Strength of Seacocks, Are Yours Strong Enough?

Maine Sail

Moderator
Feb 6, 1998
11,021
Canadian Sailcraft 36T Casco Bay, ME
For a long while I have been meaning to test the strength of seacocks using the ABYC strength requirements as a standard.. I finally got around to it, though slowly...

One installation I commonly see is a ball valve threaded directly onto a thru-hull as many production builders do for seacock installations. These builders very often display the NMMA & ABYC logo's proudly suggesting their vessels are built to meet the ABYC/NMMA safety standards.

In all fairness to the builders I suspect they believe these installations do or would meet the minimum safety standards but, as I found out, they certainly may not. Where is the in-house engineering by these builders who are violating the minimum strength requirements? Why do I have to be that guy who actually holds feet to fire? The manufacturers should be the ones ensuring these vessels meet the minimum standard before slapping the ABYC or NMMA sticker on the hull, but I digress....

I suppose we can't entirely blame the builders for installing the parts as "defined" in ABYC H-27 and then not having them meet the minimum standards? You would assume the manufacturers would have tested these fittings for the application into which they are sold but apparently they don't. ABYC leaves it up to them to prove "as installed"...

As I found out during testing a 3/4" Marine UL rated bronze valve threaded to a 3/4" Marine UL rated thru-hull fitting I purchased, does not meet ABYC H-27 Standards. It failed at 404 pounds and I was an 1 3/8" below the "innermost" hard piping so had slightly less leverage than I would have had I been able to apply the load at the absolute innermost "hard piping". This 3/4" installation should have easily handled 500 pounds for 30 seconds if it were to meet the minimum ABYC H-27 strengths standard. I have tested up to 1.25" in both bronze and Marelon. Marelon never got close. This 3/4" bronze thru-hull fitting was not the only seacock asembly I tested to not meet the standards. I also had Marelon valves also fail well below the minimum.

The good news is that the Marelon OEM / 93 Series does meet the standard as do every bronze flanged seacock I tested. In the case of an inexpensive Groco bronze flanged seacock it easily exceeded the standard by 300+ pounds.

Interestingly enough my findings are not alone, John Cly at Groco engineering concurred with me that this type of application, in multiple sizes *(I found the same), will not meet the H-27 standard. The found this in their own in-house testing too.

What does all this mean?


Does this test mean your seacocks will fail in normal use? Probably not, but we had one fail on us when a spare alternator slid across a shelf and hit the in-board end of the assembly so it can/could potentially happen. It happened to us back in the 90's. That single event is what prompted my entire diatribe on "proper seacock" installations etc.. It was a real wake up call for me..

My point here is that when buying a boat with this type of installation, for SIX to SEVEN figures, please be aware of, and know, that it is, and could be, a potential week point that may not meet the minimum ABYC or NMMA standards for seacocks and thru-hull fittings, at least in the 3/4" to 1.25" size ranges, the most common sizes....

What the ABYC definitions mean:

27.4.4 Seacock - A type of valve used to control intake or discharge of water through the hull. It is operated by a lever type handle usually operating through a 90° arc, giving a clear indication of whether it is open or shut, and is typically of the two following types:

27.4.4.1 Flanged Sea Valve
– A Seacock with an integral flange used to individually and securely mount the device directly to the boat hull structure.

27.4.4.2 In Line Ball Valve
A Seacock designed to be supported entirely by the through-hull fitting.

Some key points of ABYC H-27:

27.5.1 All piping, tubing, or hose lines penetrating the hull below the maximum heeled waterline, shall be equipped with a seacock to stop the admission of water in the event of failure of pipes, tubing, or hose.

27.5.4 Seacocks shall be designed and constructed to meet ANSI/UL 1121, Marine Through-Hull Fittings and Sea-Valves.

27.5.5 Thru-hull fittings shall be designed and constructed to meet ANSI/UL 1121, Marine Through-Hull Fittings and Sea-Valves.

