plywood bottom tank

[JNye]

I'm looking to do a plywood bottom tank. I have some leftover stuff from my last plywood tank. But this one will be rimless so it has a whole set of different variables I need to decide on. My design/solution is quite simple actually.
Dimensions 30"x30"x18" with a 16" water height. 3/8" glass.

I will cut a 3/4 sheet of plywood 30"x30". I will cut a second sheet 3/4 plywood 7/8" shorter.
That is 3/8" for glass x 2 with an extra 1/16" on each side for silicone. I will glue/screw these 2 pieces on top of each other with the 7/16", "step" all the way around. I will then frame this(aquarium bottom)with some 3/4 wood(glue/screwto bottom layer of plywood). This will create a groove for the glass to sit in.
Then I will 2 part epoxy it all as one piece. Once its all sealed I will silicone the grove, drop the glas in, silicon the vertical edges and clean it up as needed.
The only thing I may change is the "groove" size. Right now it is 7/16 for accepting 3/8 glass. This does not leave any room for 2 patr epoxy. I think my groove may need to be bigger. Also is their a better sealant I should be using in the groove that accepts the glass?
Alternatively I could glue screw 2 same size plywood bottoms and router the groove out then 2 part? The first plan seems easier and should look nicer.

 

[Rhodophyta]

I think the best thing to do would be to cut a single sheet of plywood to just under the dimensions of a glass bottom for the tank, and then put several coats (I like 4) of epoxy on it, including the edges. Once it's cured, silicone it in as if it were a glass bottom. I've seen a whole plywood tank much larger and taller than yours done this way, held together just with silicone, no screws or frame.

 

[JNye]

Thats good to hear, I actually wasn't sure if the silicone would bond good enough to the epoxy even with the groove. Since its not too much trouble I think I'm gonna stick with the way I had planned...it is rimless...and it shouldn't hurt anything. Also wondering if there might be a better adhesive to use glass to epoxy or is silicone fine?

 

[Rhodophyta]

It may be counter-intuitive, but the track record of doubled plywood and various wood batton designs is worse than simple plywood. It may be that each new seam creates more opportunities for leaks, or that screws are more likely to intersect at which point the second one bends in unpredictable directions. With care and careful adherance to a safe screw pattern, it should be possible to overcome the problem, although I have yet to see someone succeed. If I am seeing your design right, it seems more likely to work than similar ones, because you won't have as many potential clashing screws, and the seams between the plywood are not on the inside of the tank.

Silicone is the time tested adhesive, but there are others. Using clear acrylic instead of glass and embedding it in the last still wet coat of epoxy, with a primer coat on the contact parts of the acrylic, Ultimate Glue, Gorilla Glue, Permaflex, liquid rubber in a paintable form, Sanitred, and sprayable/paintable polyurea pond liners such as Pond Armor. Most cost more than silicone and would be experimental. The Ultimate Glue and Gorilla Glue should be price competitive, but still an experiment. with your shallow tank, I think the Ultimate Glue could work, but you would have tan seam lines instead of the clear or black of silicone, and they might not look good, especially in the vertical and more visible glass-to-glass seams. Plus the Ultimate glue expands as it cures, so you would need more than the casual clamping for a glass tank made with silicone.

 

[Otto2]

I would just duct tape the glass to the plywood. You can buy it in just about any color too!

 

[RichConley]

Why?



It'd probably be cheaper, and a whole heck of a lot easier to just have a glass bottom cut.

 

[JNye]

I won't be cheaper because I have the epoxy from another tank I built. Drilling a plywood tank for a closed loop and drains will be easier and cheaper as well. Plus I'm stupid. I do like the duct tape idea.

 

[just dave]

Why not do away with the inner plywood piece and just use one sheet with the the frame work around it? It would create a rim instead of the groove you were looking for but it also seems like it would do away with any concerns issued by Rhodophyta.

 

[liveforphysics]

I love my plywood bottom rimless starphire tank.

