Boatbuilding In Steel or Aluminum

We consider steel and aluminum to be the ultimate boatbuilding materials. And, anyone with determination and the right tools can build a boat in steel or aluminum.

Building Platypus in aluminum was our first boatbuilding project ever

Take as an example my son (he was about 25 when we started) and me. We benefited greatly in experience and long-term value from building the Platypus in aluminum. This was our first boatbuilding project ever. We learnt a lot and had lots of fun on the go. Now we want to build another boat, a much bigger one. And we started from a true zero. One simply could not know less about boatbuilding and working with metal than we did at the beginning. Two total amateurs who have never done anything similar before, who have never welded anything or worked on any metal-processing project, who have not even seen someone else building a boat. Both computer professionals with absolutely no relation to any trade or any kind of handwork. But we had the interest, we had the initiative, and we had the determination. And that got us successfully through all the challenges, great and small, that such a complex project inevitably produces. We very successfully built our Platypus, one of the finest boats in its class that you have ever seen.

You Can Do It Too

If we have done it, you can do it too.

Young, older, male, female, you can do it. It is your determination that matters. Even if you happen to be one of those noble characters who does not care to know how to change a light bulb, and if one blows out you just sit there in the dark and wait for your wife to come and fix the problem, even then, with interest and determination you can still do it.

Actually, you have one great advantage that we, so blissfully ignorant about metalwork and boatbuilding as we were at the start, only wish we had at the time. You have this the site where you can find all the needed information and shared experiences at one place, where you can look for solutions and guidance, ask questions and get your answers on any related subject, where you can discuss topics of interest with others as you wish. That is a unique great advantage. Use it, and you will save yourself lots of headache, make less mistakes on the go, save money and time and have a better boat at the end.

The Great Value of a CNC Pre-Cut Kit

If you can, get the plates for the boat hull you intend to build CNC (Computer Numeric Controlled; sometimes called simply NC) pre-cut. That will save you many, many days of tedious cutting, and you will get all pieces pre-cut with the highest precision which can never be achieved with simple power tools or manually.

Sometimes the boat designers are offering as an option the kit of all boat plate elements CNC pre-cut. If a kit is not available, you might be able to get the CNC files for your boat from the designer, and then you order your kit. Go for it, it will save you time and money; you will be very pleased you did.

Building In Steel or Aluminum?

This question does not have a simple black or white answer. Check the table below for characteristics of the two metals, to help you with setting your preferrences:

Feature Steel Aluminum
Strength and weight

Strong and very flexible, it can take a lot of rough beating before it fails, yet heavy (specific gravity 7.8 kg/dm3).

Yielding strength (pressure force to permanently deform the material) about 30,000 psi (pounds per square inch), and ultimate tensile strength (force to break it apart) 60,000 psi.

Specific gravity 2.7 kg/dm3, about one third of the weight of steel.

Higher elasticity than steel, yielding strength (at which it deforms permanently) about 35,000 psi, and about 45,000 psi to tear it apart. Various aluminum alloys have very different characteristics; some are even stronger than most mild steels, yet not all are suitable for marine applications or boatbuilding (follow the link for more details).

Suitable designs

Because of its weight, suitable only for larger boats. Also, beamier flat-bottomed designs may benefit from the extra weight, as the heavier hull would generally pitch and roll less. Certainly not for racing craft.

Sea-going cruisers down to 30 feet. Below that, the displacement would be either disproportionately heavy, or very thin sheets would have to be used, making the welding difficult because of distortion.

Higher elasticity than steel (it takes nearly 20% stronger impact to bend out of shape), but about 25% lower breaking strength. Yet, because it is so much lighter than steel, a hull can be designed and built to the same strength as a steel one, and still be about 45% lighter. And, if it is built to the same displacement (weight), then the aluminum hull is considerably stronger (and can take much stronger impact before it deforms).

