Through-Hull/Ball Valve Thread Mismatch -- Urban Legend?

[Dockhead]

Another thread on changing a ball valve screwed to a through-hull fitting got me thinking about this question. The popular wisdom is that you shouldn't do it, because the threads are not compatible. The popular wisdom is well described in this classic article of MaineSail's:

https://marinehowto.com/seacock-thru-hull-primer/

Apparently ABYC agrees: ABYC H-27 “Threads used in seacock installations shall be compatible (eg. NPT to NPT, NPS to NPS).”

But this clearly doesn't apply outside of the U.S., where BSP is the standard, not NP. In the Rest of the World, through-hull fittings are TAPER (BSPT), and ball valves are PARALLEL (BSPP), and male taper into female straight is specifically approved as a UNI EN 10226-1 joint.

So this made me think -- why is NPT/NPS different? NP and BSP is quite similar except that the thread angles are slightly different. So if you can screw a BSP taper through-hull fitting into a BSP straight ball valve, why not with NP fittings?

I was unable to find any engineering literature to prove the point one way or the other, so I don't really know. I did see numerous secondary sources saying that male NPT fittings are compatible with or "plausibly compatible with" female NPS fittings, e.g. https://pipeandhose.com/book/thread-type-compatibility

But the plot thickens. It seems that U.S. NP ball valves have female TAPER threads, not straight, like BSP ones are. See:

https://www.groco.net/products/valve...s-inline-valve

So maybe that's the answer. Maybe taper into straight is OK, which is why this is OK for non-U.S. boats, but straight into taper is not?

I note that Groco makes a "combination thread" through hull fitting which you can screw an NPT ball valve onto:

https://www.groco.net/products/fitti...ings/th-series

[Dsanduril]

Herewith the engineering. A picture could say it all but I don’t have one so you get the thousand words.
--------------
MS=Male straight
FS=Female straight
MT=Male taper
FT=Female taper
These abbreviations avoid any mention of thread form (British/American/DIN) as these are not germane to the topic.
-----------------
Any double-entendres and poor attempts at crude humor fully intentional.
-----------
From an engineering perspective there are two concerns with a pipe joint; sealing and mechanical strength.

Sealing is actually pretty simple. Despite all the drips, weeps, and seeps, one fully-engaged thread is all that is required to obtain a seal (*at least at low pressure) so we can discard that topic. Pretty much any combination of straight and taper can obtain a seal.

Mechanical strength is a whole other beast. With MS threads the OD of the thread is essentially the OD of the pipe. To meet specifications it is slightly less (on the order of 0.01” to 0.02” less). This is largely to account for tolerances in the manufacture of the pipe itself. The thread specs want to cut the least amount possible (both for ease of machining and to maintain strength) whilst also ensuring fully-formed threads. In practice this means that the specification for MS threads has an OD that is equal to the minimum tolerable pipe diameter.

An FT socket has an opening that is equal to the OD of the pipe. The MT fits easily in the opening and there is no reason for a tolerance greater than pipe OD. When a MT is fully home in a FT the pipe OD is still the pipe OD, no reason to have a larger opening (and some reasons not to have one).

When you fit MS to FT then, you are fitting a nominal pipe OD thread into a nominal pipe OD opening. The only room you have for mating in this configuration is the tolerances. In practice this means that you can generally get only 2 or 3 male threads of penetration before the taper of the female socket prevents further travel. This can be affected by materials. A plastic MS inserted into a metal FT can work better as the plastic can be deformed and basically create a taper on-the-fly. The converse, metal MS into plastic FT doesn’t work as well, the plastic will start to deform and then likely crack under pressure.

A MT into a FS works better. Because of the taper the male part starts easily (so easily that cross-threading is an issue) and can be fully inserted (if the female socket is deep enough) until the root threads engage at the mouth or the male pipe bottoms out at the shoulder of the female socket.

The MS->FT is mechanically very weak, only a few threads are somewhat engaged. Side-loading can easily break this connection, and axial loading (fluid pressure against a closed valve) can blow the fitting off the pipe. All bad outcomes.

