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View Poll Results: Flanged vs inline? I'm comfortable with this style of valves:
Only flanged seacocks 15 65.22%
Properly installed inline valves 7 30.43%
Other (explain in comments) 1 4.35%
Voters: 23. You may not vote on this poll

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Old 01-03-2018, 08:38   #1
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Flanged seacocks vs Inline Valve with Thruhull and backing block

I know this is a contentious topic, but for a long time I accepted the conventional sailing wisdom of flanged seacocks are the only way to go. Recently a yard manager convinced me that inline valves can be perfectly acceptable too. Doing research I found often the arguments against inline valves were either not based on science / facts, or were pointing at improperly installed inline valves.

Here's an image of an inline valve done right (scroll to Figure 2): https://www.jamestowndistributors.co...t.do?docId=538

To be clear, an inline valve done right is a bronze thruhull with the thruhull nut tightened onto a hard backing block (G10 or similar) with a marine adhesive sealant and a marine bronze ball valve threaded onto the thruhull with appropriate thread engagement. The thruhull used has combination threads designed for a compatible match with the ball valve.

I found out there are a lot of boats using inline valves, and it seems to be more common in the motorboat community (and this isn't about offshore vs not offshore, because offshore superyachts are using them too). A lot of sailors disparage motor boaters, but I'd like to learn from them - they're mariners too, and don't like dying any more than we do.

If you read a number of sailboat sources online they're advocating that flanged seacocks are the only way to go (example). I like to be open minded - I think it's always worth questioning conventional wisdom. Some people in sailing communities are very rigid and seem to do things just because that's the way it's always been done, or based on something one or two people told them.

That's fine if you have all the time and money in the world, but it's also possible we're wasting time doing stuff that isn't really necessary (and this advice gets propagated). Thruhulls seem to raise vehement opinions, understandably because a failure can be catastrophic. So to some extent it's a personal decision based on what your safety comfort level is. But I prefer to get away from hyperbole and figure out whether the arguments against inline valves have any merit.

This guy actually had a motorboat yard install super solid ball valves and was upset because he assumed they'd do flanged seacocks, and decided to redo all the work with flanges: Thru Hull and Seacock Installation To me that seems like a ton of wasted work.

A lot of the arguments I see against inline valves are lacking in supporting evidence. Here are some of the points I see made which don't present supporting evidence:
  • Inline valves have a weak point at the thruhull threads and will easily break. I haven't seen any proof of this, other than electrically compromised metals (which is an entirely different situation). Have you ever actually tried snapping a bronze thruhull? People talk about the 500 lb side load impact test, but bronze thruhulls pass that.
    Also, how on earth would a 500 lb impact hit my head or galley valves that are closed up in a small cabinet? Obviously, installation location is a factor to consider, and most valves are not in locations where 500 lb loads are possible.
  • You'll have a thread mismatch because ball valves are NPT (tapered thread) and thruhulls are NPS. This seems to be mostly false. If you buy your ball valves at Home Depot, then you'll probably have a mismatch, but with marine inline valves designed for the purpose, there isn't this issue.
  • Inline valves aren't ABYC approved. False as far as I can tell. It just specifies surviving a 500 lb impact load for 30 seconds.

One point for flanged seacocks I do see that holds is:
  • If you have severe stray electrical corrosion which compromises your bronze thruhulls, a seacock will stay attached to the hull (maybe).
    However I think even this is questionable, because if you have severe corrosion then the 3 bronze thrubolts are just as likely to disintegrate, or the seacock ball or numerous other failure points. Really if this is your one concern, it's an argument for just using Marelon seacocks, or ensuring that you never have severe electrolysis.

I think people get much too worried about the risks imposed by thruhulls, when really the risks of merely going sailing are much higher. But in the interest of truth I want to learn how prevalent inline valves are, and whether there are any inaccuracies in our reasoning.
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Old 02-03-2018, 04:45   #2
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

I will agree with your last statement that people worry too much about thru-hull connections.
That said the only one Ive seen actually break, in my fifty years in the marine business, was a ball valve screwed onto a Thru-hull fitting. The bronze thru-hull snapped off when someone kicked it or dropped a tool on it. It was a brand new fitting.

Thru-hull fittings with combination threads are realitively new. They do eliminate the thread mis match. The problem is that most people wont build up the backing block enough and leave too much of the thru-hull fitting unsupported. There should be no more than about half an inch of the thru-hull showing between the bottom of the valve and the top of the nut.

Id rather spend a few dollars more, get a real seacock, and not worry about it.
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Old 02-03-2018, 05:13   #3
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

As usual, I find myself looking to the excellent site maintained by Compass Marine...
Bruce

https://marinehowto.com/seacock-failure-testing/
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Old 02-03-2018, 07:36   #4
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

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Originally Posted by Bruce Beard View Post
As usual, I find myself looking to the excellent site maintained by Compass Marine...
Bruce

https://marinehowto.com/seacock-failure-testing/
I had somehow not seen that yet, perhaps it's a recent post? I love that Compass Marine does actual testing. I've read nearly all of his site.

