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Backstay chainplate replacement

Posted: Thu Mar 23, 2017 4:57 pm
by Allie-May
Like most of us my backstay chainplates are partially submerged. The lack of oxygen is causing corrosion on the chainplates and they will need replaced. Aside from hull extensions, has anyone figured out a way to get the chainplates out of the water? How often does a submerged chainplate typically last? Has anyone gone to titanium for the chainplate and if so what did it cost and how do you like it?

Re: Backstay chainplate replacement

Posted: Thu Mar 23, 2017 8:39 pm
by thinwater
Just curious.

Have you actually pulled a plate and seen corrosion on the backside, or is this a presumption based upon staining? I can't see where anyone on the forum has reported corrosion, in spite of the fact that this is a common issue.

The PDQ 32 owners would also like to know, since we have external chainstays.

Re: Backstay chainplate replacement

Posted: Fri Mar 24, 2017 12:55 am
by Allie-May
The corrosion is visible with the plate installed. The submerged portion was painted with bottom paint by the previous owner. Paint has flaked off where the corrosion is located making it visible. Supposedly the standing rigging was replaced three or four years ago. Everything appears to be in good shape except the backstay chainplates. Perhaps the addition of antifouling paint accelerated the problem or maybe 304 stainless was used instead of 316. Not sure which is standard but I'll check to see if mine are stamped.

Re: Backstay chainplate replacement

Posted: Fri Mar 24, 2017 9:40 am
by thinwater
Bottom paint was very likely a problem. Once SS becomes non-passive (under the paint) the copper becomes more noble and the stainless is pitted. Corrosion damage to stainless prop shafts connected to bronze props is also well known. This is something that should be better communicated--NEVER paint underwater stainless or aluminum.

If the damage is minor cleaning, polishing, and NOT painting in the future may be all that is needed. But that requires eyes-on.

Stainless should always be primed with epoxy (Interprotect or similar) before applying in antifouling. Or left bare.

Re: Backstay chainplate replacement

Posted: Fri Mar 24, 2017 11:21 am
by Allie-May
I'm going to do what I can to get the paint off and stainless cleaned up. We are in the water so that'll add to the difficulty but the corrosion I can see is now above the water line due to removing excess weight. I think I should use a plastic scraper as to not damage the metal; hopefully it'll be enough.

Re: Backstay chainplate replacement

Posted: Fri Mar 24, 2017 9:21 pm
by thinwater
The combination of copper bottom paint and proximity to water would have me thinking seriously about bronze.

* The strength is the same as 316 SS.
* No conflict with bottom paint.
* Growth will stay off.
* No crevices corrosion issues.
* Much easier to machine.

* Material is more expensive ($100 for both vs $25 in SS). One reason builders don't use it.
* Unconventional, but traditional. Another reason builder don't use it.
* Won't stay shinny. The final reason they don't use it. Of course, there is no reason you couldn't paint them.

But if you consider that that your turnbuckles are chrome plated bronze, it obviously is NOT about fatigue, strength, or corrosion. Mostly looks. down on the step, half in the water, I think it would look intelligent.

The truth is that 316 SS will outlast your ownership.

Re: Backstay chainplate replacement

Posted: Mon Jul 24, 2017 1:54 pm
by Allie-May
Has anyone replaced their backstay chainplate? If so, how easy was removing and replacing the nuts in the watertight compartment? Were you able to do it via the access panel in the stern step or did you have to make another access hole to reach the backside? It seems like the current access could be doable with a long arm and lots of patience but you'd be working blink.

I received a quote for a pair of chainplates at a local shop. They are $140 each for 1/2" polished SS with countersunk bolts. Countersinking the bolts added about $50 each. It's a cleaner look and should help protect the bolt better than an exposed hex head.

Re: Backstay chainplate replacement

Posted: Mon Jul 24, 2017 9:23 pm
by thinwater
Now that I think about it, I would ultrasound them. Easy, since they are exposed. I have done this on mine, but really it is quite a bargain for me, since I have professional UT gear and used to inspect refinery equipment do it as part of my day job. UT, in the hands of a skilled operator, is very good for evaluating this sort of backside damage.

3rd party, it would cost more than the plates, but the labor is avoided.

Re: Backstay chainplate replacement

Posted: Wed Sep 06, 2017 2:35 pm
by Allie-May
I finally got around to replacing my backstay chainplates. A local rigger inspected my boat and was in agreement that only the backstay chainplates needed replaced. He also felt these were the original chainplates. It was $300 in total for the inspection and a set of custom made chainplates. I spent another $25 on nuts, bolts, washers, and Life-Calk.
Unfortunately there was no good access to where the nuts are located so I added what every boat needs which are extra holes. I placed a 4" deck plate on the second transom step up on both hulls. A dremel tool with fiberglass bit worked well for cutting the round holes. The new access holes made this a one person job with a screwdriver in one hand and socket driver in the other. No adhesive was used on the old plates, just a sealant at the 4 bolt holes. I used Life-Calk as the replacement sealant.

Here are some before and after photos
Original chainplate - ... XYzWUdKQms
Original chainplate - ... 0tfclY5Tlk
New chainplate - ... XdFVmRIeXc
New Chainplate - ... 2Qtai0yT2s

Re: Backstay chainplate replacement

Posted: Wed Sep 06, 2017 4:01 pm
by thinwater
Was the backside similarly pitted, or only the sides?

It looks to me like the copper paint made the SS andodic and just ate into it. I've seen this before on rudder pintles. They should never be painted.

The lesson for PDQ 36 owners is to mask off the chain plates and NEVER get bottom paint on them or within about 1/2-inch. Bottom paint on any underwater metals other than bronze is generally a disaster.