Welding Stainless Through-Hulls to a Steel Hull

[David M]

Originally Posted by cooper

Stainless nipples welded on is very common...ask the metal boat people...Welcome to the Metal Boat Society

Thanks for that link cooper. I qualify!

[Maine Sail]

Just how does one weld CAST stainless effectively??? Have you tried it before? I have and cast stainless does NOT like to weld well..


I vote for how steel boats are usually built with steel nipples welded to the inside of the hull..

[Wotname]

Originally Posted by David M

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.

I have thought (but not known for sure) the reason work boats have metal fitting is survey requirements. "Plastic" fittings melt in a fire while the corrosion issues of a metal fitting can be taken care of by ongoing maintenance.

Any commercial surveyors out there who might know more?

[MidLandOne]

Originally Posted by SkiprJohn

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.

The often quoted on forums advice that you cannot use ss below the waterline is false. Two common examples where it is almost ALWAYS used under the waterline and which prove it is ok is for propellor shafts and rudder stocks. SS in sea water (assuming a correct grade is used) is only a problem in anerobic conditions - these can occur under submerged shaft bearings if the shaft is not run frequently allowing the bearings to be flushed, around ss bolts through timber where is damp or wet but isolated from oxygenation from fresh seawater, in stagnant conditions as may occur in unused filters, etc.

Another wide use of ss underwater is for seacocks and again one often sees the erroneous advice on pleasure boat forums that ss cannot be used for seacocks - that despite the fact that it is the material of choice for commercial vessels of aluminium (and other) construction. Generally not seen very often on small pleasure vessels due to cost.

A general comment regarding the use of mild steel for stand pipes and seawater services -

The major cause of flooding (and consequent loss) in small commercial vessels is due to failure of mild steel seawater pipework. In the case of fishing vessels this paper may be of interest - www.mcga.gov.uk/c4mca/research_report_493.pdf which was initiated in the UK because of the high numbers of such losses (over 50% of all losses were due to flooding). It was found that nearly all of these were due to corrosion or other failure of mild steel pipework and stand pipes used for seawater.

Of particular interest for pleasure vessels are the comments regarding corrosion of mild steel pipework and the undesirability of using galvanised pipework . Yet one often sees on pleasure boat forums offering steel boat building advice that one should take mild steel standpipes (often also said to be in galvanised - I wonder why they think the zinc will last any better than their anodes ) welded into the hull plating up to above the waterline at which point the seacock is fitted - in my view, that is very foolish advice as seacocks should be fitted either flanged onto the steel bottom plating (which raises material compatability issues if not plastic) or onto a short standpipe of ss or a reinforced plastic through hull fitting.

On the new build aluminium commercial vessels I have been involved with all seawater pipework exposed to the sea is fabricated from stainless steel at great expense but that has been for very high quality vessels and ss is too expensive to use in most work and fishing boats which resort to mild steel.

[bob kingsland]

I only have 2 underwater thru hulls on my boat... a 2" in, to the main sea chest, and 1 1/2" head exhaust... that's it below the waterline. Grey water goes to a sump tank and exits above water. I welded shedule 80 steel pipe sections to steel flanges, welded inside and out, and then welded the other end thru the hull inside and out. Then isolating gasket, and sleeves for the bolts, then a 316 ss flange to that, with a welded male threaded nip. Then 316 ss ball valves go on the end of the nip, and then the rest of it. Metal doesn't melt, and one advantage of doing it this way is that you can weld at an angle to the hull so the flange is level if that's what you want. I inserted heavier plate where the thru hulls are, and gusseted them under the flange. Very strong and stiff, and works well. I would not weld a ss valve directly to the hull. Best, Bob S/V Restless

[MidLandOne]

Originally Posted by Wotname

I have thought (but not known for sure) the reason work boats have metal fitting is survey requirements. "Plastic" fittings melt in a fire while the corrosion issues of a metal fitting can be taken care of by ongoing maintenance.

Any commercial surveyors out there who might know more?

Then what you mention poses a problem for commercial vessels that are made ENTIRELY of plastic .

Many reinforced plastic fittings such as thru hulls and seacocks meet ABYC, UL and the ISO requirements for non coded vessels. I understand that many also have classification society approvals as well, but have never had the need to check that out.

