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Hi James. I would go and talk to the guys at your local welding store. If
you are welding stainless parts onto/into carbon steel parts, stainless filler
is always used. Maybe your ship building friend was talking about putting steel
fittings into the steel hulls.
Joint design is also a critical factor. Thru-hulls are subject to some
unusual stresses.
The area of the hole should be reinforced either by adding weld beads
around the hole to increase thickness or in the case of multiple penetrations a
thicker "disk" should be welded in. ( not ON )
For a single penetration, the hole in the hull should be beveled on the
exterior, and stainless filler rod built up in the bevel, making a conical
shaped "plug" of stainless. Typical bevel is 22-1/2 degrees. If the hull is 1/4"
to 3/8" it should be doubled in thickness at the hole.
As for corrosion, you could use steel through hulls like schedule 80
couplings or nipples but they won't be painted on the inside and will rust out
more quickly than 316L-SS fittings.
Pre-heating the hull and fittings is important too, with a slow cooling (
torch work ) after welding.
Re-polish the inside of the fitting after completion to keep it's
anti-corrosion properties at maximum.
I'm using mostly 1" schedule 80 seamless nipples from McMaster-Carr at $30
each.
The really sweet way for multiple penetrations guys use now is to machine a
"port" with "O" rings like a small window and install a thick stainless disk
with threaded holes for penetrations. This way you can easily replace the disk
with more or less holes like if you wanted to add a manipulator arm or
lights/cameras at a later date. There are some photos of these available on
various member projects on the psubs site.
Frank D.
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