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Old 01-11-2008
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Installing Seacocks - Follow Up Info !!

Hi All,

Since posting the article/blog on installing seacocks, seen here, I've had a lot of emails and questions regarding the "mismatching" of different thread types such as NPS and NPT.

A guy on another forum challenged the idea that mismatching threads was OK because "so many do it" so last night I made a cut away view to show why using two different thread types is NOT a good idea. While many boat owners screw NPT threaded valves directly onto NPS threaded through-hull fittings it's clearly NOT a logical idea or even safe as the photo below clearly shows.

To do this I basically used a bronze nipple, or threaded pipe, with standard plumbing threads of NPT (National Pipe Tapered) and a bronze coupling also the industry standard of NPT thread.

I cut the bronze coupling almost in half, for a cut away view, so I could thread the NPT bronze nipple into one side and an NPS through-hull fiting into the other. I then sprayed each with a McLube, to reduce friction but save the picture quality, and threaded both the NPT nipple and the NPS through hull into the cut-a-way bronze coupling by hand and until I had an equal resistance.

The results even surprised me! As you can clearly see the NPT nipple threaded into the NPT coupling a LOT further than did the NPS through-hull. If you were to take a wrench to both you might get one more turn at best out of the NPS through hull but you may still get two or three full turns out of the NPT nipple.

If you look very closely at the picture you can also see the outer-most threads of the through hull are already NOT fitting tightly against the female threads of the coupling and the inner-most threads are quite tight or virtually bottomed out...!!! The square peg evidently does NOT fit a round hole..!

This coupling represents the threads of an in-line valve. Most ALL ball valves or gate valves have NPT or tapered threads and most all through hulls have NPS or straight threads a clear mismatch.

We finally have an answer to this question, with photographic proof, and can clearly see from the photo how dangerous it is to stick an ball valve directly onto a through hull..

Sticking a ball valve directly onto a through hull gives you about four or five threads between sinking and floating so don't do it and do use proper seacocks with bolted flanges..!!

I would have cut away an actual ball valve but I don't have a machine shop. The coupling represents and has the same exact NPT female threads as an in-line valve of either the gate or ball type...


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Last edited by Maine Sail; 01-11-2008 at 04:17 PM.
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Old 01-11-2008
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I am not sure what they say for pipe,but for bolts at least you only need to go in a few thread widths before the connection is considered to have a maximum strength of the fastener.
So, even 4-6 threads may very well be as strong as this connection will ever be.

Of course if you have marine hose above this connection, a failure in hose/clamp area is significantly more likely and going to the expense and difficulty of using flanged thru-hull (not to mention making more holes in the hull for those fastening bolts) may not provide as much safety as desired.
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Old 01-11-2008
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I wonder if a thru-hull that is NPS can be re-cut to NPT on the end since it is already the same thread pitch. I understand what you are pointing out; but many, many boats have ball valves installed onto the thru-hull and there are not lots of reports of boats sinking due to this type of installation.

With a thru-hull that is clamped to the hull via the retaining nut you have lesser chance of the fitting leaking and no extra fasteners penetrating the hull. If a flange mounted seacock leaks you need to pull the boat to fix the leak; with a retaining nut you can attempt to tighten it first. If the thru-hull fitting had NPT re-cut on the end it would be better than a flanged secock (I think).
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Old 01-11-2008
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Quote:
Originally Posted by brak View Post
I am not sure what they say for pipe,but for bolts at least you only need to go in a few thread widths before the connection is considered to have a maximum strength of the fastener.
So, even 4-6 threads may very well be as strong as this connection will ever be.

Of course if you have marine hose above this connection, a failure in hose/clamp area is significantly more likely and going to the expense and difficulty of using flanged thru-hull (not to mention making more holes in the hull for those fastening bolts) may not provide as much safety as desired.
Brak,

Your premise & comparison to bolt threads of course does not take into account the improper mismatching of threads that occurs when you screw a valve directly to a through hull. The bolt threads used in your comparison are of the same type. NPS & NPT are two different types of threads and not designed to be used together..
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Old 01-11-2008
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Quote:
Originally Posted by KeelHaulin View Post
I wonder if a thru-hull that is NPS can be re-cut to NPT on the end since it is already the same thread pitch. I understand what you are pointing out; but many, many boats have ball valves installed onto the thru-hull and there are not lots of reports of boats sinking due to this type of installation.

With a thru-hull that is clamped to the hull via the retaining nut you have lesser chance of the fitting leaking and no extra fasteners penetrating the hull. If a flange mounted seacock leaks you need to pull the boat to fix the leak; with a retaining nut you can attempt to tighten it first. If the thru-hull fitting had NPT re-cut on the end it would be better than a flanged secock (I think).

Actually approximately 50% of boat sinking's are as a result of failed below water fittings and a good chunk of this 50% is directly related to failed through hulls or valves as well as hose clamps and hoses. Boats do sink because of improper through hull configurations. A neighbor of mine lost his boat in a lightning strike right in front of our house. Why? Because the few threads holding his gate valve onto his through hull gave way when the current tried to find it's way back to the water. The valve blew right off the through hull. His insurance company tried to decline coverage because the valve was not a marine rated valve and not a proper ABYC or industry installation of a proper seacock. He got a lawyer, fought it and won. The only reason he won was because they had not requested he install proper ABYC or industry accepted seacocks when they examined his initial survey from when he bought the boat..

