The option for aluminium

The hull of a MC 41 SK is turned over

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

The first favourable feature of aluminium is the fact that it is a metal, as steel, and sometimes the obvious must be stated to reinforce a good point.(The word aluminium here is used to mean aluminium 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, aluminium 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 behaviour of an aluminium 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 aluminium has the same properties of every other portion of the same composition.

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

The energy absorbed before fracture is proportional to the area under the stress strain curve

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.

Seeing the good features of metals that make them good structural materials, aluminium alloys enjoy a good position in the marine industry since the 1950s, after the 5000 series alloys (aluminium 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

HSV Aliso carries 500 passengers, 148 cars and 112 coaches at 37 knots

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

Designing two structures, one in aluminium and the other in steel, to have the same rigidity, the aluminium 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 aluminium one extremely stronger built.

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

Aluminium’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 does not need any surface treatment other than priming and anti-fouling below the water line.

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

Among most common materials used today for boat building, aluminium 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 aluminium (of course the boat would sink if the hole was under the waterline, so no great gain here!)

A magnificent aluminium boat, Amyr Klink´s Paratii I

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.

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

Luis Manuel Pinho is a metallurgist engineer graduated in 1990  by Rio de Janeiro State Federal University, and a lifelong sailor and twice amateur boat builder. He lives on board boats with his wife Marli Werner since 1996, having sailed in excess of 30000 nautical miles, mostly in the Pacific and Atlantic Oceans. He collaborates with B&G Yacht design in some designs, producing CNC cutting files for our metal boat designs and doing some design work of his own creation.