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Old 06-03-2008, 21:17   #1
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Welding Stainless Through-Hulls to a Steel Hull

The PO used bronze through hulls on my steel yacht.

One did leak slightly until I tightened it up.

While they seem to be OK I am planning to replace them with cast 316 stainless steel through hulls.

Is there any reason why I should not weld these to the hull plating using a single bead round the outside?
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Old 06-03-2008, 21:20   #2
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If you do?

You better bond them real good and keep an eye on the zinks.

Personally, I'd stick with the bronze!
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Old 06-03-2008, 21:44   #3
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I would weld on steel nipples that are threaded at the end. You then thread on the bronze thru-hull valves. This is how it is done on my aluminum boat but with aluminum nipples of course. Large steel vessels weld on metal pipe with a flange at the end and it is then held together with bolts and a gasket in between, going to the next connection.

A steel to steel weld done correctly is not going to be questionable like a weld done with dissimilar metals. I don't mean steel pipe used for plumbing a house. I mean steel meant for a marine environment. A naval architect or a good boatyard will be able to tell you what is best type of steel for your application. Welding dissimilar metals below the waterline in a marine environment I think is asking for trouble.

Go with something that is known to work. What I described has worked on steel boats and ships for over a hundred years.
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Old 06-03-2008, 21:52   #4
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Surely stainless steel would be considered less 'dissimilar' than bronze? Can't say with certainty, but I would suppose welding in stainless thru-hulls is doable. You may want to talk to commercial shipyards, to see if it's viable. Would recommend TIG welding to reduce the HAZ to minimum. You might want to consider flame-spraying the entire welded area with zinc or other appropriate coating for corrosion-protection.
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Old 06-03-2008, 23:21   #5
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Is there any reason why I should not weld these to the hull plating using a single bead round the outside?
No problem at all.

We have a steel yacht professionally built (under my careful eye) and we have two ss tubes welded into the bilge underwater for the deck drains. If the welded interface between the mild steel and the ss is exposed to the sea then just continue the epoxy primer/undercoat up past the welded area onto the ss to protect against any susceptibility to corrosion in the ss from carbon migration into it from the mild steel during the welding process.

However, the best solution, in my view, is to use plastic (glass fibre reinforced) thru hulls with a plastic (again glass reinforced) industrial ball valve on the inside - there are no galvanic issues whatsoever then. In the particular case I mention for our own boat, we only used ss pipes welded in for the deck drains due to their fitting into the very tight turn of our bilge which made it difficult to fit a thru' hull fitting in that particular position.

John
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Old 07-03-2008, 05:00   #6
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I agree with John. I have all Forespar Marelon plumbing including all the thru hulls. They are completely stable and operate easy.
http://www.forespar.com/MarelonPlumbing/marelonAbout.shtml

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Old 07-03-2008, 09:09   #7
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Marlon

I second John advise. Marlon would not create galvanic issues. Nigel Calder has a excellent diagram on page 569 of his book that shows how to create separation between two dissimilar metals in "The Boatowners Mechanical and Electrical Manual" also in the same chapter some test of Marlon durability.

Jack
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Old 07-03-2008, 10:06   #8
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Welding Stainless to steel in an underwater application???

Oh NO! Don't do it! Just becasue they both have "steel" in their names doesn't mean they are galvanically compatable! Stainless is just as far from "steel" galvanically as bronze is!

There really are very good reasons that bronze is the metal of choice for underwater applications like this. If properly installed on a steel boat they will be fine. You also got some other good suggestions above that are workable. I'd go with welding on a steel nipple and using a Marelon valve if it was me.

Bill
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Old 07-03-2008, 14:20   #9
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Surely stainless steel would be considered less 'dissimilar' than bronze? Can't say with certainty, but I would suppose welding in stainless thru-hulls is doable. You may want to talk to commercial shipyards, to see if it's viable. Would recommend TIG welding to reduce the HAZ to minimum. You might want to consider flame-spraying the entire welded area with zinc or other appropriate coating for corrosion-protection.
Quite contrar....! see link ........ http://www.galvanizeit.org/showContent,173,215.cfm

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Old 07-03-2008, 14:30   #10
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This question is very important. I've been told not to used stainless below the waterline and definitely not welded to steel. I've also been told and read that bronze and aluminum are not compatible for mast and boom fittings. I'd really like to know if If this information was passed on incorrectly.
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Old 07-03-2008, 14:33   #11
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Steel is primarily iron. Stainless steel is primarily nickel, chromium and manganese. There are completely different elements here. The same goes for bronze which is primarily copper with some tin.

I don't understand what is wrong with going with what has traditionally worked for steel hulls. At least steel pipe nipples have a long history of not sinking steel boats. Wanting to reinvent the wheel and subsequently putting your boat at risk of sinking makes no sense.

I would not put plastic thru-hulls on a steel boat when steel is so much stronger..and cheaper as well. I have never seen plastic thruhulls or disimilar metals welded onto the hull of a ship or commercial workboat...ever.
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Old 07-03-2008, 14:51   #12
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I would not put plastic thru-hulls on a steel boat when steel is so much stronger..and cheaper as well. I have never seen plastic thruhulls or disimilar metals welded onto the hull of a ship or commercial workboat...ever.
With this in mind, welding in prefabed steel pipe threads (like in tanks) and then screwing in Marlon valves sounds like an excellent idea to me. That's something as a metal-smith that would work for me, if it were mine.
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Old 07-03-2008, 15:12   #13
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What I meant is welded on pipe (welded with a bead on both the inside and the outside of the hull) with male threads on the end of the pipe that thread in to a female valve. All it takes is a standard pipe thread cutter that you can rent or buy and cut the threads yourself...even after the pipe has been welded in if you have the clearance.

With a plastic valve, there is no chance of corrosion from dissimilar metals. The downside to plastic (Marelon) is that there is the possibility of it seizing up if it is not worked occasionally plus plastic is not as strong as metal. I have heard of plastic handles busting off as well.
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Old 07-03-2008, 15:44   #14
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Current experience has been favourable...

My current experience with stainless welded below the water line has been the studs holding on the anodes.

They seem to hold up very well.

Even if a crack appeared in the welding of a stainless through hull I would not expect it to be catastrophic.

It seems that the behavior of stainless is quite different when welded as opposed to bolted.
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Old 07-03-2008, 16:06   #15
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Stainless nipples welded on is very common...ask the metal boat people...Welcome to the Metal Boat Society
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