Selecting auxiliary tanks can become problematic as many people are unaware of what is available on the market, where they can fit tanks, and what to look for when selecting tanks for their vehicles.
If we begin with the basics we can then understand what we need to consider when selecting a tank/tanks and their capacities:
Water weighs 1 Kilogram per litre.
Diesel weighs 0.85 Kg per litre
Petrol weighs 0.95 Kg per litre
We need to consider the minimum water requirement for a person to survive and remain healthy is 2 litres per person per day, and this is only for drinking purposes, if we factor in cooking, and a person washing once per day then this rises to around 6-8 litres per person per day. From this we can calculate the volume of water we need to carry and the weight of water we may need for a trip, consideration needs to be given to any contingencies such as available water supplies, and it is always better to calculate on the high side and carry more water than may be needed. If water is unavailable then this contingency, or additional water becomes necessary. It becomes even more necessary if you become stranded through a breakdown or stuck for other reasons in a remote location.
When selecting a tank it is important to consider its construction, in the previous section we considered usage and material, so a decision should have already been made as to what material you wish your tank to be constructed from, and what it will be carrying. Tank features now become a consideration, has it got a suitably sized access on top of the tank? this needs to be large enough to get your hand in, and a light, for examination and cleaning, and how many and what size of outlets has it got? what size is the filler neck, and can it be filled quickly from an onboard food grade hosepipe. These are all considerations as too small a filler tube means it will take longer to fill, and if it does not have an inspection chamber how do you see inside it or clean out any debris which may have accumulated through normal usage.
Two other considerations are does it have a suitably sized breather pipe? and will this allow an extension pipe and filter to be fitted so it can be extended inside the vehicle, and a filter will prevent the ingress of dirt or other airbourne contamination such as dust. Too small a vent pipe will restrict the output and make it much slower in operation in gravity feed modes, and if an electric pump is used it may restrict the tanks output and overload the pumps motor.
Connections are very important, does it have adequate connection points to add both a manual and an electric pump, and does it have a drain bung for cleaning and draining, you will be surprised how many commercially manufactured tanks are missing such features.
Now we have the information to decide on what features we need, the material type, and its capacity; we need to look at locating our tank, if we add the weight of our tank to its volume we can calculate its total full weight to decide where we can locate it, and also if we have to consider uprating suspension components for example. Larger tanks must be fitted under the vehicle, or on the boot floor as these have a detrimental effect on the vehicles centre of gravity, so what type of tank will we choose? do we have one which spans the chassis rails, or do we fit two smaller and narrow tanks along the chassis rails? Both have pro's and con's but i would always suggest two smaller tanks fitted lengthways along the inside of the chassis rails, two tanks means if contaminated water is taken on then you only lose half your supply, and these narrow tanks spread the load along a much greater length of the vehicle, and are much less susceptible to being hit by debris or even a vehicles propshaft. The downside is that each tank needs fitting out individually with pumps, electrical connections if electric pumps are used, and you need two filler pipes, breather pipes, and basically twice as much installation work. In my opinion this is worth it in the long term.
Location of tanks is important, if underbonnet water tanks are used their water will be warmed from the heat of the engine, not very nice for drinking, but handy for washing yourself or the dishes. Underseat tanks may be difficult to access, but are still a useful storage tank area, and very useful in utilising space which would otherwise have been wasted. Many small tanks can be interconnected to a larger storage tank, it will require the appropriate pipe to connect them and each tank will beed an isolation valve, if one tank is filled by several others and one joint leaks you can lose a lot of water unnecessarily.
Waste or grey water tanks have not been mentioned as they are not used very often, but with tightening legislation they may become necessary in sensitive parts of the world to prevent waste being dropped in these areas, steel tanks are fine for this purpose, and one with a valved outlet lets you drop waste water elsewhere.