I just went through this operation. I have a 35 year old sailboat that
does not use the proper bolted-through through-hulls, just the kind
with a big nut on them. The have then been glassed in. There were gate
valves on all of them.

This year, having the engine out, I thought I would try replacing a
few that were in formerly "impossible to get a pipe wrench on" places
when the engine was in.

I tried two - the engine water intake and a cockpit drain. I removed
the gate valve, and got ball valves from Home Depot (I sail in fresh
water, have used them before with no problems). These thread onto the
through-hulls quite nicely - way down the thread, not just at the end.
Lots of pipe dope. No way these will come off - there is a lot of
thread.

I've no way of knowing if the ball valves are NPT or NPS - they don't
say - but I would assume NPT, since damn near everything else in the
plumbing department is NPT.

I replace a 1" gate valve and a 1/2" gate valve this way.

Now that I think about it, several years aho I replaced the gate valve
on the head intake with a ball valve, and it works fine.

YMMV

"Roger Long" wrote:

Calder and other authorities are dead set against putting NPT ball
valves on NPS through hull fittings as is often done when replacing
gate valves. The geometry of the thread engagement gives me the
willies as well because all the load is on just one or two threads due
to the taper.

My boatyard tells me that they do it all the time however as does
everyone else. The gate valves on my 1980 boat haven't broken off yet
either. I just threaded a one half inch ball valve and through hull
together and it doesn't look or feel as bad as the theory.

Except for a couple of above waterline drains, the seacocks I'm going
to put in will be flanged and bolted. I'm just curious though, has
anyone ever heard of a NPT valve on a NPS through hull breaking off or
leaking?

Larry Bradley VE3CRX
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Ottawa, Canada

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