Backstay Chainplate Discussions [pg 152 in Manual]

[mbd]

The estimate for a new one made of #316 stainless was approximately $250.00, including material and labor.

Holy mackerel! Mine cost about $100 and I thought that was expensive!

[ebb]

Put any 300 series stainless together (304 with 316, for example) under compression/tension, you will get corrosion. It happens, it's not an opinion.

Larry Pardey. "A tip for those who get bleeding or rust marks running down their hull where stainless bolts go through stainless steel fittings - use a silicon bronze bolt or screw and there will be no electrolysis if the fitting is above the waterline. There will be no more bleeding and there will be no electrolysis if the fitting is above the waterline. In fact it is common practice to use bronze turnbuckles with stainless steel wire for rigging."
August 1999 on the 'Lin & Larry Pardey site.

In fact stainless parts are screwed into bronze bodies to prevent galling. Bronze screws would certainly stop the rusting if used in the 1/2 round rubrail on the A/C.

The shroudplates taken off 338 are in good condition despite being 40 years old. It's entirely possible different bronze plate was used over the years by Pearson. Entirely possible some bronze plate was in fact brass.

[Scott Galloway]

Thanks for the quote Ebb. That sure was interesting. I've never had any rusting screws in my rub rail though. I do trust the Linn and Larry Pardy’s judgment. I also like Dan Spurr as a reliable source of lots of information. Spurr’s Boat Book “Upgrading the Cruising Sailboat” is a nice thing to own. He features the restoration and modification of his Pearson Triton and also to a lesser degree of his Pearson Vanguard. The illustrations by Bruce Bingham are worth the cover price. Dan Spurr included a discussion of stainless steel options in the appendices, and a galvanic series table also, but I find that table to be a bit confusing.

In your quote, Larry Pardy referred to electrolysis, and we are talking primarily about galvanic action in the backstay chainplate are we not? …unless of course our electrical systems are grounded to our rigs? Mine isn't.

Galvanic action being that process that causes two dissimilar metals in contact with one another to exchange ions with the resulting effect of corrosion of one of the metals, and electrolysis being the loss of ions resulting from the flow of an electric current through metal from an external source. In either case, in an Ariel we would have the entire backstay, its fittings, the tangs, and mast etc. all connected together.

So in the case of electrolysis, if there is electricity flowing through the standing rig, and to the chainplates, I am still wondering about the advisability of a zinc on the backstay chainplate, given he fact that it spends so much of its time in salt water. In the case of galvanic action, a bronze or #304 stainless chainplate would be in contact with the #316 pin and shackle or toggle above, and cover plate above, and whatever fasteners you use to tie the chainplate to the knee. We need to look at this as an entire system do we not? So in the case of galvanic action, which is unrelated to an external current, don’t we still have a problem with the contact with the rig above and the backstay chainplate below?

Also, does anyone have any thoughts on printed sources with tables listing the relative nobility of metals and particularly bronze vs. stainless steel?

By the way, I paid about $50.00 for my #304 stainless steel backstay chainplate in 2002. That price covered materials and labor for fabrication. The plate was not polished. In four years there has been no significant corrosion to the bolts, or chainplate. However, there were rust streaks, and in a small area where the chainplate came into contact with the deck-mounted cover plate, a tiny spot of crevice corrosion was observable, so that means that corrosion is indeed happening. I imagine that if the #304 chainplate were completely isolated from all other metals, and just hanging in the lazarette by a Dacron line, it would also corrode to some degree, as would all metals eventually. When I replaced them in 2004, my bronze shroud chainplates with their bronze bolts, washers, and nuts were also badly corroded. Unless you have been the sole owner of your boat, there is no telling whether the chainplates on your boat have or have not been changed at some time in the past. My boat #330 was only eight boats in the Pearson production line ahead of Ebb's #338. So it is probable that the same order of bronze was cut for chainplates on both boats...but maybe not.

I think Ebb's concept of going with bronze is a good one if you have a reliable, reasonably-priced source of bronze chainplates, but stainless steel seems to work just fine.

My bronze shroud chainplates probably lasted for forty years, provided that a previous owner didn't replace them at some point. So who is to complain about bronze? Certainly not me? But frankly after seeing the degree to which the bronze bolts on my shroud chainplates were corroded as demonstrated in the photo below, I no longer believe that bronze is corrosion proof.

I am comfortable with #316 stainless steel shroud chainplates with #316 bolts, nuts and washer, and for now at last I am satisfied that a #304 stainless steel backstay chainplate has strength and will hold up over a reasonable period in the wet salty environment of the lazarette locker, with #316 fasteners.

