Hi Peggie, I need to replace my holding tank. I found an article on the boatUS web site describing how to fabricate a custom holding tank out of fiberglass. This would be ideal, because the tank could fill all the available space. It would be a separate tank. I would not try to utilize the hull or bulkheads as part of the tank. I'm wondering if there are any drawbacks to fiberglass tanks, other than weight? Is permeation a bigger problem than with other materials?
Fiberglass holding tank
- Thread starter Alan21873
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I built mine
I built a custom tank from plywood and epoxy 2 years ago. It has worked very well so far. I included an inspestion port on the top and designed it so the hoses exited from the top of the tank. This keeps the waste from sitting in the hoses. Also had to put a pick-up extension inside the tank for the pump out hose. If your handy with wood and epoxy I'd recommend it. The West System website has an article to give you some guidelines.
I built a custom tank from plywood and epoxy 2 years ago. It has worked very well so far. I included an inspestion port on the top and designed it so the hoses exited from the top of the tank. This keeps the waste from sitting in the hoses. Also had to put a pick-up extension inside the tank for the pump out hose. If your handy with wood and epoxy I'd recommend it. The West System website has an article to give you some guidelines.
Ah-ha!
So that's why the hoses are at the top. I never could figure that out. What a great website.
So that's why the hoses are at the top. I never could figure that out. What a great website.
Fiberglass is ok, but not as good as poly
Eventually the coating inside the tank will develop hairline cracks that will allow moisture penetration and rot the wood...but that's likely to take years if you do a good job and prob'ly will be a future owner's problem. Otoh, a top quality thick-walled poly tank will last as long as the boat stays afloat. Have you checked Ronco Plastics catalog (http://www.ronco-plastics.com)...they have more than 400 shapes and sizes, including over 100 non-rectangular...and install fittings in the sizes and locations specified by the customer when they make the tank, so you could spec all the fittings on the top of the tank (requires at least 5" clearance above the tank) and an inspection port.
Eventually the coating inside the tank will develop hairline cracks that will allow moisture penetration and rot the wood...but that's likely to take years if you do a good job and prob'ly will be a future owner's problem. Otoh, a top quality thick-walled poly tank will last as long as the boat stays afloat. Have you checked Ronco Plastics catalog (http://www.ronco-plastics.com)...they have more than 400 shapes and sizes, including over 100 non-rectangular...and install fittings in the sizes and locations specified by the customer when they make the tank, so you could spec all the fittings on the top of the tank (requires at least 5" clearance above the tank) and an inspection port.
You can also use PVC sheet
for a cast in place form. Just use it like wood and glass it in for structural reinforcement. It won't rot like wood and is close to the same price. If you get some thick enough material, you can cut a hole in it and it is easy to mount and seal the hose fittings onto it. It also presents a flat surface for a custom fit gasket. Besides all of that, you can go to Harbor Freight and get a cheap plastic welder for about twenty bucks and play with that. Great fun!! Do not breathe the fumes.
for a cast in place form. Just use it like wood and glass it in for structural reinforcement. It won't rot like wood and is close to the same price. If you get some thick enough material, you can cut a hole in it and it is easy to mount and seal the hose fittings onto it. It also presents a flat surface for a custom fit gasket. Besides all of that, you can go to Harbor Freight and get a cheap plastic welder for about twenty bucks and play with that. Great fun!! Do not breathe the fumes.
I'd get a second opinion before using PVC
I don't ever recall even hearing of anyone use PVC for tankage. Reasons may be because it gets very brittle when it's cold...will crack in any temp if flexed or bulged much--or even is subject to very much shock. I'm not necessarily saying it's a bad idea...but I'd want some expert advice before using PVC. You might consider polypropylene instead.
I don't ever recall even hearing of anyone use PVC for tankage. Reasons may be because it gets very brittle when it's cold...will crack in any temp if flexed or bulged much--or even is subject to very much shock. I'm not necessarily saying it's a bad idea...but I'd want some expert advice before using PVC. You might consider polypropylene instead.
pvc sheet is not pipe
I work in the trades and use sheet pvc extensively in all thicknesses it is sold in. I would not recomend using it for a tank for a couple reasons. #1. it is not the same as sewer pipe. The density of the pvc sheet goods is considerably less than that of pipe. (my guess is that this is to keep its weight down.) #2. The pvc sheet goods sold in U.S. are in my experience Very prone to temperature expansion. I have learned this the hard way. They tend to swell and shrink with temp rapidly and dramatically. I think that this charecteristic would place undue strain at your cold welds. #3. Sheet pvc typically has one smooth side and one pouros. truthfully it is easy to mix them up. Further i only assume that sheet pvc is closed cell technology. The pouros side makes me think that there is a possibility that it could be open cell. Before building i would directly contact the supplier and ask them. but you can obtain high quality rigid plastic sheeting rated for tankage from a number of industrial suppliers.
