PVC Pipe for a holding tank? - SailNet Community

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  #1  
Old 10-22-2010
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PVC Pipe for a holding tank?

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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Old 10-22-2010
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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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Old 10-22-2010
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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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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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Quote:
Originally Posted by sailingdog View Post

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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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.


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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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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.

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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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Old 10-23-2010
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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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A 14" dia x 30" long PVC pipe would be about 19.5 gallons or so.

Quote:
Originally Posted by bljones View Post
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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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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