Chainplate pitting corrosion
joe anglim
san francisco bay, and will be replacing the standing rigging. The
upper and lower shrouds on each side attach to one chainplate. This
chainplate is located about 18" inboard of the beam, passes through
the cabintop, and is throughbolted to the main bulkhead.
That's all the background, now the problem. I have the chainplates
out, and have found an area of pitting in the stainless (grade
unknown) that is about 1" wide and 2 1/2" tall, with a few small
isolated pits outside of this area. The pits range from barely a
pinhole with almost no depth, to nearly 1/16" diam by nearly 1/16"
deep. All of the pitting is in the area where the plate passes
through the deck, and there is evidence on the bulkhead of water
getting through the deck seal.
The plate is 2" wide, 1/4" thick, and about 18" long. the shrouds
attached are 5/32" upper and 3/16" lower. The boat is about 35 years
old, but the original chainplates from Schock were 1/4" aluminum
plate, and I don't know how far back the stainless plates were put in.
I have gotten three opinions on whether the pitting is a big deal or
not:
1. YES, it's a big deal, the pitting will continue and likely extends
beyond what is visible now. Replace the plates immediately.
2. No, it's insignificant, the plate is way, way, way oversized and a
little pitting won't hurt a bit. You could lift the boat (2600 lbs
unloaded) with a plate half that size.
3. Probably not. Just paint the area of the plate that will be
sealed off from the atmosphere before reinstalling to prevent further
pitting.
The machine shop offering the third opinion also said that they could
fabricate two new chainplates for $200 us.
Relative to the cost of things on this boat, $200 is a large sum and I
don't want to throw it away on servicable parts, but I also don't want
to suffer a dramatic failure out on the bay.
In the realm of peace of mind, my wife / co-owner was present to hear
the first and most troubling opinion, but not the other more calming
opinions.
any thoughts?
thanks,
Joe
Tom Dacon
Replace 'em. I agree with (1). The chainplate with both shrouds attached
represents a single point of failure for all support against side loads.
Blowing out a chainplate on the bay could ruin your entire day. While it's
true that a 1/4" x 2" chainplate cross-section is way overkill for the size
of the rig, you can't tell by looking at the surface how deep and pervasive
the damage really is.
If the $200 is outside your budget, find a metals supplier and buy a piece
of stainless strap of the right dimensions and make them yourself. You can
cut it with a hacksaw with some difficulty, but if you can find a friend
with a metal-cutting bandsaw and a drill press and a bench grinder it'll go
a lot faster and better. Or perhaps you can do some of the fabrication and
have a machine shop do just the drilling and bending, at less expense to
you. The $200 quote seems in line with what a boatyard machine shop would
want for the job, but it seems a little high to me for the actual amount of
work that needs to be done. I doubt if there's an hour of work in the job,
and even at $75/hour (my local boatyard shop's rate) that's a little high.
Probably you got the quote from a shop that caters to the marine trades. If
you supply the materials, just about any auto parts store with a machine
shop in the back could probably do the fabrication for you at a lower price.
Get 316L stainless while you're at it, instead of the cheaper 304 or 18-8 -
the 316L is relatively immune to the crevice corrosion problem you're
experiencing. You can probably buy the metal for twenty dollars or less -
they sell it by the pound.
Before you reinstall the chainplates, look very carefully at the condition
of the bulkhead where the chainplates are fastened. If water has been
getting in for a long time, it's possible that there's localized
deterioration of the plywood. If there is, do whatever is necessary to
restore the strength of that area. It doesn't make much difference how new
and strong your chainplate is, if it pulls out of the bulkhead.
When you're done, caulk the hell out of the openings where the chain plates
pass through the deck. If you can make a slotted cover plate that slides
down over the chain plate and covers the opening and is separately fastened
to the deck, you can protect the caulking from UV degradation and it will
last longer.
Good luck,
Tom Dacon
"joe anglim" <jan...@wt.net> wrote in message
news:40da03ec.03112...@posting.google.com...
Rufus
around and ask around. You can find lots of examples and experiences.
Those sound like HUGE chain plates for that boat. Even if you postulate
that all the pitted metal is just cut out, the remaining stuff would
probably still be over kill. Since the plates are out, maybe you could
go walk into a surveyer's office and get his opinion on the spot.
However, by talking with local owners (or on the web), you can find out
what common practice is, and at least get new ones more or less sized to
that boat. The "hole in the roof" would need a little extra caulking,
though. Slotted cover plates that slide down the chain plate onto the
deck surface really make a difference to leaking.
If the bulkhead is messed up, get new metal double or triple the
original length and bolt it into something that remains solid lower down
there. I think you would want to install it so the upper shroud (the one
with the most load) gets the fair lead, but then again, ask around of
those who have done it before or take a look at a bunch of boats and see
what people have done.