I wouldn't make a tank any other way at this point. I made a 304 stainless steel frame, screwed the plywood into it, then set the glass pannels in and poured in a 2-part special poly-urea coating to seal it up. It's fantastically durable, and I have all 18 bulkheads going through the bottom. Nothing feels stronger than a bulkhead clamped down on 1" birch plywood. Extreme stregnth on the bottom where you need it, and the clarity and scratch resistance of glass on the sides, with no weakpoints from holes in the glass.

 

[JNye]

Why not do away with the inner plywood piece and just use one sheet with the the frame work around it? It would create a rim instead of the groove you were looking for but it also seems like it would do away with any concerns issued by Rhodophyta.

With the tank being rimless I wanted to have material on both sides of the glass, especially on the inside because the glass is pushin out. Without the inner plywood bottom the seams would get this extra strees, i asuume.

If I understand what you're saying LFP you also have a "groove" that the glass sat into? steel.glass.plywood?
How tight did you make the glass fit in this sanwich? Mine won't be starfire.

I was thinking about simply doing what Rhodphyta suggested and simply use on epoxied bottom as it were glass. But I also wanted this to be the step to a larger version so I think I'll go ahead and experiment with my original plan. I guess I'll be using black silicone on all joints. I found it in 10.4 oz tubes. Any clue if I need 1 or 2?

 

[just dave]

If the silicone bonds as well to the coated plywood as it does to glass the seam would be the same as one found on a glass tank. A rim around it would be the same as a glass tank that used a one piece frame. I can see many benefits from a well sealed plywood bottom tank.

 

[JNye]

Well I'll be cutting/glueing some plywood tonite, turns out I have 5/8 plywood, and its not birch, just regular cheap stuff. Its straight though and I'm gonna use it. I'll prolly leave it filled with water in the garage for a few weeks before I do anything with it. I'll be ordering my silicone tonite too. I'll order the glass once the bottom is finished and I'll measeure them to fit...just in case.

 

[Rhodophyta]

<a href=showthread.php?s=&postid=12957601#post12957601 target=_blank>Originally posted</a> by poo-tang
....
I was thinking about simply doing what Rhodphyta suggested and simply use on epoxied bottom as it were glass. But I also wanted this to be the step to a larger version so I think I'll go ahead and experiment with my original plan. ...

I think your original plan may not upsize well. It may depend on a shallow tank and limited size to reduce the odds of a leak which means upsizing it would magnify the odds. OTOH the tank I saw built with just epoxy coated plywood and silicone was about 30" by 30" by 96".

 

[JNye]

wow that's large. Are you recomending having the glass frame the plywood or sit on top of the plywood. If the glass wraps the plywood this IS easier and less work.
I already did the wood the way I said I was planning. Its drying now. If the price of glass is right maybe I'll build another with just plywood, if I can find an owner for it.

As far as intersecting screws this should not be an issue at all. I will measure for this and predrill holes also. My design is basically a single sheet with the glass siliconed to it. The bottom piece of plywood allows for a place to screw the outer frame too, thats all. The outer frame may do nothing structurally(i think it might and is does allow for more silicone to be used), aestetically it will hide a sandbed.
I'll post pics later.

 

[Otto2]

Where are you getting the glass from? I think I might have 1/4" glass at work. I will see if there is 3/8" also.

 

[JNye]

I'll be ordering locally, I've ordered from a few places in the past. Definately LMK. You guys cut glass or what? If not I can have it cut easily.

 

[JCTewks]

Any updates on this project?

 

[chriscobb]

I don't see any issue with screws clashing in the laminated bottom......I've been toying with the idea of a ply tank and was advised against setting the glass in a channel as your suggesting. Wouldn't putting the glass in a channel at the bottom and no support at the top cause more stress on the bottom of the glass panels? Being that on a traditional plywood tank the glass seals with the water exerting pressure outwards causing the seal?!?!?