1 August 2018

Safety alerts are published by Transport Safety Victoria (TSV) under section 197 of the Transport Integration Act 2010 (Vic) to promote the safe operation of transport services. Safety alerts are intended to provide information only and must be read in conjunction with obligations under relevant legislation.


  • A risk of explosion exists in spaces where underdeck fuel tanks fail
  • Underdeck fuel tanks are fitted to a variety of craft and are hard to inspect for leaks or degradation of tank material
  • Aluminium fuel tanks corrode due to moisture collecting in the tanks space
  • Spaces that house fuel tanks should be checked to be effectively draining
  • Tanks should be inspected at least every four years.



Petrol vapours collect low down in boats in spaces such as bilges. When petrol vapour mixes with air within a specific concentration range the mixture becomes explosive. Exposure to any ignition source will result in an explosion with the potential results of fatality, severe injury and/or loss of vessel.


Recently a recreational vessel owner became concerned at the smell of petrol present in the engine space of their vessel which was powered by an inboard petrol engine.

After contacting a marine mechanic, the matter was investigated and found to be due to petrol which had leaked from a corroded 5052 grade aluminium tank fitted under the sealed deck of the vessel. The tank was in its own compartment immediately forward of a partial engine space bulkhead. The compartment was fitted with a small drainage hole designed to drain excess water into the engine space.

The vessel was approximately six years of age and the tank was fitted new at time of build.

Closer inspection revealed that the drainage hole through the engine space bulkhead was blocked. When unblocked, petrol instead of water drained into the engine space. The tank was removed from the vessel utilising appropriate safety precautions to prevent an ignition source from igniting any vapour. It was found that corrosion had produced holes in the fuel tank. Corrosion was mostly evident at the forward end of the tank.

The tank plating had corroded due to a build-up of moisture collecting within the tank compartment as it could not drain to the engine space. The tank was not supported clear of the hull of the vessel. This resulted in the tank being in contact with moisture in a low oxygen environment. This atmosphere promotes and accelerates the deterioration of aluminium surfaces due to corrosion.

Fuel tank corrosion and failure of tank boundaries resulting in fuel collecting in the bottom of the tank space or the bilge is not uncommon in these situations.



  1. Check for petrol fumes emanating from the fuel tank compartment or for presence of fuel within the fuel compartment. If you detect fuel vapours:
    1. Isolate all potential ignition sources by: isolating batteries, turning off machinery and disconnecting from shore power.
    2. Do not smoke.
    3. Remove people from the vicinity.
    4. Contact a marine mechanic or other competent person for advice and/or assistance,


  1. To reduce the likelihood of fuel collecting below sealed decks, consider these points:
    1. Tank inspections. Any petrol-powered vessel with an internal fuel tank should have the fuel tank inspected regularly for damage and/or corrosion. It is recommended that where a fuel tank can be removed from the vessel it should be removed at maximum intervals of four years for a full inspection and pressure test by a competent person.
    2. Drainage. Any drainage openings designed to drain seawater from the tank space should be regularly checked for blockages etc. which may prevent seawater from draining properly. Drainage cocks/ bungs should be fitted to prevent petrol entering any other space should it be present.
    3. Vessel trim at rest. The boat should ideally be trimmed so that water collects aft where the drainage holes are likely to be ensuring sea water is removed effectively.
    4. Fittings inspections and replacement. Identify leak points such as hoses, fittings and joins and regularly inspect for signs of failure. Replace hoses and fittings which are worn, corroded or no longer fit for purpose.
    5. Checking for fumes. It is recommended that boat owners fit a petrol vapour alarm. A hydrocarbon sensor can identify presence of fumes quicker than by smelling alone.


  1. When fitting or replacing a fuel tank consider the following:
    1. Tank access. In many vessels a fuel tank inspection is difficult as the tank is in its own below deck sealed space. This would require major structural work to access the tank. Ideally tanks should be inspected as above however, if this is not possible then tank compartments should be arranged to allow sea water to be easily drained and there should be means available to regularly check whether there is sea water or petrol present in the compartment.
    2. Tank materials. There are a range of materials suitable for containing fuel. Seek advice from a marine expert, as each material will have characteristics that need to be addressed prior to installation.
    3. Tank position. Tanks should be above the hull bottom to ensure that any sea water collected remains below the bottom of the tank and to assist in draining the space more easily. This also allows air to circulate around the tank boundaries, reducing low oxygen interfaces.
    4. Tank compartments. Compartments should be fitted with their own vents and drainage systems. Vents should lead overboard. Drainage systems should not drain directly into any bilge or space where vapours may collect.