Historically stainless steel 316 has been the preferred rudder shaft material. This material was chosen as it was non corrosive and strong and widely available. At the end of last century aluminium and aluminium alloys became widely available. Driven by the aircraft and space industry, new high quality aluminium alloys were developed. Some of these alloys turned out to be perfectly suitable for rudder shafts and other parts of sailing yachts. Gradually all big boat yards making GRP yachts converted to aluminium rudder shafts. But still some people, especially some designers, doubt about the use of aluminium as rudder stock material.
The purpose of this page is to present a clear overview off all material characteristics so one is able to make a clear choice based on facts and not on rumours.

Hull material:

The choice of hull material could fix the choice of the rudder stock material. On steel hulls one should use a stainless steel rudder shaft. On aluminium hulls one should choose an aluminium rudder shaft. More in depth info about this choice is explained on our electrolysis page. On GRP or composite hulls one has the choice between aluminium, stainless steel, and carbon. As Jefa Marine's primary production is targeted on metal rudder stocks we will go in depth in comparing aluminium and stainless steel.

Important mechanical properties:

To be able to evaluate the mechanical properties of metals one should first know that four of many mechanical properties of a metal are important for rudder stocks:

• 0.2% proof stress: As soon as forces are applied to a metal it will deform. Up until the 0.2% proof stress, this deformation is called elastic deformation. This means that after the forces are taken away, the metal will come back to it's original shape (with a maximum permanent deformation of 0.2%). The value of the 0.2% proof stress is given in Newton per square mm (N/mm²). Projected on a rudder shaft, this figure will determine the point of permanent damage. When the forces on the rudder shaft will rise above the 0.2% proof stress, the rudder shaft will be permanently bend and practically unusable.
   

• Tensile strength: The tensile strength or breaking strength determines the point where the stress level in the metal has risen so high that the metal is torn. The value of the tensile strength is given in Newton per square mm (N/mm²). For rudder stocks this figure isn't very important: From the proof stress point, the metal will flow and the rudder shaft will heavily bend permanently and eventually break at the tensile strength point.
   

• Elasticity module: The elasticity module is used to calculate the elastic deformation of a metal at a certain load and is specified in Newton per square mm (N/mm²). The bigger the elasticity module, the stiffer the metal.
   

• Specific weight: The specific weight of a metal is used to calculate the mass of a product with a given volume and is specified in Kg/m³. Very dense metals (like stainless steel) will have a high specific weight, light metals (like aluminium) will have a low specific weight.

Characteristics of aluminium:

The mechanical and anti-corrosion characteristics of aluminium depend on the alloy elements. Pure aluminium is not usable for a high strength purpose like a rudder shaft. The most popular aluminium alloy for rudder shafts is AlMgSi1 (EN 6082). The addition of the alloy element manganese extremely increases the mechanical properties proof stress and tensile strength. The addition of the alloy element silicon extremely increases the corrosion resistance of the aluminium. A hard and strong layer of silicon oxide SiO2 protects the aluminium even against the most hostile seawater. We use the following types of aluminium:

• Aluminium AlMgSi1 (EN 6082)
  The tensile strength is 330 N/mm² , the 0.2 % proof stress is 280 N/mm², the elasticity module is 71.000 N/mm², the specific weight is 2.700 Kg/m³.
• Aluminium AlZnMgCu1,5 (EN 7075)
  The tensile strength is 520 N/mm² , the 0.2 % proof stress is 460 N/mm², the elasticity module is 71.000 N/mm², the specific weight is 2.700 Kg/m³.

Characteristics of stainless steel:

The mechanical and anti-corrosion characteristics of steel depend on the alloy elements and the heat treatment. By adding carbon, chrome and nickel to iron and heat tread it correctly, one achieves the alloy stainless steel. The protection against corrosion is not achieved by an oxide layer like aluminium, but the added chrome and nickel make sure the metal itself will not oxidise. We use the following types of stainless steel:

• Stainless steel aisi 316 (1.4401)
  The tensile strength is 600 N/mm² , the 0.2 % proof stress is 200 N/mm², the elasticity module is 200.000 N/mm², the specific weight is 7.900 Kg/m³.
• Stainless steel aisi 329 (1.4460)
  The tensile strength is 750 N/mm² , the 0.2 % proof stress is 450 N/mm², the elasticity module is 200.000 N/mm², the specific weight is 7.900 Kg/m³.
• Stainless steel aisi 630 (1.4542)
  The tensile strength is 930 N/mm² , the 0.2 % proof stress is 720 N/mm², the elasticity module is 200.000 N/mm², the specific weight is 7.900 Kg/m³.

