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Strength of Aluminum vs Strength of Steel

Copyright 2001 - 2010 Michael Kasten
 

I believe in metal as the ultimate boat structure, and as a result I have created quite a number of metal boat designs. To review them, please see the Sail Boats Gallery and the Power Boats Gallery. I have also created quite a number of Prototype Designs, most of which are also intended for metal structure. Sail or power - mono or multi-hull - if the structure is well-designed and well built, the result is excellence.

I am often asked about one metal vs. another - most commonly steel vs. aluminum. I do not have a distinct preference, since there are so many factors that will contribute to that decision for each boat, and for each owner. Some boats are designed for one material only, other boats can make use of either.

In general, any of my designs that have been developed for steel can quickly be re-specified for construction in aluminum. The design conversion from steel to aluminum is done at no extra cost, unless of course there are other changes to be made to the design. Where NC cutting files exist for a steel boat, they will need to be re-done in order to work for aluminum structure, and there will be a cost incurred for that conversion.

Any of my designs that were originally developed for aluminum structure are not usually so readily converted to steel, since they will have been designed specifically to save weight. Therefore, in order to convert one of them to steel structure will ordinarily require a re-work of the hull shape in order to support the extra weight of steel. For some boats, that is not much trouble. For other boats, in particular small ones, steel may not even be an option. If a conversion to steel is of interest for one of my aluminum designs, please inquire. for more information.

You will find more information about costs and other considerations between these metals among the essays linked from my Articles web page.

Below, I have presented a comparison of steel vs. aluminum on the basis of strength...
 

Is There a Difference?

The question has been asked: "What about rocks and ice and things that a boat can run into? Does either steel or aluminum provide an advantage in a real-world sense?"

As a partial answer to those questions, here are a few things to consider... In addition to these few thoughts, you may want to review my Cruising World article, Aluminum For Boats, (to which CW gave the title "To Thine Own Chines Be True"). The CW article gives a good overview of aluminum, both in terms of building, and in terms of owning an aluminum boat.

This page offers a bit of additional information regarding the relative strength of the two materials. This brief article is intended only to be a general way of viewing steel and aluminum as they are used in boat structures.

As we will see, the issues of strength are tipped somewhat in favor of aluminum, mostly for the reason of its lighter weight. Being much lighter, aluminum will permit a more robust structure within any given weight budget.
 

Built to the Same Standard

An aluminum hull structure, built to the same standards, weighs roughly 35% to 45% less than the same hull in steel. As a result, if high strength is of the highest priority, the alloy boat can be built to the same structural weight as the steel vessel, and then be considerably stronger.

This is less of an issue for larger vessels which are able to carry the necessary displacement for whatever materials choice is made. For smaller vessels however, the weight of the hull structure is very much an issue. For a small cruising vessel, say under around 35 feet or so, steel becomes less optimum, as one must resort to a large water plane and a large displacement to carry the weight of the structure.

When alloy is designed to the same standards as steel (ABS, Lloyds or other similar classification society), it is made to be higher in overall strength. The reason for this is that aluminum reaches its "endurance limit" sooner than steel in terms of flexure. Therefore the rigidity of structure (deflection) becomes the limiting design criteria for an aluminum structure, and this forces a higher than necessary overall yield and tensile strength.

With steel, one designs to the yield point of the material instead, since for steel, flexure and rigidity are not ordinarily a limiting issue. For steel therefore, the yield point of an "equivalent" structure will be considerably less, as we will see.

One advantage of steel is that between the yield point of mild steel (around 36,000 psi) and the ultimate tensile failure point (around 60,000 psi) there is quite a large plastic range (around 24,000 psi or roughly 40% of the ultimate strength), permitting a steel vessel to endure deflection without failure, so permitting considerable ability to absorb energy.
 

Scantlings

For alloy, the yield point and ultimate failure point of that "equivalent" structure (designed to the same standards of rigidity) turn out to be globally much greater for two reasons:

1) Aluminum 5083 H-116 plate, as an example, has a yield strength of around 34,000 psi and an ultimate strength of around 45,000 psi. We can see that the plastic range of aluminum is considerably less than steel (around 11,000 psi plastic range for aluminum, or roughly 24% of the material's ultimate strength).

2) Since an aluminum structure is designed to a deflection criteria, all scantlings are made somewhere around 50% or so larger than they would be for a steel structure. For the sake of an easy example, what would be one inch of plate thickness on a steel vessel would be approximately one and a half inches of plate thickness on the aluminum vessel in order to achieve the same rigidity of structure.
 

Inch for Inch

Again, for the sake of an easy to follow comparison, we might say that "one inch" of steel plate will yield beyond its ability to recover its original shape at approximately 36k psi, and will fail at approximately 60k psi.

