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?

--

Roger Long