27.6.1 A seacock shall be securely mounted so that the assembly will withstand a 500 pound (227 Kg) static force applied for 30 seconds to the inboard end of the assembly, without the assembly failing to stop the ingress of water.



27.6.1.1 The installation shall prevent any movement of the assembly.

27.6.1.3 Threads used in seacock installations shall be compatible (eg. NPT to NPT, NPS to NPS).



The Tests:


The tests were performed in my shop using the work bench as the test jig. I was planning to rig up some 5/8" thick fiberglass and bolting it to the bench using some big backing plates. After running some numbers it appeared the 3/4" plywood should handle the loads, and it did.

I initially started by just inserting the seacock into a hole in the bench but moved to an elevated sheet of 1/2" GPO-3 polyester resin infused fiberglass sheet. The elevated sheet allowed for better viewing of what was under the "hull" and any flex or other abnormalities..

I use a calibrated digital load cell to measure the load being gradually applied to the seacock. The load was measured in pounds and was initially set to capture the moment of failure or "peak hold". I later switched to actual load due to line stretch and knots slipping. I eventually got a good system using lower stretch Dynema and Vectran line, though they don't like knots and the knots slip too..

To apply the load I used an old Lewmar self tailing winch attached to the end of the work bench. The load was applied as gradually as possible. The line was attached to the seacock assembly at the innermost hard piping above the imaginary hull.. It should be noted that this was not at the innermost portion of the hard piping, as it technically should be. I was always slightly below that point as I had no way to guarantee the line would hold there. This slightly lower pull position favors the seacock assembly slightly.

TEST #1

This video, as are all of them, is straight out of the camera, unedited. I apologize for the load not being visible on camera until the thru-hull snapped. I did use the "peak hold" feature to capture the exact load when failure occurred. I shoot unedited so there are no questions.


The bench:


The materials are all "off the shelf" and purchased from Hamilton Marine in Portland, ME. To mimic what a boat owner or yard may install the materials are a mix of what was on hand at the chandlery. The bronze UL Marine thru-hull fitting was made by Apollo/Conbraco, the UL Marine valve by Groco and the bronze elbow was also by Groco.

The thru-hull was fitted to my bench and tightened down well using a spud wrench. The valve was then threaded onto it and the elbow threaded into the valve. The ball valve to thru-hull is a known thread mismatch, NPS to NPT, (another violation of ABYC H-27) but what is very often done by builders, boat yards and DIY's. I wanted to mimic a real world type installation and most, at least in bronze, are of mixed parts, not just one brand. It was not the thread mismatch that failed.

The 3/4" thru-hull failed at 404 pounds while trying to get to the 500 pounds and let it hold for 30 seconds. It never even made it to the ABYC minimum..


It failed flush with the nut.





This was taken at the 2011 New England Boat Show on a brand new vessel which is supposed to be built to NMMA and ABYC standards...

Think about being knocked down off shore in rough weather, losing the cabin sole cover board, which had no dogs to keep it in place, and stepping directly onto one of these while being tossed around in rough weather.. Food for thought any way. These seacocks were located at the base of the companionway stairs. This makes for GREAT access but a poor place for the potential to land on one if that floor board gets tossed in a knock down. Positive locking "dogs" on the cabin sole would go a long way to making this a much safer installation.


Test #2

Next I tested a 3/4" Marelon valve on a 3/4" Marelon thru-hull with a 90 degree Marelon male adapter. Many builders pumped boats out with this exact installation in the 80's and 90's. This is a late 80's higher end production boat with this exact set up:


The load or pull point with the Marelon set up was applied at 4 1/4" off the bench. The load or pull point for the bronze set up was 4 3/4" off the bench. This was a 1/2" difference in FAVOR of Marelon yet it still failed at less than half the applied load.