I guess the moral here is whatever metal you decided to choose for chainplates and fasteners, it is wise to remove and inspect your chainplates periodically, and when you do remove a chainplate it would also be wise to replace your fasteners, even if they still do look spiffy. For the backstay chainplate, new 3/8 inch #316 stainless steel fasteners cost me $15.83. Removing all of your shroud chainplates at once is a daunting task. I have done it with the mast down of course, but you could pull and inspect the chainplates on a rotating schedule. For either bronze or stainless steel, that should probably be on a more regular schedule than once every forty years.

The photo below is of the bronze shroud chainplates and bolts that I removed in 2004. I replaced these with #316 polished stainless steel chainplates. Note the bright pencil-lead-thin core of remaining bronze in the bolts. The red stuff is copper toast. I did not saw through the chainplates to see what a cross section would look like, but I could not strip through the red layer. I still have the old bronze plates. So I could cut through one and take a photo of the cross section if anyone is interested.

[ebb]

Tha Galvanic Table refers to electric conductivity. It grades metal alloys from most cathodic (platinum) to most anodic (magnesium.) You will find the 300 series (passivated) cheek by jowl with silicon bronze. In an attempt to help designers putting metals together in certain 'environments' there is an anodic index which assigns voltage separations to alloys.

Offshore sailboats are considered in a 'harsh environment' ie seawater. 300 series passivated stainless steel is right next to silicon bronze in the anodic index, with a separation of no more than .15V. More than ,15V in a cloride soup will cause electrical corrosion to kick in. "Electrolysis" is maybe the wrong word but it's the right idea. [ Might assign the word observation to the Pardey quote rather than opinion.]

Check out >the corrosion doctors< on the net

[Scott Galloway]

Hmmm...The problem is that some galvanic series charts, or electrolytic tables (as some sources refer to them) show different relationships between the various metals and alloys, or at least it appears to me that they do.

For instance, one of my printed source books, a publication of the USCG Auxiliary, lists the most cathodic metals in that order as Mercury, Monel, nickel, and bronze (silicon) in that order, whereas stainlesss steel is way down on that list behind, copper, brass (red), bronze (aluminium), gun metal, brass (yellow), bronze (phosphor) and lead. Trailing stainless steel are iron, mild steel, aluminium , cadmium, galvanized iron and steel, zinc, and magnesium.

I don't have a copy of the 64th edition of Chapman, but I took a brief look at a similar table in that edition the other day, and I recall that it showed different relationships than the above, or at least it appeared to me that it did.

Then when I consulting Dan Spurr's “Upgrading the Cruising Sailboat” I was perplexed by the relationships shown in the "Galvanic Series of Metals in Seawater" table. If someone has that book, check out Appendix B and explain the table. It has two columns that appear to run from anodic or least noble to cathodic or most noble, in otherwords from magnesium on the anodic end to graphite (above platinium) on the cathodic end. Silicon bronze is listed once on that table, but (18-8 Stainless Steel, Type 304) and (18-8 3% Mo Stainless Steel Type 316) are listed twice, once as being more cathodic than silicon bronze and once as being more anodic.

At Ebb's suggestion, I consulted the "Corrosion Doctors website at: http://www.corrosion-doctors.org/Def...es_in_Seawater

In their "Galvanic Table", the Corrosion Dotors do indeed list passive 304 and passive 316 stainless steel right next to silicon bronze. Their source appears to be: the galvanic series of metals in sea water from Army Missile Command Report RS-TR-67-11, "Practical Galvanic Series."

So: That is a long list, but the relevant part of that list reads:

#70 Silicone Bronze 655
#71 Stainless steel 304 (passive)
#72 Stainless steel 301 (passive)
#73 Stainless steel 321 (passive)
#74 Stainless steel 201 (passive)
#75 Stainless steel 286 (passive)
#76 Stainless steel 316L (passive)

With the higher numbers like 304 stainless steel (passive) (#71) and Stainless steel 316L (passive) (#76) above being more cathodic than the lower numbers like silicon bronze (#70). So that would be a good thing if you were a piece of passive #316 stainless steel sitting next to a piece of silicon bronze, and not quite so good for the bronze.

But Stainless steel 304 (active) also appears on that list in position #40, some thirty places away from silicon bronze, (and thus much more anodic). Stainless steel 316 (active) appears in position #67, three places more anodic than silicon bronze. So that would be a bad thing if you were a piece of passive #316 stainless steel sitting next to a piece of silicon bronze, but a good thing for the bronze.

