The Option for Aluminum

Being a metallurgist engineer I like metal boats, which is natural. That does not mean that other materials are unsuitable or inferior, just that aluminum is my material of choice, and the following text shows why.

The first favorable feature of aluminum is the fact that it is a metal, as steel, and sometimes the obvious must be stated to reinforce a good point.( the word aluminum here is used to mean aluminum alloys, not the pure metal, which has not much use as structural material – steel is also an iron carbon alloy, not a pure metal).

As a metal, aluminum has all those properties that make metals the materials of choice for most structural applications in the world, apart from very specific applications in niche industries such as aerospace and competition vehicles.

To calculate the structural behavior of an aluminum part or any metal part is a straightforward process because metals are isotropic and homogeneous in mass, meaning that their mechanical properties are not direction dependent and that a portion of aluminum has the same properties of every other portion of the same composition.

Aluminum alloys used in the marine industry are also not susceptible to ageing, which means that their properties remain stable with time.

Another good characteristic of metals is that they deform elastically when a stress is applied and, when the stress surpasses a certain limit called the proof stress, they will acquire a permanent deformation, called plastic deformation, and will absorb an energy proportional to the area under the stress strain curve shown here. This deformation absorbs energy from a possible impact and is a safety factor as well as an early warning for failure.

The energy absorbed up to the rupture is proportional to the area under the curve.

Seeing the good features of metals that make them good structural materials, aluminum alloys enjoy a good position in the marine industry since the 1950s, after the 5000 series alloys (aluminum magnesium alloys) were perfected, due to their unique features, as follows:

  • their light weight ( only 34.6% of the weight of an equal volume of steel )
  • outstanding corrosion resistance in a marine environment
  • the availability of  diverse semi-finished products in various shapes
  • their formability
  • their low environmental impact in marine fabrication (no blasting, less painting  etc)
  • their cost-effective recycling

From these features the most important to our analysis are the first two.

Designing two structures, one in aluminum and the other in steel, to have the same rigidity, the aluminum structure will weigh only 50% of the steel one. Or, saying it in another way, if we have two vessels weighing the same, we can have the aluminum one extremely stronger built.

Aliso carries 500 passengers, 148 cars and 118 buses at 37 knots

This weight advantage is not limited to the comparison with steel. For the same strength aluminum boats are also lighter than Fiber Glass Reinforced Plastic boats. Before the appearance of the modern cutting edge laminates with kevlar and carbon fibers, aluminum boats dominated the scene in offshore racing, remembering the famous Eric Tabarly and his various aluminum Pen Duick boats.

These cutting edge materials are many times very expensive and their production processes beyond the reach of small builders, and their longevity can be smaller than aluminum.

Aluminum´s corrosion resistance in the marine environment (Alloys 5083, 5086 and 5383 are virtually inert in salt water) means a long service life and good re-sale value. The production costs are also lowered because the material dos not need any surface treatment other than priming and anti-fouling below the water line.

Boats and other coastal structures like piers made from aluminum alloys have been in service for several decades, being sometimes replaced because their designs became obsolete rather than because any degradation due to corrosion.

Among most common materials used today for boat building, aluminum is the only one that will keep its properties if exposed to salt water without any barrier. You could even drill a hole and expose it to salt water in a boat, and no  degradation would occur to the drilled aluminum (of course the boat would sink if the hole was under the water line, so no great gain here!)

A Pop Alu 32
Parati, a Polar boat designed by Roberto Barros Yacht design, wich spent a winter in the Antarctic Peninsula with famous navigator Amyr Klink!

The net result of all this is more stable and faster boats due to better weight distribution, with few limitations to hull shape because of the good formability of the material, with good productivity when building because of the light weight and good welding speeds.

Aluminum affords safe and durable boats, features we all seek when building or buying.

A Kiribati 36