# PVC Pipe for a holding tank?



## kwaltersmi (Aug 14, 2006)

I was thumbing through the current edition of Good Old Boat magazine and noticed a quick anecdotal article about replacing and old, stinky holding tank. Interstingly, the holding tank was replaced with a 6 foot section of 4" diameter PVC pipe (roughly 15 gallon capacity). The PVC pipe had fittings fabricated into one end for the inflow from the head and the deck pump out pipe on the other end. Somewhere in the middle a smaller fitting was attached for the vent hose. 

Replumbing and possibly replacing the holding tank on my boat is on my spring "to-do" list. My current 15 gallon holding tank is under my v-berth and was factory installed before the molded cabin liner was installed over the top. Unfortunately, there is no way to remove the old tank without cutting it up and again, not much room to fit in a new tank through the small access hatch. 

Is the PVC pipe holding tank a realistic option? If so, I'm sure a similar sized PVC pipe would fit nicely under my v-berth. Do you see any odor or leakage/filling/pumping issues with this idea? My initial concern is that the PVC pipe would need to be mounted such that the in-flow pipe was elevated higher than the outflow pipe.


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## brak (Jan 5, 2007)

seems like it would have a lot of surface area relative to it's volume. I don't know if it matters - but sounds like "stuff" may have a lot of surface to stick to.

Also, it would need some way to add baffles. A long pipe without baffles, partially filled with liquid would probably behave a bit like "water hammer" - with the entire mass of liquid alternatively hitting front and back ends.


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## sailingdog (Mar 19, 2006)

Properly fabricated, the PVC pipe tank wouldn't leak, but I'd highly recommend you use Schedule 80 pipe.

However, the vent pipe and inlet pipe should be at the TOP of the pipe...the pumpout fitting should be at the lowest point of the tank. I'd also make one end of the tank a cleanout cap, so that you can clean out the tank if necessary. A good way to do this is to make the higher end of the tank a "T" fitting with one end being a clean out cap, the T itself would have the inlet and vent fittings and the pump out fitting should be at the other end and the tank should be installed at a slight incline.

Installing the tank slightly inclined would also help prevent the sloshing/free surface effects that BRAK describes.

BTW, a 4" PVC pipe six feet long would not be 15 gallons... it'd be closer to 3.9 gallons. _Going by Nominal OD, that tank would be 3.14 * 2 ^ 2 * 72 = 904.752 cu. inch. 904.752/ (12*12*12) = .5236 cu. ft. * 62.4 lbs/ cu. ft = 32.67 lbs H20 / 8.35 lbs/gallon = *3.913 gallons.* _

To get approximately a tank with a capacity of 15 gallons, you'd need:

8" diam. x 72"
10" diam x 48"
12" diam x 32"

 I hope that helps.


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## WanderingStar (Nov 12, 2008)

Might help me. I'm thinking of gray water tanks to install in a bilge that's full of heavy frames and floor timbers. I don't want to remove the sole, and I worry about bladders chafing through. Maybe side by side pipes would do.


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## brak (Jan 5, 2007)

sailingdog said:


> Installing the tank slightly inclined would also help prevent the sloshing/free surface effects that BRAK describes.


I think slight incline may not be sufficient. If incline is less than the usual pitching of a boat, it isn't too different from horizontal pipe - as long as boat pitches through horizontal pipe position.


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## sailingdog (Mar 19, 2006)

Given the absolute size of the tank, since he's looking at about a 15 gallon tank, I doubt it is going to be much of an issue. If it was an 100 gallon tank, then I'd be worried, but a 15 gallon tank is about 135-150 lbs... not that significant a weight when you get right down to it, especially on a boat the size of the OP's-which displaces about 8000 lbs. The tank would have to be well secured of course, but a fiberglass bracket would do that nicely.



brak said:


> I think slight incline may not be sufficient. If incline is less than the usual pitching of a boat, it isn't too different from horizontal pipe - as long as boat pitches through horizontal pipe position.


