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2. Properties of the Alloys

Of the range of aluminium-magnesium alloys, there are three in common use for marine purposes and approved by Lloyds. These are shown in the following table:

Grade Composition (max) % Condition 0.2% Proof Stress
Tensile Strength
  Al Cu Mg Si Fe Mn Zn Cr Other      
5083 Rem. 0.1 4.9 0.4 0.4 1.0 0.2 0.25 0.15 O 125 275
6082 Rem. 0.1 1.4 1.3 0.5 1.0 0.1 0.3 0.2 TF 240 295
5356 Rem. 0.1 5.5 0.6 0.5 1.0 0.2 0.25 0.15      

Alloy 5083 is a non-heat treatable wrought alloy and is used largely for sheet and plate. It combines reasonable strength with superior corrosion resistance. Unfortunately it does not extrude well in thicknesses below 5mm and, even above this, 5083 extrusions can be difficult to source. The material is usually used in the 'O' condition (fully annealed) and, for practical purposes, loses no strength when welded.

Alloy 6082 is a heat treated alloy and is used for the majority of extrusions. Up to one third of its higher strength is lost on welding and although this can be restored by subsequent heat treatment, this is seldom attempted on marine structures. It has good corrosion resistance, though not so high as 5083.

Alloy 5356 is used exclusively for weld filler wire to weld both the other alloys.

These alloys have a good strength-to-weight ratio compared with other boatbuilding materials, as the following approximate figures show:

Material Weight g/cm3 Tensile Strength N/mm2 Strength/Weight Ratio
Alloy 5083 2.66 275 103
Wood/epoxy 0.50 48 96
Conventional GRP 1.90 175 92
Mild steel 7.75 525 68
Ferro/cement 2.63 37 14

The latest technologies in fibre reinforced polyester and epoxies do, however, produce better strength/weight ratios than aluminium alloy and this is amply demonstrated by their having largely replaced aluminium as the pre-eminent construction material for today's race boats.

The marine alloys have a low Youngs Modulus compared with steel and this allows the material to deflect more elastically than steel when subject to impact and the energy of the impact is dissipated more gradually. Aluminium deforms plastically before it fractures and thus can absorb the energy of impact better that a more brittle material such as GRP. Thus a hull constructed from aluminium alloy is more likely to be dented in an impact, where other less ductile materials would be cracked or holed.

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© George Whisstock.