Jacking Plates and Rust

E

Ed Price

Guest
I knew that I had a little bit of rust on the inside of my hull, just above the
jack attachment tubes. The rusted area was about 2" fore & aft, and about 4"
left and right (on the part of the hull that's angled from the bottom to the
door sill). I figured that I would chip out the rust, from the inside, and then
patch the area with some steel-filled epoxy. This weekend, I attacked the rust
spots with a chisel and grinder. And I found out things about rust that I never
saw before.

I have seen previous posts that talked about the jacking plate being a
rust-prone area. Well, I sure can confirm that. And the blame goes to the design
of the Amphi. Those jacking plates were attached to the hull with about 6 bead
welds, each about an inch long. If the welds had been continuous, running all
around the perimeter of the jacking plate, we would have had no problems with
rust. But the design of the welds guarantees that water will get between the
jacking plate and the hull. And because the plate was welded, near the "inner"
edge of each weld there's bare steel. This plate will start to rust along the
inner edge of each weld bead!

When I began to peel back the hull sheet metal from the inside, I was surprised
to find a 3/8" thick layer of rust, sandwiched between the hull and the bottom
edge of the jacking plate. It looks like, as the jacking plate rusted, the rust
filled in the gap between the plate and the hull. As the rust continued, it must
have built up pressure in that gap, until the rust formed a hard filler layer.
The important thing is that the rust has no structural strength, and, as I
worked along the area with a chisel, I kept cracking out chunks of rust. It was
almost like mining a thin coal vein. I ended up with a hull-hole about 8" long
fore & aft, and almost the entire width of the angled section of the hull!

It's now a good time to say that the inside of the hull showed no indication of
the large seriously rusted area. And the bottom of the jacking plate didn't show
any bad signs either. If it hadn't been that the rust finally ate through the
hull, I don't think I would have been suspicious about the area.

My solution has been to grind off the welds that hold the plate to the hull, and
remove the jacking plates. Since I don't plan to use the Amphi jack (I prefer a
bottle jack), I'm going to simply weld flat plates over the damaged areas.
(Continuous weld, of course.)

I went into a lot of detail about this condition because I hope you will
carefully examine this area on your Amphis whenever you are doing a hull
checkout. Be aggressive about probing for rust if the paint looks blistered or
swollen!


Ed
El Cajon
67 Rust Guppy


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