glass fibre holding tank

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I am considering making a glass fibre holding tank to fit the triangular space I have available in my heads.
I propose to make a male mould with biscuit tin lid type top glassed in place and mould all the inlets and outlets in one go.
The gelcoat surface will therefor be on the inside of the tank and should therefor be impervios to smells etc. Anyone know a reason why I should not go this route.
I recon I could make the 120 litre tank for about £80 instead of the £700 quoted by specialist one off holding tank manufacturers.
 
I'm about to make one myself, having previously made a GRP diesel tank for the reasons you describe: odd shape and low cost. It isn't even necessary to make a mould. Instead, you may find it easier to make a large sheet of two-ply GRP sheet, then cut this to the appropriate sizes, glass it together (leaving the top until the rest is done), then add more GRP. If you opt to use a mould, don't rule out a female one. If you plan on having an access plate, you can glass though this if it's large enough. If the shape is really weird, you can even cover the surfaces it'll bear against with a suitable spacing material and form a two-ply section over that. (That's how I got the outside face of my tank to follow the form of the hull.)

Your proposed hose connections would need to be fairly accurately made. An alternative is simply to bond in place plastic skin fittings with the appropriate hose barb.

I wouldn't bother with gelcoat on the inside. It's essentially a cosmetic layer and I doubt that anyone's going to be admiring the finish. Bacteria are notoriously lacking in aesthetic sense, anyway. Painting on one or two flow-coats of resin (epoxy if you're feeling...er...flush, but polyester should be fine) would suffice.

The advice I've received with such jobs as this is to ensure the lay-up is resin-rich. Impermeability is the crucial element; if the lay-up is done reasonably well and the GRP is reasonably thick, mechanical strength will take care of itself. But do take care that the mountings are bullet-proof, though: 120 litres of poo & water weighs over 260lbs and would make an awful mess.
 
before you start,get a catalogue from www.caktanks.co.uk They make holding tanks for motorhomes and caravans...a fraction of the price of the marine suppliers,and just as good quality. They have a large range in stock,so you should be able to get something to fit.
 
Here's mine pre and mid installation:

P7210147.JPG


PA070290.JPG


I made a male plug from thin plywood to get the dimensions correct, and then laid-up the tank around it.
I then cut around it and removed the plug and then butt joined the tank and lid - you can see the join where the lay-up is darker.
Finally it was epoxy coated on the inside.

3 years down the line it's still absolutely fine: no odours or other problems.
 
can i recomend a book that help me get threw colage and pass with flying sucsess.
its called THE BLUE GEE GLASSFIBRE &EPOXY WOOD BOOK.it cost me £14 a wille ago and worth every penny.its from bluegee products, ring em on 01202 676612.
i would like advise one thing and that is resin is only a glue to hold the fibers together and that if you lode to much and not enuff fibers it will be very weak.
the only thing the book could not help me with is my spelling, boom boom. /forums/images/graemlins/laugh.gif
 
G'day Paul,

<<<I am considering making a glass fibre holding tank to fit the triangular space I have available in my heads>>>

Is this located in the bow? what effect do you expect the extra weight will have?

<<<I propose to make a male mould and mould all the inlets and outlets in one go.
The gel coat surface will therefor be on the inside of the tank and should therefor be impervious to smells>>>

Gel coat is not waterproof, you would be better off rolling 3 coats of epoxy resin wet on tacky to avoid any cleaning and sanding between coats

No need to make a mould, use the hull shape and plastic or remove any coating and epoxy coat the area after you have built the tank minus the lid.

Another alternative is to use sheets of closed cell foam to form the required shape, glue each sheet with Micro Fibres and resin leaving a 40 to 50mm rounded fillet and glass both sides using standard resins then epoxy the inside only, smooth the outside (after glassing on the lid with epoxy coating inside) with a few micro balloons, sand and Flo-coat for a smooth hard wearing surface. This system has the advantage of insulation, so no condensation forms on the outside.

Hope this helps.

Andavagoodweekend......



.
 
Point one, make sure you use Isophthalic resin, not the cheaper and more common orthophthalic resin which is prone to osmosis. A layer of Iso gel/flow coat on the inside will also help in this respect. (Flow coat can be made from gel coat by addition of 'wax in styrene' solution).

Point two: if the horizontal size of the tank is more than about 2 ft in any direction you would be well advised to fit baffles to stop the contents sloshing around as the boat rolls or pitches. Cut 2" holes in the baffle, especially along the bottom. They help to macerate the contents and you won't have to do silly things like having a separate bin for used paper.
 
It never ceases to amaze me what a useful source of information this forum is and how knowledgeable and helpful forumites can be.
I will try to respond to various aspects and recognise that my final attempt wil be a combination of all posts.
Making the tank of aerex foam is a great idea which I should have thought of as I built a Kelsal tri out of it some years ago.
Coating the inside in epoxy seems to be the way rather than gelcoat.
Building in fixing flanges as in the photos is a must ( what great photos .If mine turns out half as good as that I will be pleased.
The holding tank is fitted at the forward end of the heads in the starboard bow just over the bridgedeck.
I am not woried about weight as I propose to empty it as soon as I am in open water. Its just to comply with the new euro/green polution laws.
I aim to keep it simple by having the toilet pump staight into the top of the tank and in normal circumstances drop straight down and out through the seacock which is imediately below.
When in a marina I merely close the seacock opening again when out at sea.
I will have a breather pipe couple to the underside of the bridge deck with a on /off valve so that if there is a blockage I can close the valve and create a presure in the tank to blow it through
Most of these ideas come from experiences of forumites on other threads. Fitting an inspection hatch also seems a good idea for if this does not work.
Thanks again for all the help

/forums/images/graemlins/smirk.gif
Paul
 
Aha - a multihull man. That makes a difference!

(BTW what tri did you build? - I also built a Kelsall tri in the 70s and my present cat is a Kelsall)

You may be able to adapt my system - my holding tanks are inside the hollow low aspect keels - about 50 gallons per side. I pump the heads straight into the tanks and empty them using a bilge pump with a Y-valve that lets me pump either bilge or tank
 
Cant use the keels as they are totally water tank. As I described I am trying to keep it simple where I use gravity to do all the work.
The tri I built was called a chamelion. It was a 26ft centre hull built on a male wooden mould in the garage from airex foam and gps. The outriggers were a tornado catamaran split and using the tornado mast and sails. Very very quick but tended to go straight through waves rather than over them so I eventually traded up for a telstar tri. Buts thats another story.
 
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