Steel Hull Skin Fittings?

[Aussie Monty]

Did a bit of a search for this topic before posting - apologies if one exists already!

I am replacing my skin fittings on my steel hull cutter. It originally had mild steel stand pipes welded in place - onto the 5mm mild steel plate hull. Its about 30 years old.

Not terribly comfortable with them as a couple have thinned out too much.

My research indicates that 316 Stainless skin fittings are OK above or at waterline - correctly installed to eliminate metal incompatibility - but, can I use them below the waterline?

Advice on type would be appreciated.

Aussie Monty

[Lepke]

No. 316 is fine for salt water above the waterline. In salt water below the waterline it will pit. The more it pits, the faster pit get bigger. I believe 317 is what you want. Good lick finding it at a reasonable cost. I would just replace the pipes with mild steel schedule 80 - thicker walls. Or schedule 40 and wait for the next 30 years.

[Cheechako]

I'd stick with steel stand pipes if you can find a good welder for that. But I'm not that steel boat knowledgeable.

[boden36]

I know that the conventional wisdom is no stainless below the waterline. Our steel boat has had stainless prop shaft and rudder pintles for 25 yrs no problems.
A few years ago I had a machine shop turn up a thick walled stainless stub threaded to take a stainless seacock. This stub is welded to the hull plating.
I have expected to see pitting, and inspect it at each slipping, but so far no problems with stub or seacock. Must be around five years now.
Our head outlet is done differently. Four stainless bolts through the hull and with their heads welded. A marelon (sp?)flanged stub is retained against the inside of the hull by a suitably drilled stainless plate which is held down by the four studs. There is a gasket between the flat head of the stub and the hull. There is also a gasket and sealant between the plate and the hull. I have checked it once in 25 years and it was as good as the day it was installed.
These are the only two through hulls in the boat. The cockpit drains above waterline, and the sink runs into the cockpit drains. The engine and toilet share a seacock with a valve at the toilet so that the engine cannot suck air back on a long run.

Regards,
Richard.

[Aussie Monty]

Boden 36 states the obvious...replacing with mild steel, thicker walled, seal well with good finish and it should outlast me! I am in fact a boilermaker by trade so am confident i can install without issue....just that my shipwrighting skills are developing!

Thanks all for thoughts so far.

Aussie Monty

[boden36]

Aussie, what you suggest will work well, but it was not what I meant. I used 316 stainless to make the pipe stub and welded it to the hull plate with appropriate rods.

Regards,
Richard.

[Aussie Monty]

Originally Posted by boden36

Aussie, what you suggest will work well, but it was not what I meant. I used 316 stainless to make the pipe stub and welded it to the hull plate with appropriate rods.

Regards,
Richard.

Apologies for the misunderstanding Boden36. Indeed this clarifies another solution that I was eyeing off. Can I simply use stainless 316 tube/standpipes directly welded to mild steel hull and be either be free off, or avoid, metal incompatibility issues?

Aussie Monty

[boden36]

A lot of people say you can't, but it has worked for me so far. Used at least 10mm wall thickness.

Regards,
Richard.

[44'cruisingcat]

If the original stuff lasted 30 years, I'd be reluctant to try a different material, with unknown galvanic consequences.


Our steel boat had mild steel stand pipes too. Any rust happened from the inside, and was easily seen, cleaned up and painted.

[RaymondR]

Apologies for the misunderstanding Boden36. Indeed this clarifies another solution that I was eyeing off. Can I simply use stainless 316 tube/standpipes directly welded to mild steel hull and be either be free off, or avoid, metal incompatibility issues?

I have a mix of both mild steel and 316 stainless all welded into the hull plate and have not had any corrosion problems with either of them. The boat was launched in 1987.

[stillbuilding]

Always had 316 SS, but always heavy gauge and professionally welded on nipples. No problem. Almost impossible to find Duplex anyway. I could not bring myself to use weld-in mild steel. Can see that well painted it could be ok though. Probably does not matter if well blasted and painted.


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[Hartleycruiser]

Aussie Monty

The problems people have encountered seem to be with the cast skin fittings, they may have some impurities in the casting and are therefore not up to being under water .