bigger black tank?

[Stormy Weather]

The black tank (13 gals) on my 'new' '73 Tradewind was leaking so I dropped the bellypan and discovered the back 8-10 inches of the floor was completely rotted away, and the back cross member completely rusted out about half way up. I removed the black tank and now am told that I want to intall a bigger one. Since I have to remove much of the floor back there anyway, here's my plan. Someone give me a sanity check....

Bolt two 2" angle iron to the frame crosswise, 13 inches apart. Make 2 or three square 'U' shaped strap brackets about 6 inches deep and bolt them to the new cross members. Box in the new 'dropped' floor inside the strap brackets with plywood leaving a 3 inch hole at one end for the drain on the new tank....a 21 gal. tank from Accutank and which will basically be the same heigth as the original so that I can re-use all the lavatory interior, but still have that extra six inches below the floor for the additional tank space. Re-floor the back 3 or 4 inches with new plywood. I'm retrofitting a gray tank just behind the axle which hopefully hook up with the black tank dump valve, on the street side. If not I suppose I can have two dump valves. Will this work? Should I weld instead of bolt? What am I not considering?

My average needs are to support 4 boys, my wife and me taking 3 day trips, many times in places where there are no hookups. The boys and I can spare the extra tank volume for most of our 'needs', but I'm thinking that on average there will be a true "black" visit a day for 6 people for 3 days, a gallon a trip, I think I should be okay with that size of tank. Anyway, sorry this is so long but before I dive into this project I wanted to bounce this off anyone who might offer some advice and/or warning. Thanks!

[TomW]

Originally Posted by Stormy Weather

... but I'm thinking that on average there will be a true "black" visit a day for 6 people for 3 days, a gallon a trip, ...

A gallon a trip, in my limited experience is an awful lot for an RV toilet. If that is the case, do you need to reconsider the size of your potable water tank?

IMO, as far as care-free operation goes, welding is always better than bolting as long as the welds are done by someone who knows what he is doing. While I know what my goal is, I only welded non-critical areas of my Overlander. In a particularly critical area, I used grade 8 hardware to fasten the repair.

If you do weld, make sure to use nuts & bolts in the right places so the black tank can be removed at a later date.

Are you factoring in a galvanized, insulated box to hold the back tank. How about ducted heating?

Got new axles...yet? Fully loaded black & gray tanks will add a lot of weight. Andy does allow new axles with a higher load rating to be installed on Airstreams.

Sounds like you have a good plan - Good luck with the implementation!

Tom

[Chuck]

You may need to do more frame reinforcement back there to handle the extra weight. these frames flex alot, and bad things can happen if you put too much weight back there. You may read a bit on this forum about the dreaded "rear-end separation" in the later 70's models, which many attribute to the addition of more and larger tanks, without adequate frame reinforcement.

this typically happens on the larger models, though, and thinking about this...I bet that the "on the floor" black tank configuration of our land-yacht models helps to negate these forces. If a tank is suspended from the frame, as it is with many models, and in your proposed retrofit, that dead weight is going to pull the frame down away from the floor and shell when the trailer is bouncing down the road. a floor-mounted tank is going to push the floor down toward the frame, and help to keep them together. There's no place to put a grey tank, except under the floor, hanging off the frame, so that's going to be "bad enough" in that sense. but to do it twice, without additional frame reinforcement....I don't know. But still, too much weight is too much weight, whether its above or below the frame. Even if "seperation" would be helped by that configuration, too much weight way back there on that long lever could still do frame damage.

I'd like to do a grey tank retrofit on mine, as well, but I was planning on leaving the black tank in place for this reason. well, partly for that reason...it would also be more work and more $$$ to change them both. and I also wonder about weakening the floor back there, having such a large hole cut in it, so close to the back edge. I've been "under there" on my trailer, and have similar floor disintegration problems, and there isn't much room between the back of the black tank, and the back of the trailer. I would think having such a large hole in the floor would severly weaken its structual strength. As far as I know, in the larger model trailers with under-floor tanks, the only hole in the floor was the 3" opening for the toilet's output. Anyway, I don't think you can fit much of anything bigger in the original space...I haven't really investigated that closely. you might be able to fit something in there that is a little bit larger, but probably not much.

