or have any effect on corrosion over and above the moment before it was fitted?

cheers fnq
It doesn't! And i don't think i was saying that - or maybe i am confused :-/ . My shrink never thinks so...;D ;D

I am trying to say it's good to ground your battery to the hull, but it is imperative that you don't use the hull to complete any electrical circuit.

cheers

[FNQCairns]

It doesn't! And i don't think i was saying that - or maybe i am confused :-/ . My shrink never thinks so...;D ;D

I am trying to say it's good to ground your battery to the hull, but it is imperative that you don't use the hull to complete any electrical circuit.

cheers

Thanks gofishing my confusion was the ruling party:) but think I have it now.

cheers fnq

[Kerry]

NO! do not connect second or third or any more -ve cables from the battery directly to the hull, what is connected from the -ve battery to the engine block is enough as any more simply create differing paths and this is what the whole exercise is trying to prevent.

Taking this logic of multiple earths from a battery directly to the hull then no one in the right mind would connect a battery say fitted under a console (or towards the bow) directly to the hull as this then creates separate and quite different -ve connections from the battery back to the engine block,one via the actual battery cable and one effectively via the hull.

Has nothing to do with using the hull as an earth return but simply creating quite different connections between battery and engine.

Where this thinking has come from to both run a -ve from the battery to the engine as well as to the hull is quite beyond me. Perhaps there's some short circuit in the thinking between 12v and 240v boat power systems?

If anybody thinks there is absolutely no difference between a bow mounted battery connected to the hull (-ve wise) to that of a stern mounted battery connected the same way then feel free to go ahead and do it!

Regards, Kerry.

[One Day]

Thought you might like to look at this site. It has some good info on the topic that might shed some more light for you.

http://marinesurvey.com/yacht/corrosion_1.htm

[Pegasus_1]

Thanks to all who have posted a reply on this thread. OneDay, the web site you posted gives a good insight to the problem. It is obvious that this is a hot topic with individual ideas and opinions and to say I am still confused as to how to address the problem would be an understatement. I would still appreciate all thoughts and ideas to be posted so I can make an informed decision as to how to address my issue.

Many thanks


Brett/Pegasus_1

[Wear the fox hat]

Pegasus 1,
You can have 3001 different electrical solutions to any problem, but if you don't keep this area dry when the boat is in storage, you will get pitting regardless. The pitting will be caused by a galvanic reaction between salt water or tap water left in the bottom of the hull & the hull itself. If the water left in the hull is deep enough to also touch the screws/rivets that hold your bung fittings in, the problem will only magnify. Clean the area up & paint it with a quality etch primer & top coat, keep it dry when in storage & this will solve your problem. There will be people that disagree, bit this is a common sense solution, not a theoretical electrical solution that will only cost you time & money & still not solve the problem. Been there, done that & I only speak from experience.

WTFH

[finga]

As a PS to WTFH's wise post it's worth mentioning to
NEVER LEAVE ANY BLOODY HOOKS, SWIVELS OR SINKERS IN THE BOTTOM OF THE BOAT. NEVER, EVER if you get my drift :)
Cheers Scott :)