To my mind the best material for a
holding tank is polyethylene (PE).
It’s light, clean, easily worked, non-
permeable and won’t stink, unlike the
bladder tank that our boat was originally
fitted with. I’m a fan of bladder tanks for
water, but not for sewage. They must be
carefully installed and protected from
chafe; if one springs a leak I’d rather
it was filled with water than the other.
There was a definite stink about the
one I removed from our boat. Don’t use
metal—you’ll be amazed and possibly
horrified at the potency of urine as a
corrosive liquid. Fiberglass is a good
option, if you have a mind to make your
own tank.
Several companies offer ready-made PE
tanks in various shapes and sizes. It will
be up to you to work out where you want
the holes for inlet and outlet and vent in-stalled. If you have an oddly shaped space
to fill, you will have to carefully take mea-
surements to supply to the manufacturer;
I would recommend building a cardboard
mockup just to make sure your measure-
ments are correct. I had my PE tank built
by Raritan; it holds 15 gallons, and I almost
immediately regretted not having it made
a little larger.
I installed it on a plywood shelf sup-
ported by hefty aluminum angle. The tank
is retained by smaller aluminum angle
strips and secured by a pair of tie-downs.
In eight years of sailing, it hasn’t budged
at all.
I used 11/2in ID Shields sanitation hose,
which has remained odor-free. I located
the tee from the tank to the pump-out
standpipe above the waterline in case of
hose or clamp failure, but have had no
such issues. I made sure there were no
low spots in the plumbing where effluent
could pool; and since the hoses and tankare effectively rinsed out by the mere
action of sailing, odor has never been a
problem.
Note that I have a Lavac toilet—I favor
these for their simplicity and because
they break up waste like no other head—
with its own Henderson diaphragm pump.
You can use just about any sea toilet with
your gravity drain system.A gravity drain tank
on a Bavaria 45CHOOSING A TANK
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