Because of its lightness, beamier aluminum boats with shallow draft tend to pitch and roll more than steel ones. But with narrower hulls with deeper draft (sailing boats), aluminum has distinct advantages

Sea-going cruisers down to about 22 feet.

Workability Fair, stronger and sharper tools required Good, regular carpentry tools will do a good job
Price Cheaper than aluminum to buy, yet harder (and slower) to work with and weld. Requires sandblasting before painting. In use, regular maintenance builds up high costs. Higher initial price, offset by faster work processes while building the boat, and substantially lower maintenance requirements and costs in use
Main problems Rust. Protection by cleaning the rust away (sandblasting, especially a new hull) and painting. Electrolysis. Keep other metals away or electrically isolated. Electrical circuits should be properly installed to avoid stray currents through the hull. Use sacrificial zinc anodes.
Painting Essential as protection against rust. Prior sandblasting of a new hull is required. Not required, mostly aesthetical. If the hull is painted, re-painting may be required  every several years.
Maintenance Regular re-painting to keep rust away. Minimal, watch for electrolysis
Longevity All damaged parts can be replaced relatively easy. The boat may last forever. Tensile strength reduction of only 2-5% was measured for an unpainted 1.62 mm (0.064 in, approx. 1/16 in) 5xxx aluminum alloy sheet sample in a 10 year corrosion test. With a 6xxx aluminum alloy, widely used in building pleasure craft, the same test gives a 5-7% reduction. All damaged parts can be replaced relatively easy. The boat may last forever.
Resale value Lower than aluminum boats. Tends to fall rapidly if rust is visible. Higher than steel boats of the same age.

For pleasure boats within our interest range, we prefer aluminum. To us, these are the main reasons:

  1. Very solid strength and flexibility that prevents easy denting (see aluminum marine grades 5xxx and 6xxx for details).

  2. No problems with rust. Aluminum actually oxidizes quickly, but the process stops at the surface, as the outer layer of "rust" protects the inside.

  3. Softer and faster to work with.
    Except for the more expensive welding gear required (see "disadvantages" below), only very basic wood-working tools are sufficient. Of course, if you invest into better quality tungsten-carbide-tipped (TCT) blades and more advanced metal cutters, your job will be easier and faster.

  4. No need to paint.
    If you see the above table entry about longevity, for boats constantly in sea water it makes sense to protect the bottom by painting/antifouling it every 3-4 years; for trailer boats that spend significant portion of their time outside, (that is absolutely not required. Take the Platypus example: As you can verify on the pictures, the boat looks absolutely great as is.

  5. Minimal maintenance requirements.
    A huge difference, as compared to the strict demands of a steel hull.

Some "disadvantages" of the aluminum as compared to steel are actually easily answered:

  • Higher initial price
    True, aluminum is about twice the price of steel. The initial money outlay is bigger. But because aluminum is easier and faster to work with than steel, including faster to weld, and it does not require sandblasting and even painting (especially not the interior surfaces that will be covered with some aesthetic lining), many man-hours are saved during the hull building process. And, once the boat is in use, the maintenance of an aluminum hull will cost many, many times less than for a comparative steel one. All-in-all, the initial larger money outlay will be offset by simplified and reduced processes and significant savings in work-hours required.

  • Not every design is suitable for aluminum
    That is true. Due to the extra weight, wide flat-bottomed hulls in steel will exhibit a more seakindly behavior with less pitch and roll than the same hulls in aluminum. But as soon as your interest shifts towards narrower designs with deeper draft (like sailing yachts), aluminum comes to the fore. Also, for boats smaller than 30 feet LOA, especially for trailer boats, aluminum is the far preferred choice as compared to steel. The type of the hull design and the intended use of the boat are strong factors in the selection of the material for the hull.

But you will have to decide for yourself. It is one of the most important decisions to make at the start of your boatbuilding project (or before you buy a boat).

Whatever is your choice of material for the hull of your boat, aluminum or steel, you will require specific tools.

Together we know more. Together we can do more, and do it better.
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ver. Beta
Last upd: 19-Aug-10 F150806