It's the Pits

[Keeping Up]

2 years ago:

 

Boat 23 years old - check

2 years since last blacking - check

Boat in dock - check

Hull steelwork sound - check

New blacking applied - check

No stray connection from negative to hull - check

Galvanic isolator working correctly - check

Anodes OK - check

All OK

 

 

Yesterday:

 

Boat 25 years old - check

2 years since last blacking - check

Boat in dock - check

Hull steelwork sound - see below

No stray connection from negative to hull - check

Galvanic isolator working correctly - check

Anodes OK - slightly used

Not OK

 

 

But ... The whole of each side is uniformly and extensively pitted, from base to waterline, from stem to stern, to a depth of nearly 4mm in the 6mm steel. Urgent overplating is required. Dammit.

 

Now I'm wondering what went wrong. It looks like galvanic corrosion of some sort. In the intervening we've had a new engine fitted so fairly major re-wiring of the main 12v connections has taken place, which I used as an excuse to re-wire the AC supply connections, so I've examined every detail of that work, but after again checking all the boat electrics in great detail, looking for any stray connections and again measuring the isolation voltage of the GI, I can't find any answers. I wonder if the shore supply has a problem, that's the next thing (and probably the only thing) left to test.

[MoominPapa]

Any evidence of pitting on the baseplate? Where has the boat been moored for substantial lengths of time? Was it always pointing in the same direction? ie are you looking for a process that pits both sides simultaneously, or has something done one side, then the other?

 

How much do you know about conditions when the bow damage was repaired? Could a welder earth left connected for days/weeks be responsible?

 

Nightmare, and quite sobering......

 

MP.

[Alan de Enfield]

It is also important to remember that the source of the 'stray electricals' may not be your boat, but another near you using you as the route to earth :

 

An extract from an article "Types of Marine Corrosion" :

 

It is based on GRP boats, but substitute "hull fitting' for 'steel hull'

 

"............For example, your boat may be sitting between a boat leaking DC current and the best ground for that current. Rather than the DC current moving through the water to ground, your boat could provide a path of lower resistance. The DC current could enter a throughhull fitting, travel through the bonding system, and leave via your drive to the ground. Remember that corrosion occurs at the locations where DC current leaves metal and enters water.

Stray current can come from an outside source either internal or external to your boat. Internal sources involve a short in your boat’s wiring system, such as a poorly insulated wire in the bilge, an electrical accessory that may be improperly wired, or a wire with a weak or broken insulation that is intermittently wet.

External sources are almost always related to shorepower connections. A boat with internal stray current problems can cause accelerated corrosion to other boats plugged into the same shorepower line if they provide better ground.........."

 

It goes on to say that a Galvanic Isolater will help protect AC but not DC.

[WotEver]

If the galvanic voltages exceed a couple of volts then the GI would serve no purpose, unlike an IT.

 

I would use a DMM to measure voltage (both AC and DC) between the incoming earth on the shore lead and various conductive items attached to the ground - pontoon, metal posts, your boat's hull etc. I found over 70V AC on an earth feed once!

 

If you don't find anything (I suspect you will) then see if you can check out the neighbouring boats.

 

Tony

[Keith M]

In highly populated marinas I am find many more issues being caused by the Incoming earth. Highest measured AC voltage has been 80-volts.

 

Most GI will start conducting around 1.5 volts

 

Keith

[Keeping Up]

We are on an on-line offside mooring, just 4 boats are there (2 have changed during the past 2 years). The piling is plastic so there are no issues there.

