This manuscript describes a failure analysis carried out in fuel tanks of a lightweight ship that was designed to operate not far from shore.The ship under study is a high speed light craft that was entirely designed and manufactured in Portugal and has been successfully operating as a patrol boat for several years. The vessel under study was manufactured with two types of naval aluminium series alloys, namely the 5083-H111 and the 6082-T6 alloys, which are commonly used in shipbuilding. The former alloy can be strain hardened by cold work and is used in the form of rolled plates and sheets for the vessel hull, the superstructure, and the bulkheads. The 6082-T6 alloy is a high-strength alloy that can be heat treated and softens near the heat affected zone during welding. It is used in the high-stressed structural members of the ship, namely in the stiffeners of the reinforced panels.The two fuel tanks under study are located at the rear of the ship, one at the starboard and the other at the portside. Cracks have propagated from the weld toe of some T-welded joints that possess low fatigue strength resistance. The welded joints were made using TIG and MIG processes and the weld quality was assessed by visual inspection, dye penetrant examination and radiography.The fuel tank's failure due to exfoliation, hot cracking, stress corrosion cracking or fatigue are analysed in the paper and it was concluded that fatigue is the most probable cause of failure. In addition, the 3D modelling of the current fuel tank geometry and its analysis by the finite element method (FEM) allowed the behaviour of the structure to be studied and it was discovered that the highest stressed regions were located at the reinforcement stiffeners where cracks nucleated and propagated.As a result, several alternative geometries for the fuel tanks were simulated in a FE computer code and it was possible to reduce the overall maximum stresses applied in the structure in the order of 65% with an increase of 20 kg per fuel tank to the total weight of the vessel.