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## blt2ski (May 5, 2005)

I personally would not want to use PVC pipe in general, altho the green Sch 80 sewer pipe I might. You would not want to use the gray electric conduit or white PVC pipe in Sch 80 either. You could verify with stillraining as he uses the green pipe more than I at work, as to which would be better if you have to go this way. I believe the green will keep the smell out better. Altho it could be green more from a, You did up a pvc pipe, green is sewer, white is water typically.

You can get the green sch 80 in sizes bigger than 4" if you want to, ie 6, 12 and bigger. 4" is not your only option if you want a bit more volume.

marty


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## bljones (Oct 13, 2008)

I'd go with a larger diameter (8 - 12") , simply to make the required length shorter, thereby reducing the effects of "slosh" and weight transfer. Having lived with a 10 gallon holding tank, I'd look at making 15 gallons my benchmark, and play with the dimensions accordingly. I think PVC pipe CAN be a great idea, providing you a) think it through as far as vent and drain location, and b) make sure you use the right adhesives and apply liberally.


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## Stillraining (Jan 11, 2008)

Interesting Idea....IMHO Sch 80 is way overkill and schedual 40 is plenty thick @ over 2 tenths of an inch. You can basicly run over it with your car. unscathed.

As an example 12" Sch 40 has an operating pressure of around 120 psi Sch 80 around 230 ..I dont think one needs to worry about sch 40 bursting under any load a holding tank might produce no mater how rough the sea gets.

I doubt many if any poly holding tanks are that thick....as far as smell permeation it is non existent with SDR35 and greater PVC pipe...so in that regard its ideal.

I think it has possibility for the right boat.


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## sailingdog (Mar 19, 2006)

A 14" dia x 30" long PVC pipe would be about 19.5 gallons or so.



bljones said:


> I'd go with a larger diameter (8 - 12") , simply to make the required length shorter, thereby reducing the effects of "slosh" and weight transfer. Having lived with a 10 gallon holding tank, I'd look at making 15 gallons my benchmark, and play with the dimensions accordingly. I think PVC pipe CAN be a great idea, providing you a) think it through as far as vent and drain location, and b) make sure you use the right adhesives and apply liberally.


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## sailingdog (Mar 19, 2006)

Some of the better poly holding tanks are 3/8" thick or so for the wall thickness. Of course, the larger the tank, the thicker the wall needs to be.


Stillraining said:


> Interesting Idea....IMHO Sch 80 is way overkill and schedual 40 is plenty thick @ over 2 tenths of an inch. You can basicly run over it with your car. unscathed.
> 
> As an example 12" Sch 40 has an operating pressure of around 120 psi Sch 80 around 230 ..I dont think one needs to worry about sch 40 bursting under any load a holding tank might produce no mater how rough the sea gets.
> 
> ...


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## Stillraining (Jan 11, 2008)

I did not know they were that thick...my 20 gal ones sure arnt...maybe 3/16 at the most.

But equally true is that flat sided tanks would need to be by design heaver constructed then a pipe for the same strength ratio...we bury 8" SDR 35 which is standard sewer pipe and which is one grade lower then SCH 40 PVC to 16' before switching over to Ductile Iron pipe by code...there is no denying its capability to withstand pressure.

A few brown trout sloshing around in a length of pipe on board is child's play for it.


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## kwaltersmi (Aug 14, 2006)

Thanks for all the replies! Sounds like the idea has some merrit thought through. FYI - We only go wee wee in our head, not poo poo, so the contents would be purely liquid.


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## Siamese (May 9, 2007)

Hey, Kwalt, I think you're working too hard. Although the PVC sounds like a clever idea. 

I see you're in Grand Haven....howdy from Muskegon!

I have a Catalina 309 that I bought new in 07. After the first season (with some...not a lot...of odor problems), there was some additional problem with the head when we de-winterized the following Spring. The marina that was supposed to pump the holding tank apparently failed to do so, resulting in some freeze damage. So, in addition to some mild odor problems on a new boat, I was also facing some additional repairs. Leaking at the base of the toilet, etc.

