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PVC below the waterline


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Came upon this little divergence from ABYC dogma during my thru-hull replacement adventure recently.

So, we all know ABYC recommends only corrosion-resistant materials below the waterline, and most take this to mean bronze (stainless being subject to crevice corrosion) or Marelon.

But while projecting I got the impression that commercial fish boats sometimes use Schedule 80 PVC (grey, thicker-walled than white) fittings below the waterline, after the nipple. in other words, in applications where they are not mated to a fixture such as a seacock, but instead are connected to/buried in flexible hose on both ends, such as a 90-degree bend or a tee.

It seemed a little sketchy to me, so I avoided it, but I did end up using it in one place where I couldn't find an equivalent bronze fitting. In that case, bronze would have been a Franken-fitting constructed of 6 individual pieces and amounting to ~10 pounds of weight hanging on the hoses/clamps, and costing several hundred bucks -- versus 3 Sch. 80 fittings, mostly buried in hose, weighing about 8 ounces. Since the location is in fact above the waterline (just), and also extremely protected, so I went with the plastic.

I wasn't happy about it, but I also figured that since that line now has a seacock I can easily swap the fitting for a metal one if I can find something better. The next best solution I've found is buying a block of bronze and having a machinist friend mill it into the correct/ideal single fitting (a 1"-to-2" reducing Y).

Anyone seen or heard much discussion/opinion on Schedule 80 being used this way? Any clever ideas to avoid the franken-fitting short of custom milling/casting?

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I don't have a copy of standards but I think ABYC only cares about the seacock is strong enough and attached to suitable homes.

With a hose at each end I'd have no issue with PVC fittings or valves.

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Q for the group... If commercial fish boats have to be USCG approved, and the Commercial fish boats are approved by USCG, where is the hold up?

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Afraid i can't help you on the regulations, but ABS is better than PVC. Considerably more impact resistant, if you spend any time in cold waters do not use PVC. Put a piece of PVC in the freezer then throw it on a concrete floor. The Sch80 PVc is a bit of weird USA only thing, its a dimension standard copied from steel i think, there's no logic in the fittings being the shape and size they are for what's required strength wise from plastic, and in fact the sheer size of them means they get internal stresses from the injection molding process that would be better avoided. 

I'd be careful using either on anything connected to something that vibrates, engine or pump. There's glass re-inforced polyprop fittings (marelon equiv) that are better. 

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1 hour ago, Nodrog said:

Afraid i can't help you on the regulations, but ABS is better than PVC. Considerably more impact resistant, if you spend any time in cold waters do not use PVC. Put a piece of PVC in the freezer then throw it on a concrete floor. The Sch80 PVc is a bit of weird USA only thing, its a dimension standard copied from steel i think, there's no logic in the fittings being the shape and size they are for what's required strength wise from plastic, and in fact the sheer size of them means they get internal stresses from the injection molding process that would be better avoided. 

I'd be careful using either on anything connected to something that vibrates, engine or pump. There's glass re-inforced polyprop fittings (marelon equiv) that are better. 

Interesting about the glass reenforced ones. Brand or source?

Also do you have a source for more info on the sch 80 moulding stresses?

Not too worried based on what you said about temps, since this pnw 27 footer isn't going to be doing any Labrador cruising anytime soon.

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Why does PVC seem so tough and durable when handled, but look away and SNAP! I have a rural property full of that garbage. Completely untrustworthy. Low cost I suppose. I have a pile of shattered 4" Sch. 80 PVC valves and fittings headed to the dump right now. I think is gets a strain on it from earth movement then something shocks it. The city just repaired the burst 8" PVC water main in front of the PI home. Apparently a wayward stone pushed on the pipe and split it. Nutzo to use below the waterline no matter what the standards say. 

 

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3 hours ago, Breamerly said:

Interesting about the glass reenforced ones. Brand or source?

Also do you have a source for more info on the sch 80 moulding stresses?

Not too worried based on what you said about temps, since this pnw 27 footer isn't going to be doing any Labrador cruising anytime soon.

https://www.forespar.com/marelon-marine-boat-plumbing.shtml just hose connects and valves. 

the molding stresses afraid not, goog may find you something, used to work for a distributer of the stuff long time ago, feedback from the engineers.