The MT->FS is marginally more acceptable. If it bottoms out then the mating of the pipe end against the shoulder provides an additional sealing surface and significant additional mechanical strength against side-loading. The partially engaged threads along most of the length also provide more axial strength than MS-FT.

Hope that's relatively clear without the pictures.

[Dockhead]

Originally Posted by Dsanduril

. . . The MT->FS is marginally more acceptable. If it bottoms out then the mating of the pipe end against the shoulder provides an additional sealing surface and significant additional mechanical strength against side-loading. The partially engaged threads along most of the length also provide more axial strength than MS-FT.. . .

Very useful and interesting. It's always great to have a real engineer come in.



MT>FS is the European Standard UNI EN 10226-1 joint: http://www.sarm.am/docs/10226-1-2004.pdf.


Is there something about the BSP specification which makes this work better?


Maestrini (and other European) through hull fittings and ball valves are designed to be screwed together according to the UNI EN 10226-1 specification, with taper threads on the through hull fitting and straight threads in the ball valve. The through hull fitting does bottom out in the ball valve.

[George_SD]

I just went through this the hard way when raw intake barb broke on an old ball valve. I knew that thruhull and ball valve have different threads, but everyone at multiple stores told me it was OK and they would work together just fine. So I got a bronze Groco valve and it would only go as far as 1/4 turn on the thruhull. The old valve would go full 5 turns. So i went to the store and got several more - all of them would thread a varying amount (one did 1.5 turns, another did half a turn, but non would match 5 turns of the original).

I ended up buying Meralon and it went on 5.5 turns, no problem. Seems to work great so far.

[fivecapes]

FWIW, Trudesign ball valves are BSP/NPS to mate with their corresponding BSP/NPS skin fittings.

[Dockhead]

Originally Posted by George_SD

I just went through this the hard way when raw intake barb broke on an old ball valve. I knew that thruhull and ball valve have different threads, but everyone at multiple stores told me it was OK and they would work together just fine. So I got a bronze Groco valve and it would only go as far as 1/4 turn on the thruhull. The old valve would go full 5 turns. So i went to the store and got several more - all of them would thread a varying amount (one did 1.5 turns, another did half a turn, but non would match 5 turns of the original).

I ended up buying Meralon and it went on 5.5 turns, no problem. Seems to work great so far.

If you can only screw it in 1.5 turns, then you are obviously not achieving any significant mechanical strength. That is obviously no good.


The Maestrini fittings, which are designed to screw into each other, go about 6 turns before bottoming out. They do NOT form a watertight seal by themselves -- you must use hemp and pipe dope or they will leak.

[Pete7]

Originally Posted by Dockhead

But this clearly doesn't apply outside of the U.S., where BSP is the standard, not NP. In the Rest of the World, through-hull fittings are TAPER (BSPT), and ball valves are PARALLEL (BSPP), and male taper into female straight is specifically approved as a UNI EN 10226-1 joint

Are you sure through hull fittings are a taper in the UK? Not saying you are wrong but I have always believed them to be a parallel thread and ball valves the same.

This looks like a parallel thread:

https://www.asap-supplies.com/produc...mm-long-403107

The last one I did was the heads discharge, Tru Design onto the original bronze Moody through hull. It certainly went more than 6 turns, perhaps an inch of thread altogether and yes it did leak a smidgen the first time.

Pete

[Dockhead]

Originally Posted by Pete7

Are you sure through hull fittings are a taper in the UK? Not saying you are wrong but I have always believed them to be a parallel thread and ball valves the same.

This looks like a parallel thread:

https://www.asap-supplies.com/produc...mm-long-403107

The last one I did was the heads discharge, Tru Design onto the original bronze Moody through hull. It certainly went more than 6 turns, perhaps an inch of thread altogether and yes it did leak a smidgen the first time.

Pete

The Maestrini ones are taper - see the Maestrini tech literature linked above. Doesn't mean they all are.

I believe Moody used Maestrini fittings for decades, so that's probably what you have.

