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Old 21-11-2015, 03:54   #1
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Steel boat with stainless steel chainplates

I went back the other day and looked at the specifications for our boat, and came upon something that surprised me. We own an Amazon 49, a steel boat built in Surrey (outside of Vancouver) Canada.

The specifications state "Chainplates: stainless steel chainplates welded through deck with undersupports running to the frames."

I went on deck with a magnet, found that the chainplates are nonmagnetic, and, I presume, stainless. They are painted the same color as the deck, and I never knew they were a different material.

My first question is, is this normally the way it is done? I always thought the chainplates were the same steel as the rest of the boat.

Second, is there anything I should be aware of to prevent future problems? Since they are welded in place, removal for inspection is impossible. I never worried about crevice corrosion or anything else on the chainplates since I thought they were mild steel. I just made sure there was no rust anywhere around them. Now I'm wondering if I need to be looking for anything else.

Any thoughts appreciated. Cheers!

Steve
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Old 21-11-2015, 04:12   #2
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Re: Steel boat with stainless steel chainplates

Interesting. My boat is 30 years old, it had mild steel chain plates with pressed in collars where the pins go in of stainless steel. Last year I replaced the main mast chain plates with stainless, welded in place. A few weeks ago I did the same with the mizzen.

During the week I had my boat sand blasted. Surprisingly I found a number of items which I assumed were mild steel, are also stainless. The mounts for a rudder bar that sort of protects the rudder is stainless, each one welded onto the hull. The two cockpit drains are stainless pipes of two inches diameter again welded into place. The anchor roller sides are stainless, which again I assumed were mild steel as they were rusty. And there are four rope thingys (forgotten what you call them, which deck lines go through) of 1/2 inch round that are stainless. Again, I was certain they were mild steel.
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Old 21-11-2015, 04:33   #3
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Re: Steel boat with stainless steel chainplates

I am no expert, but using stainless for the drain pipes and some applications seems better than mild steel due to the difficulty of maintaining with paint (e.g. due to inaccessible inner surfaces) that would likely corrode if mild steel, or if subject to abrasion which would wear off protective paint.
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Old 21-11-2015, 04:35   #4
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Re: Steel boat with stainless steel chainplates

I guess the idea is to use stainless in areas where it's difficult to keep paint on to avoid rust...

My boat has stainless chainplates welded to the mild steel hull. In theory they shouldn't suffer crevice corrosion (as they are well above the water line).

That said a number of wooden/fibreglass boats have had stainless chainplates fail over the years.

A bigger concern on my boat is the bob-stay fitting, which is again stainless welded to the mild steel hull but this time right on the waterline...

The previous owner fitted a zinc anode to it, and it is getting eaten away over time...
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Old 21-11-2015, 05:06   #5
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Re: Steel boat with stainless steel chainplates

I have stainless chainplates / loops welded to a mild steel deck - it kinda makes sense, as making them out of mild would mean there'd be constant rust where the rigging screws attach.

As long as the area is painted correctly, I can't see how corrosion would make it's present known.....

n
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Old 21-11-2015, 06:26   #6
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Re: Steel boat with stainless steel chainplates

Our small boat, 33', has mild steel chain plates. Not sure exactly how they are attached as they are covered with foam and behind furniture.

The big boat, 44', according to the first owner, had mild steel chain plates but he had them cut out some years ago and replaced with SS. These plates appear near the gunals, the raised side plate which forms the toe rail. I had a rigging inspection done and they made me pull down the inside panels exposing the plates. They are roughly 20" long and are welded directly to the 3/16" hull plate. So there is huge cross sectional weld area.

My understanding is you can weld SS to mild steel pretty much without penalty. I'm sure I will be corrected if wrong.

I have found corrosion problems between mild steel and SS at BOLTED connections wo when the two metals are in close proximity. I've not noted it when they are welded.

