Holding tank

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Graham_Wright

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www.mastaclimba.com
I spent an age creating a three dimensional cardboard mock-up of a tank to fit in a complex curved space beneath a fore cabin berth, then cut stainless sheets to the broken down pattern pieces, had them formed to shape and then TIG welded all round (professionally). I cut all the connection holes for the Ø38mm pipework 4 for waste and one for air and a big one for a Henderson access hatch. All the stainless is 316. The whole thing is a really beautiful job.

The I bought Pegg Hall's book!

"DON'T USE STAINLESS STEEL, USE PLASTIC!"

Now what do I do?

Can I line the tank with resin or Thistlebond to prolong its life?

Perhaps Peggy will respond and if she does perhaps she would be kind enough to explain why vent connection hoses should be horizontal? I though vertical might induce a bit of circulation.
 
I think the stainless is OK, its actually the welds that cant cope in a holding tank situation. maybe there is something you can bond over the top of the weld but it sounds unlikely.
If you have to start again you might like to talk to
tek-tanks.com, they'll make a custom tank for you.
martin
 
Big ships use non stainless for all their tanks, and coat them with some sort of 2-pot paint. I'd try looking for something on those lines, presumably it would have to be more flexible than epoxy generally is.
 
On classic cars with "airtex" tanks we have used a tank sealing liquid. sloshed it around inside to form a plastic coating which seams to work. Google L&B he sells the stuff based in Cornwall.
 
Not OK in the long run. Once, we had a shiny new SS emptying basin in the lab, where all the liters of urine that we processed went. So, it wasn't all that long in contact with the stuff (.. which mostly was still "fresh" too...). After two or three years, it leaked like a sieve through tiny pinholes. Two-pack paint might help, but the inside should be thoroughly rinsed first with a solvent to remove the greasy film from the SS foil.
 
It depends upon exactly which grade of stainless steel you used. For welding applications in corrosive media, which sh*t plus seawater certainly is, you need to use a low carbon grade, signified by a L. 316L would be vastly preferable to 304L, which I would not use for the job. In addition to low carbon, the L grades have small amounts of other metals that will combine with the carbon present to form carbides, preventing the formation of chromium carbide. If chromium carbide forms it denudes the surrounding heat affected zone of chromium, causing rapid corrosion. This is known as 'knife line attack' because of its striking, sharp appearance.
 
Perhaps you could use it as a water tank.

How long do you plan to keep the boat? If less than 10 years, go ahead and plumb it in as a waste tank...just keep an eye on the welds for leaks after a year or two, and always be sure to flush it out very thoroughly with plenty of clean fresh water if it won't be used again for a while.

As for the reason for short horizontal vent lines...a vertical vent doesn't work like a chimney...it doesn't "dra." The gasses generated by waste--even odorless CO2--are about 50% heavier than air, which means they don't rise unless they're under pressure (i.e when the toilet is flushed). Nor does fresh air from outside fall down a vertical vent. But both can travel horizontally to let the tank "breathe." And, of course, the larger diameter the vent line, the better ti can breathe.
 
Lots of sympathy here for all that hard work. How easy is to get the tank in and out? I'm thinking that it would be a waste of time and money just to junk it. I would expect it to last a couple of seasons at least before the old pinholes appear, when youi could replace it with one of Mr Tek Tanks' excellent creations ( I fitted one of their custom tanks in my Westerly and cannot speak too highly of the quality of work and service). Assuming you fitted baffles in your tank ( you should have) coating the inside completely will be nigh on impossible. Keep your mock up safe and budget for an early replacement. My 90 litre tank from Tek Tanks cost £400 and was quite a complex shape.
 
Tough luck, Graham. Just a long shot possibility; "un-weld" the top plate and lay-up a GRP liner; reweld top plate. Alternatively, epoxy over the welds.
 
My son-in-law is a ship's engineer and he uses Thistlebond on all sorts of tanks for all sorts of contents. I have already used it on my glassed-in GRP fuel tanks.

I'll contact the suppliers, (Woods) who might have some experience.

Thanks Peggie (not Pegg;- sorry! typo) for the breather advice. My breather is short about 400mm and can be horizontal. Sounds like two might be even better?
 
Graham a further point. I didn't understand why you needed four outlets / inlets for waste plus a breather. I had only three holes in my tank, one for waste in, one for waste out and a breather (38mm hose up to a big shell vent emerging under the rubbing strake). My set up uses a couple of diverter valves allowing me to use the pump for our Lavac to pump waste to sea or to tank and then switch the valves to pump out the tank via the seacock when we are sufficiently far offshore. I considered including a shore pump out pipe with the appropriate deck fitting, but dockside pumpout facilities are very few and far between unless you are cruising the Norfolk Broads. In our trip from the River Orwell to the Algarve (Atlantic route) earlier this year, only one marina we visited had this facility.
 
Sorry, missed the 316. If all was 316L I would expect to get quite a few years out of the tank before you have problems. It will help a lot if the tank is flushed over winter, eliminating a line of seawater internally.
 
You are right about the low carbon in 316L (0.03%C compared to 0.08%C), but I think you will find that 316L is otherwise identical to 316.
It is 321 which contains small amounts of other elements which combine more readily with carbon as the weld temperature drops, usually titanium at 5 x C minimum, but can be niobium.
321 is better for welding than 316L, but 316 grades contain molybdenum which gives better sea water resistance (not a problem if you fresh water flush).
 
The problem with stainless

A friend of mine had the SS holding tank on his new boat perforate in 4 years. Just one experience.
Stainless is only "stainless" because of the oxide layer on the surface.
A tank of poo absorbs oxygen and the reducing environment prevents healing of the oxide film. Thus you get crevice corrosion.
Add to this chloride ions from salt water and sulphide ions in the poo and you have a dreadful corrosive mess.
I suspect that the start of corrosion may be a matter of chance. Some hard particle may scratch through the oxide layer and the rest follows. This would explain why peoples experience varies.
I'm sorry for your wasted effort but I think Tek tanks are the best way forward.
 
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