Chainplate Can-o-Worms

[Departing2017]

Well I'm in it now. The stainless chainplates on our 1976 cold molded 51 footer are original equipment. I have a surprisingly reasonable quote on titanium replacements especially in light of the added security and peace of mind they will bring to our cruising.

Yesterday I pulled the plate for the inner forestay; yup crevice corrosion and fairly pronounced pitting where the plate passes through the deck. So I'm on the right track.

Next, I moved on to one of the lower shroud plates. All shroud plates are mounted internally (inside of hull and/or bulkhead) and will be accessible with fairly minimal destruction to the joinery. The hull mounted plates are attached to studs coming through the hull. I thought I'd just pull the acorn nuts off a few of the studs for a look-see and in the process of LOOSENING the nuts I sheared off five out of a total of nine studs.

I guess I'd been in a bit of denial about the extent of this little project, and I hadn't considered what would be involved in replacing the studs. Our boat is three-ply mahogany with about an eighth inch fiber/epoxy skin. My first guess was that the studs are actually individual bolts with the heads buried in the hull material and then glassed over. I acquired a metal detector, however, and it seems like there is more metal under the fiberglass than just bolts.

My questions to the forum: Has anyone else encountered this type of construction and replaced the chainplates? Any advice on locating exactly whatever is under the hull skin? Any advice on minimizing the imminent damage to the exterior of the hull? Am I missing anything?

[Jim Cate]

Only the builder will know for sure what lies under there, so is it possible to contact either the builder or the designer for info?

One guess is that there is a s/s backing plate embedded in the hull, outboard of the wedge shaped filler shown in the pic. The studs (sic) could be bolts welded into that backer, or even carriage head screws. Either way, they will be really hard to remove without lots of destruction... and that's a bitch!

But perhaps the builder did something clever that will ease the job... it would really be good to find out, if possible. Once you get the old bolts out, the option of running the new ones clear through the hull becomes a viable solution.

As a fellow owner of a timber (strip planked) one-off, you have my deepest sympathy.

Jim

[Departing2017]

Originally Posted by Jim Cate

Only the builder will know for sure what lies under there, so is it possible to contact either the builder or the designer for info?

One guess is that there is a s/s backing plate embedded in the hull, outboard of the wedge shaped filler shown in the pic. The studs (sic) could be bolts welded into that backer, or even carriage head screws. Either way, they will be really hard to remove without lots of destruction... and that's a bitch!

But perhaps the builder did something clever that will ease the job... it would really be good to find out, if possible. Once you get the old bolts out, the option of running the new ones clear through the hull becomes a viable solution.

As a fellow owner of a timber (strip planked) one-off, you have my deepest sympathy.

Jim

Thanks, Jim, for the reply, and for the commiseration. The builder is a possibility. I did get a chance to talk to him several years ago, but have since lost his contact info. I'm still looking.

Based on the metal detector, I'm pretty sure you're correct about the embedded backing plates. I think I just need to get through a preemptive grieving process before I can bring myself to start cutting into all that lovingly cleaned and polished topside paint. Ugh.

[barnakiel]

Can you possibly take an image from the opposite angle? To show the other side of the element onto which the plate is mounted?


(I call these 'knees' but possibly there is a better word).


What is this element made of? Is it GRP? Wood?


I ask because our boat seems to have a vaguely similar solution.


Ours is of a Scandinavian designer. Who designed / built your hull?


Most interesting project. Envy.


barnakiel

[Jim Cate]

Barring detailed info, one approach would be to drill small (2-3mm) holes from either inside or outside to determine how deeply buried the backer might be. Those would not be too conspicuous in the long run, and would perhaps direct your efforts in the least destructive direction.

My first guess is that it would be directly outside of the wedge shaped filler and on the inside of the hull proper.

Jim

[barnakiel]

I ask because I assumed the wedge of wood, but looking at the image again I noticed as if it were not flush with the hull - there is a darker line above the slats where I interpret a fine gap.


To me it is somewhat hard to imagine bolts that long - imagine the bending moment with any slightest movement of the wedge.


This is a very interesting design. What I shame I am not there. I would love to help.



barnakiel

[Departing2017]

Originally Posted by barnakiel

I ask because I assumed the wedge of wood, but looking at the image again I noticed as if it were not flush with the hull - there is a darker line above the slats where I interpret a fine gap.


To me it is somewhat hard to imagine bolts that long - imagine the bending moment with any slightest movement of the wedge.


