“Is it really worth buying an aluminium yacht? Does it not suffer from terrible issues of corrosion? And what about steel, does it not rust heavily?” – these are common questions that I often hear from prospective buyers and I thought that two recents surveys that I had the opportunity to execute, one in Sardinia and one in Cadiz, may hopefully provide some answers.

The use of metal for boat construction

Europe has a long history of aluminium boat construction, with France perhaps hosting the most renowned names such as Allures, Alubat, Meta or Garcia to name a few. The Netherlands also have a wide industry of aluminium yards such as KM Yachtbuilders, Hutting, Atlantic, Vitters amongst others. In Germany some famous yards are Dubbel and Jesse, Yachtweft Benjamins, Abekin and Rasmussen, Feltz. Belgium is home of interesting sailing yacht builder Boreal and Spain has a less renowned but high quality family yard, Pita Yachts. Overseas Kanter Marine (Canada) and Mc Mullen and Wing (New Zealand) are two strong examples.

Aluminium is a very nice material for boat construction: its properties guarantee a very solid, light and stiff structure, making it an ideal material especially for sailing yachts where a high stiffness to weight ratio is desirable. Aluminium is also very ductile, which means is easily mouldable and workable (aluminium boat building often resemble the methods and tools used by wood workers) and also highly resistant to impacts (it will tend to bend and not crack as a consequence of an impact with an external object), a characteristic that is valuable particularly for long distance vessels aimed at sailing in high latitudes, where the chances of hitting ice or other floating objects are considerable.

Steel is also a common material. It is used mostly where weight is not such an issue while costs may need to be contained (steel is heavier but cheaper than aluminium). It is found mainly in larger motor and sailing vessels. It is strong, largely used in shipyards for ship construction and hence working techniques are well developed and relatively common.

What about Corrosion

The main issues with both materials are related to corrosion and paint adhesion. Aluminium may suffer especially from galvanic corrosion (is quite on the low side of the galvanic scale), while steel may suffer also from oxidation, especially in tanks and internal bilges, where water stagnates and oxidation may start without being noticed. Corrosion from circulating currents may also be an issue on both metals and it is important to ensure that no electrical losses are present between the electrical installation and the hull.

To establish the degree of protection from galvanic corrosion the best practice is to measure the electrical potential between a reference cell and the hull. These measurements, if performed with the proper tools, can give a precise degree of the level of protection and can be used to decide if corrective actions such as anodes replacement or resizing are needed

Stray currents or leaks from auxiliary systems or even from the electrical installation of the marina can also initiate a drastic and fast corrosion process. In this case a proper electrical check is desirable to ensure that no currents are flowing through the hull with the risk of corroding its plating or reinforcing structure.

How do you check for corrosion?

The above describe the processes and risks of corrosion. But what about if I want to know if the hull has corroded, not only if it is at risk? By far the the best method to evaluate the condition of the hull plating is a non destructive technique called UTM, or Ultrasound Thickness Measurements.

UTM uses sound waves to measure the thickness of the plating without damaging the hull or paint (professional tools are needed for this purpose), and can detect areas where the thickness has reduced due to corrosive action. Aluminium hulls can be successfully tested with this method, as performed on this famous design tested in Porto Corallo, Sardinia, Italy

Steel hulls in general are also sensitive to galvanic corrosion, but less than aluminium (steel is higher up in the galvanic scale). Normally the issues of corrosion on steel are related to oxidation, where oxygen reacts with the iron present in the alloy (steel is an alloy made mainly of iron and carbon), and forms what is commonly called rust (ferrous oxide). Once formed, rust being porous will let oxygen be in contact with the surface and the process will continue.

The worst cases usually involve oxidation from the inside: hidden or poorly accessible areas such as bilges under generators, A/C system, along fire extinguishing pipes or in the proximity of other systems in contact with salt water, may develop corrosion. Salt water may leak from these systems, rest on the bilge and if left unattended will corrode the plating with time. UTM is also very useful in these cases, as on this 26m explorer motor yacht, being tested with ultrasound in Cadiz, Spain.

To sum up

Yes, metal hulls are prone to corrosion, but problems, if present, can be detected by the use of dedicated tools and techniques. When considering buying an aluminium yacht or a steel vessel it is advisable to inspect for galvanic protection, verify that no stray currents are present and it is mandatory to perform a UTM campaign to evaluate the integrity of the structure, which is the soul of the vessel and what eventually will keep you afloat.

Happy and safe sailing!