I have left the video unedited from start to finish.. Please ignore the commentary as that was not planned. I was however shocked at the low failure load and apparently could not help editorializing.....;) Many boat builders used this exact installation for years. There are ten of thousands of boats out there with this set up. Please be careful if you have this type of installation.





Test #3

Forespar saw my work and sent me a 3/4" OEM Series 93 Seacock, and thru-hull, to load test. The OEM/93 Series valve passed with flying colors!:) These are the valves that come standard on the vast majority of boats today as factory installed seacocks. The are a very good valve.

Unfortunately Forespar has not made them easily available to end users as they use a completely different thru-hull fitting and threads. Suffice it to say that if you have these valves they are very robust! Be glad your builder used them. These valves can be ordered for a retrofit and would be my recommendation if using non-metallic Marelon valves.

It should be noted that the 93 Series uses an entirely different thread and thru-hull fitting than do the Marelon ball valves or the Marelon tri-flange seacocks. This thru-hull fitting is significantly thicker and much stronger.




Test #4

This test has little to do with anything but drilling and tapping the backing block rather than drilling entirely through the hull of the boat for the seacock flange. Some boaters are adamant they don't want any more holes in the hull so I offered this option.

I originally posted this as; " Alternative Method Backing Blocks" on my web site for those who do not want to drill yet more holes entirely through the hull. On my site I recommend using 5/8" GPO-3 or 5/8" G-10 polyester/fiberglass or epoxy/fiberglass sheet.

For this test I used 1/2" GPO-3 polyester/fiberglass sheet. GPO-3 is very strong and G-10 is even stronger. GPO-3 is polyester resin/fiberglass and G-10 is epoxy resin/fiberglass.

One reader of my site emailed me, with concern, that if you hit something and lost the thru-hull the bolts would simply pull right out of the fiberglass sheet and you would sink. His take was that the bolts are a weak link when drilled & tapped into a 5/8" G-10 or GPO-3 backing block. Having tested this, well before I ever recommended it as an "alternative" option, I knew that to not be the case.

To mimic this mythical event, losing only the thru-hull fitting but not punching a hole in the boat in the process, I did not install the thru-hull. Instead relied solely on the three 1/4" X 20 machine screws to hold the seacock into the GPO-3 board.

So which was stronger a 3/4" Marelon tri-flange seacock or the 1/4 X 20 machine screws tapped into the 1/2" fiberglass sheet?

Disclaimer: It should be noted that the thru-hull is an integral part of the strength of the seacock assembly. The video below is an "unfair" representation of the failure point of the Marelon tri-flange seacock because the thru-hull fitting was not installed. I do plan to destroy another valve identical to this one, only with the thru-hull installed. My guess is that it will meet the 500 pounds for 30 seconds.

Here we go:


Test #5

This test had three parts.

1- To test the strength of drilling and tapping directly into a 1/2" fiberglass board to see if it meets the ABYC standards.

2- To test a standard 3/4" Marelon ball valve to ABYC strength standards.

3- To test the strength of a Groco flanged adapter in 3/4".

Disclaimer: In fairness to Forespar this was an early Marelon valve that already had the handle fail. Why ruin a brand new valve, right? Still, in my opinion, the strength of the valve body should not have been compromised by the missing handle. With the video viewed frame by frame the valve failed & slipped the first thread at 253 pounds.. The ABYC standards call for 500 pounds at the innermost hard piping or male adapter for a period of 30 seconds. This particular valve, an older model, did not meet the standard but this is not to say a newer version would act the same.



Test #6

Again the goal for this test was to see how strong the bronze seacock was as well as the drill & tap method, just like above. This valve and the drill & tap method easily exceeded the ABYC strength standards by over 300 pounds WITHOUT a thru-hull installed.

DISCLAIMER: The valve would be even STRONGER had I been using the thru-hull fitting. Seeing as nothing failed and I stressed it to over 800 pounds no "disclaimer" is really necessary.;)



More to come, as I test them...
 
