Boats are built out of a variety of materials. In fact parts of boats are often built of different materials, choosing the best for the particular purpose. The hulls may be different to the superstructure, and the inside fitout different again. However, when we say a boat is “built out of X”, it almost always means the material of the hulls. Even then, some hulls are constructed of different materials, usually changing at the waterline: so the underwater may be solid GRP while above that may be balsa sandwich.
The most used hull materials are:
- Solid wood: built from the earliest human times, but was still built in the 20th century without fibreglass. Types are clinker, carvel. Since epoxy technology and fibreglass, some methods in cold moulding use wood plus epoxy and become sandwhiched (see below).
- Plywood: created in second half of the 20th century, it is still used today. It requires a covering of some sort for water proofing and sometimes strength by I-beam; this is usually epoxy or polyurethane, and fibreglass for strength.
- GRP: glass reinforced plastic, or fibreglass. These boats are made of solid fibreglass, usually quite thick. The method is usually using chopped strand mat (CSM) rather than woven rovings.
- Fibreglass sandwich: these boats are made from a core wood, including balsa, and foam. The “sandwich” comes from the inner core being surrounded on both sides by fiberglass – usually woven, not CSM.
- Ferro cement: these boats are made from actual cement/concrete. Seemingly a paradox, they float. Thank you Archimedes!
- Iron: not used I believe since after the iron-clad era of the late 1800’s. Indeed, there was only a short time when very large boats were clad with iron, then made with iron, then made with steel – steel being much better for naval work.
- Steel: still used today, these boats are almost always one-off, strong designs.
- Aluminium: boats were built of this since the 1890’s; aluminium has been a niche build material but with many benefits. There’s a nice history (PDF).
- Titanium: while they exist, very few boats are made from titanium although it is a good fit – apart from cost.
- Plastic: some smaller boats are made from plastic. One method is roto-moulding, which is the way some of Hobie’s small boats are built.
So what’s so good about aluminium?
Firstly, I’d better get rid of one of the problems of this earth whereby the same thing is spelt or pronounced in different ways: is it “alu-min-ium” or “alu-minum”. Fortunately, the answer is that it doesn’t matter as they both refer to the same thing. There’s a nice page on writingexplained that describes the points, with a lovely graph showing the British (red) and American (blue) usage over time:
It’s my blog, and as a scientist I’ll go with the scientific consensus of aluminium.
What is it and why use it
Briefly, and specific to boat building: it’s a lightweight, non-ferrous metal. So it’s light, doesn’t rust, and has other good and not so good properties. It’s generally not used for all boats because it’s relatively expensive, harder to shape into curves when it’s thicker, requires more skill to weld and connect properly. In Australia, the ubiquitous “tinny” has been the most obvious form of aluminium boat – a smallish monohull dinghy, often driven by a beer with a male human attached somehow.
Aluminium is one of the most plentiful elements in the earth’s crust. It is found in bauxite, refined into alumina (aluminium oxide), electricity is used with carbon to make aluminium. It’s quite a costly process, and some say the costs to make it are more than any other metal, but one advantage to the metal is that it is 100% recyclable and can be recycled essentially forever without losing any of its properties.
For yachts, motor or sail, the benefits are many:
- doesn’t rust – extremely corrosion resistant
- extremely resilient and durable, and if unpainted above the waterline, can be left in a salt or freshwater environment forever
- is lightweight compared to most other methods of equal strength (!)
- very strong for it’s thickness and weight
- bends rather than breaks, which is a benefit when (!) the boat hits something
- can be mended by welding in new pieces making it as strong as the original
- doesn’t burn (in normal operating) – which is important for fires on boats
- doesn’t become brittle in the cold
- fully recyclable
- allows lots of customization in the design and placement of boat accommodation
- can be (much) more expensive than other substrates (although the hulls are only a small part of the whole boat cost: some say 20% at most)
- can need more skill to build as the welding method is more specialised
- needs special consideration regarding electrical and metal corrosion – the electrical system, and anywhere dissimilar metals are in contact
- can need more insulation
As is clear, the benefits can be seen to be minor in comparison to the advantages. Indeed, many warships are built using it.
