Tanks are among the hardest parts of a boat to reach; often the boat was built around them. So the decision to renew depends not only on the tank's condition but on how to get at it.
Signs a tank needs renewing
- Fuel smell around the tank, fuel traces in the bilge.
- Water and sediment at the bottom; filters blocking often.
- Weeping along welds on stainless tanks; white powdering and pinholes on aluminium.
- Taste and smell in the water tank; scale and corrosion inside.
- Odour from the holding tank, leaks at hose connections.
Material choice
- Stainless steel
- Common for water and fuel. Weld quality and keeping water from pooling under the tank decide its life.
- Aluminium
- Light; contact with wet surfaces and mixed metals causes corrosion.
- Polyethylene
- Does not corrode; shape is limited, and mounting and expansion must be allowed for.
- GRP
- Integral or separate; fuel needs a suitable resin and internal coating.
The sequence
- Access plan. Which opening the tank comes out through, and which joinery must come out.
- Drain and clean. Gas-freeing and safe working on a fuel tank.
- Measure and build. With inspection hatch, baffles, fill, vent and gauge fittings.
- Pressure test. Tightness tested before installation.
- Bedding and connection. Bedded so water cannot collect underneath; hoses double-clamped.
Tank materials
Tanks aboard are made of stainless steel, aluminium, polyethylene or GRP integral with the hull. Each has its weak point. Stainless can pit where it lies against a damp surface starved of oxygen. Aluminium wastes where water collects at the bottom or where it touches another metal. Polyethylene resists corrosion but can crack at fittings and its fixings can loosen. In GRP tanks, the inner laminate surface and the tightness of connections matter.
Signs of a tank problem
- Fuel smell or a thin film of fuel in the bilge.
- Stains and residue under or around the tank.
- Fresh water going down faster than expected.
- Weeping or drips at fittings on the tank.
Why access matters
The cost of a tank replacement is usually set not by the tank but by getting to it. On some boats the tank went in before the interior was built, and removing it means taking out joinery, sole or engine. So the survey notes whether the tank could be fully seen. Surfaces not seen are recorded as "not accessible"; when replacement is planned, opening them goes into the budget.
When a new tank is made
The new tank is measured to fit the space and often made in sections or a different shape. Baffles, an inspection hatch and a way to sample from the bottom make it easier to maintain for its whole life. When it is secured, a non-absorbent layer goes between tank and bearers; the tank is not seated directly on a surface where water can gather.
Pressure test
A new tank is pressure-tested before fitting, at the maker's recommended pressure. After connection, the system is watched closely at first filling and on the first passage.
Records
The new tank's material, capacity, test record and installation date go into the handover file; the next surveyor then knows exactly what is in the boat.
It is also the time to renew connected hoses, filters and the level sender.
A photo of the tank space at the end of the job gives the next survey a reference to compare against.