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Old 27-02-2021, 20:52   #1
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DIY Holding Tank?

I can hear the groans now, but I want to play this on out.....my 1980’s 40’ sloop has a Lilliputian sized 15 gal holding tank under the v berth. Having covered the internet trying to find something in the 22-25 gallon range has turned up nothing in a size that will fit. Nigel Calder talks about building FG water tanks in his book but is silent about holding tanks. As he suggests making a mold pot of styrofoam, wrapping it in plastic and glassing over it, then cutting a hole in the top & digging the foam out. Doesn’t sound too difficult, what am I missing.

Thanks,
Lou
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Old 27-02-2021, 21:06   #2
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Re: DIY Holding Tank?

Quote:
Originally Posted by LouK View Post
I can hear the groans now, but I want to play this on out.....my 1980’s 40’ sloop has a Lilliputian sized 15 gal holding tank under the v berth. Having covered the internet trying to find something in the 22-25 gallon range has turned up nothing in a size that will fit. Nigel Calder talks about building FG water tanks in his book but is silent about holding tanks. As he suggests making a mold pot of styrofoam, wrapping it in plastic and glassing over it, then cutting a hole in the top & digging the foam out. Doesn’t sound too difficult, what am I missing.

Thanks,
Lou
Maybe his technique will work better than mine.

I layed up a large fiberglass sheet on a piece of glass, maybe 1/4" of fiberglass cloth, mat and resin, then cut out the six sides of my odd shaped holding tank box. Applied more glass on the edges and I had a finished, water tight box, a perfect fit for the space I had. Held about 25 gallons.

I cut holes for the plumbing, including one for the standpipe to the deck for pump-out.

It all worked good, until I filled it to deck level with fresh water at the pump out station to get it really cleaned out.

The four foot head on the stand pipe burst the edge seams and the holding tank broke wide open and dumped into the bilge.

So I ripped it out and reglassed the edges.

Next pumpout: same result.

Enough of that, I did a very detailed drawing of what I wanted and took it to a stainless steel fabricator. Problem solved.

I hope your glass work is better than mine.
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Old 27-02-2021, 21:14   #3
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Re: DIY Holding Tank?

Did you see B184? 25 gallons, v-berth shaped. It's what I put in my boat.
https://ronco-plastics.com/product/w...-holding-tank/
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Old 27-02-2021, 21:18   #4
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Re: DIY Holding Tank?

You can order holding tanks in many shapes and sizes, like anything you want.
I had one ordered 2 weeks ago, 18 gallons with a special shape being tall rather than wide or long, to fit in the limited space of my 28’ power boat.
Will post a picture once I get it, but use Google in the meantime, a couple different manufacturers offer custom made tanks.
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Old 28-02-2021, 05:20   #5
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Re: DIY Holding Tank?

Quote:
Originally Posted by wingssail View Post
Maybe his technique will work better than mine.

I layed up a large fiberglass sheet on a piece of glass, maybe 1/4" of fiberglass cloth, mat and resin, then cut out the six sides of my odd shaped holding tank box. Applied more glass on the edges and I had a finished, water tight box, a perfect fit for the space I had. Held about 25 gallons.

The four foot head on the stand pipe burst the edge seams and the holding tank broke wide open and dumped into the bilge.

.
This should have worked. Did you use epoxy and have good fillets in the corners? Followed by as much 'glass as in the walls?
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Old 28-02-2021, 05:56   #6
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Re: DIY Holding Tank?

You don’t need a mold. It’s a bunch of flat panels in any dimensions you choose. Just make panels out of whatever (plywood/glass, foam core, etc).

Then cut panels to represent all required sides of your custom tank.

Round over any exterior corners where 2 panels come together with a router so the glass goes on nicely over the curve rather than a sharp edge.

Start joining the inside cirners of the box together using fillets and glass over the fillets using biaxial glass. On the outside do the same with biax around the rounded corners.

Maybe put in a partial wall or two inside the tank as a baffle. Put holes in the wall to allow fluid to move.

Leave your panel with the inspection/clean out hole or senders in it for the last panel.

Reach inside the tank through that hole to do the inside fillet and corners that would be impossible to reach.


Done!

Now put your access hatch or senders or whatever into that last hole you left for access and seal it up stink tight.

It’s that easy.

Let me know if you need more details.
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Old 28-02-2021, 06:18   #7
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Re: DIY Holding Tank?

Exactly!