May 24, 2004
5,855
CC 30 South Florida
Re: Load Testing A Seacock/Thru-Hull To Failure

Maine I agree with you about the proper installation for a seacock but if I were to to thread a valve directly to the thruhull I would cut the stem down to size perhaps leaving 1/2 inch between valve and holding nut. This would significantly reduce the leaverage torque force and could easily meet the the 500# input force requirement before breakeage. I believe this is a shortcut and not the proper way of doing it but allows the manufacturers to claim they are within guidelines. If you repeat your test with a trimmed down thruhull you should see that it will likely withstand in excess of 500#. Keep up the good work.
 
Feb 14, 2005
4,775
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Re: Load Testing A Seacock/Thru-Hull To Failure

This test illustrates that building an extended 'cantilever' at perpendicular to the hull makes the assembly 'more vulnerable' to breakage ... keep everything as close to the hull and not subject to 'cantilever' or moment loads and the structure will be able to bear more stress.

Good test however, interesting.
 
Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Re: Load Testing A Seacock/Thru-Hull To Failure

One should also keep in mind that these were new fittings with no corrosion to compromise the integrity of the materials. This was as strong as it would ever be.
 
Jan 4, 2006
2,500
Hunter H-310 West Vancouver, B.C.
27.6.1 is Not Clear At All ..................

................. as to what the "inboard end" of the assembly actually refers to. :confused:

27.6.1 A seacock shall be securely mounted so that the assembly will withstand a 500 pound (227 Kg) static force applied for 30 seconds to the inboard end of the assembly, without the assembly failing to stop the ingress of water.


Firstly, the "assembly" referred to seem to refer only to the through-hull fitting and valve itself.

27.4.4.2 In Line Ball Valve A Seacock designed to be supported entirely by the through-hull fitting.

No mention is made to any fittings or rigid pipe after the valve. If this is the case then:

a) your attachment point is about an inch too far out thereby adding additional bending moment due to the 404 lb. force.

OR

b) if the "assembly " does include the barbed 90 deg. fitting, as you've said, you're reducing the bending moment due to the 404 lb. force.

Either way, an excellent demonstration.

Now, having put all that aside, just what does one screw on to the end of an NPS thru-hull fitting ? I'm not aware of any NPS ball valves which accepts the male NPS thread. Every production sail boat boat I've seen at boat shows that uses this thru-hull arrangement has the NPS thru-hull to NPT valve arrangement. Just ram it in with loads of pipe dope ......... that'll hold it 'til she's outa sight. :eek:
Or maybe I'm just myopic.

Has anyone ever seen anything acceptable used on this type of thru-hull ? :confused:
 
Jan 27, 2008
2,904
ODay 35 Beaufort, NC
Mismatched threads

In the past the discussion has been around the mismatched tapered thread in the valve to the straight thread of the thru hull. In this experiment I was surprised that the failure did not occur at the thread mismatch. So in short all this experiment did was test the strength of a thru hull fitting in a cantilever beam bending test. You could just as easily build an argument that the thru hull fitting should be a slightly thicker wall to make it stronger (at the expense of flow capacity). Or if the threads were "J" threads with controlled root radii the stress concentration would be lower at the root of the thread giving it slightly more strength.
 
Mar 26, 2011
2,371
Corsair F-24 MK I Deale, MD
The solution is simple, and it is not stronger fittings.

This is a common problem in the chemical industry. Not seacocks, rather but piping carrying very hazardous chemicals, often made of weak materials (often the compatible material is PVC--many chemicals will eat through stainless or bronze in hours).

Support or guard the pipe. In the examples of the pipe on the floor, a simple tab sticking up, perhaps with a u-bolt or perhaps not, would solve it.

I do API tank, pressure vessel and piping inspections. If the pipe were in a potential impact zone and a failure was life-threatening (the same reasonableness test as a through hull), I would require support or reliable guarding. Particularly on small pipe, it is difficult to make it strong enough.