Temps, at 5deg C think you have a problem with rigid PVC becoming brittle

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I have spent much time working with PVC schedule 80 and 40.  I don't even like to see schd 40 in my house.  We had a schedule 80 fitting fail at work that hit a concrete wall and damaged the concrete!  All this however is in the range of design working pressures.  With inches of water column  on a boat I would not worry a whole lot, but I would advice only schedule 80 and wouldn't use it in extreme cold conditions.  Use it only between stress and vibration free points.  It is impervious to salt water and in my opinion is superior to running any type of hose any significant length- many less hose clamps to deal with maybe and cheaper too.  Use the glue in accordance to its instructions and properly ream pipe sections.  

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https://www.trudesign.nz/marine/certifications

Full range of valves and fittings, wouldn't use anything else these days.

https://www.trudesign.nz/marine/products

7 hours ago, Zonker said:

With a hose at each end I'd have no issue with PVC fittings or valves.

Quite agree, its pretty popular in commercial boats and yachts. Very easy to use, light, doesn't rust. The GF  stuff is pretty useful.

https://www.plasticsystems.co.nz/sites/default/files/pdfs/Brochure - Metric PVCC%2C PVCU.pdf

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12 hours ago, El Boracho said:

Why does PVC seem so tough and durable when handled, but look away and SNAP! I have a rural property full of that garbage. Completely untrustworthy. Low cost I suppose. I have a pile of shattered 4" Sch. 80 PVC valves and fittings headed to the dump right now. I think is gets a strain on it from earth movement then something shocks it. The city just repaired the burst 8" PVC water main in front of the PI home. Apparently a wayward stone pushed on the pipe and split it. Nutzo to use below the waterline no matter what the standards say. 

 

This is my gut feeling about PVC in general. It's been nothing but delicate where I've worked with it (only with sch40 though), and seems to shatter at the slightest provocation. I am surprised to hear this about schedule 80 though - seems well-regarded even by the working boat guys, and is tangibly heavier-duty than sch 40 - but I will definitely take it on board along with what Nodrog was saying about molding stresses.

 

7 hours ago, BOI Guy said:

https://www.trudesign.nz/marine/certifications

Full range of valves and fittings, wouldn't use anything else these days.

https://www.trudesign.nz/marine/products

 

Like I said in the OP, Marelon is popular but doesn't make the fittings I need. Unless I missed something on their website, they basically only sell seacocks, ball valves, and straight elbows and tailpipes (king nipples). They don't sell anything I could find for more complicated systems like Tee's - let alone reducing nipples or bushings.

TrueDesign looks like they make some super cool products - they've got a position-monitored ball-valve with a collar for hull mounting that they are sellingfor ~125 - definitely would have considered that if I had known about it, although probably overcomplicated. But again, while they sell reducing nipples, they don't sell tee's.

 

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I didn't explain my specific situation at first but it seems relevant now...

In a nutshell, I have to join a pair of 1.5" scupper drain hoses above a 2" thru-hull. That dictates either a "bull head tee" and regular king nipples or a regular 2" tee with either reducing nipples or reducing bushings + straight nipples on two legs.

The bull head tee would be the very best - one fitting to rule them all. Actually, a bull head barbed WYE would be the ideal thing. But I even bullhead tees (the opposite of a reducing tee) are a relatively uncommon fitting, even in black pipe. Finding a bullhead tee in bronze to say nothing of a bullhead wye was impossible, at least where I looked (mcmaster, various plumbing warehouses, fisheries, SeaMar, amazon, internet plumbing sites).

A shipwright friend suggested just upsizing the cockpit drains to 2" - which is admirably simple but would require some fonnnky glass work b/c the drains are in the corner of the footwell.

Anyhow, like I said I'm not super thrilled with the solution I came up with - definitely appreciate everyone's ideas. Sch 80 or ABS seem like the best worst options. Might rig up a dummy of each set up in some fixed hose and stress-test both for fun.

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2 hours ago, El Boracho said:

That is why nobody makes your "bull head tee": because simply using bushings in a 2" tee is obvious, no?

A bullhead tee would be the ideal, is all I was saying.  And plenty of people make them, just not in bronze. And yes, you could obviously do it by Daisy-chaining half a dozen fittings together. Heck, with reducing nipples you don't even have to do that much Daisy-chaining, if you can find them in marine bronze (McMaster has them in some kind of bronze but???)

