Historically stainless steel 316 has been the preferred rudder
shaft material. This material was chosen as it was non corrosive and relatively strong and
widely available. At the end of last century alternative rudder stock materials
like aluminium and high strength stainless steel
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 rumors.
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 three 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.
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:
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:
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.
Comparison between shaft materials relative to aluminium AlMgSi1 (6082)
|Al. 6082||Al. 7075||St. St. 316||St. St. 329||St. St. 630|
Aluminium 6082: 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. Due to this properties,
70-80% of the boat builders worldwide use this material as rudder stock
material. The AL6082 rudder stocks can be anodized for extra protection. The
downside if this material is the relative thick rudder stock an therefore blade,
so it is mainly used for cruising rudder blades.
Aluminium 7075: 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 relative thin rudder shafts will bend more than usual under loading, it is vital to use self-aligning bearings. It's an ideal metal for boats which are not in the water all year long.
Stainless AISI 316: When comparing Aluminium 6082 and stainless 316, the first thing that catches one's eye is the low proof stress of stainless 316. In fact it's the weakest material in the list. As result of this the rudder stocks made from this material will be tick and heavy and consequently expensive. The corrosion resistance is the best of all metals.
Stainless AISI 329: When analyzing the figures for stainless 329, it's obvious that this material is a much better choice over stainless 316. The price is only 10% higher than stainless 316 and the proof stress is 225% higher with the same corrosion resistance. Due to the high proof stress, one can make the rudder shaft thinner so 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, and the thickness of the rudder blade is not an issue, one should use stainless 329.
Stainless AISI 630: When the rudder blade thickness is an important issue and a carbon rudder shaft is not an option due to the much higher price, stainless steel 630 (also called 17-4 PH) is the best choice. It's 4½ times as strong as stainless 316. It withstands corrosive attacks better than any of the standard hardenable stainless steels and the amount of corrosion is comparable to stainless 304. So in time a brownish corrosion layer will cover the rudder stock and when submerged in seawater for length of time, some small pitting may occur. This corrosion is harmless and doesn't significantly influence the strength of the rudder stock. When this is undesirable, a thin (2½ mm) stainless 316 sleeve should be used around the rudder stock to achieve a corrosion free running surface. See this web page. This metal is very popular in France where thin rudder blades tend to be used and has been successfully used over 25 years.
Aluminium 6082 rudder shafts are strong, light and economic and ideal for cruising production yachts..
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.
Very thin rudder blades can only use the ultra strong stainless AISI 630 which has some small corrosion issues. (See the above text how to avoid this).
All measurements to avoid corrosion can be found on our electrolysis page.