easy holding tank improvement?

[markxengineerin]

I recently did some work on the holding tank that involved removing the plastic lid. When I re-assembled, I used only silicon to seal the flange. Bad move, silicon is permeable to gases, 2,000-20,000 cc/m² per 24 hours and water vapor transmission of 100-150 grams/m² per 24 hours according to one source. It certainly smelled, especially on hot days.

Pacific Seacraft appeared to have origninally used a combination of silicon (outer layer) and some kind of white sealant for the inner layer. The white sealant must have been better, it didn't smell as much, but it's right below the v berth and the area has always smelled like a "boat", not necessarily like a holding tank, but more bilge-y.

In looking for solutions that didn't involve removing the lid again and making a proper gasket, I came across aluminum foil tape, which I had on board. "A typical water vapour transmission rate (WVTR) for 9 µm foil is 0.3 g/m2 per 24 hours at 38 °C and 90% RH." So, assuming these were tested the same way (not sure what "thickness" of silicon was used or if they normalize for that) , that's a 300X improvement over silicon, for water vapor. Water vapor is obviously not the issue, but not easy to find data on other gases.

Anyway, day 1 after taping the seams has been a night and day difference, huge reduction of smell. I still need to air out the mattress, but I can no longer smell the boat odor directly from the tank lid seams. Hopefully this continues, I will report back after some time, but might be worth a try for anyone with a similar setup.

Photo with 1 seam taped- I used 3 layers.



As an aside, what would be the best type of material to make a gasket from sheet? How thick? And what caulking/sealant type products are most impermeable to gas?

[peghall]

Only one POSSIBLE drawback to using aluminum tape and I'm not sure of that:
Urine is so corrosive that it'll turn an aluminum tank into a colander in just a few years. I understand that you're using it as a gasket, so urine itself should never come in contact...but I'm wondering what kind of impact--if any- sewage gasses might have on the tape that would shorten its life as an odor blocker.


As for the best rubber gasket material: rubber is fine. It needs to be thick enough to run screws through it without creating "puckers" around them...they leak. As for caulk material...what do you want to calk? There isn't much in a sanitation system for which caulk is good solution.


--Peggie

[markxengineerin]

Thanks, I suppose I can easily replace the tape each year if it becomes an issue. Another downside is that I would not want to buy a boat if I saw something like this going on, but I'm not selling : )

I was thinking of caulking the seams with something before I put the tape on. There was a small gap on the edges of the lid, as I had placed the sealing bead of silicon more "inside" the screw holes and any overflow on assembly would have oozed into the tank rather than out into the side gap. I decided against this because I didn't know what product to use, but if a non-permeable caulk that could stick to fiberglass and gelcoat existed, I think that would be a good place for it.

[peghall]

How 'bout a top quality bedding compound?

[markxengineerin]

I looked for permeability info, and did not find it on the datasheets of the usual marine bedding compounds used. Maybe someone has used something in the past with success?

[Bowdrie]

Originally Posted by markxengineerin

but if a non-permeable caulk that could stick to fiberglass and gelcoat existed, I think that would be a good place for it.

Not sure about permeability, but of all the stuff commonly available I would be tempted to try this.
https://www.boatlife.com/product/life-calk-cartridge/
It's a Polysulfide, as opposed to the much more common Polyurethanes.
While it does have adhesive properties, it's much less than the Urethanes.
Just thinking out loud, I wonder about "Dolphinite", a wonderous stuff with far more uses than what its label suggests.