NPT to NPS through hulls

[Roger Long]

Note: This post turns out to be based on erroneous information (MaineSail does it again). The original thru hulls in my boat are evidently not NPT. Worth reading though for a full discussion of the issue. You won't have the same experience I did if you buy new through hulls off the rack at the typical marine store.

(Begin original post)

First let me say that I don't recommend doing this. It especially should not be done if there is any chance of the through hull or attached pipe or hose turning into a hand or foot hold or having something heavy fall on it. In view of the large number of boats that have Groco or similar ball valves threaded on to through hull fittings, some perspective is probably in order because there are lots of things that can be improved and you sometimes have to prioritize.

My boat came with gate valves which should definitely be replaced yesterday if you have them. That was when I first learned about the thread mismatch issue. I even put a piece on my blog about the expensive (I thought it was expensive at the time but I hadn't yet heard the term "boat buck") pickle I was in. I even made up a nice little CAD drawing showing why the thread mismatch is so horrible.

The yard manager, who was a very knowledgeable guy, told me that they had been doing it for 20 years and he had never heard of a problem. I took a pair of NPT and NPS fittings, screwed them together hard, and knocked on them a bit. They felt solid. So, I did as the yard manager suggested and ended up with four on my boat. Two are a few inches above the waterline and small so I’m not too concerned about them. Another is in a very protected space and, like the previous two, is only half inch. Groco doesn’t make seacocks that small.

The fourth one however, is inch and a half. That one could easily overwhelm the boat before I could get it plugged. I decided to put a proper Groco adapter on it. This also gave me a chance to assess the condition of the mismatched threads.

The valve was quite hard to take off. I could just barely move it with a pipe wrench in the confined space. I had only used Teflon tape on the threads but it still needed the wrench for most of the removal. The threads can be seen below. There appears to have been pretty good engagement and contact and no sign of thread distortion. The Groco adapter turned easily onto the through hull after the threads were cleaned up.

I don’t think the thread mismatch is as much of an issue in these installations as the lack of a flange and having the valve often way up at the full length of the through hull where there is a lot of leverage and only the small nut area to support it. If I’d seen the condition of these threads before I was committed to the project, I might not have been so quick to do it myself although I would, of course, recommend that anyone else do it.

It will be nice to have the valve easier to replace anyway. This is a holding tank pump out line and, as you can see by the third picture, the stuff really does a job on the stainless ball. The seawater side is shiny.

Here's how I'm installing the Groco adapter without removing the thru hull to cut it short:

http://www.rogerlongboats.com/09Winter.htm#January27

 

[gettinthere]

Groco has another way of doing it. The "seacock" has the mismatch to the thru-hull.

But they have a 3 piece solution that matches perfectly with no mismatched threads.
You use a thru-hull, a flange that screws down on the thru-hull and bolts to the hull, and then add a 'valve' instead of a 'seacock'.
The valve screws onto the flange with matching threads...NPT. IBVF is the flange part number. take a look at groco.net, they have a picture that illustrates this. it's hard to find so look for the IBVF flange adapter.

 

[Rick Sylvester]

It'll work great, until it doesn't

Hey Roger,

I just replaced two thru-hulls and glassed over three more so I went through what you're dealing with. If you haven't seen it, Mainesail has a good website dealing with this very issue.

A thread mismatch will tighten securely, but once backed off a bit it becomes very loose. If you encountered resistance all the way off then it must have been sealant or dirt in the threads causing that. Not a great way to ensure a secure connection.

Your concern about flange mount surface area is a valid one. As you wrench a valve-mushroom combo open and closed it will eventually torque the thru-hull loose from it's sealant and will begin leaking (I had this happen.) The nut won't provide any real resistance to any turning force and you sure don't want to subject your sealant to turning loads.

Frankly, I think your yard manager is an idiot. If the boat is already hauled, why on earth would you ever avoid the relatively small expense of installing an adapter which would solve all these problems? Other than hauling and blocking under my supervision I tend not to let yard personnel work on my boat. I've found many do not give a crap about doing a job properly and prefer to do what they've done in the past. I live on my boat and, occasionally, my life depends on it.