So why aren’t most recreational vessels made of aluminium? One reason is that aluminium doesn’t lend itself to being made in a mold. Each boat is built from the keel up rather than being popped out of the mold, so it takes longer and economies of scale are harder to create. This is likely the main reason, along with initial cost, why aluminium remains a niche material.
For yachts, 5083 or 5086 aluminium are usually used. This means they are aluminium alloys of the 5-series with magnesium. Specifically:
(taken from an interesting discussion on BoatDesign).
5083 Weldability of this alloy is very good by conventional means. When filler rod is required it should be the same alloy, 5083. Shear strength in O temper is 25 ksi.
5086 This alloy is readily weldable by conventional methods. Use of electric arc welding in particular produces excellent results. Shear strength for O temper is 23 ksi, for H34 it is 27 ksi. Condition O Temperature 68 Tensile Strength 38 ksi, Yield Strength 17 ksi, Elongation 22.
5454 All of the commercial welding methods will work satisfactorily with AL 5454. The TIG or MIG process, with AL 5554 or 5356 filler rod, works especially well. Shear strength varies slightly from 23 ksi in the annealed condition to 26 ksi in the H-34 temper. Condition O Temperature 75 Tensile Strength 36 ksi, Yield Strength 17 ksi, Elongation 25
5456 Arc welding is best for this alloy and this makes it useful for structural applications. Gas welding may be used, but results are less favorable than for arc welding. Shear strengths for various tempers are: O temper, 28 ksi. H321, 30 ksi. H311, 28 ksi
The StrongAll® build method
StrongAll® (from hereon, I’ll just use the shorthand SA so I don’t have to pepper this text with ® for the registered trademark!) is a form of building boats in aluminium. The innovation appears to be using thick sheets instead of thin: it seems that SA boats have double or so the hull thickness of the usual aluminium built boats.
A translation of the Meta boatyard’s page is:
A word first about the benefits of aluminum.
The universal and perfect material does not exist: every material has its qualities and defects.
We chose aluminum to succeed steel because:
- Aluminium boats are solid: they are homogeneous constructions: no joints between the welded elements, no weaknesses by composite combinations of various materials.
- Aluminium boats have a long lifespan.
- Aluminium boats are a good investment when budgets are under control.
- Aluminiums of types 5083 and 5086 require little maintenance.
- Aluminium boats retain a good price when they are resold if they have been well designed…. well-built.
What’s the Strongall®?
It is an aluminium construction method developed and patented by META Chantier Naval since 1977 in Tarare (France).
The concept is that of a self-carrying aluminum deck hull, based on the use of thick sheet metal for the development of aluminum boats.
The cross-sectional structure is then minimized: integrated tanks, ber engine, mast belt, crash-box or waterproof bulkhead depending on the size of the boat.
This type of construction is synonymous with robustness and durability guaranteeing the significant investment that a boat represents over many years.
The Benefits of Strongall®:
Stronger than a steel construction with equal weight.
- About 30% lighter than the same steel hull, with comparable strength.
- The use of thick sheets allows for greater intensity in welding, thus eliminating the risk of “gluing” during welding with the guarantee of a better fusion of the metal.
- Little or no deformation of the edged sheets.
- The thickness of the sheets increases corrosion resistance, and significantly decreases the risk of metal fatigue (repeated twist-bending cycles: a hull is a hollow beam that moves on a complex phenomenon such as waves and swell!).
- The hull is treated with Silicaty Inorganic Zinc.
- Painting of hulls and bridgedeck is only necessary for cosmetic reasons, they can remain in the raw state without consequence for the material.
- A significant advantage for the shipowner, the absence of binding interior structures makes the design of the facilities more flexible.
The hulls are necessarily with developing edge surfaces [chines], which does not prevent them from being elegant and efficient!
More than 300 sailboats, motorboats and trawlers have been built in Tarare since 1977.
Their extreme robustness allows them to navigate particularly difficult areas: Arctic, Northwest Passage, Antarctica… and elsewhere!!!
At the same time, the young subsidiary PROMETA – headed by François FEVRE – also dropped steel for the benefit of STRONGALL®
So essentially the reasonably famous Meta shipyard in France builds in StrongAll Aluminium. They licensed it to a subsidiary ProMeta. Between them, they’ve build many, many aluminium pleasure craft that have sailed the oceans of the world.