Stainless steel screws under epoxy !?

[DanishBagger]

I just bought Water Craft (the UK-one, that is) #69, because of an article-seris of building a strip-plank Roxane (the Nigel Irens design).
The author-cum-builder (Dick Phillips, who's building it for Charles and Gillian Taylor) writes this about strip-planking the hull:

At the plank ends, the strips were fixed permanently to the backbone with 316 stainless steel countersunk screws.

– He's not only mentioning he's using stainless, no he's making a point of it being 316.

Now, am I being paranoid or what? When I build mine, I used bronze screws for that part, simply because of fear of oxygen starvation even though I knew it would be very unlikely that I will ever get water in there - but then again, it's in the "corner", with loads of end-wood (can't remember the term in english), and a place where, if hit or abraded the epoxy/cloth will quickly be "holed": Enter crevice corrosion. Hell, epoxy isn't even completely water proof.

Am I too paranoid?

How about you guys? Which way would you choose to do it? Stainless or bronze for this part?

[masalai]

I have seen several boats being built and a couple removed the screws but in strip plank - how? look up some of the building boat books and or email the designer...

[Meanz Beanz]

316 stainless is suitable for below waterline use so what would the problem be?

[DanishBagger]

Originally Posted by masalai

I have seen several boats being built and a couple removed the screws but in strip plank - how? look up some of the building boat books and or email the designer...

Well, I have looked at several books, and I build my own small boat. That's my point here – why would someone choose to use 316 below the waterline?

[Edit:How, you ask? It's for use at the ends where the strip-ends meet the stern and bow respectively. Using permanent screws there are the usual way to go around this task. Edit 2: I am not asking "how" to do this. Nor are we talking about the screws that one removes when the epoxy in the strips is hardened. I (and he) am talking about the permanent screws]

Originally Posted by Meanz Beanz

316 stainless is suitable for below waterline use so what would the problem be?

No, it's not. Only on boats living on trailers. Otherwise you don't want stainless below the waterline where it will starve for oxygen (= crevice corrosion).

[masalai]

Danish, Maybe I did not express myself well - my How was how does one remove those screws at the ends etc..... since it is not structural what about monel?

[DanishBagger]

Originally Posted by masalai

Danish, Maybe I did not express myself well - my How was how does one remove those screws at the ends etc..... since it is not structural what about monel?

Well, you don't remove the screws at the ends. They're supposed to sit there under the epoxy and the cloth. The "intermediate" screws, used in between the ends are simply unscrewed and filled with epoxy before the "primer-layer" of epoxy is put on, just before the cloth is the cloth is laid on.

The end-screws are not structural in the end-product, but until you have cloth and epoxy wrapped around the "corners", they are. And when you have the lot wrapped around it, you don't want to puncture the layer to get the screws out. It's usually not worth the ekstra work.

I guess monel would be an even better choice than bronze, all things considering.

I still can't wrap my head around why a professional boatbuilder would use 316 SS somewhere that will mean it'd be starved for oxygen and thus be prone to crevice corrosion.

Monel would be perfect in this application, methinks.

[masalai]

The only way I can think of removing the "end" screws is to glass the inside first, that leaves the option of removing the end screws and easier to fill and seal from the outside... - just a dumbass throwing ideas around?

[DanishBagger]

Originally Posted by masalai

The only way I can think of removing the "end" screws is to glass the inside first, that leaves the option of removing the end screws and easier to fill and seal from the outside... - just a dumbass throwing ideas around?

Very true. But then it would suck building it upside-down, methinks. And you won't be able to glass it on the inside, without removing the frames, and thus everything that holds the thing in shape. You could certainly do it, using silica-thingies in the ends, but you would still have to do it in a somewhat small workarea, climbing underneath the hull. And it won't be as pretty as if you wait until the hull can be turned right-way-up.

