Disintegrating rivets

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DCS30

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Hey folks. We have a 1978 Starcraft Holiday 18 Elite that my dad bought new when it came out, and for the last few years we've been battling rivet issues; heads chipping off, disintegration...it's been a mess. So we've been doing the basic DIY approach that no one should do (ie - drill them out, replace with sealed pop rivets, marine epoxy over them), and just slapping marine epoxy over the ones that aren't that bad yet. Realizing this was probably electrolysis, I took the zinc plate off the boat and ground it down to clean it, and it seemed to work temporarily. Now things are hitting the proverbial fan again, and we have zero idea what's causing it. Can't be the water because the rivets on our other boat are just fine, and it's even older. Is the plate just beyond repair and needing to be replaced? I've attached a few pics I took while the boat was in the water showing our hack epoxy job and what's happening to a couple specific rivets that looked fine before they were under water. I should note, that this isn't the case with EVERY rivet that's under the water line, which makes it more confusing, in my mind. Thanks in advance!
 

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Chris1956

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So you keep the boat in the water right? Is it at a marina or your dock? What bottom paint do you use? Did you recently install a SS prop?


Marinas can have stray currents in the water that cause such corrosion. Of course, your dock could have the same as well, but less likely.

A shotgun approach is to add more zincs. Your motor should have at least 2. Prop nuts will accept them as well.
 
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DCS30

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Chris1956 said:
So you keep the boat in the water right? Is it at a marina or your dock? What bottom paint do you use? Did you recently install a SS prop?


Marinas can have stray currents in the water that cause such corrosion. Of course, your dock could have the same as well, but less likely.

A shotgun approach is to add more zincs. Your motor should have at least 2. Prop nuts will accept them as well.
Click to expand...

when it's not at our dock, it's in the driveway, then into indoor storage. in the water for a few months of the year only. i'm not sure what paint is on the bottom, as it was repainted about 30 years ago, but it's different from the side paint, and the rivets are doing the same thing both on the side (at the rear..pics), and on the bottom. it's the original aluminum prop, and this boat, and drive, only came with the one large plate.
 
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Chris1956

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OK, to be clear, you keep it at your dock for a few months of the year. You do not paint it with antifouling paint, and it has one zinc anode.
I assume it is freshwater, or you would have serious growth build up.

You might look up to check for stray electrical current at your dock. You also might check to see that your battery cable or other wires do not have the insulation rubbed off. If bare wire touches the hull, it will conduct thru the hull, into the water which is grounded. This can corrode the metal of the hull.
 
Scott Danforth

Scott Danforth

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did you add any electronics or electrical? if so, did you ground anything to the hull?
 
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DCS30

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Chris1956 said:
OK, to be clear, you keep it at your dock for a few months of the year. You do not paint it with antifouling paint, and it has one zinc anode.
I assume it is freshwater, or you would have serious growth build up.

You might look up to check for stray electrical current at your dock. You also might check to see that your battery cable or other wires do not have the insulation rubbed off. If bare wire touches the hull, it will conduct thru the hull, into the water which is grounded. This can corrode the metal of the hull.
Click to expand...
oh yeah, it's freshwater..sorry, i should have mentioned that earlier

Scott Danforth said:
did you add any electronics or electrical? if so, did you ground anything to the hull?
Click to expand...

we didn't add anything in the last couple years, no. my dad is a retired electrician, and he went through it a couple years ago and couldn't find any shorts. that being said, i wonder if it's more internal than not. like maybe in the drive or something. he wants to try putting a substance between the zinc and the drive that's supposed to help conductivity, but i'm not sure how well it will do. it's pretty crazy to see these disintegrating before our eyes. very reassuring feeling knowing that your boat is being kept afloat by epoxy and that the rivets are weakening pretty much by the minute haha
 
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Chris1956

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Your dad might take a galvanometer and check the water around the boat for stray currents. with his training, he may know how to detect same.

You might also see if an impressed current system like MerCathode would help your situation.
 
Scott Danforth

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there should not be any connection between the electrical system and the boat.
 
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Chris1956

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You also might look to see if any electrical wires have fallen to the bottom of the boat, where they could be covered in water. Water could find a nick in the insulation and allow electricity to contact the hull, completing the circuit.


Scott, wouldn't you expect the engine to be connected to the battery negative, and therefore the entire hull is grounded?
 
Scott Danforth

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Chris1956 said:
You also might look to see if any electrical wires have fallen to the bottom of the boat, where they could be covered in water. Water could find a nick in the insulation and allow electricity to contact the hull, completing the circuit.


Scott, wouldn't you expect the engine to be connected to the battery negative, and therefore the entire hull is grounded?
Click to expand...
generally no, not directly. there usually is a volt or two potential difference between the hull and the battery ground. power heads and engines are usually rubber mounted, so there is that isolation.

it when people assume they can use an aluminum boat as a ground that most of the time galvanic corrosion eats the hull.
 
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Chris1956

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Scott, Are you sure about that?

My Merc has a block ground and 4 1/2" mounting bolts. There is no way the engine is not grounded to the boat and water. my boat is fiberglass, however, a lot of motors have parts in the water, even with the motor tilted up.
 
Bondo

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Ayuh,..... My ole tin barge did that many years ago, back when I put in lights that grounded through the hull,.....
I disconnected those lights, 'n the corrosion stopped,.....
 
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DCS30

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Chris1956 said:
Scott, Are you sure about that?

My Merc has a block ground and 4 1/2" mounting bolts. There is no way the engine is not grounded to the boat and water. my boat is fiberglass, however, a lot of motors have parts in the water, even with the motor tilted up.
Click to expand...
post a pic?
 
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DCS30

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it's possible the drive/motor is grounded to the hull. we don't think the battery is.
 
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