40'x4'x0.67' Plywood Shrimp Farming Tank, thickness plywood?

[fullmetal7777]

I'm a senior at my college in Texas and our senior design is building a shrimp farming system. Were building an 800 gallon 40'x4'x0.67' tank. We figured that the most cost effective way was to build this as a plywood tank, so I'm wondering what size plywood do y'all recommend? I've seen most builds use 3/4", but since our tanks is significantly different in shape then others, I figured I should ask for you guys opinions first! Obviously we would also be reinforcing this tank with 2x4's along the sides and most likely along the top as well.

Also, since the aesthetics of the tank isn't a concern, is there any other type of plywood that we could use rather than oak and birch? 3/4" of that stuff isn't cheap, and I assume most people buy those because they look good.

Thanks!

 

[fullmetal7777]

Also slightly concerned about water proofing the tank. There seems to be a ton of products out there, and pond shield would cost us close to $1000 alone if we were to go that route. The liquid rubber waterproof sealant does look like a good option.

 

[Jexnell]

Have you thought of getting one of those 300gal + Rubbermaid stock tanks? Would be way cheaper.

 

[Oughtsix]

Tank material thickness is usually determined by depth. At 8" deep you are not going to have a lot of water pressure that you have to contain.

If I were building that tank I would use 2 layers of 1/2" shop grade CDX plywood and stagger the joints.

Start with a 4 x 4 sheet on the bottom. Butt another 4 x 8 sheet on the bottom. Cover the first 4 x 4 sheet and half the 4 x 8 sheet with a 4 x 8 sheet on the top. I would glue the top sheets to the bottom sheets using a water proof PVC glue (Titebond) and use stainless steel screws to hold them together while the glue is drying. Hopefully some one on your team played with Lego's as a kid and understands the concept of overlapping joints.

I have never built a plywood tank. I have worked with fiberglass and wood a fair bit though. The approach I would investigate first is fiber glassing all the seams and corners then using what ever water proofing is necessary to seal the wood. The great thing about fiberglass is it adds considerable structural strength along with water proofing and it has some give and flexibility to it so it should keep its integrity as the tank settles once it has been filled with water.

 

[fullmetal7777]

Have you thought of getting one of those 300gal + Rubbermaid stock tanks? Would be way cheaper.

Yeah, we shape of the tank for our application is crucial, so we cant go that route unfortunately.

Tank material thickness is usually determined by depth. At 8" deep you are not going to have a lot of water pressure that you have to contain.

If I were building that tank I would use 2 layers of 1/2" shop grade CDX plywood and stagger the joints.

Start with a 4 x 4 sheet on the bottom. Butt another 4 x 8 sheet on the bottom. Cover the first 4 x 4 sheet and half the 4 x 8 sheet with a 4 x 8 sheet on the top. I would glue the top sheets to the bottom sheets using a water proof PVC glue (Titebond) and use stainless steel screws to hold them together while the glue is drying. Hopefully some one on your team played with Lego's as a kid and understands the concept of overlapping joints.

I have never built a plywood tank. I have worked with fiberglass and wood a fair bit though. The approach I would investigate first is fiber glassing all the seams and corners then using what ever water proofing is necessary to seal the wood. The great thing about fiberglass is it adds considerable structural strength along with water proofing and it has some give and flexibility to it so it should keep its integrity as the tank settles once it has been filled with water.

I was definitely planning on fiberglassing all the seams since those are the places most prone to leakage. So I understand the concept of staggered joints, but what is the point of building the bottom of the tank with 2 1/2" CDX plywood sheet? And from what i understand about staggered joints, the sheets would be staggered when stacked side by side and not on top of each other.

Just curious, but I like the idea, thanks for the reply!

 

[Oughtsix]

Is the tank going to be directly on the floor? Is there going to be any plumbing running under the tank?

If it were me I would build the tank so it is 30" off the floor to allow for the initial plumbing and any repairs down the road. 30" should also provide a nice height for working on the tank without having to get on your hands and knees.