If you're interested, Dick Phillips has the first installment on his website:

http://www.dickphillips.co.uk/docs/32-36RoxaneW68.pdf

I didn't do it quite like that - he's had his frames cnc-cut and whatnot. If I ever build another boat, this will be the way I'll do it, instead of using off-sets from a book, battens and whatnot, lol.

[Meanz Beanz]

Originally Posted by DanishBagger

No, it's not. Only on boats living on trailers. Otherwise you don't want stainless below the waterline where it will starve for oxygen (= crevice corrosion).

316 is fine below the water in my experience, most prop shafts for a start I think are 316. I have used 316 on rudder fittings with no drama at all, 304 is to be avoided below the waterline but 316 is fine.

How deep do you have to go before the water is actually oxygen starved anyway?

Some one better tell these guys....

http://www.bosunsupplies.com/product...?product=S3815

[DanishBagger]

Originally Posted by Meanz Beanz

316 is fine below the water in my experience, most prop shafts for a start I think are 316. I have used 316 on rudder fittings with no drama at all, 304 is to be avoided below the waterline but 316 is fine.

I have no idea about engines - I doesn't matter, though, if you trailer your boat. You can use all the stainless you want. And you're wrong about 304 being worse than 316 when oxygen starved. Oxygen starvation is a real thing, and 316 stainless is just as bad as 304, if not worse. Simply because it has a higher content of chromium.

How deep do you have to go before the water is actually oxygen starved anyway?

It doesn't matter. We're talking about water getting "in between", becoming stagnant, and thus essentially "rotting" the stainless from beneath, because there's no oxygen there.

Some one better tell these guys....

http://www.bosunsupplies.com/product...?product=S3815

Ah, yes, by the same token, there's nothing wrong with using this:

http://www.binksonline.com.au/store/prod2783.htm

(it's a nylon through-hull, if you're wondering).

Btw, you may want to read what Bosun Supplies quote Brion Toss for themselves:

http://www.bosunsupplies.com/Corrosion.cfm

And this (begin half way down):

http://www.coletech.net/Resources.htm

Most importantly, though, this:

http://www2.sandvik.com/sandvik/0140...256B49004451B2

It says much more, but if nothing else, extrapolate from this:

Stainless steel should normally not be painted, because a crevice will result if the paint is damaged.

So, unless you're sure there's no way salt water will enter behind the rudder fitting (you're not), stainless steel is far from being a good product for fittings below the waterline in saltwater.

[catsketcher]

Hello Danish

I think you need oxygen or as Meanz says some form of cathodic protection in the water or the oxide layer goes and pitting occurs. Is this a problem under epoxy? I have not had any problems with stainless totally immersed in epoxy as I guess there can be no electron transfer through the totally encapsulating insulator.

I don't understand why you have to leave the screws in with strip plank though. Just put some dabs on the inside of the planks to the mould to hold them. If you are worried do the test - put a SS and a Monel screw in a bit of composite and smother in epoxy. Leave in water of a jetty or such for a year or two. Then pull apart. My friend did this a bit and it gave good data on antifouls.

cheers

Phil Thompson

[Meanz Beanz]

Bob's right. There is a great book called "Metal Corrosion in Boats" by Nigel Warren that goes into great detail about dezincification of brass, crevice corrosion of stainless steel, electrolysis, etc.
The mechanism that keeps stainless steel stainless, is a protective oxide layer that forms on its surface. Stainless needs to be exposed to Oxygen it in order for that layer to form. A fastener in a wooden boat is continuously having its oxide layer rubbed off by the action of the wood around it. When in air, the necessary Oxygen is taken from the air; when in water, it is taken from the MOVING water. However, when water enters the sealed cavity of a bunged fastener, the stagnant water's reserve of Oxygen is quickly depleted. Since no water movement can occur, thereby replenishing the Oxygen, then no protective oxide layer can reform. This is the beginning of crevice corrosion.

There you go... for fastenings below the waterline in saturated timber that see's movement its not so hot. Keep in mind that it needs to be in a situation that the timber will move enough to wear the surface of the fastening. So basically I think we are talking not suitable for below waterline fastenings in traditionally constructed timber boats.