Until a while back I managed around 35 surveyors who did statutory surveys of commercial vessels (ie periodic and other surveys required by regulation) and had no problem with plastic fittings where appropriate eg in pleasure boat derived vessels such as used for sailboat charter, fishing charters, etc where all is protected by the cabin sole, etc. In the end such vessels typically have plastic thru hull fittings for speed log and sounder so what the hell with a couple more . Probably not a good idea in the engine room of a big fishing boat with fishermen stomping around in the exposed engineroom bilges though .

[cooper]

...if you want to see how to build a boat properly, have a look at Bobs web site. The boat is a work of art.

[Wotname]

Originally Posted by MidLandOne

Then what you mention poses a problem for commercial vessels that are made ENTIRELY of plastic .....
Until a while back I managed around 35 surveyors who did statutory surveys of commercial vessels (ie periodic and other surveys required by regulation) and had no problem with plastic fittings where appropriate eg in pleasure boat derived vessels such as used for sailboat charter, fishing charters, etc where all is protected by the cabin sole, etc. In the end such vessels typically have plastic thru hull fittings for speed log and sounder so what the hell with a couple more . Probably not a good idea in the engine room of a big fishing boat with fishermen stomping around in the exposed engineroom bilges though .

Never confuse regulations with rationale
This makes sense - thanks for enlightening me.

[delmarrey]

Originally Posted by Acoustic

Just how does one weld CAST stainless effectively??? Have you tried it before? I have and cast stainless does NOT like to weld well..


I vote for how steel boats are usually built with steel nipples welded to the inside of the hull..

Cast SS welds fine. It's pipe fittings one has to be careful of. They are usually cast iron which takes special (expensive) rod/wire and preparation before and after the weld, which I would not use on a boat!!!

If castings have impurities in them it could present a problem. Also steel has to be clear of any rust by grinding/blasting before a quality weld can be applied.

I don't think anybody said "You can't use SS under water" What was said was "don't weld SS to Steel and then put the assembly underwater!"

Certainly if you isolate the seacock electrically from the dissimilar metal hull, SS makes a great valve materal, if a bit pricey!

[MidLandOne]

Originally Posted by GreatKetch

I don't think anybody said "You can't use SS under water" What was said was "don't weld SS to Steel and then put the assembly underwater!"

If referring to me - then a poster in this thread did say "I've been told not to use stainless below the waterline and definitely not welded to steel" and asked for that to be clarified. I was just clarifying (for his eyes only perhaps ) that one can use ss underwater.

[Lodesman]

Originally Posted by David M

Steel is primarily iron. Stainless steel is primarily nickel, chromium and manganese. There are completely different elements here.

AFAIK stainless steel is primarily iron - generally 50-70% iron, with the rest being nickel, chromium, manganese, copper, molybdenum and/or others.

I have no idea of what constitutes the "best" material/method for through-hulls, but logic and a quick google search reveals that stainless can be welded to steel in underwater applications, as long as proper cathodic protection is used. http://www.nsc.co.jp/gikai/en/contenthtml/n87/n8711.pdf

[David M]

Originally Posted by Lodesman

AFAIK stainless steel is primarily iron - generally 50-70% iron, with the rest being nickel, chromium, manganese, copper, molybdenum and/or others.

I have no idea of what constitutes the "best" material/method for through-hulls, but logic and a quick google search reveals that stainless can be welded to steel in underwater applications, as long as proper cathodic protection is used. http://www.nsc.co.jp/gikai/en/contenthtml/n87/n8711.pdf

Sure, there are lots of different grades of stainless steel.

Stainless steel - Wikipedia, the free encyclopedia

[bob kingsland]

I said above that I would not weld a stainless valve directly to the hull, but I should clarify that I wouldn't hesitate to weld a 316 ss nipple to the hull, and then tread a 316 ss ball valve to that. I chose to do it with flanges, using sched 80 steel pipe through the hull, but they are only about 3" lengths of heavy pipe between the hull and the flange, sand blasted and epoxied inside then all 316 ss after that, with isolating gaskets and bolt sleeves. Seems to work well. Best, Bob S/V Restless

[camaraderie]

Since we seem to be coming to the consensus that stainless is OK for through hulls I think it is worth noting that it is NOT ok for anchors and chain or mooring gear that will be sitting in an anaerobic situation on the bottom for extended periods of time. This is when crevice corrosion can and will begin to act (and over the SHORT term) and pose a real danger. Galvanized remains the best solution for chain. I won't getinto what is the best material for an anchor...but it ain't stainless!