NPT ball valves on through hulls
 

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

"Roger Long" wrote in message
...
has
anyone ever heard of a NPT valve on a NPS through hull breaking off or
leaking?

--

Roger Long



Or, as another possibility, has anyone rethreaded a NPT to NPS?

Facing replacement of a 2" and wondering...

L8R

Skip

--
Morgan 461 #2
SV Flying Pig
http://tinyurl.com/384p2

"Twenty years from now you will be more disappointed by the things you
didn't do than by the ones you did do. So throw off the bowlines. Sail
away from the safe harbor. Catch the trade winds in your sails. Explore.
Dream. Discover." - Mark Twain

Or, as another possibility, has anyone rethreaded a NPT to NPS?

I asked both a machinist friend and the shipyard and they said it
couldn't be done.

However, looking at the two half inch fittings on the desk in front of
me at this very moment, I'm damned if I can see why running a pipe die
down the first few threads of the through hull wouldn't taper it and
let it engage more than the half depth it does now.

Anybody out there got a pipe die set and want to invest $5.39 (price
of half inch bronze through hull at local store) in edifying the
boating world?

--

Roger Long

"Roger Long" wrote in message
...
Or, as another possibility, has anyone rethreaded a NPT to NPS?

I asked both a machinist friend and the shipyard and they said it
couldn't be done.

However, looking at the two half inch fittings on the desk in front of
me at this very moment, I'm damned if I can see why running a pipe die
down the first few threads of the through hull wouldn't taper it and
let it engage more than the half depth it does now.

Anybody out there got a pipe die set and want to invest $5.39 (price
of half inch bronze through hull at local store) in edifying the
boating world?

The local Lowes and Home Depots will cut threads reasonably cheaply, if you
don't have one. Unless, of course, the threads (not the taper) are
completely different in the two (which hardly seems likely, as people are
putting valves on NPS all the time).

I was thinking more along the lines of the other way - as, surely, given
that it starts that wide, the valve has enough body to support removal of
some material - taking an NPT valve and machining/tapping/whatever the entry
to NPS.

I'd think that taking material away from the TH would weaken it
unnecessarily, yielding the same end result of a small portion of the
full-depth pipe section being available to support any side loading, whereas
taking away the material in the valve shouldn't have any deleterious effect
on the strength. But then, I'm not an engineer, nor even play one on TV so
can't really comment on that.

L8R

Skip

--
Morgan 461 #2
SV Flying Pig
http://tinyurl.com/384p2

"Twenty years from now you will be more disappointed by the things you
didn't do than by the ones you did do. So throw off the bowlines. Sail
away from the safe harbor. Catch the trade winds in your sails. Explore.
Dream. Discover." - Mark Twain

I've been looking closely at these ½ inch fittings and measuring them,
comparing the NPT tail piece with the NPS through hull. The threads
are identical to the eye and the taper is only visible with the
parallel sides of measuring tool. Interestingly, the NPT tail piece
goes only as far into the valve as the NPS through hull with hand
force. The NPS fitting hits bottom more solidly however and I'm sure
it would feel differently when honked on.

Relieving the first half diameter of the through hull with a NPT pipe
die would clearly taper it enough to let it go deeper in the fitting.
The threads farther back, left untouched by the die, would then do the
same job as on the unmodified fitting but there would be more
penetration and more threads engaged. This would clearly be a good
thing.

The amount of material that would have to be removed to taper the
first half pipe diameter would be insignificant and there would be
untapered threads buried the same distance in the valve as if the
through hull was not modified.

The shipyard was right, however because they were responding to a
different question. The whole point of the exercise is to avoid the
labor of removing and rebedding the through hulls. Trying to re-thread
them in place usually wouldn't be practical because of space and the
risk of breaking the bedding. Once you have to take them out, you
might as well put in a proper seacock and do the job right.

Tapping the valve out to NPS isn't practical because the straight
threads and only be depended on when they are squeezing something so
as to provide the force to lock the threads.

Since my half inch through hulls, which are replacing plastic fittings
that I'm removing, are not in the boat yet, I may try to get a die run
on the end to see what happens. I want to cut them shorter anyway
later to minimize the leverage on the through hull when the valve is
actuated. If the re-threading looks good, I'll have it repeated. I'd
be using proper seacocks incidentally if these were larger seahulls
but ½ inch doesn't seem to be available.

It's amazing that, after over 30 years in the boat business, there is
still so much to learn about something so basic. I'm pretty sure now
that just putting ball valves on old through hulls is a reasonable
thing to do and this is another case of "don't believe everything you
read". If there were gate valves on there before, the force fit of the
old fittings has probably already somewhat modified the through hull
threads anyway.


--

Roger Long

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.

Both the male and female threads are tapered so thread engagement is
taken by more than "one or two threads". Remeber that threaded joints,
even in fractional inch sizes, are adequate for working pressures in the
thousands of pounds per square inch.

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?

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
Remove "removeme" from my e-mail address for direct mail
Ottawa, Canada

(use the e-mail address above to send directly to me)

Why not just do it the right way? NPS thruhull to NPS valve.

Doug

"Roger Long" wrote in message
...
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

Is a"gate valve" the same as the outdoor faucet on most homes? If so
what is the problem with them?

Allen

Doug Dotson wrote:

Why not just do it the right way? NPS thruhull to NPS valve.

Doug

"Roger Long" wrote in message
...

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

"Allen McCann" wrote in message
...
Is a"gate valve" the same as the outdoor faucet on most homes? If so what
is the problem with them?

Allen

Doug Dotson wrote:

Why not just do it the right way? NPS thruhull to NPS valve.

Doug

"Roger Long" wrote in message
...

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


Answer is yes. You turn the round handle and this screws down a gate across
the diameter of the pipe, sort of like a guillotine.

There are several potential problems with them and I'll just mention a
couple:

-not really intended for marine use so you don't know the type of alloy used
-the attachment to the hull is a weak point and they can be broken off
-you cannot easily tell if the valve is open or closed by looking at it
-they tend to seize up if you don't exercise them frequently and people
don't tend to do this because it is a pain to turn the valve a dozen time to
close/open it

Lots of older boats have them. If you maintain them you can live with them
but lots of folks replace them with proper seacocks.