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From: Stefan Mochnacki (no email)
Date: Mon Sep 24 2001 - 19:33:33 EDT
From: "Tom O'Meara" <>
At 03:09 PM 9/24/01 +1100, you wrote:
>Hallo all,
>My husband is planning to build a holding tank in our wooden boat (H28).
>Has anybody had experience doing this themselves, and could offer any tips?
>He has read an article saying you can build one using plywood coated with
>epoxy resin, with internal baffles. The only place to put it will be under
>the forward V-berth. Holding tanks are not common where we live, but will
>be required as we go further up the Australian coast.
>cheers, Margie
>
The original "St. Brendan's Isle", a 50 ft steel schooner, had ply tanks
for fuel, water and holding tank duties. When I built it, I did NOT want
any integral steel tanks (as most people were doing then on steel
boats). The reasons for this were many but are not the subject of your
question.
I built mine by constructing the bottom, sides and baffles first. All this
was West System epoxy coated, then covered with two layers of epoxy filled
cloth. It has been years since I did this and I do not recall the cloth
weight (it was scrap I had around from other jobs) but it was not very
heavy. I doubt it matters. The cloth just stabilizes the ply grain so
that it does not "open up" later and it also gives a surface you can smooth
MUCH easier than just ply alone. It adds little appreciable strength.
Once I had the tank and lid complete and both 100% covered and
multi-coated, I permanently affixed the lid by giving it a last coating of
epoxy and setting it on top of the tank assembly. I "buttered" the top
surfaces (edges) of the tank sides and baffle tops with thickened epoxy to
bed the top firmly with no gaps, then screwed the top on with drywall
screws. Before anyone gasps with horror about the use of drywall screws,
they were later backed out while the epoxy was still 'green'. The holes
were then filled with epoxy and the job was done. If you don't want to
mess with taking the screws out, use bronze and leave them in. Actually,
you could probably use any material at hand as they are/would be sealed in
and do not come in contact with any metal or liquid at all.
I probably do not have to add this, but all holes and inspection covers
need to be thoroughly coated and sealed. I do not need to elaborate on
what things would start smelling like if sewage waste started permeating
the ply.
Apply thought to this. You are not building fuel tanks, but on mine I had
one fairly large hole on top of each of the three 50 gallon diesel
tanks. I took great care in sealing the wood in these openings and epoxy
glassed cloth over them to boot. Over this hole I bolted down the 1/4"
thick aluminium cover plate (about 12" in diameter if I recall
correctly). This single cover contained all the tank plumbing. Fuel
pick-up, fuel return, fill hose and vent. It was much easier to do all
this in the aluminium cover than have myriad little holes in the tank lid
itself. Water and waste tanks were also fitted with a "one shot"
inspection and plumbing cover.
While being built, the ply tank lids were drilled and fitted with stainless
epoxy bedded bolts, thread upwards, every 3 inches around the perimeter of
the access hole, acting as studs. Water and waste tanks were similarly
fitted. I used truck tire inner tube gasketing material (probably not the
best choice but it was what I had) and insulated the studs, washers and
nuts from the aluminium cover with nylon inserts commonly available... but
I believe this was overkill, especially if you can keep the tank tops
really dry. At the time (steel boat, remember) I was fixated on stopping
all corrosion, real or imagined. :)
All tanks were set on angle iron mounts so air could get at all sides and
the tanks were not sitting in a permanent "wet spot" (I also wanted to be
able to see all portions of the hull interior to combat rust). I used
heavy rubber, again from truck inner tubes, cut into strips to set the
tanks on to keep chafe and rubbing at bay. They were all FIRMLY bolted
down with angle iron "straps" (three per tank) and threaded rod across the
tops. Movement is your enemy.
I painted the exterior with epoxy swimming pool paint and as far as I know,
those tanks are still in service.
To arrive at a size (in gallons), measure thus (all dimensions in inches):
Inside HEIGHT times WIDTH times DEPTH = x
Calculate the volume of the baffles (Outside height x width x depth) = y
(x-y) divided by 231 = gallons
I remembered this simple formula not from my boat tank building but from my
aquarium building days. By the way, you can make ply aquariums too.
The great benefit to building your own tanks is that they may be of almost
any size and shape you like.
Fair Winds,
Tom
Tom & Jackie O'Meara
Searunner 40 Trimaran, "Sea Skate"
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