I would use 2 x 4's laying flat and doubled (two stacked on top of each other) on 16" centers for a frame under the bottom of the tank. This should allow the seams on the plywood bottom to hit the center of the 2 x 4 frame which will support the plywood seems. With the largest unsupported span being 16" by 16" the doubled 2 x 4's should be plenty of support for the tank filled with water and a couple of guys standing in the tank working with the critters.

When you build the tank start with a 2 x 4 grid laying on the ground. Cut the first row of upright studs 26" long. I would use 3 upright studs across the 4' width so the studs will be on a 24" x 16" grid... this should allow some on to crawl under the tank and work.

DO NOT CUT THE REMAINING ROWS OF STUDS 26" LONG!!!! All the remaining rows should be indexed off of the first row so the the tank bottom will be level! This is how you will compensate for an uneven floor! So the row set of studs will be 16" down from the first set and might be 26 1/8" or 25 7/8" or what ever is necessary for the tops of the second row of studs to be level with the tops of the first row of studs.

Once you have the bottom 2 x 4 grid on the ground and the ~26" studs coming up on a 16" x 24" pattern start building the doubled 2 x 4 tank bottom frame on top of the studs.

To me this seems like it would be a strong, usable and maintainable tank!

 

[Oughtsix]

what is the point of building the bottom of the tank with 2 1/2" CDX plywood sheet? And from what i understand about staggered joints, the sheets would be staggered when stacked side by side and not on top of each other.

Just curious, but I like the idea, thanks for the reply!

two sheets of 1/2" plywood will be considerably stronger than 1 sheet of 3/4" plywood. Not only will the bottom of the tank be 1" thick but there will be no joints that run the entire thickness of the bottom.

When the weight of the water is added the tank bottom will want to bow. The seam where the 2 sheets of 3/4" plywood butt together will be the weakest part of the bottom and the top of this seem will want to pull apart... springing a leak. By having two sheets of 1/2" plywood one sheet will support the seam of the other sheet greatly reducing the affect of the seam wanting to pull apart and spring a leak.

I think you will find doubled 1/2" sheets of plywood not much more expensive and possibly less expensive than a single layer of 3/4" plywood. I would recommend pricing out your material at a lumber yard! In my neck of the world lumber yards are usually cheaper than Home Depot or Lowes and usually carry a better quality product than the big box stores.

 

[Oughtsix]

Have you considered using a sheet rubber liner in the tank instead of the roll on sealer? I am not saying a rubber liner is necessarly the best way to seal the tank... but I would consider the pros / cons of a rubber liner in your planning process.

Since your tank is only 4' wide you will be able to get a single sheet of rubber liner (with no seams) to seal the tank.

 

[fullmetal7777]

If it were me I would build the tank so it is 30" off the floor to allow for the initial plumbing and any repairs down the road. 30" should also provide a nice height for working on the tank without having to get on your hands and knees.

Woah, you read my mind! So actually were putting the tank on top of a workbench in our sponsors greenhouse. The workbench itself is 40'x4' so thats how we came up with the size of the tank. The work bench itself is built with sever 4x4 legs, should is should be strong enough to hold the tank. Sorry I didn't explain that earlier, would have saved you a lot of time, but I appreciate all the help regardless.

two sheets of 1/2" plywood will be considerably stronger than 1 sheet of 3/4" plywood. Not only will the bottom of the tank be 1" thick but there will be no joints that run the entire thickness of the bottom.

When the weight of the water is added the tank bottom will want to bow.

Will I have to worry about the bottom bowing even though it'l be on top of a workbench? Also, I'm pretty sure the surface of the workbench is made up of what looks like a thick welded wire mesh.

Have you considered using a sheet rubber liner in the tank instead of the roll on sealer? I am not saying a rubber liner is necessarly the best way to seal the tank... but I would consider the pros / cons of a rubber liner in your planning process.

Since your tank is only 4' wide you will be able to get a single sheet of rubber liner (with no seams) to seal the tank.

Haven't really considered it. I'm not too sure if that would work well for our application, but it's definitely something I'll ask our subject matter expert on saturday

 

[fullmetal7777]

Also, I'd post a link to what we have designed in solidworks but I cant post a link for 2 days unfortunately.