Boat Water Tanks –  out of sight and often out of mind

Since one of the most precious commodities on any boat is drinking water, it follows that the containers used to store the stuff have to be of exceptional quality. Unfortunately, this isn’t always the case.

A variety of materials are used to make tanks:

• Early tanks were often moulded in place, using the hull itself to form one of the sides. Although convenient from the construction point of view there are at least two serious problems. Firstly, that the water would soak into the GRP laminate, promoting osmotic blistering on the inside; and, secondly, that soluble compounds within the laminate would leach out to taint the drinking water. This technique is very rarely used today.
• Galvanised steel – a choice borrowed from the header tanks many of us have in our attics. Rust is a perennial problem.
• Aluminium tanks were also popular but suggestions that aluminium cooking utensils might be implicated in the development of Alzheimer’s disease somewhat took the edge off their popularity. Whether this is true or not has yet to be established.
• Stainless steel. Still widely used and an excellent choice. Flexural failure or pin-hole corrosion to welds can be an issue, but these are generally reliable.
• Welded food quality polyethylene – another worthy material. Because the material is fairly thick (10mm being common) the sides are exceptionally rigid and therefore resistant to the flexural fatigue problems you can have with stainless steel.
• Rotary moulded high density polyethylene. These are invariably standard off-the-shelf items, available in a number of shapes and capacities but not customisable like the previous types on our list. Compared to the others, they are economically priced.
• Flexible tanks. These are available in standard sizes or made-to-measure. Being light and adaptable to awkward spaces, this is an attractive concept but they are susceptible to abrasion and subsequent leakage. On a delivery trip to the Mediterranean, I had a flexible tank burst on me. Fortunately, the boat was a catamaran with an identical tank in the other hull. Strength and durability depends greatly on quality. The best flexible tanks are double skinned, having a polyester reinforced outer skin and a food grade polyurethane liner.

Manufacturers often adopt a fit-and-forget philosophy, meaning that tanks are often buried into unreachable spaces fit for nothing else. The irony is that most have access hatches so you can open them up and clean them. But that assumes that you can access the access hatch which is far from always being the case.

Maintenance:

• If possible, open the tank and remove any obvious debris. With the tank dry, a vacuum cleaner is a useful tool.
  
• Tanks should be sterilised periodically with a sterilising product (such as Milton). The manufacturers’ recommendations should be followed. In Milton’s case this calls for 30 millilitres of solution for every 5 litres of water.

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Related reading: Pressure water system