Screw it. I took out the toilet, and have it stored in the box that my new PORTA POTTI came in. 75 bucks....done.

-it looks nice in the head
-very easy to empty...REALLY...there's s a swing out spout so you can easily dump the thing in any toilet onshore. No mess.
-odor problem is GONE
-if a passenger drops a big one in your porta potti it's not a problem. If that happened with my old toilet...I don't even want to think about it!
-It's easier for passengers to use.
-Frees up storage space where the holding tank used to be.
-no lines to give off odor or leak
-no pumping fees or trips to the gas dock just for a pumpout.
-no finding yourself 10 miles out on lake Michigan and discovering the holding tank is full.
-no repairs. In four years of use, I've had nothing to fix. If I should have a repair, it'll be cheap and easy, and can never exceed the cost of a new unit.
-I can tell you that we don't have to empty the thing any more often than we used to pump out the holding tank.
-Installation on our boat consisted of velcro on the bottom and a webbed hold-down strap around the pottie to a fitting installed behind it...easy and looks fine.
-in addition to normal cleaning and wiping, once or twice a season I put the whole thing on the dock and blast it and scrub it and let it dry in the sun. We're talking CLEAN.

We use our boat every weekend with a couple longer trips every season. It fits out usage just fine.

I've posted this solution to head problems before and there's never been a response to it. If people have an objection to this solution, they aren't saying what it is. I can only guess.

-not something that belongs on a REAL boat?
-worried about emptying it?

I dunno. I can tell you sincerely that it's the perfect solution on our boat.

I've talked to just a couple of other sailors who have switched to the porta potti solution and they are enthusiastic in their agreement.


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## kwaltersmi (Aug 14, 2006)

Siamese - I like the idea. However, my reservations stem from the fact that my marina doesn NOT allow dumping of porta-potti's and has this posted on each restroom entrance. I think they're concerned about chemicals, or losing the $23 pumpout fee!

My only other concern would be capacity. My current holding tank is only 15 gallons and I find it to be too small four our family of four, even though we only use it for urine (no poo!). 

If I could eleviate those two concerns, I'd be willing to go with a porta-potti.


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## bljones (Oct 13, 2008)

Stillraining said:


> I did not know they were that thick...my ones sure arnt...maybe 3/16 at the most....
> 
> ... brown trout sloshing around ...


Once they are in the holding tank, they are on the endangered feces list, no matter what size they are.

(oh, c'mon, this thread has been far too mature and civilized for far too long- somebody had to take a run at the subject matter.)


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## Stillraining (Jan 11, 2008)

Figured some one would latch onto that eventually. 

We had a porta potty in all our power boats...the one in the sea ray was plumed for pump out, macerating and everything...worst smelling head we have ever owned.

Way to small of holding tank at 5 or 6 gallons or what ever they were...we had to pump out every other day with our family of 5.

Sometime down the road I will build custom fiberglass non permeating ones with a gasketed inspection ports on top to replace the poly tanks that are in the boat now ... I have no idea how old they are but I know they are permeating a bit...Poly is a nice product for some things...holding tanks is not one of them.


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## brak (Jan 5, 2007)

Stillraining said:


> I did not know they were that thick...my 20 gal ones sure arnt...maybe 3/16 at the most.


I had a poly tank custom-built for my previous boat from 1/4" material, 35 gallons. Good tank, high quality hoses - and it worked great. Cost me a lot of $$ though.