Make a blind fillet for the last, preferably the top panel, so you can do a better job. Clamp a thin strip of plywood that has been covered with clear packing tape across the top edge (do two sides opposite each other at a time) and you will have plenty of room to work underneath to make the fillets. After curing, remove the plywood strips and do the other edges the same way. This will result in a nice wide face to epoxy the top panel to. Since it's on top, you probably won't need 'glass, but you could lay some in through excess hole as Choctu says.
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Old 28-02-2021, 06:28   #8
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Re: DIY Holding Tank?

Stupid builder installed an aluminum waste tank. It was a bugger to cut out.
We simply glassed in the same 50 gallon cavity using epoxy, glass covered a plywood top and set it on top of a thickened epoxy putty around the perimiter edges. Easy, cheap, couple of (long) days work.

For us full time liveaboards it's been working fine for 6 years now.
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Old 28-02-2021, 07:33   #9
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Re: DIY Holding Tank?

Quote:
Originally Posted by CSY Man View Post
You can order holding tanks in many shapes and sizes, like anything you want.
I had one ordered 2 weeks ago, 18 gallons with a special shape being tall rather than wide or long, to fit in the limited space of my 28’ power boat.
Will post a picture once I get it, but use Google in the meantime, a couple different manufacturers offer custom made tanks.
Whom did you use, was it not terrifically expensive?
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Old 28-02-2021, 08:04   #10
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Re: DIY Holding Tank?

Quote:
Originally Posted by LouK View Post
Whom did you use, was it not terrifically expensive?
Don't know who he used but Ronco of one of the big ones. They build tanks for the boat builders and still have the molds for every one they ever made. They'll have one that fits. And it doesn't have to fit exactly, if it's a little smaller you make up the difference with spray foam. Just can't be bigger than the space.
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Old 28-02-2021, 08:26   #11
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Re: DIY Holding Tank?

Ronco Plastics Ronco Plastics marine Tanks is your best source for a tank. They make TOP quality thick-walled water and waste tanks for a very reasonable price and have more than 400 shapes and sizes, over 100 of which are non-rectangular, and they install fittings in the sizes and locations specified by the customer when they make the tank. There are retailers who sell Ronco tanks, but Ronco sells direct for a much lower price...and they're great to work with.

When looking at drawings it's important to know that there is no top or bottom until the fittings go in, and YOU decide where they go. So rotate, flip in any orientation to find the tank that fits your space.

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Old 28-02-2021, 12:16   #12
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Re: DIY Holding Tank?

LouK,
build a full size cardboard tank first and see if it will fit through your hatches and doorways!
I almost made the mistake of ordering a tank that would not pass into the cabin!
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Old 28-02-2021, 13:00   #13
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Re: DIY Holding Tank?

I just finished my DIY holding tank build. I first mocked up the space with a cardboard template, then transfered the dimensions onto 1/2 plywood (a good quality, but non-marine-grade). The hard part was getting all the beveled angles to line up but if I had better access to things like a table or bandsaw it would've been much easier. Once I was satisfied with the dry fit, I glued all the panels together sans the lid using thinkened epoxy. I sanded off any excess and then laid down two thick coats of laminating polyester resin on the inside to fully saturate the plywood (this is an important step, do not skip!). Once these coats were tacky I continued the layup with a thick fillet in every seam, then 1 layer of 1.5 oz csm (2-3 layers over each seem) throughout. Then, 1 layer of 1708 throughout, again with additional 1708 taping at the seams, and 1 more layer of 1.5 oz csm. I then faired the inside and sanded it smooth. For the outside, I used the same laminate schedule and fairing process. the top piece was also laminated and once everything was fully cured, I glued the top down with thickened epoxy and 2 layers of 10 oz cloth on the outer seems (using epoxy). The top was made from 2 layers of 1/2 inch ply glued together for a full thickness of 1 inch but with a 5/8" rabbit around the edge so that the lid would seat nicely in the tank and increase the gluing surface. The added thickness also creates more room for threading the fittings. On the lid, I drilled all the fitting holes oversized and then placed the fitting in the holes and filled the gap with thickened epoxy. My fittings are all ABS and are all epoxied in place with the exception of the diptube kits (Dometic ~$35), those come with a gasket that is a really tight press fit. The inside of the tank was painted with 4 coats of InterProtect 2000 epoxy barrier coat, then 2 coats of epoxy.

