Bad Prep, Electrolysis or Something Else?

Had the boat hauled about a month ago for bottom work. It had been almost 3 years and several of the yard workers said it looks pretty good for that amount of time except for the area around the swim platform supports and trim tabs. They also commented that they had seen this regularly on Flemings, and since my boat is made in the same yard, it made sense and they knew how to deal with it. Of course, it would also cost an additional several hundred dollars to deal with it though. Picture 1

Picture 2 is after paint - Pettit Ultima SR-60. Looks good to me!

Turns out that they had installed the bow thruster props on backwards, so I had to haul the boat again 3 weeks later to get this corrected, and discovered that things didn't quite look as expected as shown by pictures 3-7.

They immediately said that this was due to an electolysis problem, so I'm having the bonding system tested next week. They may be right and I'll know more then. But, in the meantime, and because I don't know jack about this, I'm also wondering about perhaps bad prep work, or maybe something else I can't even think of.

TF experts, what is your opinion?

Hard to tell from the pictures but is that aluminum (trim tabs) and steel (hull) ?

The zinc has come off the prop nut on the bow thruster. Was this removed or did it come off on its own? Suggest you use a longer Allen bolt than supplied with the zinc and use lock tight. Some thing happens to me and I think they need to be inspected after a season's use. Same happened with the zinc on the prop nut for the propeller on our boat. I don't think I used lock tight on the bolt last time. Used the blue lock tight this time but may have to go with the red lock tight.

As for the swim step and trim tabs, they don't look like they were properly bonded with the rudder Inc, or, that the rudder zinc wasn't bonded with the rudder itself. Several possibilities. Others on the forum will have better views.


Jim
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Fiberglass hull, and, as embarrassing as it is, I don't know if the trim tabs are steel or aluminum. Never thought about it before.

JDCAVE said:

The zinc has come off the prop nut on the bow thruster. Was this removed or did it come off on its own? Suggest you use a longer Allen bolt than supplied with the zinc and use lock tight. Some thing happens to me and I think they need to be inspected after a season's use. Same happened with the zinc on the prop nut for the propeller on our boat. I don't think I used lock tight on the bolt last time. Used the blue lock tight this time but may have to go with the red lock tight.

As for the swim step and trim tabs, they don't look like they were properly bonded with the rudder Inc, or, that the rudder zinc wasn't bonded with the rudder itself. Several possibilities. Others on the forum will have better views.

Jim
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Good catch. The rudder and bow thruster zincs were put on after the photos were taken, so that wasn't the problem.

Second to the bottom pic looks like a situation that I have experienced and other experienced people have commented on being from too much zinc and certainly bottom paints with copper in them.

The haze in the paint radiating out from where it is actually failed, is why I say this.

I have read other discussions on this issue, but never really read a full tech description of the failure.

This will lead to poor paint holding on metal..but bottom line on metal is it should be primed before bottom paint. With something like Interlux Interpret 2000.

I would see Petit paint manufacturer tech support would take a look at some emailed pictures for a better evaluation.

Any halos around through hulls that are bonded?

Sorry if I'm hijacking this thread a bit....but, could somebody explain this "bonding" thing to an naïve newbie boat owner? Please use layman's terms so I can understand. Thanks!

wyoboater said:

Sorry if I'm hijacking this thread a bit....but, could somebody explain this "bonding" thing to an naïve newbie boat owner? Please use layman's terms so I can understand. Thanks!

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I think about bonding this way: you need to protect your metal parts on the boat from corrosion and use of sacrificial anodes is the most common way of achieving that. But you cannot put a zinc on every part exposed to salt water. Those that are difficult to put a zinc on are bonded by wire to those that can accept zincs such as the shaft and rudder. Perhaps I'm wrong but that's how I think about it.


Jim
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Ah, ok and thanks JDCAVE....kinda makes sense to me now....I was really confused. So, you're basically having the installed zincs do double duty by connecting them to areas that you can't get zincs to....WOW, I've learned something here AGAIN! I'm so glad I subscribed to this forum!