On that same web page and from a different source, the Corrosion Doctors provide a table titled "Galvanic Series in Seawater" about which they say,

"A galvanic series has been drawn up for metals and alloys in seawater, which shows their relative nobility. The series is based on corrosion potential measurements in seawater. The relative position of the materials can change in other environments. The further apart the materials are in this series, the higher the risk of galvanic corrosion."

The Corrosion Doctors are using "The Handbook of Corrosion Enginnering" by Pierre Roberge as a source.

This how that table reads:

Begin Quote from that source

"Most cathodic, noble, or resistant to corrosion

Platinum
Gold
Graphite
Titanium
Silver
æ Chlorimet 3
è Hastelloy C
æ 18-8 Mo stainless steel (passive)
ç 18-8 stainless steel (passive)
è Chromium steel >11 % Cr (passive)
æ Inconel (passive)
è Nickel (passive)
æ Silver solder
ç Monel
ç Bronzes
ç Copper
è Brasses
æ Chlorimet 2
è Hastelloy B
æ Inconel (active)
è Nickel (active)
Tin
Lead
Lead-tin solders
æ 18-8 Mo stainless steel (active)
è 18-8 stainless steel (active)
Ni-resist
Chromium steel >11 % Cr (active)
æ Cast iron
è Steel or iron
2024 aluminum
Cadmium
Commercially pure aluminium
Zinc
Magnesium and its alloys

Most anodic or easy to corrode"

End Quote from that source

In this list you also see stainless showing up twice with active stainless steel scoring worse than silicon bronze and passive stainless steel scoring better in the degree of nobility or cathodity.

So does the key to understanding the relationship between silicon bronze and stainless steel in these charts depend on the definition of the words "active" and "passive" related to 304 and 316 stainless steel?

I was feeling much too passive to look this definition up for myself, so I asked my chainplate, but it didn't answer. Does that mean that it is passive?

Anyone know the definition?

[ebb]

Passivation is about steel alloys. The ones that concern us are the 18-8 or 18% chromium 300 series stainless steels. Metalurgy is still close to the alchemical experiments of the middle ages. imco. Like turning lead into gold, stainless steel is an attempt to turn iron into bronze. Alloys and methods are always being tweeked - and I just know I will never GET IT! Either my beard or my white coat are too short.

There are various 'galvanic' lists from different sources generally agreeing BUT also showing important differing placements around iron alloys. I think we should gather our intelligence from sources specific to bluewater. {How come we take greenwater over the bow???}

Passivating stainless steel involves soaking fabricated pieces (usually) in acids at certain temperatures that remove minute carbon machining and milling debris from surfaces. Dilute and very toxic nitric acid is used - followed by equally nasty dilute alkaline baths - otherwise the acid would keep eating the steel. Followed by much plain water to neutralize both. Environmental laws hamper the process. Some manufacturers use citric acid (lemon juice?) to avoid the heavy duty chemicals. But this has not been entirely succesful. It means that there could be 316 passivated on your ship that will flash rust. Nice, huh?

Passivation is also selfcreated by stainless steels (and metals in general). It is a chromium oxide film so thin it can't be seen that forms in the presence of oxygen. A warehouse can destroy the passivation by dragging a sheet over another.
The industrial passivation process is evidently an attempt to perfectly clean the surface so that the selfcreated film is perfect. There's a lot of buffing and electro-polishing goin on around here. That's why you don't want to ever scratch or even clean stainless with scrubs and solvents and scotchbrite pads, etc. Even so, it can in the presence of oxygen repair itself, but has to have that airy opportunity.

What is peculiar to me is that soaking a metal in acid can radically change its position on the anodic index. That's alchemical to me. There is a problem with the help too - eg, I ran into a professor who was saying you coat dissimilar metals with siomething to keep them separate, you know like rubber - but WE all know full well that you need air for the passivation process to work right on our stainlesses. Who is the ultimate authority remains to be revealed. It's obvious steel alloys are a bit unstable - even hysterical. This country boy wants good ole uncomplicated silicon bronze ever time!

[Scott Galloway]

Thanks for the book report on passivity, Ebb. I am feeling much less passive now... but much more in favor of passivity than I ever was.

Well actually, as I understand the tables above, the passive 18-8 stainless steel in either #304 or #316 stainless would be more cathodic than silicon bronze. Their evil twins over on the active side of the stainless family would be more anodic. So it would appear that passive 18-8 steel ain't a bad choice. Now the question is how does one know whether the stainless one purchases for ones boat is passive or active?

