Fuel Tank (Portable/Separate)

What sets a separate fuel tank apart

A built-in integral tank forms part of the hull. A separate fuel tank is its own vessel, fitted into a compartment or stowed on deck, and that independence brings its own set of responsibilities. It has to hold fuel without leaking, withstand vibration and the movement of a small boat in a seaway, and stay safe when petrol vapour is involved. Typical users are tenders, rescue boats, fast rescue craft, workboats, sailing yachts and motor yachts, plus the portable tanks that supply outboard engines. Because the tank is a standalone component, it usually comes with its own certificate and a defined set of fittings that the installer has to use as intended.

Main designs and materials

• Rotationally moulded polyethylene tanks - light, corrosion-free and available in shapes that fit awkward hull spaces; static charge and fire behaviour must be considered for petrol
• Aluminium tanks - welded, robust and fire-resistant, but sensitive to corrosion at contact points, bilge water and certain fuel contaminants
• Stainless steel tanks - the long-life choice for permanent installation, with welded baffles and fittings
• Portable tanks for outboards - small, hand-carried, with a bayonet or quick-release coupling and a vent that is closed when the tank is carried

Fittings and details that matter

• Filler neck and deck filler, arranged so that spilled fuel and vapour cannot enter the boat
• Vent line with a flame arrester or a goose-neck, led to a position where fuel cannot run back on a heeled boat
• Pick-up pipe, return connection and shut-off valve, plus a drain or sump point for water and sediment
• Level sender or sight gauge, bonded and sealed against the tank wall
• Internal baffles to limit surging and mounting feet or straps that spread the load without trapping water against the tank

Selection and sizing

• Fuel type: petrol and diesel place different demands on materials, venting, bonding and fire protection
• Capacity against the range and consumption of the engine, with usable volume smaller than the nominal volume
• Material for the installation: plastic for easy fitting, metal where fire resistance or permanent installation is required
• Dimensions and port positions to match the compartment and hose runs
• Stowage and fixing, including how the tank is secured against movement and how condensation is kept away from the surface

Regulations and class

For leisure craft in Europe, fuel tanks are covered by the Recreational Craft Directive and the related ISO standards for installed fuel systems, which is why the certificates in this category typically refer to that directive. Rescue boats and workboats follow the rules of the flag state and the classification society, and tanks in lifesaving appliances have their own requirements for fuel type and fire safety. In every case the tank, its fittings and its installation should be traceable to a certificate that names the exact model or variant.

Typical faults

• Water and microbial growth in diesel, collecting at the lowest point and clogging filters
• Corrosion of aluminium or steel tanks at straps, supports and wherever bilge water can sit
• Leaks at fittings, sender flanges and welded or moulded seams, often traced back to vibration or loose screws
• Blocked vents causing fuel starvation or a pressure build-up in the tank
• Cracks or deformation in plastic tanks from ageing, heat or incorrect mounting
• Fuel hoses and clamps at the tank connections that have hardened or been replaced with unsuitable types

What to look for in a supplier

• A certificate that names the tank model, material and fuel type, rather than a general maker certificate
• Clear marking of capacity, material and manufacturing date on the tank itself
• Fitting kits and spare senders available for the same model, since an unusual thread or flange makes a repair into a replacement
• Dimensional drawings with port positions before ordering, especially for replacement tanks in an existing boat

Open the lowest drain or take a sample from the bottom of the tank at least once a season. The first sign of water or sludge is easier to deal with than an engine that stops in a swell.