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Discussion Starter · #1 ·
I figured out that I could make a custom designed holding tank that will provide 80% of the capacity of the "standard" tank I have but take up less than 50% of the space. So the question is how to build it. I'm thinking that a male mold would be easiest for building in fiberglass. But then got to thinking, maybe it could be cored with 1/8" plywood and fiberglassed on both sides. The stitch and glue system might be the way to go.
Three sides plus top and bottom will be flat and one side will be curved to match the hull. I would built the tank and top separately and join them to make a sealed unit.
Does this approach sound right? And what schedule should I use for laying up the fiberglass. Will odor permeate/penetrate the fiberglass? What do I need to consider in this project.
John
 

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I made a holding tank for my boat recently. I made it in two halves using female molds made from 3 mm melamine coated particle board hot glued together and with play-dough for the rounded edges. I avoided using plywood core and instead made it solid fibreglass. I used vynalester with mostly CSM but also some roving and mat. I made it thicker at the bottom and top. The two halves joined together using a bell and spigot joint. I was going to glass the joint afterwards, but the fit up of the halves was so neat that the glue alone was sufficient. The benefit from this approach is that the outside of the tank looks very neat and I could get the exact external tolerances I wanted which was important for my installation location. The solid glass construction also has the added benefit that the tank is translucent so I can shine a torch up from the bottom of it to see the contents level.

I have to say that this method was time consuming and fiddly, although one half of the tank has a relatively complex shape. A simple method of construction is to use florist's foam glued together and sculpted to the desired shape and then fibreglass around it. A hole is then cut into fibreglass and acetone or fuel sloshed into the opening which rapidly dissolves the foam.

As for smell, I used vynalester for strength and resistance to absorbing the liquid contents. I had researched that coating epoxy on the inside of the tank once complete (like a barrier coat) helps ensure the tank won't absorb odours. I opted not to do this, so I guess time will tell!
 

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I would be concerned that the glass would crack and those liquids would quickly absorb.

Why not have a fabricator make your design out of poly?
 

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Discussion Starter · #4 ·
I would be concerned that the glass would crack and those liquids would quickly absorb.

Why not have a fabricator make your design out of poly?
Like who?

What I have appears to be a custom tank but does not fit the available space very well.

John
 

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Holding tank acids can be caustic to metal.

I had mine custom fabricated by Triple M plastics in Maine. Even with shipping, they were way below others. Service and product were great too.

There is also a place in Annapolis that has hundreds of pre designed molds. If you can find one close, you can really save a bundle. Name escapes me at the moment. Someone here will know.
 

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Suggest you go to the Gougeon (WEST SYSTEM - epoxy) website and look up holding tanks in their 'projects' section. You'll find the correct barrier coating material also listed to prevent VAPOR permeation plus the need for internal baffles for larger tanks.
For holding tanks you really want to avoid ANY metallic components due to the acidity of the 'contents' ; Stainless or Aluminum would be among the worst choices for a holding tank.

Other - you really dont want any 'core' as this will only become a reservoir for 'stink' to permeate - resin-rich solid laminate will better control vapor and liquid permeation.
I havent looked in the WEST SYSTEM projects files lately; but, an epoxy-bondable inner liner/surface such as polyethylene would be a plus. Many manufacturers have been experimenting with polyethylene sheet with an integral surface 'scrim' that is bondable to epoxy.

Good luck

Epoxy by the Leading Epoxy Manufacturer | WEST SYSTEM Epoxy
 

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There is also a place in Annapolis that has hundreds of pre designed molds. If you can find one close, you can really save a bundle. Name escapes me at the moment. Someone here will know.

These guys are in Seattle, not Annapolis, but also have a huge selection:
Holding Tanks & Accessories

I bought my holding tank there, but it is a very normal boring shape and size.
 

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Just thought I'd pass this on. I was on the verge of trying to build the entire holding tank system into my boat when CLOSECALL recommended I look into an Air Head toilet.

I cannot vouch for either as I've never had a holding tank and have not ordered an Air Head yet though I plan to. And I can't say one or the other would be better for your needs, but based on what I have heard on both sides, and after spending hours upon hours researching what I needed for a holding tank, reading about Air Head blew my mind. I'd recommend at least considering it.
 

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If you decide on glass/epoxy or similar..... Cut a block of styrofoam the size of the inside of tank. Smooth coat with a layer of clay. . coat with mould release. A .Install thru hull and sump pipe (heavy sched plastic) on the top .coat with appropriate tank type gel coat. Glass the bejeesis all over ,turn and turn. Or B..Drill the holes and glass on the threaded fittings Pour in a dollop of acetone and a handfull of pebbles,Shake a while. Pour the mess out and install.I've made motorcycle tanks like this. Works well.
 

