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Old 03-21-2022, 07:20 PM   #1
Wavery
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Default Black Water Tank Plan

I'm going to start building the mold for a fiberglass black water tank.

Before I start building the mold, I thought I'd brainstorm with some of you guys.

The current plan is to build a single female mold 18" wide x 19.5" long x 5" deep.
My thought (at the moment) is to use one mold to build 2, 5" deep tanks with a 1" wide joining lip. The 2 halves will be glassed together after the top mounting plate is installed in the top half and a 3" F/G drain pipe in the lower half.

The 1" lip is designed to support the weight of the tank (mainly) on the upper aluminum floor plate as apposed to just dropping it down on the, weaker, bottom floor plate. A 1" wide x 2" high square aluminum tube will surround the hole in the floor and the tank lip will rest on that tubing. The tank lip and tubbing will be drilled through to accommodate 16, 3" long S/S machine screws. I'll use nutserts in the aluminum floors as apposed to just using sheet metal screws.

The finished tank will be 18" wide x 19" long x 10" tall. That should net me about 14G capacity.

I'm open to any/all comments. Please don't be shy about criticizing the plan. I'm looking for any positive, negative and corrective comments.
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Old 03-21-2022, 10:30 PM   #2
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That looks like a nice design! Good for you. Let me offer two quick observations.

1. Your diagram shows a wedge in the lower half of the tank, to make room for wheel well clearance. The wedge appears to be entirely below the finish floor. However, the wheel well extends several inches above the finish floor. Are you sure your tank will clear it?

2. I ask this next question out of complete ignorance. I have seen only a few black tanks, and they all seem to have a floor that slants downward toward the exit opening. Presumably this is to ensure complete emptying of the tank (no puddling). And perhaps more important, the slant ensures an energetic WHOOSH when the dump valve is opened. This is important because the WHOOSH must sweep away all the solids that are lying on the floor of the tank.
The tank of the Thetford recirc toilet doesn't slope, I don't think, but they ensure that the WHOOSH sweeps away solids by trapping those solids right near the exit opening. Should your design have a slanted floor?

This is a real nice project. I'm anxious to see how it comes to fruition.

Bill
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Old 03-21-2022, 11:15 PM   #3
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Quote:
Originally Posted by Bill View Post
That looks like a nice design! Good for you. Let me offer two quick observations.

1. Your diagram shows a wedge in the lower half of the tank, to make room for wheel well clearance. The wedge appears to be entirely below the finish floor. However, the wheel well extends several inches above the finish floor. Are you sure your tank will clear it?

2. I ask this next question out of complete ignorance. I have seen only a few black tanks, and they all seem to have a floor that slants downward toward the exit opening. Presumably this is to ensure complete emptying of the tank (no puddling). And perhaps more important, the slant ensures an energetic WHOOSH when the dump valve is opened. This is important because the WHOOSH must sweep away all the solids that are lying on the floor of the tank.
The tank of the Thetford recirc toilet doesn't slope, I don't think, but they ensure that the WHOOSH sweeps away solids by trapping those solids right near the exit opening. Should your design have a slanted floor?

This is a real nice project. I'm anxious to see how it comes to fruition.

Bill
Thanks for you input Bill.

The wedge in the mold creates a void in the finished tank. That wedge will simply be taped into the mold for laying the F/G in the bottom half. When I lay out the top half, that wedge will be removed.

As for the bottom being flat....... I haven't decided yet. I could actually do as I did with the wedge for the wheel well. I could tape in some sloping pieces, sloping to the middle of the tank. That would probably cost me about 1.5G of
storage and complicate the tank installation. I'd have to set the tank in expanding foam. My current plan is to install a "Camco Tornado Rotary Tank Rinser" to flush the tank out after dumping.
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Old 03-22-2022, 12:20 AM   #4
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Actually, when I draw it out, it really does look better and I just lose about 3/4G. The mold is always the most time consuming part and it has to be perfect. Once I have the mold, I could make lots of tanks. I actually enjoy working with fiberglass. I've designed and built more small (F/G) boats than I care to remember.
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Old 03-22-2022, 10:50 AM   #5
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Quote:
Originally Posted by Wavery View Post
Thanks for you input Bill.

