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Holding tank Gasket?

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Hi all, while we're discussing holding tank hoses....... I'm building a sewage holding tank out of glassed, epoxy on ply. Half way through it - b&$ch of a job - why can't boat designers make at least some sections in straight lines with right angles???


Anyway, I'm thinking of making the top piece of ply removable - but that means I need a gasket to seal the top well to the box - for obvious reasons.


Any thoughts on what material to use as a gasket???


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Made mine out of 20mm PU foam.
Release tape on hull to get shape.
Then glassed back panel insitu.
Made bulk panel, glassed both sides
Templated other 5 sides ,cut out , glue gun, fillet & tab. At appropriate time glassed in stack, and pumpout tube that is directly above dump valve for rodding.
Tedious to make, but works great and Max vol/space.
Oh & 2pac gloss. Yep....OCD.
BTW, have spare sheets of 20mm PU medium density if any one wants to make something...free to good home. (non-structural)

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My black tank is formed with 6mm ply (and some hull) that has been glassed (inside and out) and has been flowcoated the crap (excuse the pun) out of.


The top is ply (6mm) that has been glassed and flowcoated. It was then put down on a well and carefully laid big thick bead of a MS (Quilosa brand Modified Silicone) and screwed while that went off. That bead will be amlost gasket like now, probably 17-20mm wide and about 2mm thick and it is a bit flexible so will not crack. I did make sure it was a good solid continuous bead, no gaps or breaks.


In the lid is a small (6mm odd) vent up and out the cabin side. This line has a valve in it I can open or close at will. The valve is placed directly over the black tank outlet just in case I need to get a stick thing in to 'free things up'.


After a week or so to let the MS cure I closed the vent valve and pumped the dunny (exit to tank, not over the side as I can also do) up which pressurised the system/tank. I left that pressure, or so I hoped, in the tank for a day and then cracked the vent valve.




My seal works and works well.


In the plan is if some 'tank contents' become reluctant to leave I can close that valve pressurise the tank and then crack the bottom outlet hopefully firing the contents out like a bad case of explosive diarrhoea.


My only concern is getting the lid off should I need to later. That Quilosa stuff sticks far better than sh*t to a blanket so removal may involve some ply carnage. But I have lots built in to get to and around possible issues that taking that lid off will be the last resort nuclear option.


I do have 2 right angles in the pooh travelling tunnel, one downside of having to retro a 50fters worth of gear into a 30fter, but one is early in the system so things will be well lubed and slippery plus have a pump encouraging them along. The other is the flush mount thru hull valve and I'm hoping by the time things get there all there is will be a slurry. Fingers are crossed but noses are living in terror that the plan is a f*ck up.

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I think if you ever seen intestines you will understand sh*t actually goes around corners....


The problem is toilet paper, great wadges of it, or worse - the white mouse.  There is no surprise that neither of these forms part of the human diet.

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Thanks KM - that's what I was planning on doing but wondered if there was, literally, a gasket material of some sort out there.

Decision made!! Yay.

However, several have raised the 'flowcoat' finish. I've read lots of stuff that says 4 to 6 coats of epoxy is fine as the protection and as the finish.

Why use flowcoat on a holding tank? And does it need a finish over itself in a holding tank?

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And TL - I went everywhere trying to find a plastic one but nothing fitted the space(s) I wanted to use - without remodelling the forard 1/3 of the yacht..................

AndDtwo - trust me - at my age I know a thing or 2 about bowels and how they (sometimes don't) work!! Sigh.

LATERAL - thought about that but decided against it as I haven't a bloody clue how to do it - I understand plywood much better...............

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Make sure your port cover has as much overlap as possible so the MS 'Gasket' has as much area as possible to bond.... more is better. Also tighten it down lightly so you don't squeeze the sealant out, tidy up and come back and nip up the fastenings a fraction after 24 hours.

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