water tank

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sutton sailor

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I am planning to make a GRP tank. I was advised to make a plug, then gel coat the plug and lay up the GRP on the outside so that the gel coat is on the inside of the tank. As this will produce a water proof tank. I had intended to make up the tank with just lay up resin and glass then coat both inside and out with epoxy resin to produce a water proof tank. However some one said to me that some grades of epoxy resin are not suitable to be used with potable water. Can the forum advise me. The tank is to be approximately 54 x 16 x 20 inches, volume 40 gallons or 400 lbs. It will be supported underneath and strapped down, but sides and ends will be unresupported. can any one advise on thickness or lay up weight?
 
Have you considered buying a ready made plastic tank? Much easier than making one - try Vetus or caktanks or other firms available. Or caravan suppliers. Probably cheaper too.
 
I got a quote from Tec-tanks, and they wanted over £900 to make this tank in HDPE which is quite a complicated shape to fit in the hull. The dimensions I gave are only the approximate size of a rectangular cube to indicate the overall size. An estimate of the cost of materials for the GRP tank are £150 to £200. As I am retired I have plenty of time to make the tank.
 
Without calling the Secret Police of Laminate Schedules to design your layup, I'd recommend a test piece. Lay up a strip like a batten and see how it bends under load, then adjust your schedule to suit.
 
Seems to me there's two issues here. First is whether it's best to build it on a plug, from the inside out, or (as I think you are saying you first intended), from the outside in. Second is whether it is best to finish the inside of the tank in epoxy or gelcoat. (Only if you're going down the epoxy route does the question arise of which epoxy.)

I don't know the answers to those questions, but would be interested to hear them for a similar project I've pondered.

I would guess you would want ribs to support/strengthen the sides at the size you mention, unless the shape is such that it provides some of that.
 
LittleSister said:
Second is whether it is best to finish the inside of the tank in epoxy or gelcoat. (Only if you're going down the epoxy route does the question arise of which epoxy.)

I don't know the answers to those questions, but would be interested to hear them for a similar project I've pondered.
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I do remember that on a small 1970's cruiser there was a water tank formed in the forward bunk moulding. It was a "pond" with a fibreglass lid riveted on. Removal of the lid revealed quite a lot of blistering to the gel coat. Perpetual wet and damp with fresh water was certainly detrimental to the gel used in those days. There was absolutely no blistering elsewhere on the boat.
 
Having made a few tanks in my time I can assure you the only material to use a foam core.

No mold required, easy to cut easy to bend, easy to lay up and will not sweat of allow condensate on outside.

Depending on the size you will also need bracing inside to stop the content sloshing around.

if the hull is to form one side or the base or both it will have to properly treated before you start the actual build.

Most of the solvent free epoxy resins are OK for drinking water water, just down load the 'safety data sheet' for the product before ordering.

When making tanks try to have rounded corners on all sides, stronger and safer. If you can not do this on the outside, do it on the inside (even better on both sides).

Any and all fittings should be reinforced and glassed in, not screwed or bolted later. Cut oversize holes and create a layer of Resin and Micro-Fibres at least 10 mm wide, (between edge of fitting and edge of tank, this means that it will clamp onto the filler rather crush the foam.

To make a curve simply make a number of cuts 90% of the thickness of the foam and bend to check the curve is right, then add some resin mixed with Micro fibres and bend to required angle and support it till cured (24 hours).

Because the internal is all epoxy, you could standard resins and cloth on the outside.

Well within the average DIY scope of works.

Good luck and fair winds. :)
 
sutton sailor said:
I am planning to make a GRP tank. I was advised to make a plug, then gel coat the plug and lay up the GRP on the outside so that the gel coat is on the inside of the tank. As this will produce a water proof tank. I had intended to make up the tank with just lay up resin and glass then coat both inside and out with epoxy resin to produce a water proof tank. However some one said to me that some grades of epoxy resin are not suitable to be used with potable water. Can the forum advise me. The tank is to be approximately 54 x 16 x 20 inches, volume 40 gallons or 400 lbs. It will be supported underneath and strapped down, but sides and ends will be unresupported. can any one advise on thickness or lay up weight?
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Building a water tank of that size in GRP is not a trivial undertaking.As suggested lay up over a box shaped plug which is made to fit the space allowing for thickness of layup and any reinforcing. You might use foam to stiffen the large panels. The top can be just a lid that can be bonded on afterwards with inspection hatches for cleaning or removable if you have clear lifting space. You will need to fit baffles and think through positions of inlet outlet and breather positions. Coat the entire interior with food grade epoxy. When you see how a Tek Tanks custom tank is built you will appreciate why they cost so much. However their tank design guide will be useful in deciding on your layout.
 
In common with many other boats I guess, the water tank in my Sadler is built in, made of GRP. I think there is a possibility that it is epoxy coated internally, mainly because it does not suffer osmosis, whereas several other internal parts (engine bilge, heads basin, shower tray) that are frequently wet have become blistered. I imagine that when the boat was built the concern that epoxy might not be suitable for potable water had not been raised. I can only state that the water coming from it seems to be perfectly OK: we have been drinking it for 20 years without apparent harm.

Having built many GRP items over the years I would not attempt to build one from scratch over a mould, which is a time-consuming method that lends itself to fitting problems that may only become apparent when the job is complete. The oldsaltoz method would be my choice but there are a couple more material options. Nowadays it is possible to buy ready laid-up GRP flat sheet, or you could lay your own up beforehand using Formica as a mould. The only baffles in my 60 gallon tank are a few ribs, about 2 inches square section, although I guess a few full-height ones might be a little better. Also relatively easy with pre-made sheet.
 
I have made a water tank in the past, used plywood, then GRP inside and gelcoated. Did two separating walls inside, important for strength and to prevent to much water motion inside. Did the top and screwed it on, added inlet, outlet, and air, then sealed everything with GRP outside.(This was for a safari truck, not a boat, but I went back and forth across Sahara several times and never had any problems with the water tank. It is a strong and inexpensive solution if you know how to work with GRP.
 
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