Finally, although Groco doesn't make a 1/2" seacock, they do make a 3/4" unit and reducing from 3/4" to 1/2" would be no trick.

Many production boat builders avoid installing proper seacocks (or adapter equivalents) to save a buck or two. I know you said didn't advocate others doing it, but it sure sounds like you're suggesting that a thread mismatched piece of equipment keeping the water out of the boat isn't that big of a deal.

 

[Ross]

From the bright finish on the thru hull threads I wouldn't be very concerned about that failing. It is amazing just how bad plumbing can get and stilll function as built. Even better than teflon tape is brush on pipe dope. Pipe threads are either tight and they don't leak or they are loose and they do leak. No middle ground.

 

[Roger Long]

A thread mismatch will tighten securely, but once backed off a bit it becomes very loose.

It may depend on the fittings and manufacturing tolerance. After the threads were cleaned up, this valve and through hull were quite tight. Felt tighter actually than the NPS adapter on the through hull.

Frankly, I think your yard manager is an idiot. If the boat is already hauled, why on earth would you ever avoid the relatively small expense of installing an adapter which would solve all these problems?

I don't think these adapters were available at the time.

I know you said didn't advocate others doing it, but it sure sounds like you're suggesting that a thread mismatched piece of equipment keeping the water out of the boat isn't that big of a deal.

Both times I've looked at critically at mismatched fittings it hasn't seemed like as big a deal as it's made out to be. It certainly isn't best practice but I think the lack of a flange is a more critical issue. It may well be though that some combinations of individual NTP and NTS fittings do not go together as well as the ones I've worked with. This joint was pretty damn solid when I took it apart and the valve lever has been very stiff so there was plenty of force on it.

 

[Ross]

My boat passed forty years of service this past year and when I bought it, it had gate valves on all of the nps thread thru hulls. I sawed them off because I absolutely couldn't unscrew them. If they are out of harms way they should be fine. No pipe fitting should be placed, unsupported, in a position where it will be subject to mechanical stress not related to its function.

 

[Maine Sail]

Anyone..

Anyone who has done a fair amount of plumbing knows the inaccuracies in pipe threads.

Just this past Sunday night I was adding a hydronic heating zone to my brothers house up the street. We had two copper male adapters bottom out into female fittings and they were not yet tight. Despite the use of both Teflon tape and Pipe Dope it still likely would have leaked so we swapped out the fittings and found others that fit correctly. On the other side we had a couple that barely threaded in four or five turns before becoming very tight.

Many times I have had a valve bottom out when the threads were not yet tight and many times I have had NPT to NPT fittings only thread in four or five threads because both tolerances on male and female were to tight.

Risking tight tolerances when mixing NPT to NPS threads is a dangerous game. Sure, in some instances you will get four five or six threads but in others you may only get three or four.

This is a cut away I did to showcase NPS to NPT on the left and NPT to NPT on the right. These were straight off the shelf at Hamilton Marine then lubed with Sailkote and threaded in tight by hand to where they stopped and I would have needed more than my hand strength.

Problems do occur with mis-matched threads. Groco warns of it in their literature and most recently the voting members of ABYC added a section to H-27 Seacocks and Drain Plugs that say all threads must be compatible meaning either NPS to NPS or NPT to NPT. The ABYC does not make changes without merit and is made up of all type of individuals in the marine and even insurance industry. This wording was not done because there are no issues with this type of thread interface it was done because there are.. ABYC is not a mandatory standard, though most builders do adhere, but their guidance is the best we have in the US.

Here is another hypothesis on your thru-hull. I hope you don't mind that I took the liberty of editing your photo. If you do I will remove it.

In this photo I have marked the two threads that would be the tightest fit in red. It is apparent that these threads, when cranked down on with a wrench, had major thread to thread impingement and removed the teflon tape interface. With the teflon ripped away and removed and you had a metal to metal seal leaving behind the verdigris (green color) between the metals.