______________________

I've seen some bad accidents resulting from corrosive pipe failures. Both were from someone falling against a pipe while the guard was removed for maintanance! Granted, these were weak pipes. since then we have required tank flange mounted valves in certain services, 3/4-inch minimum. Anything smaller is too weak.
 
Last edited:

Maine Sail

Moderator
Feb 6, 1998
11,021
Canadian Sailcraft 36T Casco Bay, ME
................. as to what the "inboard end" of the assembly actually refers to. :confused:

27.6.1 A seacock shall be securely mounted so that the assembly will withstand a 500 pound (227 Kg) static force applied for 30 seconds to the inboard end of the assembly, without the assembly failing to stop the ingress of water.


Firstly, the "assembly" referred to seem to refer only to the through-hull fitting and valve itself.

27.4.4.2 In Line Ball Valve A Seacock designed to be supported entirely by the through-hull fitting.

No mention is made to any fittings or rigid pipe after the valve. If this is the case then:

The standard is very clear on this but I can't post the FIG #1 which shows it more clearly. I am an also ABYC member so do understand H-27 quite well. The load is to be applied to the "innermost" hard piping or the very end of a hose adapter whether plastic or bronze. I was already about 1" below that. with the elbow and would have been more had a straight hose adapter been used.


Now, having put all that aside, just what does one screw on to the end of an NPS thru-hull fitting ? I'm not aware of any NPS ball valves which accepts the male NPS thread. Every production sail boat boat I've seen at boat shows that uses this thru-hull arrangement has the NPS thru-hull to NPT valve arrangement. Just ram it in with loads of pipe dope ......... that'll hold it 'til she's outa sight. :eek:
Or maybe I'm just myopic.
A "combination thread" thru-hull is to be used in those applications. The problem is this is a band-aid of a "thread-match"... You CAN NOT cut down a combination threaded thru-hull fitting and the full length MUST be used to get a somewhat closer "thread match".


A combination thread is a half arsed way of making a ball valve thread onto a standard straight threaded thru-hull. Essentially the tips or peaks of the threads are machined off so the ball valve can thread over them and get more thread purchase. At least one manufacturer, Groco, does not recommend in-line valves on thru-hulls for below waterline use even if a combination thread has been used. Why? They don't pass their in-house testing to H-27 standards.

While this is clearly better than using a regular straight threaded thru-hull with a ball valve it is still a "shortcut" that may not meet the minimum standard depending of course on size. One major draw back is that you can not shorten the thru-hull to minimize interior height with a combination thread. The longer the stem inside the hull the more leverage there will be to potentially snap it off. Though the real reality is that very few builders, DIY's or boat yards every cut anything off a thru-hull when threading a ball valve to it. If you must use a thru-hull with a ball valve threaded on a combination threaded thru-hull fitting gets you a lot closer to thread match than NPS to NPT...

You can clearly see in the pic above how the first few threads are shaved down. This does NOT make it an NPT thread just a Band-Aid approach to get some more thread contact area. This design was likely driven by large production boat builders in an attempt to save a few pennies. While certainly better than mixing NPS to NPT, I would think hard about it before using a combination threaded thru-hull with a ball valve as you can't shorten it. It is still not true thread-match as the thread valleys are not cut any deeper just the peaks are shaved off..
 
Feb 14, 2005
4,775
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Has anyone ever seen anything acceptable used on this type of thru-hull ? :confused:
When replacing, I usually prefer Groco flanged valves and then special order the thruhulls through my favorite supplier .... an example: go to the online Groco catalogue, choose the correct thread profile thruhull and special order (you advise them of the correct part number) through your usual supplier.
 