A few things I find interesting: the inline valve assemblies still held 404 to 458 lbs, which is over 80% of the standard. Not as good as 100%, but I get the sense 500 lbs is somewhat arbitrary? I think ABYC picked that because it's a nice round number and it's pretty high.

I'd like to have a better understanding of what 500 lbs of force is comparable to. It seems very high to me - I imagine it's like hitting the thruhull with a very large sledgehammer? Or perhaps having a group 31 battery drop on it from a height of 2 feet? And that's a dynamic load, not a static load.

Also note: Compass Marine was testing mismatched threads with a 7/8" gap between the valve and thruhull nut. He was testing "common bad installer practices" rather than the properly installed inline valve I outlined here. I'm not saying I disagree with his test, it's still interesting.

Compass Marine says they once saw a thruhull break when a spare alternator fell on it. I don't understand that. That's what, like 50 lbs of force at most? Maybe the thruhull had some major defect?

I think my perspective on this is coming from the installation location of our thruhulls. In our engine space where the seacocks are out in the open, I prefer flanged, but in our head and galley cabinetry, the seacocks are in a very confined space with shelving and hoses surrounding them. Even if we had a spare alternator sitting in there, it would be physically impossible for it to get close enough to touch the thruhulls.

I believe many thruhulls on modern boats are like this. On newer boats at boat shows I've seen many of their thruhulls are under stout floorboards with only 2-3" of gap above and around the thruhull.
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Old 02-03-2018, 07:48   #5
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

Also, Compass Marine's commentary is pretty interesting. He says "The builders get away with installations like this, because we the buyers allow it."

Exactly. People are voting with their wallets and saying they feel comfortable with the level of safety provided by inline valves. Of course it's possible some people are just ignorant, but probably not all of them are.

It's not relevant whether flanged seacocks are stronger than inline seacocks. It's obvious that they are. The relevant question is whether inline seacocks are sufficiently strong for the location and service they're used in. Ie, whether they provide a sufficient margin of safety.

You can always make something stronger by over-engineering it more. Buyers of new boats don't want the most over-engineered boat possible. That would make costs and weight ridiculous. They want a boat engineered with a sufficient margin of safety. The same applies to rigs. Airplane builders and even space businesses (satellites, shuttles, rockets) understand this - you don't build the strongest, heaviest airplane possible, you build one with a sufficient margin of safety.

Determining where the correct margin is is difficult of course, but that's why we have science and experimentation.
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Old 02-03-2018, 08:21   #6
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

I also agree that people worry too much about this. But if you cut a thru hull fitting longitudinally, you will find that the thickness at the root of the threads is actually very thin. Not only that but it is a perfect stress riser to start a crack. Yet it seldom happens. A flanged seacock can continue to function even if the mushroom threads were to break.

In line valves have pipe threads, mushrooms have straight threads. This can leave very little thread engagement, possibly even just 2+ threads. I've done enough pipe fittings over time to realize that the tolerances in the thread cutting, being on such a shallow taper, can add up to minimal engagement even with both pieces having pipe threads. Let's face it, home plumbing isnt required to do much but sit with no external pressure after installation.

I've never seen a mushroom thru hull with combination threads....? First I ever heard of it in the link you provided. With the variance of hull thickness from maybe 1/4" to 1.5", not sure how that combination thru hull would work, as the guide says no more than 1/2" gap between the thru hull nut and the valve.
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Old 02-03-2018, 09:17   #7
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

I don't understand why so many are stuck on metal thru hulls and valves with the potential issues.
Trudesign thru hulls with ball valves are ABYC compliant and you'll never have to worry about corrosion. The thru hulls become part of your hull, if it's a composite hull, and the valves will never seize.
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Old 02-03-2018, 09:39   #8
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

You bring up several issues. The ABYC standards indicate that a thru-hull/valve assembly must withstand a static load of 500 lbs for 30 seconds on its most inboard aspect, in other words the end of the tailpiece.
The flange on a seacock, does distribute the load, as does the nut if backed against a suitable backing block. The holes in the flange are there for fasteners that, if installed, prevent the seacock or thru-hull from twisting if the handle proves recalcitrant.
The bigger problem resides with the threads. For a threaded fitting to have full strength, a minimum of 5 fully engaged threads are necessary. Fitting tapered pipe threads onto straight threaded thru-hulls only allows 1.5, maybe 2 threads to be engaged. But there is hope. Most contemporary thru-hulls, at least those made by Groco have the last couple of threads tapered, thus allowing a much better join, as compared to thru-hulls wherein all of the threads are straight. Groco also makes a flanged bronze thru-hull ball valve adaptor- straight threads on the inside and tapered threads on the outside which resolves that issue of mismatched threaded and possibly a too-weak base.
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Old 02-03-2018, 09:40   #9
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