I work in the trades and use sheet pvc extensively in all thicknesses it is sold in. I would not recomend using it for a tank for a couple reasons. #1. it is not the same as sewer pipe. The density of the pvc sheet goods is considerably less than that of pipe. (my guess is that this is to keep its weight down.) #2. The pvc sheet goods sold in U.S. are in my experience Very prone to temperature expansion. I have learned this the hard way. They tend to swell and shrink with temp rapidly and dramatically. I think that this charecteristic would place undue strain at your cold welds. #3. Sheet pvc typically has one smooth side and one pouros. truthfully it is easy to mix them up. Further i only assume that sheet pvc is closed cell technology. The pouros side makes me think that there is a possibility that it could be open cell. Before building i would directly contact the supplier and ask them. but you can obtain high quality rigid plastic sheeting rated for tankage from a number of industrial suppliers.
Josh and Peggie
You need to reread what I said. Use it "INSTEAD OF WOOD FOR A CAST IN PLACE FORM". I did not say to make a complete tank out of it. Pvc can be cut and glued into a basic form easily or welded too as I am told. I've never done that kind of welding myself so some guidance is warranted. Glass the outside of the form (inside too if you want)to seal it. I used and would advise epoxy resin. You can use thick material for the top and it gives you a flat surface which you can glass into place. You cannot use polypropylene if you want to glass it in because none of the resins will adhere to that. You can weld polypropylene though and you can use it (thicker material) for a bolt on top fitting. I have made a tank this way and it works fine. I glassed both inside and outside of the pvc so it is really just a filler which provides the original basic shape and stiffens the sides. That is why wood is used in some tank designs. I bet it will outlast any aluminum tank made - bleach or none. I did get my material from an industrial supplier in Seattle - 1/8 inch thick 4 X 8 sheet was about $30. What do you think the plastic access covers are made of? Pvc is my best guess.
You need to reread what I said. Use it "INSTEAD OF WOOD FOR A CAST IN PLACE FORM". I did not say to make a complete tank out of it. Pvc can be cut and glued into a basic form easily or welded too as I am told. I've never done that kind of welding myself so some guidance is warranted. Glass the outside of the form (inside too if you want)to seal it. I used and would advise epoxy resin. You can use thick material for the top and it gives you a flat surface which you can glass into place. You cannot use polypropylene if you want to glass it in because none of the resins will adhere to that. You can weld polypropylene though and you can use it (thicker material) for a bolt on top fitting. I have made a tank this way and it works fine. I glassed both inside and outside of the pvc so it is really just a filler which provides the original basic shape and stiffens the sides. That is why wood is used in some tank designs. I bet it will outlast any aluminum tank made - bleach or none. I did get my material from an industrial supplier in Seattle - 1/8 inch thick 4 X 8 sheet was about $30. What do you think the plastic access covers are made of? Pvc is my best guess.
giving patrick his due
Im, sure that your pvc tank performs admirably. I was aware that you were using it as a core. I simply meant it would not be my first choice. It is a simple opinion. much like your original post. I should have added that i love the pvc sheet as a product and believe it has many great uses. As it does not rot like wood. IF your glass work is on par then your pvc tank should last longer than the rest of us i would imagine. Also i should have stated that my pvc sheet expansion problem. is with a mechanical fastening application. And i have no idea how it would behave. When encapsulated in high quality glass. However i will stick with my opinion that it would not be my first choice. That is reserved for a cast poly tank, in the dimensions of my choice. but the pvc option could easily come in second.
Im, sure that your pvc tank performs admirably. I was aware that you were using it as a core. I simply meant it would not be my first choice. It is a simple opinion. much like your original post. I should have added that i love the pvc sheet as a product and believe it has many great uses. As it does not rot like wood. IF your glass work is on par then your pvc tank should last longer than the rest of us i would imagine. Also i should have stated that my pvc sheet expansion problem. is with a mechanical fastening application. And i have no idea how it would behave. When encapsulated in high quality glass. However i will stick with my opinion that it would not be my first choice. That is reserved for a cast poly tank, in the dimensions of my choice. but the pvc option could easily come in second.
Plastic acess covers are either polyprop or
acrylic, Patrick. And I wonder if you're confusing sheet PVC with polypropylene...'cuz it's polyethylene that nothing will bond to. In fact, C.C. Tech uses PP instead of PE their one-off welded tanks because it CAN be "glued." After reading Josh's comments about the high expansion/contraction of sheet PVC, I'd want to investigate a lot more before using it for a waste tank. If a water tank cracks and leaks, you only have a wet bilge...but if a waste tank does, it's a mess.
acrylic, Patrick. And I wonder if you're confusing sheet PVC with polypropylene...'cuz it's polyethylene that nothing will bond to. In fact, C.C. Tech uses PP instead of PE their one-off welded tanks because it CAN be "glued." After reading Josh's comments about the high expansion/contraction of sheet PVC, I'd want to investigate a lot more before using it for a waste tank. If a water tank cracks and leaks, you only have a wet bilge...but if a waste tank does, it's a mess.