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## YARDPRO (Aug 3, 2009)

blt2ski said:


> I personally would not want to use PVC pipe in general, altho the green Sch 80 sewer pipe I might. You would not want to use the gray electric conduit or white PVC pipe in Sch 80 either. You could verify with stillraining as he uses the green pipe more than I at work, as to which would be better if you have to go this way. I believe the green will keep the smell out better. Altho it could be green more from a, You did up a pvc pipe, green is sewer, white is water typically.
> 
> You can get the green sch 80 in sizes bigger than 4" if you want to, ie 6, 12 and bigger. 4" is not your only option if you want a bit more volume.
> 
> marty


why in the world would you not use white pvc? Pvc is used for municipal water lines all the time? green pipe is used more for drainage and sewer lines.. it is a thinner wall and is more prone to cracking and breaking. there is no difference in smell between the green pipe and others... the green is merely a color used so people can more easily identify the pipe and it's contents in the future... ie. purple pipe for reclaimed non potable water..... same pipe, just a dye used for ID purposed...

Grey conduit is also the same PVC with a dye so it can be identified ad an electrical conduit.

any pvc would work fine, i like the schedule 80 idea as it is stronger. any of the drainage pipe (green, etc) would be a thin wall and more prone to cracking or chafe than the thicker 80, although the volumes would be reduced...


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## roline (Apr 7, 2000)

I designed a custom shape holding tank to fit just behind the head to minimize all of the hose lengths. The boat came with a bladder tank under the V-berth, bad idea...... SOlution was 15 gallons, 12 layers of cloth and epoxy, 3/8" thick with fillets on all inside corners and a final slick coat for easy cleaning.
The form for the tank was made from the 1/4" paper covered foam boards cut and taped, then covered in release plastic. Epoxy and cloth covered..
When completed, I pulled the form through the top access port. 

I used aquarium through fitting, much better ( heavier wall thickness and improved gasketing) quality than the boating stuff, and used the heavy vinyl hose recommended for the application.. 6 yrs later still as new. NO, yes that's NO smell at all. There are rules, pump when 3/4 full, winterize prior to freezing, use ororlos treatments when in use..


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## mitiempo (Sep 19, 2008)

Aquarium through fittings - can you please expand on this.


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## waterdog52 (Oct 10, 2010)

Here is my 2 cents on PVC. Based on construction experence, The fittings do not like strain, if they are in a bind they will crack. Pvc swell just slightly when exposed attic temps over 140 degrees. As PVC ages it becomes brittle. Lastly, PVC is can stand no UV light. This applies to all scedules. I have seen air line ran in PVC last 10 years till some bumps them then you have an air leak.


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## robotopdx (Oct 8, 2010)

YARDPRO said:


> why in the world would you not use white pvc? Pvc is used for municipal water lines all the time? green pipe is used more for drainage and sewer lines.. it is a thinner wall and is more prone to cracking and breaking. there is no difference in smell between the green pipe and others... the green is merely a color used so people can more easily identify the pipe and it's contents in the future... ie. purple pipe for reclaimed non potable water..... same pipe, just a dye used for ID purposed...
> 
> Grey conduit is also the same PVC with a dye so it can be identified ad an electrical conduit.
> 
> any pvc would work fine, i like the schedule 80 idea as it is stronger. any of the drainage pipe (green, etc) would be a thin wall and more prone to cracking or chafe than the thicker 80, although the volumes would be reduced...


I think just about all Grey PVC has UV stabilizer in it. that is why you can use it for stand pipes and conduit. I cant say Ive seen Grey PVC over 4".
Not sure about the other colors. but I would steer clear of white.


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## jskrypek (Sep 19, 2008)

Schedule 80 is overkill. Schedule 40 would be fine. I would be more worried about sloshing. You could get an amazing amount of backpressure in a rough sea. This would be magnified by the diameter and the length of the pipe. Think of it like a blowhole.


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## WanderingStar (Nov 12, 2008)

It won't get much UV light in the bilge.


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## tdw (Oct 2, 2006)

jskrypek said:


> Schedule 80 is overkill. Schedule 40 would be fine. I would be more worried about sloshing. You could get an amazing amount of backpressure in a rough sea. This would be magnified by the diameter and the length of the pipe. Think of it like a blowhole.


Surely anyone heading offshore would void their tanks and go to direct pumpout at the first opportunity ? Holding tanks are, after all only for inshore use.


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