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Originally Posted by bcboomer View Post
I don't understand why so many are stuck on metal thru hulls and valves with the potential issues.
Trudesign thru hulls with ball valves are ABYC compliant and you'll never have to worry about corrosion. The thru hulls become part of your hull, if it's a composite hull, and the valves will never seize.
Many of us have had bad experiences with plastic fittings of all sorts. My experience is plastic valves all become very hard to rotate over time. It's not a very stable material. It reacts to temperature and vibration poorly and readily changes shape over time. Has low strength in any part of the design that must be small. It can be break and is often crack prone. Not just in Marine use but other uses too. I have had Schedule 80 grey pvc fittings 2"- 3" catastrophically break in industrial use . Those things are super thick.
Hopefully Tru Design are getting somewhere with using the material, but I'm not holding my breath.

I'm not sure what your " The thru hulls become part of your hull" means though?
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Old 02-03-2018, 09:44   #10
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

An alternative view.

Don't worry about corrosion - use Marelon through hulls and sea cocks.

I have 13 of them on my boat. Most of them are flanged sea cocks, some are the original Marelon valves threaded onto the through hulls. The boat is now 33 years old and because they are reinforced plastic they are as good as new.

About ten years ago I had to pull an original through hull to fit a larger one. after pulling it I did some destructive testing. Even at 20+ years old it was very strong. Smashing it with a hammer did not crack it.

This is how I install a through hull sea cock combination.

Take two pieces of plywood, one 1/4 inch and one 1/2 inch thick, square and large enough for the sea cock flange to sit on. In the 1/4 inch ply I drill a hole large enough for the thru hull. In the 1/2 inch piece I drill a hole large enough for the nut that came with the through hull. the two pieces of ply are epoxied together and epoxy coated.

Then put the through hull into the hull, bedded with 5200 caulk. Also adding 5200 under the backing plate to fill any gap formed by the hull curvature. Screw on the nut. It does not have to be torgued down very much - just enough to hold everything firmly in position.

Let it all cure. Note: wetting the 5200 accelerates the cure.

Once that has cured attach the sea cock using your favorite pipe sealant - I use a little 5200.

Then the flange can be screwed to the plywood.

To those that are concerned about the water pressure you can relax. The pressure difference at three feet under water is only 1.5 pounds per square inch.

Disclaimer: I have no connection with Forespar or 3M, just a satisfied customer.
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Old 02-03-2018, 09:48   #11
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

I had Marelon's. The handles broke off. The material was super tough though... as I found out having to replace those secocks that broke. They also get to where they will not turn. Plastic deforms. It's unstable, even when glass filled.
With a redesign they might be better. Go to West Marine. Pick one up and try to rotate the handle. You will often find you cant.
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Old 02-03-2018, 10:02   #12
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

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I had Marelon's. The handles broke off. The material was super tough though... as I found out having to replace those secocks that broke. They also get to where they will not turn. Plastic deforms. It's unstable, even when glass filled.
With a redesign they might be better. Go to West Marine. Pick one up and try to rotate the handle. You will often find you cant.
1) The handles have been redesigned, decades ago.

2) The valves are much easier to turn when the sea cock is secured to the hull.

3) As for the plastic being deformed I have not seen this.
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Old 02-03-2018, 10:26   #13
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

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Originally Posted by Tessellate View Post
...the inline valve assemblies still held 404 to 458 lbs, which is over 80% of the standard. Not as good as 100%, but I get the sense 500 lbs is somewhat arbitrary?...
Quote:
Originally Posted by Tessellate View Post
...It's not relevant whether flanged seacocks are stronger than inline seacocks...
If you're going to ignore facts and standards, why bother to read them at all?
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Old 02-03-2018, 10:27   #14
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

Quote:
Originally Posted by bcboomer View Post
I don't understand why so many are stuck on metal thru hulls and valves with the potential issues.
Trudesign thru hulls with ball valves are ABYC compliant and you'll never have to worry about corrosion. The thru hulls become part of your hull, if it's a composite hull, and the valves will never seize.


I installed all Trudesign seacocks while reconstructing my 70 y/o Alden ketch. Finding all the bits needed was a challenge - the folks @ Raritan were quite helpful. Quite reasonable pricing compared to bronze.
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Old 02-03-2018, 10:27   #15
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Re: Flanged seacocks vs Inline Valve with Thruhull and backing block

Quote:
Originally Posted by bcboomer View Post
I don't understand why so many are stuck on metal thru hulls and valves with the potential issues.
Trudesign thru hulls with ball valves are ABYC compliant and you'll never have to worry about corrosion. The thru hulls become part of your hull, if it's a composite hull, and the valves will never seize.
Id be a lot more comfortable with plastic fittings if only they made some effort to reduce the height of them to some more low profile design, having less leverage and less able to be hit and break. But they dont
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