Rufus
Dazed and Confuzed
why not take it to a welder, and have the damaged spots welded up and then
grind them down to a flush surface? might be cheaper than having a new one
made.
just my $0.02.
--
An amateur built the ark ....professionals built the Titanic.
Brian Whatcott
here in order of cheap:
1) Take the plate to a plating shop and have them grind and buff.
2) Buy a mild steel plate, and have a local shop drill to your
drawing. Paint.
Repeat 2) in seven years.
Try for under $30
Brian W
Jim Conlin
www.schaefermarine.com makes chainplates as a stock item. Cheap. Their website seems a bit bent at the moment. Google for schaefer chainplates.
joe anglim
It's true that there are lots of these boats around here, and lots of
collective wisdom. Opinion 2 came from a respected rigger with years
and years of Santana experience. He replaces the stock aluminum
chainplates with 1/8" stainless.
Schock also sells new chainplates now that they are selling new
santana 22's again, but the bolt pattern is different on the new hulls
so I would have to mess with the bulkhead. Same case for the standard
Schaefer plates.
I'll look more closely before buttoning everything back up, but the
plywood seems to be in pretty good shape - some varnish has
dissappeared, but no apparent softness in the plys or raised surface.
I do have slotted caps and will caulk well.
I'm not wild about going to mild steel/paint or spending much money or
effort improving the ones I have. If I'm going to mess with it, I
want less worries, not more.
The $200 quote was for 316 at a generic machine shop - I had to
describe what a chainplate is and does, I take that as a good sign but
I probably should have just said it was for farm equipment. I didn't
ask them about 316L.
My gut feeling is that if I don't replace them, I'll probably worry
about them enough that even if there is no spectacular failure on the
bay, the $200 would have been worth it.
I'll have to check with a friend to see if he can handle stainless in
his limited backyard shop.
McMaster-Carr's price for a 2" x 36" x 1/4 flat bar of 316 is $42
plus shipping. They don't have the 316L in 2" bars so I would have to
cut down a larger plate, for $65. Local would be a better bet anyway
to avoid shipping...
Thanks again. I appreciate all the fast responses.
Joe
David Flew
I think that if you stand back a little from this and read your own words,
you will clean it up and put it back. From where I sit you are saying that
original construction was 1/8 aluminum, and that you have a replacement made
from 1/4 SS. I've no idea of the loads or stresses, but if the original alu
was anywhere near good enough, then 1/8 SS would have been overkill. Or if
the original was under-sized, the 1/8 is OK. So even if you had lost 50%
thickness all over ( which you don't sem to be suggesting) seems to me you
are still well into the OK range. Just remember that SS requires oxygen for
it's corrosion resistance and don't go overboard on sealing/bedding which
might exclude oxygen and accelerate corrosion.
If you are accepting the advice of the rigger who replaces the 1/8 alu in
1/8" SS, why are you planning to go back with new 1/4?. What thickness are
the new Schock ones? What grade?
I suggest you could either stay with the 1/4 SS and inspect it again in a
few years, or make the replacement out of 1/8" and be pretty comfortable
either way .....
Regards
David
"joe anglim" <jan...@wt.net> wrote in message
news:40da03ec.03112...@posting.google.com...
joe anglim
The original aluminum chainplates were 1/4" thick. Enough of those
failed in the early years on the bay that most of the fleet was
converted to stainless. At least one prominent local rigger was using
1/8 stainless in the swap (with apparently no reported failure after
the swap.) There was also at least someone out there using 1/4"
stainless in the swap, because that's what my boat ended up with.
You're right, I should ask Schock what size and grade the new
chainplates are.
joe
Panama
http://www.onlinemetals.com/
What's to discuss?
On Fri, 21 Nov 2003 12:40:17 -0800, "Tom Dacon" <To...@t-dacons.com>
wrote:
Rufus
They let you prospect the yard. Advice given freely, but I suggest you
go for the older counter help if you want advice. Also sell new stuff.
Your quote come from a shop in SF? That still sounds pretty high. If
you like, you could get it cut by the supplier (+$5 if I recall), drill
it yourself, then get a shop to bend it on their roller machine. You'd
need to either supply a sample for the angle or spec it yourself.
Chamfer the edges _slightly_ yourself with a dremel tool - also the
edges of the holes. Of if the shop will give you a good price, have them
finish it as well as bend it. The hardest part would be rounding the
ends - a grinder would do it, but I think you'd need to finish with a
file to get it clean.
If the bend is small, you could do it yourself with a real heavy shop
vice and a small sledge hammer. Drill the holes after since you can
locate them exactly.
Rufus