For equal structural and impact strength, a well designed aluminum boat is lighter in weight and stronger than either it’s FRP of steel counterparts. Typically the hull is 2/3rds the weight of a well built FRP boat and less than 1/2 the weight of a steel hull of similar dimensions.

Since the HP required to attain a given speed is roughly proportional to the cube root of the HP to weight ratio, speed is gained more easily by reducing weight than by increasing HP. A significant increase in HP, depending on how the increase is attained, will likely increase engine weight and consequently the weight of fuel needed for a given range, since HP and fuel flow go hand in hand.

Impact strength of an aluminum hull is the order of 3 times that of steel assuming similar size panels. The short explanation is that the maximum stress in the panel during impact is less because the energy of impact is dissipated over a great distance, i.e. aluminum  deflects more than steel (it has 1/3rd the Modulus of Elasticity of steel).

Aluminum as an Engineering Material

Aluminum (Al) was discovered, i.e. reduced to metal from the ore known as Bauxite by a Danish physicist, Hans Oersted, in the early 1800’s. His technique was prohibitively expensive for most applications. During the 1880’s the Hall-Heroult process was developed. This led to the formation of the Pittsburgh Reduction Co which later became the Aluminum Company of America (Alcoa). The first significant uses took advantage of the electrical and thermal conductivity of pure aluminum. Aluminum transmission cables supplanted copper which carries only 64% of the current per pound. Al cooking utensils became common because of thermal conductivity, and light weight. Structural applications came along as high strength alloys were developed.

An interesting early use were the pre WWI dirigibles which had aluminum structure. Aircraft engines began using aluminum. . The Al-Cu alloys (duralumin) discovered by an Austrian, Albrecht Dürer, had good mechanical properties, but poor salt water corrosion resistance. The 5000 series Al-Mg alloys often referred to today as the marine alloys, are refined versions of those developed in the ‘30’s. Unlike the Al-Cu alloys which had poor salt water corrosion resistance, they work well in salt water or around it. For small boats and numerous not highly stressed parts, e.g. fuel tanks, control consoles, the most common alloy is 5052 H32. Greater strength is available using 5086 H32 or H116, or 5083, the former being more popular, and the later being more popular in Europe. Extruded shapes such as flat bars, angles, tees, channels, I beams and pipes are usually produced from 6061 T6 alloy, or 6063 T5 in the case of pipes requiring ease of bending.

Although several small aluminum boats were built in Switzerland, France, and also in the US prior to 1900, large scale usage in good sized boats and small ships awaited the practical development of inert gas welding.

Welding Aluminum

Aluminum is easily welded using the MIG or TIG process which flood the weld area with and inert shielding gas. The inert gas, Argon or Helium/Argon mix, keeps oxygen away from the joining thus overcoming aluminum’s high affinity for oxygen. Prior to WWII this was done with a flux which itself was highly corrosive and therefore had to be carefully removed. The advent during WWII of inert gas welding becoming practical in the shop was a substantial break through. The recommended filler allow for welding a 5000 series to itself or to 6000 series to 5000 series plate in boats and fabrications exposed to salt water is 5356.