Tank Progress By Michael

Eleanor and the 3-year-old tanks at the machine shop.

Presumably because they were leaking, the previous owners of our boat replaced both the original, 30-year-old stainless steel water tanks in 2008, in Guaymas/San Carlos and used the services of reputable welders. When we bought the boat in June 2010, one of the tanks was leaking slowly and on both tanks, epoxy was smeared along the inside seams. So the tanks we just tore out of the boat enjoyed a lifespan in the 12- to 18-month range. That’s shocking. And while I’m still not sure why they were so short lived, I now know where they failed.

·    All of the interior baffle welds were rusting and weeping on the outside of the tank.

·    The welded seams were corroded and leaking in places.

·    Pin-holes were near the welded seams and appeared sporadically at other places.

My theory is that the tanks failed due to a combination of electrolysis and poor welding.

Electrolysis: All of the tanks and thru-hulls and mast step and even a chain plate, were bonded with a combination of heavy-gauge wire and copper strapping. Experts still argue amongst themselves over whether or not bonding is a good practice, or a bad one. In my reading, I learned that even proponents of bonding concede that there is a lot of room for error in terms of conduit size and adequate attachments, and that error can serve to magnify and even create problems. Whereas by not bonding metal components in a boat, you lose the theoretical advantages of bonding, but avoid the pitfalls. I don’t think bonding is necessary and I removed all of the bonding aboard Del Viento. Hopefully isolating the new tanks will contribute to their longevity. As an aside, when I was in the water cleaning the hull and zincs the other day, I happened to reach up and noted current in the port-side cockpit drain thru-hull. It is above the waterline and obviously attached to something inside that is producing the current. I have to tear Eleanor’s bed apart to determine the cause, so I haven’t done this yet.

A pin hole is clearly visible here, just adjacent to the crimp weld. There are
many others along this seam, a half-a-dozen or so visible in this picture.

Poor welding: There is no reason the interior spot welds on the baffle tabs should be apparent on the outside of the tank as they are. But every one of these welds is corroded on the outside of the tank and leaking. My understanding is that it takes an experienced welder to not “overdue” things. I think these welds were overdone. The tank seams were crimped and then welded. I think this was not the best approach, resulting in crevices where crevice and pitting corrosion can occur—and it looks like it might have.

This is how all of the interior baffle welds look on the outside of both tanks.

I’ve got the new tanks in the boat and installed. Tomorrow I am going to fill them with water to ensure there is no settling or position changes that will happen when they weigh 450 pounds each. Then I will continue working to get the floor joists in their exact positions before I glass them to the hull. From there, attaching the flooring and replacing all of the furniture will be time-intensive, but straightforward.

--MR

Here is how things looked just before I dropped the
new tanks in: all freshly painted with BilgeKote.