On the big boat they finished the tops of the side plates/toe rails with SS angle before mounting the teak. Also the cleats are welded to SS plates, then welded to the deck. And the scuppers and line holes are edged with SS. I've noted no adverse situations in any of this.

Our mild steel bow sprit rusted out and I replaced that with SS welded to the deck.
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Old 21-11-2015, 07:10   #7
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Re: Steel boat with stainless steel chainplates

The only place I really have any rust weeping is where the SS Stanchions as welded to the deck.

Its driving me crazy

LD
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Old 21-11-2015, 07:24   #8
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Re: Steel boat with stainless steel chainplates

Quote:
Originally Posted by steve77 View Post
I went back the other day and looked at the specifications for our boat, and came upon something that surprised me. We own an Amazon 49, a steel boat built in Surrey (outside of Vancouver) Canada.

The specifications state "Chainplates: stainless steel chainplates welded through deck with undersupports running to the frames."

I went on deck with a magnet, found that the chainplates are nonmagnetic, and, I presume, stainless. They are painted the same color as the deck, and I never knew they were a different material.

My first question is, is this normally the way it is done? I always thought the chainplates were the same steel as the rest of the boat.

Second, is there anything I should be aware of to prevent future problems? Since they are welded in place, removal for inspection is impossible. I never worried about crevice corrosion or anything else on the chainplates since I thought they were mild steel. I just made sure there was no rust anywhere around them. Now I'm wondering if I need to be looking for anything else.

Any thoughts appreciated. Cheers!

Steve
That is good practice, assuming all the scantlings are sized correctly for the loads. I built my own 48' steel cutter 20+ years ago, and I used heavy SS chainplates let through the mild steel decks, and welded to the mild steel frames and stringers on the hull.

When welding SS to mild steel, a SS rod is used. Thus the new blended weld metal is 75% SS, due to the addition of the SS rod metal. All of it was painted with Devoe products, and it's perfect today like it was perfect 20+ years ago. When you weld small bits of SS to large masses of mild steel, the two metals are quite happy together. No corrosion, electrolysis, etc. I also used SS plates right in the bow where the anchor and chain would strike. Also used SS rings/eyes etc where shackles would be attached.

No problems where the SS transitions to mild steel plate. None at all. I paint right over all of it, SS and mild steel. No electrolysis under the hull, around the shaft, etc. The only area I'm keeping an eye on is the rudder, because the shaft is a very big piece of SS welded inside of mild steel. New small zincs at every haulout handle this so far, 20+ years. Nowhere else on the hull do I have any concern. I would do it all the same way today.





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Old 21-11-2015, 07:27   #9
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Re: Steel boat with stainless steel chainplates

Quote:
Originally Posted by lamadriver View Post
The only place I really have any rust weeping is where the SS Stanchions as welded to the deck.

Its driving me crazy

LD
Stanchion bases are problem areas for other reasons besides dissimilar metals. They get torqued around a lot in normal use, with loads falling on lifelines, being bumped by docks and especially travel lifts etc. Once the paint is cracked, it's a matter of time before a little weeping rust appears. I just chip it out, wire brush/dremel tool it, and apply new Devoe Bar-Rust 235, and the problem is solved for a few years, until they are torqued around again by bumps.

Where I have SS welded to mild steel, but the parts are not subject to being jolted around, (heavy chain plates, block/shackle rings etc), there has never been any rust appearing at all. Never.
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Old 21-11-2015, 07:35   #10
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Re: Steel boat with stainless steel chainplates

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Originally Posted by Steady Hand View Post
I am no expert, but using stainless for the drain pipes and some applications seems better than mild steel due to the difficulty of maintaining with paint (e.g. due to inaccessible inner surfaces) that would likely corrode if mild steel, or if subject to abrasion which would wear off protective paint.
My cockpit scupper is a SS pipe welded at the top to the back corner of the cockpit sole and at the bottom to the hull. I did put in thicker plate in a "donut" where the pipe exits the hull. No through-hulls on the cockpit scuppre pipe at all. You can drop a coin or pebble down that pipe and it's gone. I keep a close watch on it, after 20+ years, no problems and I would do it the same way today.

