Aluminium is the way to build a hull nearly as tough as steel at far lower weight. It can be left unpainted and is favoured for fast boats and expedition sailing yachts. But aluminium is much more sensitive than steel to other metals and to electricity.
Material
For marine use, 5000-series alloys (for example 5083) are used for plate and 6000-series for extrusions. In 6000-series alloys strength drops in the weld zone; the design allows for it.
Critical points in aluminium
- Welding
- Clean, dry conditions and surface preparation; porosity is the most common defect. The welder's aluminium qualification must be documented.
- Galvanic isolation
- Every stainless, bronze or copper-bearing part is isolated from the hull; a single unisolated fitting can start rapid corrosion.
- Electrical system
- The hull is not used as part of any circuit; stray current does serious damage.
- Paint
- No copper-based antifouling; pretreatment and a suitable primer are essential.
- Cathodic protection
- Anode selection and avoiding over-protection, confirmed by measurement.
Why aluminium?
Aluminium is far lighter than steel and, with the right alloy, forms its own protective surface in seawater. So it is chosen for fast boats, expedition boats and some large yachts where lightness and toughness are wanted together. Some owners leave the hull unpainted; that reduces maintenance but changes the look.
Galvanic sensitivity
Aluminium's greatest weakness is contact with other metals and stray current. A stainless bolt, a copper-based antifouling or a wrongly connected circuit can waste aluminium quickly. So insulation goes between dissimilar metals, antifouling suitable for aluminium is chosen, and the electrical system's relation to the hull is designed with care.
The electrical system
On an aluminium boat the electrical system is installed so that the hull is never used as a current path. On the shore-power side, a galvanic isolator or isolation transformer is especially important. Hull potential readings are repeated at the end of the build and at every survey.
Welding and the shop
Aluminium welding needs a clean shop, dry conditions and experienced welders. Strength loss in weld zones is allowed for in the design; dye penetrant is widely used for inspection.
On survey
An aluminium boat survey looks particularly for corrosion and points where dissimilar metals meet; anode condition and hull potential values are read together.
For owners
Any new fitting on an aluminium boat, however small, should be checked for metal compatibility before it goes on. Many costly corrosion problems start with a single well-meant addition.
Likewise, galvanic current from neighbouring boats via shore power damages aluminium faster, so the isolation arrangement is never bypassed. Even on an unpainted hull, the bottom is protected with antifouling checked as suitable for aluminium.
A long life, if protected
A well-protected aluminium boat is known for a very long life. That life depends less on the metal than on three habits: a correctly installed electrical system, separation of dissimilar metals and regular anode renewal. Those three are the most important items on an aluminium boat owner's maintenance list, and each is easy to check at every haul-out.
Results go in the maintenance log at each haul-out, so a developing problem is caught early, while it is still small and cheap to put right.
With that care, an aluminium boat serves trouble-free for many years and holds its value well at resale.
Small habits, big savings.
Check them at every haul-out without fail.