Well that's as far as my research has taken me at this point and it explains to me why I have never had trouble where I have used 316 below the water...

Anyone for anymore...

Nylon through hulls are shocking, talk to any surveyor...

Use 304 below deck level and you are going to get trouble.

[DanishBagger]

Originally Posted by catsketcher

Hello Danish

I think you need oxygen or as Meanz says some form of cathodic protection in the water or the oxide layer goes and pitting occurs. Is this a problem under epoxy? I have not had any problems with stainless totally immersed in epoxy as I guess there can be no electron transfer through the totally encapsulating insulator.

It's not about electrons at all. It's about lack of oxygen. However, I'm not worried about having stainless below the waterline, if everything is perfect. No, I'm worried that because these are placed in the "corners", there's a bigger chance of water intrusion (Take a small block of wood and a piece of sandpaper on a block - the corners will be rounded first).

I don't understand why you have to leave the screws in with strip plank though. Just put some dabs on the inside of the planks to the mould to hold them. If you are worried do the test - put a SS and a Monel screw in a bit of composite and smother in epoxy. Leave in water of a jetty or such for a year or two. Then pull apart. My friend did this a bit and it gave good data on antifouls.
cheers

Phil Thompson

Antifouls? You don't want to cover stainless in antifoul. Nor do I want to do tests that takes years, when crevice corrossion is all well understood. Besides, I got around the worry, and the suggested work-arounds by using bronze.

Originally Posted by Meanz Beanz

There you go... for fastenings below the waterline in saturated timber that see's movement its not so hot.

Hate to say so, but your rudder fittings see movement as well.

Keep in mind that it needs to be in a situation that the timber will move enough to wear the surface of the fastening.

No, all it needs is a scratch for water to enter, but not being able to come out again.

So basically I think we are talking not suitable for below waterline fastenings in traditionally constructed timber boats.

No, we most certainly are not. You didn't read my posts properly, and you certianly didn't read the links I provided for your benefit. If so, you would never make such a statement.

Well that's as far as my research has taken me at this point and it explains to me why I have never had trouble where I have used 316 below the water...

Yes, quite a bit of "research" there, Beanz

Anyone for anymore...

I'm always up for more ;-)

[TeddyDiver]

I've been thinking the same issue a bit, but I left them (316) in place. Anyways I made oversized holes and filled with epoxy so they should be totally capsulated and thou protected.
If one wants to be more on the safe side with 316 maybe preoxidation could do the trick? I mean thats the real problem here that SS's aren't preoxidized and without oxygen around won't get the protective layer..

[DanishBagger]

Originally Posted by Meanz Beanz

Use 304 below deck level and you are going to get trouble.

Under some circumstances, but please do read the links I have provided you with, instead of just spewing that nonsense. Crevice corrosion is a well-understood factor, and when it comes to such matters, the high contents of chromium works against it in places with crevices where stagnant sea water can sit.

Originally Posted by TeddyDiver

I've been thinking the same issue a bit, but I left them (316) in place. Anyways I made oversized holes and filled with epoxy so they should be totally capsulated and thou protected.

Yup, sounds like the best solution, if you don't want to remove them.

If one wants to be more on the safe side with 316 maybe preoxidation could do the trick? I mean thats the real problem here that SS's aren't preoxidized and without oxygen around won't get the protective layer..

Nope, preoxidation doesn't work. Apparently SS needs a "supply" of oxygen, so to speak.

Oh, you added a bit, Beanz:

Nylon through hulls are shocking, talk to any surveyor...

I know, that's my point. Just because you can find somewhere that sells an item, doesn't mean you have proven the point that there's no problems with stainless 316 under the waterline. Hence me using the same argument to show why it doesn't work, and you can't conclude like you do, by way of linking to a SS through-hull, just like I can't prove that nylon is a great material for through-hulls by way of linking to such an item.