Why not have a poly tank built?
Alan: Why not have a poly tank built? You can get a welded tank built at a reasonable cost. Obviously you need to evaulate all of the Ronco tanks. You really do not care about 2-5 gal. of difference unless you are getting a 10 gal. tank. Shop around before you decide, but you know that POLY is your best bet and you will not need to worry about your ability to build it properly.
Alan: Why not have a poly tank built? You can get a welded tank built at a reasonable cost. Obviously you need to evaulate all of the Ronco tanks. You really do not care about 2-5 gal. of difference unless you are getting a 10 gal. tank. Shop around before you decide, but you know that POLY is your best bet and you will not need to worry about your ability to build it properly.
Alan, as I read your question,
You are asking if it is practical to build a custom tank to fit an available space most efficiently. Having done that myself, I suggest that you use something other than wood to create the basic shape and interior baffles, etc. I used pvc but other plastics are certainly an option. I would suggest that if you use a wall of pvc encased in epoxy fiberglass with very thick and well reinforced seams, you would not have as much a permeability (odor) problem as fiberglass alone. All common plastics are somewhat permeable to gasses such as those of concern in a holding tank but pvc works well for sewer lines and the sheet form that I have seen is not really much different than the pipe material. You can get it in all kinds of thicknesses and densities. It does suffer from some mechanical weakness as others have pointed out but that is not an issue if you use it for a form reinforced with fiberglass which compensates for its mechanical drawbacks. The lower gas permeability of pvc is an added benefit. There are many other kinds of plastics which will work for this purpose but pvc is common, cheap and bonds very well with the epoxy resin. You can heat it with a heat gun and form it to curves, etc. My opinion is that it is simply the best choice for the application you describe. I agree that a polyethylene or similar material might be the best tank if you can get one that fits the space. As to welding the stuff, that just sounds like a fun method of making something cool like maybe a fire extinguisher/beer can launcher. You could even the make the tank form itself that way but I agree that I would not count on a welded or glued seam to seal a large volume of the stuff you are talking about. Peggy is right, I was thinking of polyethylene instead of polypropylene. They are one carbon atom different and similar in properties. Polyethylene is more slick but I do not think polypropylene would be as good a choice as pvc for the project you have in mind. My impression is that it would be less permeable but also less able to bond with the fiberglass resin and therefore would have less strength. There are all kinds of access covers as I have now discovered including pvc, acrylic, etc. If you are adventurous, all of these plastics can be welded but I have no personal experience and thus no real world advice on that. If you like to play with this kind of thing, the equipment is cheap and simple and like I said earlier, it might be fun. Here is hoping you all have a lot of that.
You are asking if it is practical to build a custom tank to fit an available space most efficiently. Having done that myself, I suggest that you use something other than wood to create the basic shape and interior baffles, etc. I used pvc but other plastics are certainly an option. I would suggest that if you use a wall of pvc encased in epoxy fiberglass with very thick and well reinforced seams, you would not have as much a permeability (odor) problem as fiberglass alone. All common plastics are somewhat permeable to gasses such as those of concern in a holding tank but pvc works well for sewer lines and the sheet form that I have seen is not really much different than the pipe material. You can get it in all kinds of thicknesses and densities. It does suffer from some mechanical weakness as others have pointed out but that is not an issue if you use it for a form reinforced with fiberglass which compensates for its mechanical drawbacks. The lower gas permeability of pvc is an added benefit. There are many other kinds of plastics which will work for this purpose but pvc is common, cheap and bonds very well with the epoxy resin. You can heat it with a heat gun and form it to curves, etc. My opinion is that it is simply the best choice for the application you describe. I agree that a polyethylene or similar material might be the best tank if you can get one that fits the space. As to welding the stuff, that just sounds like a fun method of making something cool like maybe a fire extinguisher/beer can launcher. You could even the make the tank form itself that way but I agree that I would not count on a welded or glued seam to seal a large volume of the stuff you are talking about. Peggy is right, I was thinking of polyethylene instead of polypropylene. They are one carbon atom different and similar in properties. Polyethylene is more slick but I do not think polypropylene would be as good a choice as pvc for the project you have in mind. My impression is that it would be less permeable but also less able to bond with the fiberglass resin and therefore would have less strength. There are all kinds of access covers as I have now discovered including pvc, acrylic, etc. If you are adventurous, all of these plastics can be welded but I have no personal experience and thus no real world advice on that. If you like to play with this kind of thing, the equipment is cheap and simple and like I said earlier, it might be fun. Here is hoping you all have a lot of that.
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