 

I do suspect the electricity supply. Some years ago one of the moorers installed a washing machine in a shed and it had enough earth leakage to generate a current through the GI, but it was being fed directly from the supply and not through an RCD so nothing tripped (I fixed their shed supply for them). That suggests that our shared supply earth may not be all that it should be. I will be taking a look at it as soon as I can (although I will have more than a month before I get the boat back there)

[Bee]

Sorry to hear that. 4mm is a deep, deep pit, pits of 1 -2 mm can look alarming, have you measured? are they silver at the bottom, i.e. still fizzing away? Have to say it does sound electrical which I do not understand as well as others but if you do have to overplate it you need to find out what the problem is, anybody else have problems? I always turn the battery master switch off whenever I leave the boat and unplug any shore lead just because I fear hot wires etc. Don't worry about all the scary stories about overplating, its perfectly ok in my experience and it gives you a chance to apply the best modern coating available (whatever that is this week) Good Luck

[LEO]

Hi,

 

Nothing to do with work carried out when your boat was repaired?

 

L

[Keeping Up]

The pits are almost certainly still active. The surveyor measured them accurately and inspected them closely for me. He said that the baseplate appears to be fine.

 

I don't think it could be anything to do with last year's repair work as the boat was out of the water for that.

 

My only reservation about the overplating (apart from the cost) is that the boat will sit about 1" lower in the water, so the skin fittings will only just meet the minimum requirements for freeboard and also the gas locker floor will be permanently underwater.

[alan_fincher]

My only reservation about the overplating (apart from the cost) is that the boat will sit about 1" lower in the water, so the skin fittings will only just meet the minimum requirements for freeboard and also the gas locker floor will be permanently underwater.

 

I'm confident you will have considered this closely already, but any chance of removing any significant amount of any ballast you currently have?

 

(Or perhaps you have already factored that into your "one inch extra draught" estimates?)

I have to say I'm staggered this could happen so quickly, so I'm sure we will all be very interested if you can unravel any more on what may have caused it.

 

Where are you having the plating done?

[Keeping Up]

 

I'm confident you will have considered this closely already, but any chance of removing any significant amount of any ballast you currently have?

 

(Or perhaps you have already factored that into your "one inch extra draught" estimates?)

I have to say I'm staggered this could happen so quickly, so I'm sure we will all be very interested if you can unravel any more on what may have caused it.

 

Where are you having the plating done?

 

Removing ballast is not an option - we don't have any! All the necessary weight is provided by the baseplate itself.

 

The work will be done by WFB Co - the boat is in the Kayes arm now, awaiting the spare slot in their diary.

[alan_fincher]

 

Removing ballast is not an option - we don't have any! All the necessary weight is provided by the baseplate itself.

 

The work will be done by WFB Co - the boat is in the Kayes arm now, awaiting the spare slot in their diary.

 

So I suppose you could argue there can ultimately be a downside to the uber-thick baseplate approach then, should over-plating ever become a necessity? I'd never really considered that.

 

I have no doubt Graeme will do the highest possible quality job for you. "Sickle" has just had some fairly specialist work done at Stockton Dry Dock, and his skills seem to have no bounds - we are really pleased with the outcome, and he is a thoroughly nice person to deal with. I hope it goes equally smoothly for you - I can't possibly imagine it would not.

[Guest]

and also the gas locker floor will be permanently underwater.

 

Is there no scope height wise for raising the gas locker floor? and re fabricating the 'spill vents' at a higher point?? or do you not have the clearance above your gas bottle tops??

[nicknorman]

Does your GI have some sort of meter or LEDs? Ours has a meter which just barely moves when I plug in shore, which at least tells me the diodes aren't short circuit.

[Mike Tee]

So, out of interest as I'm now boatless, how is it possible to 100% be sure the hull is protected from outside influences that we have no control over?

(Sorry to hear Denmark's sibling is suffering with pitting)

[Alan de Enfield]

So, out of interest as I'm now boatless, how is it possible to 100% be sure the hull is protected from outside influences that we have no control over?

(Sorry to hear Denmark's sibling is suffering with pitting)

 

You have no control over anything that you have no control over.

[Mike Tee]

I have no control over potential burglars, but I do have control over my home's defences.