Mar 16, 2010
5,943
Beneteau 411 Oceanis Annapolis
Well that is scary! There is small comfort in seeing the fitting slow deform before failure. I'd like to think that my linebacker nephew would see he was breaking the boat and jump off the thru-hull before it snapped! Bent thru-hull = replacement. I was also impressed that the much-fretted backing plate area showed no damage. Makes me wonder how important the backing plate is to overall fitting strength.
 
Jun 9, 2010
5
Mirage 33 Vanc
Re: Load Testing A Seacock/Thru-Hull To Failure

But, seems to me, we are missing the big picture here in this test - I'm assuming the "goal" was to show that the dreaded pipe thread mis-match is the weak link in the system - but apparently not! I think the question still remains - is this thread mis-match a "real" problem or perceived. It may still be "bad", but if the through-hull always fails before the valve assembly, not as big as deal as first thought.
Don't get me wrong, I still agree it's not the "ideal" way to make an assembly, but this test has shown it may not be as bad as perceived.
BTW, I really appreciate the time/trouble MS puts into these evaluations and all the information is invaluable.
 

Maine Sail

Moderator
Feb 6, 1998
11,021
Canadian Sailcraft 36T Casco Bay, ME
Here's the ABYC diagram for load testing.


I actually used less "lever" than what the ABYC calls for.


If you look at the video you'll see where the "load" was applied to the elbow which is a full 1 3/8" inward of where the ABYC load is to be applied at the "innermost" portion of the assembly. My original statement of "1" less" was off by 3/8" when actually measured.

I don't have a good way to apply the load to the "innermost" portion of a male hose adapter so used an elbow instead. In the process I gave up 1 3/8" of lever which actually favors the "seacock" when testing to ABYC H-27. Despite this 1 3/8" of favorable advantage for the seacock assembly, the thru-hull still failed at 404 pounds and never got close to 500 pounds for 30 seconds. A straight male hose adapter is even "taller" than the elbow and would thus provide for more lever.


On the opposite end there was just 7/8" of exposed thru-hull which, as originally stated, mimics installations I see on a daily basis. When I see a valve on a thru-hull I rarely if ever see them cut shorter. It would certainly help but I just don't see it very often in the real world.

Even if I managed to get the valve within 1/8" of the thru-hull nut, and then moved the load to the proper positioning for ABYC H-27, the thru-hull would still fail at well below the standard of 500 pounds at the innermost position of the assembly for 30 seconds..


This test also illustrates that building an extended 'cantilever' at perpendicular to the hull makes the assembly 'more vulnerable' to breakage ... keep everything as close to the hull and not subject to 'cantilever' or moment loads and the structure will be able to bear more stress.

Good test however, interesting.
Yes keeping it closer to the hull definitely helps but a flanged seacock would be significantly stronger and should meet the 500 pound standard with "typical" adapters etc..


How about this builder.....???;) Someone paid over $200k for this boat.....


And they do not always cut or shorten, even from the factory, as recently as 2011!
 

Maine Sail

Moderator
Feb 6, 1998
11,021
Canadian Sailcraft 36T Casco Bay, ME
But, seems to me, we are missing the big picture here in this test - I'm assuming the "goal" was to show that the dreaded pipe thread mismatch is the weak link in the system - but apparently not! I think the question still remains - is this thread mismatch a "real" problem or perceived.
That was not the goal at all. The problem with the thread mismatch is that every thread cutting die has tolerances, and they dull and change over many cuts. Different factories use different machines and different dies from different manufacturers. As a result there are small variances. Anyone who's done a lot of plumbing would know how even matched fittings, from different manufacturers or even different lots from the same manufacturer, fit together quite differently. Some times you can get four turns, as I was able to to on this mismatch, and sometimes you can get just 2.5 turns because of tolerance differences. This test was to simply see if a 3/4 valve threaded to a 3/4" thru-hull would meet the ABYC standard. I didn't care where it failed I just wanted to see if this very "typical" installation meets ABYC H-27 in the 3/4" size. it doesn't, but also does not mean you're going to sink...