My through hulls are threaded SS pipe welded to the mild steel hull, with Marelon valves screwed on. I purchased SS "pipe nipples" (threaded each end) and cut them in half. I welded the non-threaded ends to the hull. This has been very satisfactory. I give the hull a bit more attention around these welded SS fittings at haulouts, but the metal appears to be fine. These are the only places where the paint is a little unhappy, but it's not been a big maintenance issue. I'd still do them the same way.
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Old 21-11-2015, 07:36   #11
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Re: Steel boat with stainless steel chainplates

Quote:
Originally Posted by hpeer View Post
Our small boat, 33', has mild steel chain plates. Not sure exactly how they are attached as they are covered with foam and behind furniture.

The big boat, 44', according to the first owner, had mild steel chain plates but he had them cut out some years ago and replaced with SS. These plates appear near the gunals, the raised side plate which forms the toe rail. I had a rigging inspection done and they made me pull down the inside panels exposing the plates. They are roughly 20" long and are welded directly to the 3/16" hull plate. So there is huge cross sectional weld area.

My understanding is you can weld SS to mild steel pretty much without penalty. I'm sure I will be corrected if wrong.

I have found corrosion problems between mild steel and SS at BOLTED connections wo when the two metals are in close proximity. I've not noted it when they are welded.

On the big boat they finished the tops of the side plates/toe rails with SS angle before mounting the teak. Also the cleats are welded to SS plates, then welded to the deck. And the scuppers and line holes are edged with SS. I've noted no adverse situations in any of this.

Our mild steel bow sprit rusted out and I replaced that with SS welded to the deck.
Your experience pretty much mirrors mine.
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Old 21-11-2015, 07:45   #12
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Re: Steel boat with stainless steel chainplates

ok stainless steel is iron based. mild steel is iron based. where are the dissimilar metals. sure the alloys. but who has seen the dissipation of lalloys from stainless when welded to mild steel as time goes on and how long tdid that take to occur.
some schooners i know use galvo rigging. no rust under those..mebbe is the maintenance sche-- err, regime practiced.
i lived on a dutch built steelie with ss chainplates and affixments. what is the worry. same basic metals. if ye has a problem, go galvo. or bronze. or rope and wood.
seems folks donot worry about affixing a stainless steel anchor to galvo chain, why worry about ss chainplates on a treated steel hull?? even untreated, how long do you expect to go on without checking the joints for corrosion and leakage and security???
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Old 21-11-2015, 09:09   #13
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Re: Steel boat with stainless steel chainplates

The best solution is to have silicone bronze or monel chainplates bolted to the hull. The spot where they bolt to the hull can be doubled up so you have extra hull thickness where the tension will be at its max. Strong, long life, and easy to inspect. If your worried about galvanic corrosion, put a silicone strap between the chainplate and the hull. Correctly sizes, that will last 40-50 years at least.

for modern rigs, mild steel flexes too much while stainless will be subject to stress corrosion.
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Old 21-11-2015, 09:47   #14
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Re: Steel boat with stainless steel chainplates

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Originally Posted by Rustic Charm View Post
And there are four rope thingys (forgotten what you call them, which deck lines go through) of 1/2 inch round that are stainless. Again, I was certain they were mild steel.
They are called fairleads.

You mentioned SS cockpit drainpipes....are there valves on them, or just straight through? My Pearson 30 had no valves on the cockpit drains, and it always worried me, although they were made of fiberglass as thick as the hull.
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Old 21-11-2015, 09:53   #15
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Re: Steel boat with stainless steel chainplates

If your SS stancions are rusting at the bottom, I would worry that they have water inside the stancions. You might check to see if that is even possible on your type of stancions. In that closed environment, the SS would corrode.
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