This is a very interesting design. What I shame I am not there. I would love to help.



barnakiel

There is indeed a gap behind the wedge, but the studs (or more likely bolts) are set in solid wood because the hull was built on three-inch wide stringers with around three-inch spacing. The stringers are hidden behind the ceiling in the photo, but the wedge has full contact with each stringer, and the bolts are drilled through the wedge at the stringers. That's the reason the nuts are paired. The total thickness of the hull at the stringer would be just over two inches. Add another 2 3/4" for the thickness of wedge at its thickest part, the top, and you are fastening through, at most five inches of solid wood.

I don't know if that description gives you any further insight, but I appreciate any mental energy you're willing to expend. And I wish you could help too.

[Toccata]

You could unscrew the lining boards, remove them and "have a poke" around

[Departing2017]

Originally Posted by Jim Cate

Barring detailed info, one approach would be to drill small (2-3mm) holes from either inside or outside to determine how deeply buried the backer might be. Those would not be too conspicuous in the long run, and would perhaps direct your efforts in the least destructive direction.

My first guess is that it would be directly outside of the wedge shaped filler and on the inside of the hull proper.

Jim

Great idea on the small test holes. I think that will be the way to start.

And I did find an email for the builder. Hoping for a reply.

Thanks, again.

Jon

[Uncle Bob]

For what it's worth, rather than wholesale demolition at the chainplates, would it be possible/worthwhile to drill out the offending studs, probably all of them and install through bolts to the exterior? you have established that there is a backing plate there so strength would not be an issue. Difficult, yes but possible, I believe so.

[Jim Cate]

Originally Posted by Uncle Bob

For what it's worth, rather than wholesale demolition at the chainplates, would it be possible/worthwhile to drill out the offending studs, probably all of them and install through bolts to the exterior? you have established that there is a backing plate there so strength would not be an issue. Difficult, yes but possible, I believe so.

Why Rob, I reckon that you are a true optimist! Imagine drilling out a >5 inch long bolt without deviating from its axis... several times in a row. Merlin would have a hard time with that, and mortals don't stand a chance!

But I agree that if one could get the bolts out of the way, through bolting to the outside of the hull would be structurally fine, and with some care, not too cosmetically obtrusive from the outside with no significant change inside.

But that's the problem... getting the old studs/bolts out without a wrecking ball.

Bugger of a job!

Jim

[TeddyDiver]

If I were you I'd replace them with composite chainplates. That would require new wedges and maybe some stripping of the interior but not too much if the other chainplates are similar to the one in the picture.

[Uncle Bob]

Originally Posted by Jim Cate

Why Rob, I reckon that you are a true optimist! Imagine drilling out a >5 inch long bolt without deviating from its axis... several times in a row. Merlin would have a hard time with that, and mortals don't stand a chance!

But I agree that if one could get the bolts out of the way, through bolting to the outside of the hull would be structurally fine, and with some care, not too cosmetically obtrusive from the outside with no significant change inside.

But that's the problem... getting the old studs/bolts out without a wrecking ball.

Bugger of a job!

Jim

Have to agree, bugger of a job, however I have seen it done by a pro using a diamond core drill, cutting around the outside of the existing failed stud, BUT, it was only one.
If it was doable the bolts would need to be the next size up.

[Dsanduril]

Beg, borrow, or rent an IR camera. Wait for a reasonably warm day. Fill a sock or stocking with ice, then place that in some plastic to catch drips. Use tape or other means to hold the ice-filled sock against the chainplate, trying to get the best contact at the stud locations. Pile some more ice on deck around the head of the chainplate. Wait 5-10 minutes.

Take the IR camera outside the boat and image the chainplate area. Set it up for only a few degree range around the hull temperature to give you the most contrast for small temperature differences. If you see distinct spots at the stud locations, with the area between close to hull temperature then you probably have individual bolts embedded. OTOH, if you see indistinct blobs that blend toward each other then the studs are likely welded to or installed in a carrier. You should be able to estimate its width from the temperature blobs.

I’ve never done this with chainplates, but we use similar techniques in the field all the time to delimit embedded metals in many substrates. It will all depend on the amount of “cover” the metal has from the outside. Lots of cover and you may not be able to see what you want. Little cover and your hidden metal should stick out like a sore thumb.

[barnakiel]

Idea 1) - not all SS is actually a-magnetic, so how about a small magnet test from the outside of the hull ? perhps it will add to our undertstanding of what is hidden.


Idea 2) - an IR test (mentioned) - ask around and grab an IR imager, many engineers have them, small handheld units, work wonders,



Idea 3) - destructive (moderately - lacking a drawing - remove the plate, then REMOVE the lowest-most portion of the wedge - only up to the lowest bolt. This way with minimal damage you can tell if the bolt does disappear into the hull skin.


I will re-read the thread again and re-post if I have any further guesses.


It is very interesting.


b.