It may still be "bad", but if the through-hull always fails before the valve assembly, not as big as deal as first thought.
Having had a thru-hull fail on us, what actually got me fired up over all this seacock stuff many years ago, I can say without a doubt that when it happens to you, it becomes a big deal pretty fast....


Don't get me wrong, I still agree it's not the "ideal" way to make an assembly, but this test has shown it may not be as bad as perceived.
My point is to bring caution to this situation and bring awareness. If folks understand the potential risks they can keep what they have an still minimize risk by such things as securing floor boards covering seacock assemblies, or making tankage hold downs close to seacocks "bomb proof" etc. etc... There are hundreds of thousands of boats out there with these installations and the vast majority never have an issue. It still never hurts to know these things so that when it comes time to replace your seacocks you will at least know of other options.
 
Feb 14, 2005
4,775
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Yes keeping it closer to the hull definitely helps but a flanged seacock would be significantly stronger and should meet the 500 pound standard with "typical" adapters etc..


How about this builder.....???;) Someone paid over $200k for this boat.....


And they do not always cut or shorten, even from the factory, as recently as 2011!
YIKES !!! That installation, with or without flanges, should be 'protected' and with 'nipples shortened' to avoid what myself and Thinwater have previously described ... just good 'piping practice' to do so. Such is not always found in 'specs'.

The real benefit of such discussion is this 'expands common knowledge' for all of us :)
 

Maine Sail

Moderator
Feb 6, 1998
11,021
Canadian Sailcraft 36T Casco Bay, ME
YIKES !!! That installation, with or without flanges, should be 'protected' and with 'nipples shortened' to avoid what myself and Thinwater have previously described ... just good 'piping practice' to do so. Such is not always found in 'specs'.

The real benefit of such discussion is this 'expands common knowledge' for all of us :)
The real scary part was people were looking at me cross eyed that I'd even want a picture of a seacock. Out of sight out of mind.. They were more concerned about whether the drapes came "standard equipment"...;) The builders only get away with this shoddy construction because we the buyer allow it..

At least one builder at the 2011 show went for rugged...
 
Jan 27, 2008
2,904
ODay 35 Beaufort, NC
Threads

Just a few notes. Threads are designed to come in contact at the pitch diameter. While MaineSail is correct that threads have tolerances they should not be coming into contact at the root of the female thread and the tip of the male thread. So machining off the male thread should not have any effect at all on engagement of the parts since the pitch diameters will still engage the same whether the tips are there or not. If the tips came in contact and not the pitch diameter then the cutting action of the tips on the mating part would cause a thin wall condition and add a really high stress concentration to the joint.
This practice of shaving down the tips is a common practice when dealing with aviation "J" threads. These threads have a controlled root radius to reduce fatigue susceptibility and the tips of the mating part thread are machined off so they do not make contact. Since these are not "J" threads in this experiment the contact should only be at the pitch diameter.
This is why you need to use teflon tape or plumbers putty on a threaded joint. The teflon fills the tiny space that exists between the tips of the threads and the root of the mating piece so you don't have a leakage path through that space.
 
Apr 14, 2009
627
Sabre 28 NH
Re: Load Testing A Seacock/Thru-Hull To Failure

Those aren't seacocks, there just ball valves & some pretty sleezy looking ones at that.

Bob
 
Jan 15, 2006
388
Macgregor 22 Silverton
Re: Load Testing A Seacock/Thru-Hull To Failure

Been burned by that same principle myself: "A UL part does not make it a UL installation."
 
Dec 15, 2011
103
Oday 20 SF Bay Area/Monterey Bay
"A UL part does not make it a UL installation."
Truer words have never been spoken.

Thank you MS for bringing this topic into focus for the masses. I am new to sailing but shopping for boats as a plumber, on-board plumbing techniques and sea cock installation have baffled me. Lack of proper bracing technique drives me insane. ~Craig