My point was just that doing it that way seems like it has its own drawbacks: it's unweildy/bulky, it's expensive, and it would be heavy enough that to do it right send like it would require some kind of strapping or other external support.

Thus I was curious what people thought of the only apparent alternative (plastic). Which is why I created a post asking about PvVC below the waterline, not a post asking 'is it possible to do this in bronze, if you're willing to spend several hundred bucks to make a Frankenfitting?' I already know the answer to that one.

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4 minutes ago, El Boracho said:

No worries. I thought PVC was the subject. Yes, in bronze that would be a monster. 

Eh, I'm probably just bring a cheapskate baby and should just pony up for the bronze and fabricate some sort of adjacent support member while I'm at it

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9 hours ago, Nodrog said:

If you're not comfortable with plastic it sounds like fabricating the right shaped Tee, or Y, out of s/s tube is going to be cheaper / lighter / better? 

Ooh I'd heard as was a no-no though. I had wondered about it -cockpit scupper sees a lot of water movement, dries out sometimes, so theoretically gets oxygen, won't crevice corrode - but I've heard people say it's bad to use at/below the waterline

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Typically class requirements focus on the shell connection or hull connection and are more lenient after.  Sch 80 PVC is very common is sw applications it's also one of the only good materials for high chemical ones like sodium chloride dosing. CPVC sch 80 does help the fracture issue as it's a bit stronger.  There are a quite a few commercial plastics used with class approval.  Welded HTPE, same stuff they make gas cans out off is very common.  Aqua is another welded one heavy wall.  Mepla is a composite aluminum core crimp.  PVC stuff tends to be pretty bulky so I don't use much, but it's usually available everywhere so it is a good backup.

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If you shop around there are quite a few marlon options for tricky fittings as well.  PVC hose Barb's can be a bitch on marine hose to get matched correct.  Something I have seen on our boat and other older wood boats for close stuff is sweated copper.  Heavier wall but simple sweated copper with stubs for hose connections.  Some of the stuff I pulled out was quite old with no wear or issues and seemed to work fine.  For new build or large instillations I would probably use CuNi crimp.  Viega is the standard and Mapress is the metric.  Same tooling to crimp both different dies.  In the last several years they have a pretty huge fitting line to make for some very nice clean installs.  The o ring crimp fittings allow for a little flex and the union combinations for easy removal.  The tooling is a lot more than most would want to invest for a small job, about $2k for a 1/2" to 2" set.  But it's super easy to run just need a tube cutter and off you go.

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12 hours ago, SASSAFRASS said:

If you shop around there are quite a few marlon options for tricky fittings as well.  PVC hose Barb's can be a bitch on marine hose to get matched correct.  Something I have seen on our boat and other older wood boats for close stuff is sweated copper.  Heavier wall but simple sweated copper with stubs for hose connections.  Some of the stuff I pulled out was quite old with no wear or issues and seemed to work fine.  For new build or large instillations I would probably use CuNi crimp.  Viega is the standard and Mapress is the metric.  Same tooling to crimp both different dies.  In the last several years they have a pretty huge fitting line to make for some very nice clean installs.  The o ring crimp fittings allow for a little flex and the union combinations for easy removal.  The tooling is a lot more than most would want to invest for a small job, about $2k for a 1/2" to 2" set.  But it's super easy to run just need a tube cutter and off you go.

Sweatted copper is an interesting thought. The rest of what you said might as well be Latin to my Luddite ears.

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12 hours ago, SASSAFRASS said:

If you shop around there are quite a few marlon options for tricky fittings as well.  PVC hose Barb's can be a bitch on marine hose to get matched correct.  Something I have seen on our boat and other older wood boats for close stuff is sweated copper.  Heavier wall but simple sweated copper with stubs for hose connections.  Some of the stuff I pulled out was quite old with no wear or issues and seemed to work fine.  For new build or large instillations I would probably use CuNi crimp.  Viega is the standard and Mapress is the metric.  Same tooling to crimp both different dies.  In the last several years they have a pretty huge fitting line to make for some very nice clean installs.  The o ring crimp fittings allow for a little flex and the union combinations for easy removal.  The tooling is a lot more than most would want to invest for a small job, about $2k for a 1/2" to 2" set.  But it's super easy to run just need a tube cutter and off you go.

Where did you find the marelon fittings? I looked on the catalog on their website and they didn't have much, mostly just different sizes of tailpiece.

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