On my most recent boat purchase (CS-36) my insurance company (Amica) made me replace all my through-hull's w/valves with marine rated seacocks. They did not require, however, that they be through bolted but did require flanges and marine rated. After I installed them, and changed a bunch of other stuff they requested too, such as standing rigging, the surveyor had to come back and sign off. It's getting tougher and tougher to insure an older boat and they are mandating ABYC spec on many items...!

I personally had an improperly installed through hull crack on me when a heavy item under the sink, a spare alternator, fell & hit the valve in rough weather. My boat did not sink and I found the problem in time, thank god for bilge alarms, but I could have lost it.

There is a reason I'm passionate about this subject and it's because I've personally seen the consequences..

P.S. You can have NPS threads cut to what's called a "combination thread" and while still not NPT they are better than stuffing an NPS into an NPT fitting. You will incur machine shop expense or have to custom order the fittings pre-machined that way though and still not have the benefit of the strength of a flanged seacock.
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Last edited by Maine Sail; 01-11-2008 at 05:15 PM.
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Old 01-11-2008
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Is that a brass or bronze pipe nipple? Very important to never use brass on a seawater system.. but seems like you can't find bronze anywhere!
Edit:
Did some quick searching and found that "red brass" sold at marine stores isn't really brass at all

Quote:
Gunmetal is a kind of bronze, an alloy of copper, tin, and some zinc, originally used chiefly for making guns, but later superseded by steel. It is also called red brass in America.

Gunmetals produced for different purposes vary slightly in composition. In some cases, the alloy may be composed only from copper and tin, or from copper, tin, and lead. It has many uses in industry, and is used for statues and various small objects, e.g. buttons.

U.S. Government bronze spec. G is a gunmetal composed of 88% copper, 10% tin, and 2% zinc. U.S. Government bronze spec. H is composed of 83% copper, 14% tin, 3% zinc, and 0.8% phosphorus.

Gunmetal can also mean steel treated to simulate gunmetal bronze. Bushes made of this metal are used in machinery

Gunmetal is also a name for a shade of matte gray/silver, aka machine finish.
So what gives?? Is "red brass" really bronze and suitable for saltwater application???

Also, regarding proper seacocks (and I DO believe a seacock bolted to the hull is the best way to do it) Nigel Calder has this to say: "At one time the ABYC required seacocks to have flanges that could be securely fasted to the hull. This is no longer part of any standard..."
So, a mushroom with a marine grade valve seems to be acceptable according to Calder. Thoughts?

Last edited by sailboy21; 01-11-2008 at 05:25 PM.
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Old 01-11-2008
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ABYC H-27 was last revised in 1997 but they want $50.00 to download the standard. I don;t know when that Calder book was originally written but my insurance company claimed to be working of ABYC standards and did not want a valve on a through hull..

I'd actually like to know the wording of that standard but not for $50.00..


Oh and yes the nipples are red brass not yellow brass..



I also found this little tid bit.

"Around 1987, the A.B.Y.C. established a set of “standards” for recreational watercraft that included guidelines for plumbing components. This was the early stage of the H-27 “Recommended practices and standards covering seacocks, thru-hull fittings, and drain plugs.” By 1989 this was published by the A.B.Y.C. and builders were encouraged to comply. By 1991 the Marine U.L. began incorporating the A.B.Y.C. standards into their standards. The biggest change from the original U.L. standard was the A.B.Y.C. addition of a 500 lb. load test applied to the “outermost fitting” (the tailpipe) of the valve “system”. This was the first time a seacock was viewed as a “system” including the thru-hull and tailpipe (or 90 deg. elbow) for hose attachment."

So if your flange-less through hull, with valve screwed onto it, can support a 500lb load test at the outermost fitting I guess your safe?? Mine was hit by an alternator hardly 500lbs..
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Last edited by Maine Sail; 01-11-2008 at 05:41 PM.
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Old 01-11-2008
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For an older boat, with many fittings installed prior to ownership, I always have that feeling "what did they use???"
I had two dangerously corroded GATE valves. One of which the gate was completely missing when I bought my boat. I also have two thruhulls where it is physically impossible to install a seacock. I'm seriously considering the groco NPS-NPT flaged adapter next time I do a serious haulout... It just makes sense to know every 1 1/2" hole is backed up with something substantial.

Those thoughts aside, I neglected to properly cite my Calder quote above. That is from "Boatowner's Mechanical and Electrical Manual," 3rd ed., 2005. In that book he does detail the rigorous testing that seacocks and thruhulls must endure. Lets just say I'm not willing to perform those same tests on my boat while in the water

500lbs alternator??

Last edited by sailboy21; 01-11-2008 at 06:03 PM.
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Old 01-11-2008
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In my opinion, Halekai 36 is entirely correct in his assertment that you should not interchange NPS with NPT threads. I have some machining experience, (I have a Bridgeport at my house) and while it may be possible in some cases to re-machine one part to match the other, why would you do this when you can buy the correct stuff in the first place? If you want to hear horror stories about mis-matched thru hulls and sea cocks, call Groco direct and speak to one of the thru hull techs on this subject... they're pretty passionate about it and can tell quite a few stories of new boat sinkings, law suits, etc.
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Old 01-11-2008
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Quote:
Originally Posted by steelboat View Post
they're pretty passionate about it and can tell quite a few stories of new boat sinkings, law suits, etc.
While your on the phone ask them about their latest product recall too.
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