[GordMay]

Originally Posted by Dsanduril

Herewith the engineering. A picture could say it all but I don’t have one so you get the thousand words.
--------------
MS=Male straight
FS=Female straight
MT=Male taper
FT=Female taper
These abbreviations avoid any mention of thread form (British/American/DIN) as these are not germane to the topic.
-----------------
Any double-entendres and poor attempts at crude humor fully intentional.
-----------
From an engineering perspective there are two concerns with a pipe joint; sealing and mechanical strength ...

Thanks Dsanduril!
This may be one of the best technical explanations, that I've ever read, on any subject. Concise, yet comprehensive.

[Pete7]

I suspect Maestrini make both, depending on were in the world they are selling them.

The ASAP website says BSP thread under the specification for the through hull and they are normally a pretty reliable supplier, rather than the 17 year old in the Marine Super Store.

Pete

[Joli]

Originally Posted by Dsanduril

Herewith the engineering. A picture could say it all but I don’t have one so you get the thousand words.
--------------
MS=Male straight
FS=Female straight
MT=Male taper
FT=Female taper
These abbreviations avoid any mention of thread form (British/American/DIN) as these are not germane to the topic.
-----------------
Any double-entendres and poor attempts at crude humor fully intentional.
-----------
From an engineering perspective there are two concerns with a pipe joint; sealing and mechanical strength.

Sealing is actually pretty simple. Despite all the drips, weeps, and seeps, one fully-engaged thread is all that is required to obtain a seal (*at least at low pressure) so we can discard that topic. Pretty much any combination of straight and taper can obtain a seal.

Mechanical strength is a whole other beast. With MS threads the OD of the thread is essentially the OD of the pipe. To meet specifications it is slightly less (on the order of 0.01” to 0.02” less). This is largely to account for tolerances in the manufacture of the pipe itself. The thread specs want to cut the least amount possible (both for ease of machining and to maintain strength) whilst also ensuring fully-formed threads. In practice this means that the specification for MS threads has an OD that is equal to the minimum tolerable pipe diameter.

An FT socket has an opening that is equal to the OD of the pipe. The MT fits easily in the opening and there is no reason for a tolerance greater than pipe OD. When a MT is fully home in a FT the pipe OD is still the pipe OD, no reason to have a larger opening (and some reasons not to have one).

When you fit MS to FT then, you are fitting a nominal pipe OD thread into a nominal pipe OD opening. The only room you have for mating in this configuration is the tolerances. In practice this means that you can generally get only 2 or 3 male threads of penetration before the taper of the female socket prevents further travel. This can be affected by materials. A plastic MS inserted into a metal FT can work better as the plastic can be deformed and basically create a taper on-the-fly. The converse, metal MS into plastic FT doesn’t work as well, the plastic will start to deform and then likely crack under pressure.

A MT into a FS works better. Because of the taper the male part starts easily (so easily that cross-threading is an issue) and can be fully inserted (if the female socket is deep enough) until the root threads engage at the mouth or the male pipe bottoms out at the shoulder of the female socket.

The MS->FT is mechanically very weak, only a few threads are somewhat engaged. Side-loading can easily break this connection, and axial loading (fluid pressure against a closed valve) can blow the fitting off the pipe. All bad outcomes.

The MT->FS is marginally more acceptable. If it bottoms out then the mating of the pipe end against the shoulder provides an additional sealing surface and significant additional mechanical strength against side-loading. The partially engaged threads along most of the length also provide more axial strength than MS-FT.

Hope that's relatively clear without the pictures.

That's a good explanation but builders today are using MS skin fitting with FT ball valve with minimal thread engagement. A seacock is FS on the recieving end of the skin fitting, which is MS, the opposite end is MT to FT with full thread engagement.

For how little it costs to install proper seacocks versus the cost of the boat it's beyond me why anyone accepts this.

[Dockhead]

Originally Posted by Joli

That's a good explanation but builders today are using MS skin fitting with FT ball valve with minimal thread engagement. A seacock is FS on the recieving end of the skin fitting, which is MS, the opposite end is MT to FT with full thread engagement.

For how little it costs to install proper seacocks versus the cost of the boat it's beyond me why anyone accepts this.

