2 or 3 waste tanks?

[Jim Blackwood]

First the geometry. My fresh water tank is about 40 x 55" and has roughly an area around it that is 14" on the sides and 10" at the front, still allowing room for the HVAC door to open fully. (rear bay) The fresh tank is 275g.

So does it make sense to have 3 roughly rectangular waste tanks or two "L" shaped ones? The combined volume of the waste tanks is about 230g.

The side tanks can be 57 or 73g depending on whether they are full length or the end tank is full width. The end tank can be either 87 or 107g, same reason. The tanks will be linked with dump valves between tanks, meaning that, say if the configuration was either 57/107/57 or 73/87/73 (still approximate numbers at this stage) the end tank could be selected to expand the capacity of the gray tank, the black tank, or make a single combined tank depending on how the valves are opened. Gray water will flush from the left tank to the right to dump.

Or, if the tanks were "L" shaped there would be only one dump valve between the tanks but I'd have to decide  how much of the front went with each side and it would probably need a remote operator on the connecting valve. (A single tank would not be possible to install btw.) If evenly split each tank would be about 115g.

I'm sort of leaning towards the 73/87/73 split but it makes dumping more awkward and install of the left tank a little harder compared to the 57/107/57. Given this scenario what would you guys do?

Jim

[richard5933]

Shape of the tanks shouldn't matter, as long as they slope to the drain point. For the black tank though, there will be a time you need to flush things out and an L-shaped tank is going to make that more difficult.

Two tanks is certainly less of a hassle to empty than three.

One consideration, especially since these are huge capacities you're talking about, is weight distribution & weight shifting. What I'm talking about is planning out how the weight will change/remain the same as water moves from your fresh water tank to your waste tanks.

On my bus, I have two tanks (fresh and waste) which are each 90-gallons. They are relatively tall and stand side by side in one bay. As the water leaves the fresh tank, most of it returns to the waste tank. The total shift is about two feet forward as the waste tank fills and the fresh tank empties. They are both on the same side, so the side-to-side balance remains the same. I typically travel with about 90 gallons total between the two tanks, and it is counter-balanced on the other side of the bus by the generator which weighs about the same.

If I remember correctly, you're planning to have over 200 gallons of fresh water capacity, and depending on how you situate the waste tanks you have the potential to be shifting 1600+ pounds side-to-side and/or front-to-back. That would be my bigger concern than the shape of the tanks.

[someguy]

275g + 230g = 505g  @ 8.33 lbs / gallon = 4200 pounds in one bay.  Plus the next bay has 180g x 7 pounds = 1260 pounds of diesel fuel in it.

These bus frames were never designed for that kind of concentrated loading.   If you run a frame stress analysis you'll see what I mean.

The bus frame was designed for 48ish passengers distributed evenly from front to back on the main floor and their luggage, which is fairly light,  distributed in the bays.

The bays aren't designed for a 4200 pound load, plus whatever else you put in that bay. 

You'll want to look at reinforcing things if you proceed with that plan.

[richard5933]

275g + 230g = 505g  @ 8.33 lbs / gallon = 4200 pounds in one bay...

Usually the freshwater is emptied as the waste tank is filled - they are rarely both filled at the same time, if ever. Under that scenario the most would be 275g x 8.33 = bout 2,300 lbs. A lot of weight, but doable. Would certainly be capable of upsetting the balance of the bus if it shifted from side-to-side or front-to-back though as it's used.

[Jim Blackwood]

Correct Richard. And I made that same mistake originally of calculating the combined weight but 275g will be the absolute max since once the waste tanks are full you are done, no matter how much fresh water capacity you have. The exception might be with hookups where you bypass the water tank and fill the waste tanks but those would be dumped before leaving so I don't imagine that would pose a problem. Because the waste tanks will surround the fresh tank weight shift will be minimal. Maybe 5" forward at most, and from inboard to outboard. Fresh water tank is well baffled and the tall profile of the waste tanks means little baffling will be required. Thinking about water usage I am suspecting that the 57+107/57 configuration is going to result in the least change in balance. with the right hand tank being black water. I think it might be possible to install that large of an "L" shaped tank on the left. there is a 28v breaker box I would have to remove and swing out of the way but I think that could be done.

Jim

[chessie4905]

Those bays can hold a hell of a lot more than you think. The bay walls are attached to the floor structure, which in turn is attached to the sides and roof structure. A lot like an airplane. Greyhound carried a lot of heavy freight in their baggage compartments and havent heard of issues from doing so.

[DoubleEagle]

Wasn't there a story about how Greyhound put four engines in the bays for shipping in a Scenicruiser and serious stress cracks developed in the sidewalls? I lot of retrofitting went on to address the weakness of the structure. Tom McNally would surely know about it.

[windtrader]

A consideration for 3 vs 2 is easier and more balanced placement. Also smaller tanks might slosh around less. Personally, I'd place tanks where it maximizes easy bay storage access and most flexible use of the remaining space. If a smaller tank can be tucked into an otherwise not very accessible or usable space, do that. Naturally, more tanks, more  plumbing and more maintenance/problems.

[Jim Blackwood]

Thanks guys, I think I've settled on a plan. If you'll recall I had been working up two waste tanks of about 60g each but following our discussions on gray water I looked at the available space again and saw I could add the end tank pretty easily. Now I'm thinking of one side tank for the black water of around 60g and an "L" shaped tank for the gray that takes up the other side and the end. That solves several problems like putting the gray dump valve in a convenient location right inside the bay door, and making it easy to route drains from both sides into the gray tank, plus easy access for a small gray water drain. Anyway I now have the detailed sketches done all except for the vent lines and the drains into the gray tank and hope to finish that up tomorrow. This setup has evolved quite a bit over the summer and it looks like most of the improvements have now been made. The advice was definitely helpful. Of course nobody can really advise on the relative volumes of the gray and black tanks, but I think the ~170/60 or roughly 3:1 ratio should be in the ballpark based on past experience. In any case it should be adequate for my needs, and it will all fit behind the swing of the HVAC door in the rear bay. Access to the plumbing should be very good as well, plus leaving space just inside the doors and a cross space 2ft wide and 30" high for temporary storage. That seems to me a pretty sensible use of the rear bay. The vent lines might be right much of a challenge but I'll figure something out. I want to bring fresh air into the black tank to encourage aerobic decomposition.

Jim

[silversport]

275g + 230g = 505g  @ 8.33 lbs / gallon = 4200 pounds in one bay. 

This scenario is very, very unlikely. Most RVers dump and fill at the same time, having both tank full at the same time, I don't think so. In the real world you have a total of 275 gals (give or take 25 gals) in your rig at anyone time.