[GUMPY]

There is no way to be a 100% safe, however disconnecting all cables to and from the boat will go a long way, failing that if you need power connected then an IT covers most bases and is pretty much equivalent to disconnecting the power cable.

[WotEver]

There is no way to be a 100% safe, however disconnecting all cables to and from the boat will go a long way, failing that if you need power connected then an IT covers most bases and is pretty much equivalent to disconnecting the power cable.

I would argue that an IT is identical to disconnecting the shore cables. There is no connection between shore and boat when using an IT.

[GUMPY]

I would argue that an IT is identical to disconnecting the shore cables. There is no connection between shore and boat when using an IT.

Provided its wired correctly I agree. I was just being cautious with my previous statement

[Bobbybass]

Have you got a Travelpower/ Electroluix.....?

[Keeping Up]

Have you got a Travelpower/ Electroluix.....?

 

Good thought, but no I have an inverter. I have checked, there is no connection between input (12v) and output (240v including the "earth")

[cuthound]

So, out of interest as I'm now boatless, how is it possible to 100% be sure the hull is protected from outside influences that we have no control over?

(Sorry to hear Denmark's sibling is suffering with pitting)

I suppose having the hull painted in 2 pack, inside and out and bringing all electrical connections to insulated (from the boats hull) busbars would mean the hull was immune from stray currents until the 2 pack got scratched through to the bare metal?

[gazza]

The pits are almost certainly still active. The surveyor measured them accurately and inspected them closely for me. He said that the baseplate appears to be fine.

 

I don't think it could be anything to do with last year's repair work as the boat was out of the water for that.

 

My only reservation about the overplating (apart from the cost) is that the boat will sit about 1" lower in the water, so the skin fittings will only just meet the minimum requirements for freeboard and also the gas locker floor will be permanently underwater.

Minimum requirement for skin fittings is for non private boats. If you are well below 10" it matters not in terms of BSS.

 

Our sink and bilge outlets are on the waterline and cause no issues at bss time - provided you get a competent inspector....!

 

As for the gas locker, do you use much gas? If no, can you accept dropping a bottle size and raising the locker floor and drain?

 

ETA, our exhaust exits under water

Edited September 24, 2016 by gazza

[bizzard]

Mains!! It seems by the boats here that the more powerful mains appliances are used, electric kettles, washing machines ect the worse electrolytic pitting is, where or not the boats have GI's. We have boats here which have no mains land line, only occasional mains leads to carry out repairs and they remain pit free. Boats like mine that only use low powered mains appliances like my little telly, laptop charger and 12 amp battery charger only remain pretty well pit free. I think I mentioned in another thread that recently we had two early Springers docked and hulls surveyed, both had excellent reports, neither had full time mains landlines, just 12v.

Another thing I've noticed whilst working on docked boats was that wherever there were large powerful appliances inside the boat there was corresponding outbreaks of pitting on the hull on the other side, on and below the water line. As I've said GI transformers don't seem to make a scrap difference in my opinion.

Another thing I don't care much for is the blue plugs and sockets (Hella and whatnot) for landlines, mainly because most are outside getting wet or at least damp even if sheltered. They're pretty naff things and not very water tight. Probably no bearing on pitting though but I prefer a mains lead to be run through gap to the inside the boat where plugs and sockets are kept warm and dry. Not into the dampness engine space of a cruiser stern boats though.

I have another theory. Electrical experts? How about lining behind things like washing machines and big electrical appliances with large L shaped sheets of lead. These lead sheets to be fully behind and fully underneath the appliances. Would this help to prevent what I said in my first paragraph. Come to that might as well line the whole boat out from gunwales down and the whole floor with thin lead sheet, if it works. Radio active barrier. Problem might be is, that containing and imprisoning all the electrical waves within the lead sheet lining they will buzz around zapping folk and giving them severe headaches instead of escaping and damaging the hull on the way out, or in. Folks might have to have anodes fitted just in case.