Total materials and cost:
(1) 4x8 1/2" plywood---$40
~2 gallons laminating polyester resin (~$40 when bought in bulk)
~1 quart of West System epoxy (~$45)
(4) yards 50" 1.5 oz CSM ~$12
(3) yards 50" 1708 $35
1/2 Quart of Interlux InterProtect2000 epoxy barrier coat (~$17)
1 Quart of Rustoleum gloss enamel for the outside ($10)
Misc. brushes, rollers, sanding discs (~$15)
Total cost ~$214 sans all the fittings, so in the same range as what you'd pay for a Ronco tank. It took me 3 weekends to build but mainly this was due to low temps in my garage waiting for everything to cure.

This tank is extremely overbuilt but I was in the same position, Ronco didn't have anything that would fit my space under the v-berth without wasting a lot of room. Overall, I'd say the tank weighs about 35 lbs and will hold 17 gallons. This tank will outlive the boat. I don't know why anyone would build a holding tank to a lesser standard. Do you really want this tank failing?
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Old 28-02-2021, 13:41   #14
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Re: DIY Holding Tank?

Quote:
Originally Posted by rma View Post
I just finished my DIY holding tank build. I first mocked up the space with a cardboard template, then transfered the dimensions onto 1/2 plywood (a good quality, but non-marine-grade). The hard part was getting all the beveled angles to line up but if I had better access to things like a table or bandsaw it would've been much easier. Once I was satisfied with the dry fit, I glued all the panels together sans the lid using thinkened epoxy. I sanded off any excess and then laid down two thick coats of laminating polyester resin on the inside to fully saturate the plywood (this is an important step, do not skip!). Once these coats were tacky I continued the layup with a thick fillet in every seam, then 1 layer of 1.5 oz csm (2-3 layers over each seem) throughout. Then, 1 layer of 1708 throughout, again with additional 1708 taping at the seams, and 1 more layer of 1.5 oz csm. I then faired the inside and sanded it smooth. For the outside, I used the same laminate schedule and fairing process. the top piece was also laminated and once everything was fully cured, I glued the top down with thickened epoxy and 2 layers of 10 oz cloth on the outer seems (using epoxy). The top was made from 2 layers of 1/2 inch ply glued together for a full thickness of 1 inch but with a 5/8" rabbit around the edge so that the lid would seat nicely in the tank and increase the gluing surface. The added thickness also creates more room for threading the fittings. On the lid, I drilled all the fitting holes oversized and then placed the fitting in the holes and filled the gap with thickened epoxy. My fittings are all ABS and are all epoxied in place with the exception of the diptube kits (Dometic ~$35), those come with a gasket that is a really tight press fit. The inside of the tank was painted with 4 coats of InterProtect 2000 epoxy barrier coat, then 2 coats of epoxy.

Total materials and cost:
(1) 4x8 1/2" plywood---$40
~2 gallons laminating polyester resin (~$40 when bought in bulk)
~1 quart of West System epoxy (~$45)
(4) yards 50" 1.5 oz CSM ~$12
(3) yards 50" 1708 $35
1/2 Quart of Interlux InterProtect2000 epoxy barrier coat (~$17)
1 Quart of Rustoleum gloss enamel for the outside ($10)
Misc. brushes, rollers, sanding discs (~$15)
Total cost ~$214 sans all the fittings, so in the same range as what you'd pay for a Ronco tank. It took me 3 weekends to build but mainly this was due to low temps in my garage waiting for everything to cure.

This tank is extremely overbuilt but I was in the same position, Ronco didn't have anything that would fit my space under the v-berth without wasting a lot of room. Overall, I'd say the tank weighs about 35 lbs and will hold 17 gallons. This tank will outlive the boat. I don't know why anyone would build a holding tank to a lesser standard. Do you really want this tank failing?
Wow that is one nice looking tank!
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Old 28-02-2021, 14:49   #15
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Re: DIY Holding Tank?

I have done the styrofoam male mold trick for a tank. It works well.
  1. Carve styrofoam chunk (glued up if need be) to the final shape.
  2. Check fit.
  3. Check fit AGAIN.
  4. Wrap the mold in multiple layers of glasscloth and epoxy (or polyester, if you prefer)
  5. Whlie the resin is still wet, bed in pieces of 4mm plywood onto each flat side to improve stiffness.
  6. Add a few more layers of glass and resin.
  7. Add a few extra layers to reinforce corners and other places of stress.
  8. Drill out the holes where you are going to attach your fittings.
  9. Pour in a bit of Acetone (not much is needed!) The styrofoam will collapse and dissolve in the acetone, and you just pour it out.
It's not rocket science (except maybe that trick with the acetone). Just make it too strong, because nothing too strong ever broke.
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