The components of this discussion that are missing, or underrepresented here are relative strength of the materials, cost, and availability. So here are five more thoughts about the virtues of bronze and steel respectively from one who is unschooled in metallurgy:

1. The prime reason given to me by certain riggers and other marine professionals for using #304 stainless over #316 stainless is that #304 is stronger than #316.

2. The prime reason given to me by certain riggers and other marine professionals for using #316 stainless over #304 stainless is that #316 is less subject to corrosion. I chose #316 for the shroud chainplates and stayed with #304 for the backstay.

3. The prime reason given to me by certain riggers and other marine professionals for using either #304 stainless or #316 stainless over bronze is that stainless steel is stronger than bronze.

4. The prime reasons that I went with stainless steel on all of my chainplates are:

a. The professionals told me that steel was stronger than bronze, and the literature that I have read backs this up. (Yes I know, if it corrodes then it isn’t as strong as it used to be, but you need to do preventive maintenance on any system on your boat, whether mechanical or structural.)

b. The bronze shroud chainplates and their fasteners on my boat were badly corroded, so no metal is impervious to corrosion. (see earlier photo)

c. None of the above sources even carried bronze stock suitable for making my chainplates, and I would have had to pay for an entire sheet of bronze, although I needed only part of that sheet, since the above sources had virtually no market for bronze stock. The yards and other suppliers to whom I spoke simply aren’t making bronze chainplates anymore except by special order. So price and time to complete the order were certainly factors in my decision.

5. Despite all of the above, them bronze cannons on them ol' Spanish galleons shine up pretty nice after being in the bottom of the ocean for a few centuries. Stainless steel would not fare so well. So it all depends on your application, your intended use, and whether you intend to remove and inspect your hardware from time to time. All the stuff that goes below water on my boat is bronze, and all the stuff that stays above water is stainless. Of course my backstay chainplate can't make up its mind whether it is above or below water.

I really do think that either choice (stainless or bronze) for chainplate stock is acceptable. A sound maintenance schedule is prudent in either case.

Time to go sailing!

[mbd]

An anternative improvement to the Pearson backstay on the Ariel would be to have a chainplate on both sides of the knee. IE TWO chainplates of exactly the same pattern.

That is exactly what I have on #226. Prior owner's 'beter idea' that seems quite a bit stronger then the original.

Found a picture. Nice. A-226 Aft Chainplate

[Bill]

Found a picture. Nice. A-226 Aft Chainplate

Uh, don't see it . . .

[Scott Galloway]

Bill, I think what what we see in the photo posted by MDB is the tops of two separate chainplates protruding thorugh two separate slots in the taffrail separated from one another by a distance equal to the width of the knee, with a single coverplate with two matching slots set down on top of those two chainplates. The turnbuckle is attached to a pin run between the two chainplates.

If that is not what we are seeing, then whatever we are seeing looks like what we might see if we were looking at an installation similar to the double chainplate installation described above.

[Bill]

Bill, I think what what we see. . . is the tops of two separate chainplates protruding thorugh two separate slots in the taffrail separated from one another by a distance equal to the width of the knee, with a single coverplate with two matching slots set down on top of those two chainplates. The turnbuckle is attached to a pin run between the two chainplates.

Ok, guess I was looking for a couple of "seperate" chainplates . . . such as are on one of the Bay Area boats (which is temporarily "lost"). We may soon have the backstay chainplate setup suggested by Ebb. Photos will follow.

[c_amos]

Bill, I think what what we see in the photo posted by MDB is the tops of two separate chainplates protruding thorugh two separate slots in the taffrail separated from one another by a distance equal to the width of the knee, with a single coverplate with two matching slots set down on top of those two chainplates. The turnbuckle is attached to a pin run between the two chainplates.

If that is not what we are seeing, then whatever we are seeing looks like what we might see if we were looking at an installation similar to the double chainplate installation described above.

Sorry,

Not sure how I missed this discussion of the twin chainplates aboard 'Faith', I must have not been paying attention.

THis mod was done by Herb Tucker, prior to my purchasing the boat.

Hope these pictures make it a little more clear (did not have a good one).





There are twin chainplates, bolted to either side of the knee. That was not enough, there are two angled pieces of stainless that attach to the middle hole on either side and then are throughbolted to the transom with 4 5/16" bolts (in case the knee should fail). Pretty solid set up.

Even so, I had spoken to a rigger about going with a dual backstay....

Commnader Pete posted this;

Here is the Port Townsend denunciation of double backstays.

http://briontoss.com/education/archive/miscapr99.htm


.......

This link makes sense to me, and I think I will stick with what I have.