Quality builders are NOT building underwater fittings with "minimal thread engagement". That is not inherent to MT > FS. This is the EN 10226-1 joint:



As you can see, there are standards for thread engagement etc.

This joint has certain advantages over straight to straight or taper to taper. It could be that this clash of consciousness between Americans and Europeans results from BSP threads being specifically designed to be screwed in MT > FS, whereas American standard threads are not designed for this.

From the literature:

"BS EN 10226-1:2004 Pipe threads where pressure tight joints are made on the threads. Taper external threads and parallel internal threads. Dimensions, tolerances and designation
BS EN 10226-1 specifies the requirements for thread form, dimensions, tolerances and designation for jointing pipe threads, sizes 1/16 to 6 inclusive. It applies to joints made pressure tight by means of mating of the threads. These threads are taper external and parallel internal, and are intended for use with pipes suitable for threading and for valves, fittings or other pipeline equipment interconnected by threaded joints.
An appropriate thread sealant or jointing compound should be used on the thread to ensure pressure-tight joints." https://shop.bsigroup.com/ProductDet...00000030235769

I would guess that MT>FS is chosen over MP>FP or MT>FT so that you can have BOTH (1) ability to adjust the angle; AND (2) ability to have watertightness at the threads (with the help of sealant). Only MT>FS will give that.

As to why builders, including all the best yacht builders in the world, choose this method vs "proper sea cocks" -- the answer is NOT that the engineers at Nautor Swan, Oyster, Halberg Rassey, et. al are idiots, or that bean counters at those companies are saving a couple hundred euros on a multi-million euro boat. The answer is that there are different advantages of this construction compared to "proper seacocks" -- fewer holes in the hull meaning less risk of core saturation, leaks, etc.; vastly easier to replace the valve when you need to; simplicity. And done right with proper thread engagement this construction is plenty strong.

I've seen boats where through-hulls were hidden behind cabinetry or equipment; where you couldn't see or reach them; or where on the contrary through-hulls are placed in a position where heavy things can fall on them or you can step on them. I guess you would care more about ultimate strength of the seacock in such cases, but quality builders don't build them like that. In my boat every single underwater through-hull (and there are 15 of them) is located either in the engine room or in a separate compartment which is immediately accessible through a separate hatch and where there is nothing else -- nothing which can fall on them, no way to step on them. The ones in the engine room are right at the doors where they are immediately visible and accessible. It's like this with other quality boats; surely this is much more important than whether the device itself can withstand a 400 pound or a 500 pound side force.

[Etalon]

Post by dockhead is key.

BSP seems to have been designed around the idea of a male taper to female straight. I have *very* little experience with NPT, but more experience with BSP than I care to count. The thread engagement with BSP MT to BSP FS is not just a couple of threads, its a very positive interaction over numerous turns.

The BSP male parts though by design therefore also need to be quite thick so that there is enough material to actually taper.

Pre-threaded pipe stubs that are advertised as a cut to length thing obviously wont be a taper fitting, they will often be a thin wall parallel male. Those are the fittings we don't like in the (chemical) industry as then our fitters mess them up and they leak! We would prefer to have a plain pipe, cut it to length and cut a taper male thread onto it.

See photos:
2" BSP MT FS, from a single thread barely engaged to HAND TIGHT. There would still be another half turn in it at least.

https://imgur.com/a/ngvippE

[Pete7]

Originally Posted by Dockhead

I would guess that MT>FS is chosen over MP>FP or MT>FT so that you can have BOTH (1) ability to adjust the angle; AND (2) ability to have watertightness at the threads (with the help of sealant). Only MT>FS will give that.

MS>FS would also give you that. I suspect the builders pre-assemble to see how the line up to give access to the handle, then take part and fit to the hull.

If a replacement doesn't line up, a touch with an angle grinder on a MS soon solves the problem.

Did you fit FS valves to your through hulls?

[donradcliffe]

The last time I replaced thru hulls, I used bronze male NPS thru hulls into bronze female NPT ball valves. Many threads of engagement, no leaks, and takes some major torque to unscrew. The ABC recommendations are inferior, and the Euro brass stuff is a joke.