Swim platform bonding

[jwag956]

We just got hauled (South Puget Sound) and the yard noticed that our swim platform supports had corrosion around where they attach to the transom - EXACTLY the same as another GB in the yard (both GB 42CL late ‘80s’). According to the yard - the large zincs mounted on the transom are precisely to help with this - inside the rear lazerette the zincs and platform mounts are all bonded (independent of any other system). But they have no explanation for the corrosion - both boats are same harbor, different marinas.
Anyone solved this? Thoughts?

Pictures attached.

 

[mvweebles]

Question: are your swim step brackets bonded to the zinc anode? Should be.....

Peter

 

[Comodave]

Either bond the brackets to the hull anodes or add individual anodes on each bracket. If the brackets are through bolted then just add a heavy green wire from the bolts to the hull anodes. On my swim platform extension that I added I put an anode on each bracket.

 

[jwag956]

Thanks - as I mentioned - all the brackets are tied together inside the boat with the zincs.. (and nothing else - i.e. completely separate system/bonding than any of the other thru-hulls)

 

[Comodave]

Thanks - as I mentioned - all the brackets are tied together inside the boat with the zincs.. (and nothing else - i.e. completely separate system/bonding than any of the other thru-hulls)

When you say tied together inside with zincs, they should be wired together and then to the anode on the outside of the hull. Do you have a bonding system in the boat? All underwater metal should be either bonded or unbonded. If you bond some then you should bond all metals. Are you having any other corrosion issues? I prefer to have everything bonded and have adequate anodes on the hull. I would check to make sure the brackets are indeed connected to the anodes and actually have a good connection if they are connected. Break out the ohm meter and it should be 1 ohm or less between the brackets and the anodes.

 

[jwag956]

Yes - sorry - inside the rear lazerette the support bolts go through as do the anode bolts. Those are all connected together.
Yes - the boat has all engine, thru-hulls, shafts, etc, all bonded.
The most interesting thing here is that the identical corrosion pattern is on 2 boats - same design/era, same harbor...

 

[Comodave]

Ok, so they are connected but are the connections actually good? Put an ohm meter and make sure that there isn’t more than 1 ohm resistance between them. Also it is preferred to have each connection make a home run and not to have them daisy chained together.

 

[psneeld]

I have been told and researched that too much zing can cause copper bottom paint "burn" around underwater protected metals.

The only way to tell for sure is to do a bonding system survey with a silver-chloride electrode or other methos to make sure you have not overzinced certain underwater fittings.

Research/Google silver-chloride electrode use/measurement.

 

[Fleming]

You're overprotected, too much zinc for those small supports. That's not corrosion, that's "haloing" of the paint.

https://www.practical-sailor.com/blog/can-you-have-too-many-zincs

Edit: I was writing at the same time as psneeld

 

[mvweebles]

Will leave diagnosis to Fleming and Psneeld, but here is a silver-oxide reference electrode for $139 plus shipping from BoatZincs.com. not a lot of money but since it won't get used much, seems like something best purchases by a group of friends or a yacht club.

https://boatzincs.com/categories/tools/corrosion-reference-electrode.html

Peter

 

[Portage_Bay]

Suggest starting with Comodave's test. Easy to do and hopefully solves the problem. Given that your problem seems to be limited to the swim step supports it's a good chance he's right. A bonding system like any system on the boat requires maintenance.

Then if corrosion is still a problem as psneeld suggests a full bonding survey. That's the only way to really figure out what's happening.

Flemming may be correct you are over zinced burning the bottom paint. It happens. On my last boat at survey several areas showed looking like yours. The surveyor called out over zinced. The problem continued and turned out to be a near complete failure of the bonding system.

Ok, so they are connected but are the connections actually good? Put an ohm meter and make sure that there isn’t more than 1 ohm resistance between them. Also it is preferred to have each connection make a home run and not to have them daisy chained together.

I have been told and researched that too much zing can cause copper bottom paint "burn" around underwater protected metals.

The only way to tell for sure is to do a bonding system survey with a silver-chloride electrode or other methos to make sure you have not overzinced certain underwater fittings.

Research/Google silver-chloride electrode use/measurement.

You're overprotected, too much zinc for those small supports. That's not corrosion, that's "haloing" of the paint.

https://www.practical-sailor.com/blog/can-you-have-too-many-zincs

Edit: I was writing at the same time as psneeld

 

[SteveK]

Is this the first year this has occurred to your boat and to the others. All things being the same over the years then I would look for another reason this occurred like stray AC current.

