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Hi Brian. My thoughts on this are.......
A nipple with external threads is easier to repair than a coupling with
internal threads.
Unfortunately, in the case of schedule 80 nipples, the interior diameter is
less than the actual pipe size so a 3/4" ( ID ) nipple has a 1" OD. Good to fit
a standard size 1" ball valve.
A regular coupling ( schedule 80 again ) is short so it means a short
nipple on the inside and another on the outside for connecting whatever you've
got.
I call out schedule 80 because to use thinner schedule 40 fittings when
welding them into the hull, they will warp and then you gotta ream out the
threads. On a nipple, the threads are farther away from the weld area and with
proper use of a heat sink the threads don't need to be repaired after
welding.
Next, a regular ball valve has female ends, so it threads right onto a
nipple, where a coupling will need a short nipple between the valve and
thru-hull. One more threaded connection to weep.
As Cliff said earlier today, if you can configure all your thru-hulls into
one location the best bet is to make a thick stainless disk in a port hole type
ring with "O" rings to seal it up. The disk can be drilled and tapped
for any size fittings your penetrations require. This method is really good for
a device like an articulating arm where several thru-hulls are needed in the
same location. With the disk, you can add as many holes that will fit as new
devices are added, or you can simply make a new disk and bolt it in if a
re-design is done.
If a female thru-hull is used, I'd suggest using a thick SS piece of
bar stock with enough length and thickness to minimize any
warpage of the threads. It's pretty straightforward to drill and thread a hole
into it.
I got some 2" SS bar stock locally for various pieces on my sub, and at
$1.75 a pound it's pretty economical. The two inch stuff fits just nice inside a
2" SS pipe which I used for my dive plane swivels. Real beefy and with a little
grease inside makes a nice smooth swivel connection with no machining.
For the thru-hulls, I'm using schedule 80 nipples at 4" long. This keeps
the threaded portion far enough away from the weld so no problems with the
threads. I pre-heat the hull and nipples, wrap a wet rag around the threads and
weld them up. So far so good.
Frank D.
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