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Vinyl ester. That just hurt to look at.

Polyethylene. Poly what?

Urine is corrosive to metals, not caustic (though it is generally alkiline).

The holding tank on my current boat is vinyl ester, is a built-in part of the boat, and has not permiated in 17 years. However, I would go with Ronco if they had anything that would fit. If I were building with FRP (I've built FRP fuel tanks) I would go with no core, resin-rich, and would use veil coats suported with very light glass cloth. I would need to research the resin, but I suspect there are epoxies that would be best.
 

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.I've made motorcycle tanks like this. Works well.
Thanks all for the information presented in this thread. This is one of the rare teachings that I will copy into a word document and file for reference.

As I was reading and concentrating intently on the topic I couldn't help but think, how does Capt Len discretely use the head on a motorcycle? :eek:

Easier I suppose if he rides two-wheeled Winnebagos.
-CH
 

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Discussion Starter · #16 ·
Update,
Well I've made a mockup using foam board and masking tape. I've studied the ronco and sealand product lists but did not find one that would fit and be as efficient (space wise) as a truly custom tank.
I looked for styrofoam, but have not found any of a size that would work for making the mold. And, it is really expensive! Why did I throw out all those styrofoam blocks if had over the years?
So, capt Len, can I use that spray foam in a can sold at hardware stores? Will it dissolve it in acetone? I think if I leave one side off of the mold it will not explode the mold and then I can free hand the compound side, which is the problem side anyway.
Then the real problem - do you work three sides at a time? How do you keep from getting all stuck up?
And how thick of a layup would be required for an 8 gallon tank?
What kind of wax could smooth out the foam? I assume I will need the inside surfaces as smooth as possible.
Should I use starboard to reinforce where the fittings go?
I appreciate everyone's input and helping me grope along.
John
 

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Update,
Well I've made a mockup using foam board and masking tape. I looked for styrofoam, but have not found any of a size that would work for making the mold. And, it is really expensive! Why did I throw out all those styrofoam blocks if had over the years?
John,

Just a heads up, no pun intended:)
Your mold wouldn't hold up if it was Styrofoam. The resin would melt it down before it had a chance to set up. Can't speak for foam board.

Why not use sheet rock for the mold?

Bob
 

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Discussion Starter · #18 ·
The foam mock-up is intended to mold a form that would function as a "lost wax" process. I assume the foam is covered in wax to insulate it from the epoxy resin. Once the lay up schedule is completed and set, the foam is dissolved.
Thanks for the advise. This is all new ground for me.
John
 

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I figured out that I could make a custom designed holding tank that will provide 80% of the capacity of the "standard" tank I have but take up less than 50% of the space. So the question is how to build it. I'm thinking that a male mold would be easiest for building in fiberglass. But then got to thinking, maybe it could be cored with 1/8" plywood and fiberglassed on both sides. The stitch and glue system might be the way to go.
Three sides plus top and bottom will be flat and one side will be curved to match the hull. I would built the tank and top separately and join them to make a sealed unit.
Does this approach sound right? And what schedule should I use for laying up the fiberglass. Will odor permeate/penetrate the fiberglass? What do I need to consider in this project.
John
John,

I built one using plywood, fiberglass and west systems, and NO ODOR as of yet! my old tank just stunk, so i decided replace mine. After researching costs, pros and cons. I decided to build one... at the recommendation of another sailor, I installed 2 vents to promote additional venting/ breathing. this additional vent is supposed to help eliminate odor, so far so good. After a year or so of use...no issues. I had a 6 gallon tank when I bought the boat and in that same space I was able to fit my custom made tank that holds 14 gallons.

here are some pics;
Adventures of Salacia: Crap! Alberg 30 Holding Tank


good luck!
 

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I considered the foam idea, but in the end opted for a tank built in two halves. I'd built a baffled cylindrical diesel fuel tank in two halves previously and not had any problems with it in service.

I made the top and bottom about 1/4" thick and the sides around 1/8" to 3/16" thick or so. I think it is way overbuilt, but I had a tin of vinyl ester that was starting to "turn" and I figured better thicker than thinner.

Here's some pictures of my tank which is around 10 gallons in capacity.

The template made out of corrugated plastic:


One half of the tank in it's mold. The mold was made from mdf with a white plastic coating on one side, some pvc pipe and I stole my grand-kid's play dough for the rounded edges. Mold wax and PVA were used as release agents. I did have minor problems with the white plastic adhering to the finished fibreglass when (destructively) releasing the mold, but a quick hit with a pressure water cleaner solved that problem so I assume that the PVA bond was similar to the plastic coating bond to the mdf:


Holes for fittings all cut out:



All assembled:



Installed:
 
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