Through-hull for steel sailboat - how to do it ?

Hello,
I'd like to ask about one think I couldn't find anywhere

I'm building steel sailboat (40 ft, we bought unfinished steel hull) and I don't know what is the best way how to do through-hulls (below and above waterline).

Someone told me that id can be done by welding threaded stainless steel pipe directly to the hull, but someone else told that this is not good idea because stainless steel have no place below waterline because of crevice corrosion. I'm confused

Please can you give me advice how to do it or where I can find information about it? How do you solve it ?

thanks a lot
Dodo

 

turn up a flange(ring) make the hole in plate, 5mm smaller than ring hole this will give a fillet weld
Then tap the ring, and using a 3 piece ss valve, ball type and throwing away the screwed flanges that normally come with such a valve, use hex machine screws to fasten to your ring, the teflon seal will seal against the ring

 

Thanks for advice,
but I still can't imagine how exactly to do it.
Please, do you have some photos of result or instalation procedure?
And isn't it necessary to insert thicked plate (I have plate thickness 4mm) in place of valve welding?


...and with 3 piece ss valve, ball type you mean somelike this one ?

http://scplumbing.manufacturer.globalsources.com/si/6008824074697/pdtl/Ball-valve/1003141028/Ball-Valve.htm

And what about SS below waterline, isn't it problem ? what about "oxigen starvation"

on http://sailboat.creatica.org/material.html
I read :

For example, the stainless steel area with the broken passivated film, which is often caused by oxigen starvation in stagnant seawater, will have its potential significantly lower than the rest of the material. The large cathodic area will cause rapid destruction of small anodic spots.

thanks
Dodo

 

On my steel hull I use flanged silicon bronze seacocks. To mount it, I weld about two inch of a properly sized pipe, ending it with a welded flange matching the seacocks flange. We obtain an upside down hat on which to bolt the valve, but we don't bolt it directly on. We isolate the two dissimilar metals from each other by having a piece of isolating material (we use a thin fiberglass sheet and goop) between the two flanges. The fasteners are isolated as well with plastic inserts and washers. You want to make sure to use a good quality bronze (not brass) seacock. Sorry I don't have a pic of the assembly, hope the description is somewhat clear.

 

like this, the bronze is further away on the galvonic scale so I would use ss 316 cheaper too, I used to do all my alloy boats too like this, and if you use a sleeve ocer the screws and a nylon washer you have total isolation

 

Thanks a lot Lazeyjack and M&M Ovenden, now I have idea how to do it.

The idea with seachest is also very good. From what material you use to made it ? SS Steel? painted mild steel ? plastic ? anythink else ???

thanks
dodo

 

yes you could use painted ms, weld in a bunch of sockets, or nipples(heavy wall) and sand blast and paint 300 microns epoxy or you can get a peice of flat bar about 3 inches wide, drill, weld the sockets or nipples in and then fasten to the box using a cork gasket I'm glad I switched to alloy boats in 86)

 

Welding a threaded stainless pipe directly the hull is by far the best way to put a thru hull in a steel boat.
Brent Swain

 

thru hull in steel boats

Braint is right,

weld a heavy SS niple or pipe directly to the hull. Make sure thye inboard is treated so you can screw your SS ball valve on to it. Make sure to use a doubler on the inside around the pipe. this can be mild steel. this is how it is done commercial in canada.

John

 

So Mr grace
You recommend sticking a peice of nipple tube through the plate?
Say you have a 2 inch npt, or here bsp pipe, the wall is say 4mm, the depth of thread 2mm, what does that leave you?(I can not remember exact measures
Plus ss is not entirely compatible with mild steel, , you also can get crevice corrosion, come time to unscrew your valve and you could end up with a busted thread
Sure it can be done usually with HEAVY wall pipe, min 6mm wall, ,schedule, not you crap from the local fence supp;ier, remember pipe is hard to maintain
but its for amateurs
The way I described works, esp on Alloy yachts

 

Disimualar metals should always be isolated. The chemical reaction which causes SS to have a protective coating requires oxegen. I'll disagree with my good friend LazyJack and go with MM on this one; I'd use a bronze (but isolated) fitting. That said, LazeyJack has built steel & alloy boats, where as I have not.

Good fortune to you in your building endevours.

Tim

 

ah Tim but the way I do it there is total isolation, the three piece valve has teflon seal,one screwed piece of the valve are disgarded and the teflon seats against the flange, the thru screws that tap into flange are aso isolated
With steel no need isolate the screw
Ships have flanges, toys have pipe, and old crappy fish boats)

 

Gotca Stu

 

I've been using welded in stainless type 316 pipe nipples for 35 years and dozens of boats I've built, with no problems . Most of the over 200 boats built to my designs have used the same , with no problems. Skip the doubler. It's totally unecessary, and offers no advantsge.
There may be a slight blistering, with tiny pinhead sized blisters around the thru hull, only if your paint there is not thick enough, but not a serious problem.
I always find it entertaining when someone claims that something that has been working well for decades "Won't work".
Brent