But whatever you decide, take pictures and post them, so I can steal your ideas for myself. do you have a particular make and model grey tank in mind? I've browsed around on the internet a bit, and have found it hard to find one that would fit in the existing frame spaces, and still be big enough to be bothered with the expense. I can get a custom tank, but that is cost prohibitive.

[Stormy Weather]

Thanks guys. TC (great initials btw) I'm not a welder and one of my goals is to do all this myself, so I think I'll stick with high quality hardware for fastening. Great suggestion on the insulated galvanized tank wrap. I'll do that for sure.
Chuck, excellent insight. The weight thing does scare me. I need to figure out a way to mount it in a way that it transfers some weight to the floor above and not the new cross members....hmmm, something to keep me up tonight. Also I'd like to beef up the frame in the back, but don't know how right off the bat, but I'll go read up, as you suggest, on the backend sag on the forum. You're right that that seems to be more of a problem with the longer models, but I do want to take every precaution.

Regarding the rusted out cross member in the back...should I try and replace that? Does Andy or someone sell those? Of course that would require welding....maybe I'll have to break my code of independenc! The back edge of the frame that supports the shell is also in bad shape. Is there any fix for that other than a shell off repair?

Thanks for you help!

TClay
a.k.a.

[markdoane]

Stormy,

In your plans, make sure that you build a fairly substantial enclosure for the top of the tank, to support the weight of the toilet. It sounds like a fairly large tank and a fairly large floor penetration.

How old are your boys? Have any of them started to bulk up for football? A high school football player can easily weigh 200 lbs. That much weight on a toilet will need a pretty well built base. The Accu-tanks are made of polyethylene and can't support any of the weight, so you will need to transfer the weight to the floor somehow.

[Stormy Weather]

Don, the boys are all smaller than their 200 pound father! Thanks for the heads up on the lavatory support. You may have just saved me the embarassment of a lifetime. lol I didn't even think that the new tank wouldn't function as the old one. In the 73 Tradewinds, the toilet is basically supported by the tank.
Thanks again,
Tom

[Chuck]

you know, this is exactly what I was picturing when I first saw the rotted edge of floor under my trailer. "me", falling through to the ground after assuming the throne. So I made temporary repairs right away. upon further reflection, though, I think that even w/ that back edge of flooring essentially "missing", the plywood under the tank itself is in good shape, and is supported by frame on 3 sides, and has a relatively narrow span, so I don't think I'm in "critical" danger.

I haven't disassembled my bathroom as of yet, (it should be identical to yours, Tom), but from the diagram in the service manual, it looks like there's another piece of plywood on top of the tank, and that is what supports the toilet. this is hidden by the abs plastic tank cover, but when you look at the bathroom, it appears that the toilet is being supported by only this cover and the tank below. I don't have the manual in front of me, but I seem to remember the plywood on top is attached to the back wall somehow. I don't remember seeing "legs" for the front edge, though. perhaps it is another strip of plywood sitting vertically on edge, making a sort of "half-box" ?

[Stormy Weather]

Chuck, you're right there is a piece of plywood about the size of the top of the tank directly under the toilet between the lavatory base (what I think you're calling 'abs plastic tank cover') and the tank, with a hole in it for the tank flange. I'm sure it's there to distribute the weight from the toilet and its occupant, to the tank so that the weight isn't on the connection. It wasn't connected to anything, it just sat there. I've decided to build a 4-sided (three sides and a top) plywood box for support to spare me the fear. I'll post some pics when i get that far.

Tom

[Chuck]

Interesting. Don said that polyethlene tanks can't support the weight...I'm not seeing how an otherwise unsupported piece of plywood is making much difference. It has to be there to give you something to which you can attach the toilet and vanity with screws/bolts. but is it really doing anything to take the stress off the top of the tank? aren't these tanks polyethlene, too? It looks like it from below.