Because the female threads of the ball valve are tapered the top two or three threads on the seacock will be the tightest fit and th majority of your holding power. The threads marked in green did not even tear away the teflon tape leaving protected bronze underneath. If this was an NPT to NPT the teflon tape would have been removed in a more uniform manner and all threads would have been evenly tight not just the top few.

Photo by Roger Long

 

[Ross]

One of the reasons for the tapered threads is to bury the weakened portion of the pipe inside the fitting. A perfect pipe joint has almost 100 percent of the pipe diameter at the fitting. They could be straight threads with gaskets but they wouldn't be mechanically as strong

 

[Rick Sylvester]

Of course the mismatched threads can be made tight, but then, that's not the salient point of the issue. It's all about what happens if they become loosened. There's also the previously undiscussed matter of there only being a few turns holding the whole thing together. How big of a deal is it? I dunno, but I know I don't want it on my boat.

I confess I don't know when adapters came on the market but I imagine proper seacocks have been around longer than your yard manager has been in business. We can certainly agree that an installation without a flange base is a serious problem and I find the manager's lack of insight on that one troubling indeed (his 20 years of installations notwithstanding.)

Finally, and again, those threads may hang together but that's not going to matter much when water begins seeping from around sealant that's been wrenched loose. Yes, I know, it didn't happen with yours and the yard manager's never seen it, but it's happened to me. Maybe I'm just unlucky but I'd rather not depend on luck in this situation.

Best of luck to you.

 

[Maine Sail]

If you must

If you absolutely must use a thru-hull with a ball valve threaded to it please us a thru-hull with what is called a "combination thread" thru-hull.

While a combination thread is NOT a true NPT thread they essentially shave the tops off the first few threads and create a pseudo taper that will bury deeper into the female fitting. Groco thru-hulls with combination thread can be ordered..

This is directly from the Groco web site:


"In-line valves have NPT threads,
which are not compatible with
NPS threaded thru-hull fittings
(unless the thru-hull fitting is
machined with “Combination
Thread”. Installing an in-line
valve onto a thru-hull fitting will
create a mismatch of threads
resulting in minimal thread
engagement between valve and
fitting, and an unsafe installation.
Property damage, personal injury,
or both could occur. If you
choose to utilize an in-line valve
as a seacock, the thru-hull fitting
used must have “Combination
Thread”."

 

[Roger Long]

Groco thru-hulls with combination thread can be ordered.

I'll bet that is the key to this whole discussion. Those 1980 through hulls were originally intended to have gate valves on them. I'll bet they are combination thread. It should have occured to me before. The through hull I tried a NPT fitting on back when I first got the boat was also one I took out of the hull to replace with a larger one.

I was thinking of replacing this through hull with an new on but noticed that it is considerably thicker than the new Groco's. In addition to the difficulty of getting it out, why end up with a thinner one?

I'll bet that if I was to pick up a through hull down at the marine store and try it on a NPT valve I would find it just as insubstantial and problematic as you have been warning people about.

Those are certainly combination threads in my boat.

 

[Roger Long]

One of the reasons for the tapered threads is to bury the weakened portion of the pipe inside the fitting.

Well, the valves have only about 1/4 pipe diameter of thread anyway. See pictures above. I had full as much bury as the valve design allows although the last two threads weren't quite as tight as the others as MaineSail points out.

 

[sailingdog]

The problem with the Groco "combination thread" through-hulls is that there is absolutely NO BENEFIT to them if you shorten them to the appropriate length for your boat's hull. The only section that is partially tapered is the very end, and if you've shortened them to make them the appropriate length for your hull's thickness, you've eliminated the combination section. If you don't shorten them, you are often left with a more vulnerable installation, due to the longer lever arm you've left on the through-hull as well as one that requires more space.