 

[tiltrider1]

What was the bottom paint? 70% copper paints have a tendency to halo when over zinced. 55% copper paints are much less likely to halo.

 

[porman]

The way I read post 4, he has two separate systems. One for the supports and anode, and another for the rest of the underwater metals. Shouldn't they all be bonded together?

 

[Fleming]

The way I read post 4, he has two separate systems. One for the supports and anode, and another for the rest of the underwater metals. Shouldn't they all be bonded together?

That's the way I read it, and yes it should all be tied together to that big hull zinc. It's too much zinc for just the supports. It's not hurting anything to be overprotected except the paint.

What is protecting the through hulls if not that zinc?

 

[fgarriso]

Bond everything to everything! Add more zincs everywhere!

 

[tiltrider1]

I make a point to test continuity of all items that are bonded. You would be surprised how often you will find no continuity.

 

[jwag956]

Thanks for all the input - looking back at the GB Bible - it implies that all the thru-hulls, engines, etc should all bond back to the transom zincs... so I got some bad info...

I have checked the transom zincs to swim platform resistance and all were at or around 0 - so those all seem to be bonded together - next up - start from first principles and check from engines to transom...

 

[SteveK]

with boat out of water you can also check continuity from the outside since bonding should complete circuit. Zinc to all metal parts. Then last check zinc to engine block.

 

[Duvie]

I am of the do not bond camp. Galvanic corrosion requires two dissimilar metals to be connected together and submersed in the water. I don't see it being very likely that any other under water metal will be electrically connected to your swim step brackets unless they used stainless bolts to fasten a bronze bracket or vice versa.

When you bond you set up exactly what is needed for galvanic corrosion to occur and then use the zinc to protect it. Then if you have failures in your bonding system you can have set things up for galvanic corrosion to occur.

My guess on the current issue, as others mentioned above, is that it is over zinced. I think on wooden boats this can cause deterioration of the wood around under water metals also!

 

[Steve DAntonio]

You're overprotected, too much zinc for those small supports. That's not corrosion, that's "haloing" of the paint.

https://www.practical-sailor.com/blog/can-you-have-too-many-zincs

Edit: I was writing at the same time as psneeld

Agree, this looks like paint failure, however, based on the photos I see no corrosion per se. Is there actual pitting of the metal?

More on over-protection and paint haloing here https://stevedmarineconsulting.com/...al-do-you-need-wi-fi-or-texting-smoke-alarms/

More on testing corrosion protection levels with a reference electrode here https://stevedmarineconsulting.com/...rosion-protection-level-editorial-old-vs-new/

These appear to be stainless steel, more on stainless corrosion here https://stevedmarineconsulting.com/stainless-steel-miracle-metal/

 

[Steve DAntonio]

I am of the do not bond camp. Galvanic corrosion requires two dissimilar metals to be connected together and submersed in the water. I don't see it being very likely that any other under water metal will be electrically connected to your swim step brackets unless they used stainless bolts to fasten a bronze bracket or vice versa.

When you bond you set up exactly what is needed for galvanic corrosion to occur and then use the zinc to protect it. Then if you have failures in your bonding system you can have set things up for galvanic corrosion to occur.

My guess on the current issue, as others mentioned above, is that it is over zinced. I think on wooden boats this can cause deterioration of the wood around under water metals also!

There are two camps, bond and protect, or isolate. Bonding, however, offers, I believe, far more advantages than disadvantages, chief among these are mitigation of stray current corrosion, in that the fault current is returned to the source with low resistance, which is far more likely to trip a breaker.

Bonding reduces the possibility of electrocution and fire in that if a bonded metal is energized with shore power, a circuit breaker will almost certainly trip.

Albeit anecdotal, in my experience bonded vessels often suffer leas damage than non-bonded vessels in the event of a lightning strike or nearby strike.

Over-protection on timber and aluminum vessels can cause damage, on fiberglass vessels it is only harmful to paint.

More on bonding systems and their many benefits here https://stevedmarineconsulting.com/wp-content/uploads/2014/03/BondingSystems138_05.pdf

 

[boatnbump]

What are the brackets made of. If their aluminum, they could be acting as Zincs.