The wedge in the mold creates a void in the finished tank. That wedge will simply be taped into the mold for laying the F/G in the bottom half. When I lay out the top half, that wedge will be removed.
Wavery -

I failed to make my point. Your drawing shows a wedge indentation in the lower half of the tank, so that the tank will clear the wheel well liner. But the wheel well liner extends above the upper floor skin - you can see it, next to the wall near the rear corner of the sink cabinet. It looks like a raised flap of the vinyl flooring. Most owners are unaware of this, and those who do see it often refer to it disdainfully as “sloppy construction”. But if you lift that flap, you can see the wheel well liner poking through the floor. My concern was to be sure that the upper half of your new tank, including the square mounting tube on which you will set the tank, will clear this obstruction.

Bill
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Old 03-22-2022, 11:37 AM   #6
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Quote:
Originally Posted by Bill View Post
Wavery -

I failed to make my point. Your drawing shows a wedge indentation in the lower half of the tank, so that the tank will clear the wheel well liner. But the wheel well liner extends above the upper floor skin - you can see it, next to the wall near the rear corner of the sink cabinet. It looks like a raised flap of the vinyl flooring. Most owners are unaware of this, and those who do see it often refer to it disdainfully as “sloppy construction”. But if you lift that flap, you can see the wheel well liner poking through the floor. My concern was to be sure that the upper half of your new tank, including the square mounting tube on which you will set the tank, will clear this obstruction.

Bill
Thanks Bill.

I think that I actually got the point. I won't actually make the wedge until the floor is cut away and a 10G steel plate is installed on the trailer to protect the tank from "exploding tire" syndrome. It will definitely have no impact on the upper tank or tank lip. The wedge in the mold will create a 10" x ? X ? void in the finished tank. Maybe this drawing will make it clear..... (or not.).

Thanks again for helping me brainstorm. As I make these drawings, it helps make it more clear in my head.
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Old 03-22-2022, 04:58 PM   #7
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You mention flat vs tapered for the bottom of your tank. Count one vote for tapered. The one complaint I have with my Dometic SeaLand toilet is the bottom of the tank is flat.

Another thought, incorporate a flush/rinse device into the tank. I have a Camco Tornado waiting to go into the SeaLand tank.

I lied I do have another complaint. I wish I could’ve gotten it in white instead of “bone”.
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Old 03-22-2022, 06:39 PM   #8
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Quote:
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Wavery -

I failed to make my point. Your drawing shows a wedge indentation in the lower half of the tank, so that the tank will clear the wheel well liner. But the wheel well liner extends above the upper floor skin - you can see it, next to the wall near the rear corner of the sink cabinet. It looks like a raised flap of the vinyl flooring. Most owners are unaware of this, and those who do see it often refer to it disdainfully as “sloppy construction”. But if you lift that flap, you can see the wheel well liner poking through the floor. My concern was to be sure that the upper half of your new tank, including the square mounting tube on which you will set the tank, will clear this obstruction.

Bill
I just saw a picture of your bathroom floor, on another thread. Now, I understand your concern.

I'm not sure why your TM has that big wedge on the floor. Mine doesn't have that. It has that small cabinet brace (easily removed) but the floor is nearly flat all the way up to the cabinet. The distance from my cabinet to the tub is exactly 20" also.

Picture 1 is your bathroom #2 is my bathroom.
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Old 03-22-2022, 06:54 PM   #9
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Remove that wedge and you’ll have a bit of wheel well under that raised vinyl.
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Old 03-22-2022, 06:59 PM   #10
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I love our idea of making your own toilet to fir your needs. But reading Bills inputs, I got confused. The wheel well in my smaller rig is hidden under the bathroom cabinet.
I believe that wedge is to protect the wiring, not the wheel well?
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