A "strength-equivalent" aluminum structure, having used deflection (stiffness) as the design criteria, will have been built using roughly 50% greater plate thickness. We might then say that this strength-equivalent "one and a half inch" thick aluminum plate will yield at around 51k per square inch of surface area (around 29% greater yield strength than the "equivalent" region of steel plate), and will fail at around 67.5k psi (around 12.5% greater ultimate strength than the "equivalent" region of steel plate).

Of course these broad generalizations are intended only as a way of illustrating the approximate relative strengths of the materials. However, from these considerations we can see that the aluminum vessel will have a greater overall strength than the steel vessel per square area of plate. The reason for this is that the aluminum plate will, for the sake of stiffness, be 150% the size of the steel plate.

The result in practical terms is that a boat built in aluminum will be far less easy to dent by running into stuff (roughly 29% higher regional yield strength), and will have a slightly higher resistance to ultimate failure (around 12.5%). As an added bonus, this means that the aluminum yacht will resist distortion all the better while being welded during construction. As an extra added bonus, the aluminum structure will weigh considerably less than the equivalent steel structure.

Dave Gerr has equated the two materials similarly, referring to a material's structural efficiency. By this, he means the ratio of a material's stiffness to the density of that material. Per those equations, aluminum is shown to have a "structural efficiency" much greater than steel. In more precise terms, for columns that are designed to an equivalent stiffness an aluminum column will weigh 57% of the equivalent column in steel. For beams and panels (frames and plating) designed to the same stiffness, an aluminum structure will weigh 48% of the equivalent structure in steel.

In my view, Gerr has made an excellent comparison. In every day terms, an aluminum structure will often end up weighing a little more than those percentages might indicate, mainly due to accounting for the "as welded" strength of aluminum...
 

"As - Welded" Strength

The plot thickens somewhat when you consider that the as welded strength of 5083 H-116 aluminum plate (in the heat affected zone) is 23k psi yield, and 39k psi ultimate strength. The as-welded strength will vary, but those are the values permitted by the ABS for structural calcs.

For that same "sample" plate region (having made the aluminum plate 1.5 times as thick as the steel plate) the as-welded one and a half inch aluminum plate will have roughly 34.5k psi "per square area" yield, slightly behind steel.

In terms of ultimate strength in the as welded condition, the 150% thicker aluminum plate comes to roughly 58.5k psi "per square area" which is again very close to steel for a structure designed to an "equivalent" standard.

One might then argue in favor of steel which has very slightly higher values, except that the resulting structure will still be roughly 1/3 lighter in aluminum than in steel.

For aluminum plate, to compensate for the loss of strength in the weld zone, all butt joints are planned so that they may have backing bars and extra longitudinal reinforcement. Additionally, butt joints are placed so that they are around 25% of the distance between supports (frames). The additional long's and the backing bar are intended to give back the majority of the lost strength, and the location of the butt places the weld at the point of least bending stress in the plate.
 

Fatigue

In a steel structure, fatigue is normally not considered for general structure (fatigue is not usually the limiting criteria). The exception of course is around engines and chain plates.

Aluminum however, is subject to fatigue failure (referred to as its endurance limit) more readily than mild steel. So an alloy vessel must have its endurance limit considered more carefully wherever there will be vibration, again primarily at the engine, but also at chainplates and other high stress points.

The obvious design solution is to increase the scantlings so that deflection is kept within the allowable range. With aluminum, this is very effective, and does not incur much of a weight penalty.
 

Corrosion

It should be kept in mind that over time the probability is that corrosion may diminish the scantlings of steel more rapidly than with alloy, although when you throw electricity into the water, an alloy hull has the potential to lose thickness at a much more rapid rate. As a result, one must design the vessel's electrical system correctly, and then manage it with vigilance.
 

Strength

The short answer to our original question is that in terms of strength, presuming an alloy and a steel vessel of the same design have been engineered correctly, they will have very nearly the same strength, with the balance tipped somewhat in favor of aluminum, both in terms of overall yield, and in terms of ultimate failure.

In terms of weight, the balance is tipped very significantly in favor of aluminum.

Of course, taking that one step further, if the aluminum vessel is designed to have exactly the same weight of structure as any given mild steel vessel, the aluminum boat will have considerably greater strength than the steel vessel.
 

What About Cor-Ten Steel?

In steel, without increasing the weight of structure, one can achieve roughly a 40% greater yield strength over that of mild steel by simply using Cor-Ten steel, AKA "Corten" an alloy developed by US Steel for increased atmospheric corrosion resistance. For boat building, Cor-Ten offers no corrosion advantage. Cor-Ten needs the same rigorous paint protection as mild steel. Cor-Ten has its main advantage in lighter weight structures, where one cannot increase weight, but greater strength is required.

A typical example might be to make use of Cor-Ten where hull plate thickness must be less than around 3/16 inch. In this case, one can make good use of Cor-Ten in order to have better control over distortion during fabrication and better dent resistance in use. Roughly speaking, a 10 gauge region of Cor-Ten plate, having roughly 40% greater strength, will behave more or less similarly to a 3/16 inch region of mild steel plate. The practical result is that a Cor-Ten hull of a given thickness will have a greater resistance to distortion during welding when compared to a mild steel hull of the same thickness, and will be less likely to be dented by running into stuff.

