Why should you be very cautious of buying a Welded Aluminium Trailer? 

 August 13, 2021

By  Petra Tripmaker

More and more Aluminium trailers are seen under boats these days. Why? Because there are many performance and longevity benefits to be had including the resistance to corrosion and tare weight advantages. This is not the same with many ‘pretend’ Aluminium trailers which are a hybrid of galvanised and Aluminium materials… here we are discussing 100% aluminium trailers.

But when making enquiries about Aluminium trailers… not all Aluminium trailers are created equal! In this edition of ‘Trailer Tips’, we highlight why Aluminium trailers with any welds on their chassis or structural members, including chassis brackets and bolting plates, should be avoided – or at the least very carefully considered prior to purchase.

Aluminium is an incredible material – perhaps most visibly used to make aircraft. But have you ever seen an aircraft that was welded together? Of course not… and that’s a strong clue that you should be very cautious when buying a welded Aluminium boat trailer.

Aluminium is tremendously convenient for so many applications. It’s clean and easy to work with and lends itself to fabrication. It’s 1/3 of the weight of steel, does not rust and generally, depending on what grade and how it is treated, is as strong as, or even stronger than many steels. And when anodised it lasts for decades and resists all types of pollution without corrosion.

A natural and most obvious characteristic of Aluminium is the ability to safely bend and flex. The most visible demonstration of this is when viewing the up-and-down bending movement of the wingtip of an aircraft while the plane is moving. Trailers carrying a load and being driven at speeds on our roads will experience significant movement and flex as well. So all trailers built from Aluminium must be designed to allow and accommodate the flexing of its chassis and structural components – because this is the way Aluminium naturally relives its stress.

Airline manufacturers successfully relieve the stress by bolting or riveting the structural parts of the aircraft together, which allows just enough play for the flexing stress to dissipate. So a fully bolted Aluminium trailer is the only credible construction method that will relieve stress in the same way.  

Similar to the Stainless Steel grades of 304 and 316, the ‘marine’ grade equivalent in Aluminium is 6061. This is the minimum grade that should be used in your trailer on account of its excellent corrosion resistant properties as well as high structural integrity if hardened from T4 to T6.

When Aluminium is extruded into a ‘mill’ finish, it has a hardness grade of T4. This T4 grade is most commonly used for trailer chassis fabricators because T4 is easily bent, shaped, and drilled and cut, without destroying the base properties of the material. If you harden the Aluminium to T6…which increases its strength more than twice, then it’s also more difficult to work with, however is very resilient and very strong. But it cannot then be welded or bent.

Welding of Aluminium results in a form of hardening – though more uncontrolled compared to process hardening of entire sections to T6. A weld creates a hardened zone around the weld while the area around the weld remains softer at T4. And in addition, welding also significantly reduces the yield strength properties of the material in that zone – especially so with 6061 Aluminium which is also crack-sensitive to welding.

In a trailer the movement and flexing of the chassis, which will always occur because it’s a natural characteristic of Aluminium, will transfer all stresses to the next weakest point (which is the weld), resulting in structural failure and cracking. Those of us who have seen cracked Aluminium welds will recall it is never the weld that cracks; it’s the metal beside the weld that cracks. This is where the soft metal meets the hardened weld.

Manufacturers who weld their Aluminium trailers can only use the softer, less resilient T4 grade Aluminium, and therefore often beef up and increase the welds and also their chassis sections… to make them artificially stiffer and stronger. But this constraint exactly opposes the natural characteristic of Aluminium to bend and flex… and thereby not allowing the material to relieve stress. The only alternative is for the stress to migrate to the weakest area…and that’s the nearest weld…around which cracks will appear. 

A typical weld crack


A welded boat trailer suffers a similar stress and turbulence that a commercial airliner endures – that of bending and flexing. We suggest that when you can safely board a welded aircraft, that may be the time to consider buying a welded boat trailer. In the meantime, we suggest you seek out Aluminium trailers with only a fully bolted construction, no welds on any parts of the chassis and structural members, and utilising T6 hardened and anodised 6061 material.   

Happy Boating

Trailer Tips by Spitfire Boat Trailers SA
Andrew van Ryneveld – Adelaide
0450 731 952

Petra Tripmaker

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