We built a custom off-road boat trailer to tackle the trails

Taking the Tinnie

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To the outdoor adventurer, there is quite possibly nothing more quintessentially Australian than the humble tinnie. Everyone seems to have one (or know someone who does) and if it’s anything like mine, it has been gathering leaves sitting idly in the backyard for quite some time.

It was probably somewhere around the fourth or fifth beer whilst sitting on the dunes on a typical clear and starry Fraser Island night that the comment was made, “Geez it’d be nice to have a boat out here.” It was at this point that the conversation swung from the usual budget-less hypothetical vehicle build banter to concepting our very own custom off-road boat trailer.

By the time the last whiskey nightcap was poured, planning was already well underway for what would easily rank as our most ambitious backyard engineering project to date. As a frustrated fabricator trapped in the body of a hydraulic engineer, my mate Brett would play the role of point man. Armed with a cut-off saw, a MIG welder and a fully-licensed version of Autocad, his nous and motivation for the project would prove invaluable.

Over the coming weeks each and every element of a typical OE roadgoing boat trailer was analysed and weaknesses were identified in an attempt to reverse engineer a reliable outcome. Member sizes, bracing (quantity and size), slipper springs and the keel roller assembly would all require thoughtful upgrading. Along with understanding weaknesses it was also important to identify other ‘nice to have’ elements based upon the destinations we would likely tow the trailer to. With planning underway for trips to Moreton Island and Cape York, engineering outcomes would be required for the rolling stock, track width, walkways, fuel storage and a drawbar extension.

Doubling the OE member size from 38 x 18mm to 75 x 50mm would give the trailer the necessary strength while also adding weight to help make the suspension ‘work’ (more on that later). The 75 x 50mm main A-frame member size would be replicated for the bracing; and the quantity increased from two braces to three to add rigidity.

While slipper-style leaf springs work well for light loads on bitumen roads, they are simply not up to task or the demands of places like the Peninsula Development Road. Fitting up a set of eye-to-eye springs was not so much of a challenge as engineering weight into the trailer to get some flex into these minimum 700kg (rated pair) leaves, which were the lightest we could find at the time. With a hull weight of around 100kg and an outboard tipping in at 50kg it was important to try and have the trailer weigh as much as reasonably practical. An additional structural bracing detail was added to the main A-frame using 50 x 25mm section for this purpose. Caravan shocks were added to assist with dampening out any rebound.

Keel rollers on a factory boat trailer are quite literally few and far between. To reduce the transfer of stresses to the hull from pounding along corrugated roads, a full-length keel roller bed was engineered using 50 x 50mm angle and a set of 12 keel rollers were spaced at 200mm centres. It was quite conceivable that at some point we would find ourselves having to ‘skull drag’ the boat from its sandy resting place up onto the trailer – and for this reason the roller bed was extended past the rearmost bracing member and angled to help lift the hull up and onto the trailer. In wrapping up the main A-frame, all major joints would be gusseted using 5mm plate for peace of mind.

The beauty of customisation is that it gives full control and artistic license to the individual to produce a completely tailored outcome specific to one’s needs. In considering where and how the boat would be used, a list of additional requirements was created and factored into the build.

From Day 1 we were particularly keen on matching the track width of the trailer to that of my GU Patrol. As a finished product the entire package would find itself on the islands of south-east Queensland at least once a year… and it just made good sense that the trailer wheels should exactly follow the line of the tow vehicle (at least in a straight line, anyway). Boat trailers are typically engineered to have the hull as low to the ground as possible for variety of good reasons. Manufacturers achieve this, in part, with a wider wheel track that sees the hull nestle in between the guards. Making our trailer narrower would see the hull sit above the guards – which would throw up a few challenges to ensure clearances were adequate between guard and hull and guard and tyre.

We considered independent suspension to be overkill on such a small trailer, when a beam axle assembly would get the job done in almost all situations. To increase height under the axle and provide a better overall outcome, we settled on a 15-inch wheel and tyre combo. A significant amount of discussion took place on this subject before a decision was taken; with serious consideration given to matching the rolling stock on my GU Patrol. As it would turn out, the decision to run with the 15-inch wheels was a good one… as it simplified the construction of the spare wheel bracket and the clearance and spacing of the guard to the top of the tyre and the guard to the underside of the hull (a by-product of the narrower track width we identified earlier).

Keeping the tow vehicle safe and dry on open beach launches has been assisted with the addition of a simple slide-out draw bar extension. Fixed in place by a pair of tow hitch pins, an additional 1,800mm is available for days where a small rolling swell is likely to lick at the vehicle’s rear wheels. With the snapping handbags of Cape York and the ever-present threat of Irrukanji along Fraser Island’s western shore during the summer months, the ability to remain dry while traversing the full trailer length during launch and retrieve was considered fairly important for obvious reasons. Chequer-plate walkways on either side of the central roller bed presented neither the most expensive (nor the most difficult) challenge we faced with the build; but that enhancement remains one of the more functional outcomes in terms of safe launches and retrievals.

Since the invention of the internal combustion engine, the scourge of the long-distance traveller has been fuel storage. In our situation, be it diesel for the Patrol or petrol for the outboard, lengthening the draw bar slightly to accept twin 20L jerry can holders has increased our range to a much more respectable level while also allowing the entire winch post assembly to shift forward to accept a slightly larger hull down the track.

Wrapping up the build were several other nifty details that helped make for a more functional product. Each guard and leaf spring assembly has been fabricated to allow the entire axle and wheel assembly to shift forward or rearward to enable balancing of the whole rig based upon the results of road testing. We chose to fit an Alko off-road ball coupling to improve off-road ability, while allowing for differing tow vehicles utilising a standard 50mm tow ball. A pair of recovery points have been factored into the rear structural arrangement of the trailer to allow a winch to be attached for any manoeuvring or straightening exercises on the tracks. It is probably important to mention here that these recovery points will serve for winching of the combined boat and trailer weight only and they’re not for snatch recovery.

While very little of this planning and building was straightforward, it was by no means overly difficult either. Patience was without doubt the key ingredient during the process which allowed for all reasonable ideas to be brainstormed, design problems to be identified and engineered out and practical outcomes that we weren’t satisfied with to be dismantled (or ground back) and redone. The work was completed in a domestic garage with a reasonable quality cut-off saw, a MIG welder and a grinder… and the enthusiasm of one bloke who was particularly up for the challenge. For a fraction of the cost of a new off-road boat trailer (but with complete customisation) we have added an extra dimension to our touring travels. Whether it makes us better fisherman… only time will tell!