External Chain Plates

[hilbert]

I take it with good will and I am grateful for the discussion. I may be wrong ( it is not unheard of ), but the crazing exactly corresponds with the rust in the pocket on the inside. I believe you about the direction of force. I still think that fatigue will overtake me (and probably the tang), before the 3/8" x 1.5" cross section of the bar.

[gates_cliff]

Frist let me preface my remarks with, I have considered the idea of external chain plates. Although I don't have the technical knowledge to attempt a DIY job like this. One of the reasons I like the idea is strictly convenience, since on my CD 27 the side decks are fairly narrow and the inboard chain plates do get in the way of going forward and aft. But, that's not a suitable reason to make the change. However, I am concerned about the longevity of the current design, especially in light of the post showing how someone used Home Depot turnbuckles!

The mention of naval architects does present a good point. To go to that sort of effort would it be reasonable to suggest that one employ a good naval architect to calculate the loads, sizes, etc? I can't imagine the cost would be prohibitive to have someone do those kinds of caluclations. But, as I said, I don't have the technical knowledge to be comfortable with just winging it and thinking, "oh, that should be plenty strong enough". But clearly the orginial post shows someone with seirous experience and knowledge.

[Kevin Kaldenbach]

My bad. That last pics tells a different story as far as the crazing goes. On the CD31 the chain plate does not extend down the side like that. I still like to original design and as I said earlier provided the chain plates were not rusted out.

[Steve Laume]

Jonathan, You definitely needed to do something.

As far as having a naval architect run numbers on the job, I would resort to major over kill. It usually works out, Steve.

[Duncan]

I don't think there's any doubt that external chainplates can do the job - they've been used for centuries, after all.

With respect to the rusty backing plates on Cape Dorys, I've seen several reports of this, but no reports of rig failure.

If you look at the manuals (dwg. 2.1-4 (a) for the 27', 28', 30' models), I think you can see why. The chainplates in the CD design are actually the cast bronze deck fittings on deck, not the mild steel backing plates and the rebar glassed to the hull underneath.

These bronze chainplate fittings are bolted through the fiberglass hull and deck flanges. The backing plates and the rebar help to spread the load. When the backing plates and the rebar rust out, the rig doesn't come down, but there is more concentrated strain on the hull and deck flanges.

So far, I have never heard of a CD rig failing because the cast bronze chainplates on deck failed, or because they ripped up out of the deck and hull flanges. I guess that means that they can take the strain, even when the reinforcement fails.

It's not great to know that the backing plate and the rebar can rust out, but apparently that's not enough to fail the rig?
I am thinking that adding big square washers might be a perfectly seaworthy precaution here?

[Sea Hunt Video]

With respect to the rusty backing plates on Cape Dorys, I've seen several reports of this, but no reports of rig failure. . . .It's not great to know that the backing plate and the rebar can rust out, but apparently that's not enough to fail the rig? . . .

Duncan:

Check out this thread. At page 2 it references a failure reported by Ralph Naranjo on a CD Ty Weekender.

http://www.capedory.org/board/viewtopic ... it=Naranjo

[hilbert]

There are other boats that have worse issues with chain plates. An Island Packet lost it's mast at the marina where I have my boat. It has stainless bars that come through the deck and are secured by welded cross sections that are glassed into the side of the hull. It is impossible to inspect this construct, which is susceptible to severe anaerobic corrosion.

[Duncan]

Check out this thread. At page 2 it references a failure reported by Ralph Naranjo on a CD Ty Weekender.

http://www.capedory.org/board/viewtopic ... it=Naranjo

Thanks for the link, Robert. It's interesting to note that both of the reports there were of broken bronze bolts on Typhoons. Sharkbait reported corroded bronze bolts, too, when he restored his Ty.

I'm grateful for this thread, as a reminder to check the backing plates, rebed the chainplates, and probably to replace the bolts.

(As an aside, I recently took all the hardware off my CD 10, and I noticed that about half of the bronze screws had corroded.)

[bamabratsche]

Speaking of Typhoons, I was browsing sailboatlistings.com and came across this ad for a Typhoon that looks like it's had external chainplates installed. I thought it was an interesting idea, considering that it is really difficult to get a wrench on the underside of the factory ones, and completely impossible to visually inspect them without a mirror. A previous owner had installed thick, very confidence-inspiring stainless backing plates on mine, though, so I am pretty satisfied with my current setup.

I do agree with Russell that the external ones look very "salty", and I think this one looks pretty cool.

http://www.sailboatlistings.com/view/31205

[Adamhagan]

I am favoring external chain plates because they can be routinely inspected. The current design, while functional does not allow easy access.

After the replacement is it necessary to deconstruct and remove all the mild steel, or will sealing the deck reduce the moisture and arrest/slow corrosion?

[hilbert]

I chose to leave the metal plates that are under the deck. They did not interfere with the adding of external chain plates.

[S/V Necessity]

While the origional arrangement has obviously withstood the test of time, I suspect some of us are wondering "how much longer?" I have most definitly seen cape dories with issues regarding the chainplates. I recall seeing a CD30 in a Alameda CA marina that had clear distortions in the hull that could easily be seen from quite a distance away.

My CD28 has a significant amount of corrosion on all the mild steel chainplates I can gain access to, this does not seem to be ideal.... I can certainly sympathize with anyone who wants something new, and more easily inspected and serviced. (although I agree that the origional arrangement is quite attractive, and apparently quite sufficient for 30 years of service)

[Adamhagan]

Well said! Nothing last forever.

[gates_cliff]

Now you guys have me worried! Hadn't considered the prospect of doing this work. Oh well, as someone just said, "nothing lasts forever".

[moctrams]

I can easily inspect Gabbiano's plates and they are as sound as the day they were layed up.