"Cor-Ten" is simply a brand name for one of the High Strength Low Alloy steels (HSLA). The HSLA steels that are suitable for boat building are usually referred to by their ASTM alloy number.

"Cor-Ten A" is also known as ASTM A-242, which is an older specification for the current ASTM A-606 (usually for sheet under 3/16") and ASTM A-588 (usually for plate over 3/16" thickness). ASTM A-588 is also known as "Cor-Ten B" and is the more commonly encountered current spec for Cor-Ten, since it has a minimum yield strength of 50k psi in plates of greater thickness.

An alloy sometimes specified for low temperature applications is "Tri-Ten" also known as ASTM A-441. An alternate (newer) spec for this alloy is A-607 when referring to sheet, or A-572 and A-572-M when referring to plate. The "Tri-Ten" alloys contain a small amount of vanadium (A-572), or they may contain both vanadium and manganese (A-572-M).

The addition of these alloying elements to HSLA steels allows them to achieve greater strength by producing a more refined microstructure as compared with plain carbon steel (mild steel). The alloying elements provide a smaller crystalline grain size and a fine dispersion of alloyed carbides, thus provide higher yield strength without sacrificing ductility.

High Strength Low Alloy Steel Common Names & Properties

HSLA STRUCTURAL STEELS

ASTM A-572-50 EX-TEN 50 Offers 50 KSI minimum yield.
ASTM A-441 TRI-TEN Offers 50 KSI minimum yield. Resistance to atmospheric corrosion twice that of carbon steel.
ASTM A-242 COR-TEN A Resistance to atmospheric corrosion four times that of carbon steel. Excellent paint adhesion.
ASTM A-588-A COR-TEN B Similar to A-242. Modified chemistry offers 50 KSI minimum yield. Resistance to atmospheric corrosion four times that of carbon steel.

Regarding Cor-Ten, in terms of either yield or ultimate strength, if we're comparing an aluminum vessel having a hull structure of the same weight as the same vessel design having a Cor-Ten steel structure, it is interesting to note that the aluminum vessel will still be the stronger of the two. This becomes obvious when you consider that aluminum weighs only around 168 lb. per cubic foot, while steel weighs around 490 lbs. for the same volume, permitting the aluminum components to be nearly three times the size of the steel components within the same weight budget.

Naturally, it would be highly unusual to do that, mostly due to cost. Instead, the aluminum boat can be optimized in other directions, such as reducing overall displacement, or keeping the same displacement and getting a greater range under power by being able to carry more fuel within the same displacement, and so on.
 

The Plastic Range

An advantage in favor of steel, as mentioned, is the larger spread between the yield point of steel and its ultimate breaking point, referred to as its >plastic range. Practically speaking, this means that while a given structure in steel may be comparable in terms of ultimate tensile strength to its "equivalent" aluminum counterpart, the mild steel structure will yield more readily.

The down side is that the "equivalent" steel vessel will dent more readily. The up side is that a steel vessel will absorb more energy on impact (broader plastic range). An aluminum vessel of "equivalent" design strength will be considerably less easy to dent (a plus), and will have approximately the same ultimate failure point.

Aluminum is said to be less "plastic" in that regard; another way of viewing its lesser resistance to flexure and of understanding the primary requirement for rigidity of structure (to avoid flexure) when designing and building in aluminum.
 

Abrasion

Of course, one big advantage of steel is its superior abrasion resistance when compared to any other boat building material, even aluminum. This turns out to be an advantage for steel in use, but an advantage for aluminum during construction, since aluminum can be cut using ordinary power tools. Rather than requiring a torch for cutting, instead the saber saw, circular saw, router and the band saw become the tools of choice.

These tools make construction not only easier, they also speed construction immensely. Along with the very fast speed of welding aluminum, this combination often provides a cost advantage for aluminum in the hands of a capable builder.
 

The Bottom Line

In choosing between steel and aluminum, the deciding points are mainly in the realms of:

Does all of this make steel 'inferior' to aluminum? Not at all..!

For example, steel is much more abrasion resistant than aluminum, and is more ductile in terms of fatigue resistance and in terms of having a broader 'plastic range.' Yes steel is heavier, however that translates into improved comfort. Yes steel must be painted, but if properly done the paint system will not be any more trouble than maintaining a fiberglass boat.
 

Other Articles on Boat Structure

Metal Boats for Blue Water | Aluminum vs Steel | Steel Boats | Aluminum for Boats
Metal Boat Framing | Metal Boat Building Methods | Metal Boat Welding Sequence | Designing Metal Boat Structure
Composites for Boats | The Evolution of a Wooden Sailing Type