Okay, I'm a bit of a newbie myself and I probably jumped the gun on an explanation. Best to read about it yourself and make up your own conclusions. This article looks interesting.

http://www.bristol27.com/projects/bonding-grounding-system


Jim
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And another article that looks interesting...

http://www.kastenmarine.com/_pdf/mbqCref.pdf

'Bonding is defined by the ABYC as “the electrical interconnection of metal objects in common contact with water, to the engine negative terminal, or its bus, and to the sacrificial anodes or impressed current system.”'


Jim
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You cannot use copper-based bottom paint on aluminum without a very thorough barrier coat application first. Even a zinc won't protect it.

Stainless steel is an incredibly bad electrical conductor. Instead of using those wires to connect the trim tabs, why not just attach a zinc to the tabs? Or even better, use aluminum anodes.

Your rudders and tabs appear to have been coated with zinc paint - why? The rudders should have zincs attached. Why not bottom paint the rudders and tabs? How did the rudders look when you hauled her, they aren't in the photos.

I'm posted here for interest sake... i.e. to stay in the loop.

It looks to me that the swim platform brackets are made of aluminum. I say this because the color where they don't touch water look too 'white' for stainless. If they are aluminum I would think they should not be attached to the bonding system at all. They should definitely not have copper laden bottom paint on them, at least not without a very thorough barrier primer.

You should be using aluminum anodes

Your in brackish water, zink is not a anode for your conditions, aluminum is. Trim tabs are stainless as are the brackets. I really like your rudders, the wedges on the trailing edge and dam on the bottom look like a damn good attempt to improve slow speed maneuverability. I don't think you have a electrolysis problem with your paint. Usually the black bottom paint will begin to have a pinkish hue. The large divers plates bolted through the transom are there for your bonding system. The bonding system is usually a green or green with yellow striped wire connecting all of the metal including you machinery that somehow can come in contact with seawater. Some of your electronics may be grounded to this bonding system as well. If the bonding wire connection is corroded at the bonded through hull or whatever that piece will be subject to corrosion, not the bottom paint. I think you have a paint prep or paint problem.

Thanks all for the replies so far.

The rudders and trim tabs were coated with Pettit Barnacle Barrier. And, supposedly, the areas that were looking bad when hauled were first coated with a special primer before paint because of that familiar problem to them.

Rudders have zincs, they just weren't installed when the photo was taken.

And oops, I forgot to mention, we keep the boat in Anacortes, so it's salt water and not brackish.

JDCave: Those are some pretty interesting articles..seems to be some mixed feelings on the bonding issues. Seems to me that complete isolation on certain items when possible is the way to go. I've saved those articles to my faves so I can peruse them at length later. Thanks!

Nsail said:

TF experts, what is your opinion?

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It is called "burn-back" and it happens when your copper-based anti fouling paint reacts with improperly primed metal.

Xsbank said:

Stainless steel is an incredibly bad electrical conductor. Instead of using those wires to connect the trim tabs, why not just attach a zinc to the tabs? Or even better, use aluminum anodes.

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While they may do some conducting, I think those wires/cables are to keep the tabs from unintentionally over extending when you back down.

some pix look to show some haloing from previous paint failure. There is something electrical going on galvanism or stray current I cant tell.

Fastbottoms is saying what the uneducated but experienced me is alluding to.

BUT....I really can't say anything definitive from just those pics....I was just offering a direction of recourse if other leads failed to explain what was going on.

Good chance fastbottoms is going to have good info on this as well....

psneeld said:

Good chance fastbottoms is going to have good info on this as well....

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I see this phenomenom with some frequency, but most of what I know about its cause I learned from this document:

www.yachtpaint.com/LiteratureCentre/antifouling_101_usa_eng.pdf

fstbttms said:

I see this phenomenom with some frequency, but most of what I know about its cause I learned from this document:

www.yachtpaint.com/LiteratureCentre/antifouling_101_usa_eng.pdf

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fst - TY for great link. I have listed in my marine folder and will fully read. New btm paint for our Tolly is not too far in the future. - Art

My original though was a dissimilar metals problems but that may not be the case.
dk

wyoboater said:

Sorry if I'm hijacking this thread a bit....but, could somebody explain this "bonding" thing to an naïve newbie boat owner? ...

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First up, it`s nothing like bondage. You need a different site for that.

AH! THAT explains it!