 

[Roger Long]

The problem with the Groco "combination thread" through-hulls is that there is absolutely NO BENEFIT to them if you shorten them to the appropriate length for your boat's hull.

Quite true and I can't think of any reason to use them. I decided not to try replacing mine (Not a Groco but something with thicker walls. Wilcox Crittenden?) because I couldn't even budge the nut in the confined space and, why go to all the work to end up with a thinner fitting? I didn't want to cut it short in place because it would have been a nightmare job and I would have been cleaning bronze filings out of the bilger forever.

This was the solution:

http://www.rogerlongboats.com/09Winter.htm#January27

It's massively strong but I'd better hope I never do need to replace that thru hull!

 

[Maine Sail]

The threads..

The threads on the left are on a Groco 3/4" thru-hull fitting and are machined with a combination thread end. You can tell a combination thread because the tips or peaks of the first few threads look squared off as they have taken the tip off to try and mimic a true tapered thread. this is not a true tapered thread but will burry into an NPT fitting deeper than it would with no combination threads.

The thru-hull on the left is made by Apollo and has no combination threads. You can see by the pointed thread tips all the way to the end of the fitting that is has not been machined to be used in an NPT female fitting..

 

[Roger Long]

You can see by the pointed thread tips all the way to the end of the fitting that is has not been machined to be used in an NPT female fitting.

Now I'm flipping back. Looking at a close up digital camera picture of my old thru hull, I don't see any sign of such machining or any taper on the fitting. I now don't think they were combination threads. Possibly having NPT gate valves threaded on them hard when the boat was built made the threads a little more amenable but they don't look distorted.

The ball valve went on to the thru hull right to the bottom of the threaded portion. The end two threads were not as tight as the rest as you point out but they still would have been engaged. It was tight with just Teflon tape and very solid. The threads were clean with just tape and it took a lot of force to get the valve off. Good thing the thru hull is in sold. I suspect it was done with 5200.

I can believe that the yard manager who advised me back when I was new to this never had any problems doing it but also agree that it's not a good idea on something so critical. I'm glad to have the big one done properly. I'll probably try to do the remaining below waterline one as well now.

Say, how do you get those big pictures to show up in the middle of your posts?

 

[Hermit Scott]

Wonderful assortment

When I first looked at my through hulls and sea cocks, I thought "I think they'll be ok"
After 6 months on this site I think differently.
These are the sea cocks out of my boat. There is some copper, galvanized, plastic and bronze. And probably some brass. THe big flanged seacock is a groco, it wasn't installed on my boat, it was in a box but it has a rubber ball valve and a compression plate inside to tighten it up when you close it.
The other new ones came in the mail today. On maine sails recommendation I am sending the flanged seacock back. I ordered the flanged adapters, they'll be in next week.
I wonder how people learned about sailboats before the internet???

 

[Maine Sail]

Say, how do you get those big pictures to show up in the middle of your posts?

None of those photos are uploaded to SBO they are simply hot linked to my web site using this tool

..

It works really well if you use Mozilla Firefox and use "right click" / "copy image location" then paste it into the box after you click the little yellow icon above..

I can even do it with a picture of you..

 

[sailingdog]

Say, how do you get those big pictures to show up in the middle of your posts?

The difference is that your photos are attached to the post, and his are using the IMG tags. I believe you could also use the IMG tags for the attached images if you know the URL for them.

Appears to work with attached images too.

 

[Taylor]

Groco SV seacocks

The one in the upper left of the photo above is a Groco SV series seacock, my boat has about a dozen of them. Info about them is available here: http://www.groco.net/SVC-MAN-07/Sec4/PDF/SV.pdf. They have rubber plugs and a plate with wingnut to squeeze the rubber until it seals. I take mine apart when I haul for paint and lub 'em with some superlube.

Besides a dozen of the SV's I have one newer ball valve threaded onto some sort of through hull and one classic tapered brass seacock that is very stiff and needs a tap with hammer to prersade it to move. I worry about the old brass one, but until now I was not worried about the ball valve. Time to take a closer look.