Comparing aluminium and stainless steel:

Another important comparing factor, besides the mechanical properties, is the price of a rudder stock. In order to make a complete comparison between the four types of materials we will take an example of a complete rudder stock. A typical rudder stock has it's maximum diameter at the bottom bearing area, is tapered down to about 50% and up to about 60% of the maximum diameter, has a keyway, 3 or 4 spokes, and an emergency tiller connection.

When comparing the two most used materials, Al. 6082 and stainless 316, the first thing that catches one's eye is the low proof stress of stainless 316. The proof stress is the most important strength figure for a material used for rudder stocks. As soon as the stresses in the material rise above this limit, the rudder shaft will be bend permanently. If self-aligning bearings are used, there is a chance the rudder will still rotate. If non self-aligning bearings are used, the rudder will be stuck in it's last position. Stainless 316 combines this low proof stress with a high tensile strength. Often people think this is a big advantage over aluminium (61% higher), but actually this is a big disadvantage. I think we all can agree that a bend rudder stock is useless, so the real important figure is the proof stress, but what is the influence of the tensile strength on the rudder shaft behaviour? Our opinion is that a high tensile strength is undesirable: as soon as the rudder stock will start bending permanently, a high loading will work on the bottom bearing. The rudder shaft will try to bend the bearing out of the hull. The aluminium rudder shaft will snap off at a loading 18% above the proof stress loading, the stainless 316 shaft will continue deforming up until the loading is 300% higher than the proof stress. The bottom bearing will torque out of the hull creating an enormous hole in the hull, mostly underneath the waterline with all unwanted consequences.
When analysing the figures for stainless 329, it's obvious that this material is a much better choice over stainless 316. The price is only 20% higher than stainless 316 and the proof stress is 225% higher. Due to the high proof stress, one could make the rudder shaft thinner so it is possible that the end price of the rudder shaft in stainless 329 is actually lower than the end price of the shaft in stainless 316. So one may conclude that when stainless steel is the preferred rudder material, one should always use stainless 329.

One could wonder why so many boat builders choose aluminium as preferred rudder shaft material: Aluminium 6082 combines a high proof stress with a relatively light weight and a low price. On top of that it is fully seawater resistant. In fact, the only downside of aluminium is the lower E-module. The E-module is three times smaller than the E-module of stainless. In practice this means that an aluminium rudder shaft with the same physical dimensions as a stainless rudder shaft will bend three times further under a certain loading. This is absolutely not a problem (take an aluminium aircraft wing for example, it's also extremely strong, but could bend heavily under loading) but one should consider using self-aligning bearings to guarantee a smooth rotation under these high loads. 
The reason for the extreme price difference of aluminium 6082 and stainless steel is the fact that the raw material has nearly the same price per kilogram. As stainless is 3 times heavier than aluminium, the raw material price of a rudder shaft is three times higher in stainless compared to aluminium. The machining costs of the stainless rudder stock are also higher compared to aluminium as the latter material machine's much easier and faster.
Another big advantage of aluminium over stainless steel is the three times lower weight. The lower weight is a big advantage in the construction phase; it's much easier to handle the rudder shaft for the rudder blade production and the installation in the hull is also much easier. The low weight is also in the finished yacht a big advantage as the weight is very far away from the centre of gravity and will not influence the yacht's dynamics to the extend than a much heavier stainless steel shaft would do. If a light rudder blade core material is used, the finished rudder will float and will help lift the back of the yacht and therefore positively influence the water flow at the back of the yacht.

On high performance racing yachts with very thin shaped rudder blades one could use the high strength aluminium 7075. The use of this material will give the opportunity to minimise the shaft diameter and thereby minimise the maximum rudder blade thickness. As this material is not seawater resistant, the complete rudder shaft will be anodised after production, making it completely electrically neutral, but a full proof guarantee on corrosion can not be given as any damage on the shaft will lead to corrosion. As these rudder shafts will bend under loading, it is vital to use self-aligning bearings.

Conclusions:

• Aluminium 6082 rudder shafts are strong, light and economic.

• If stainless steel is preferred, one should use stainless steel aisi 329.

• Stainless steel 316 rudder shafts are NOT stronger than aluminium 6082 rudder stocks.

• High performance yachts should use aluminium 6082 or 7075. With aluminium 7075 one should check the rudder shaft yearly for any signs of corrosion.