Towing Big Rigs

by Holly Dukeson

Shane Kite looks at some of the things you should know if you are towing a seriously large boat/motor/trailer package.

The issue of the dangers of driving large boats around on our roads was brought home to me with a thump when a friend of mine was involved in a life-threatening accident while towing a large cruiser along the motorway. It was raining and there were associated high winds, and the rig started to sway uncontrollably, pushing the vehicle and boat right off the roadside and into a ditch. That was one instance, which was then backed up only a matter of weeks later when a boat dealer was delivering a boating package to its new owner, he got into a bit of strife, and the boat ended up parked in the middle of the road. We hear about only a fraction of such incidents, for there are a lot more that don’t get reported for fear of retribution in one way or another. In other words, the problem is more prevalent than perhaps we realise. The shock of someone being in such a perilous position allows us to think of the ease with which these situations arise, with seemingly no thought of the potential hazards that exist when towing large boats. The number of large (over 7m) trailer boats has grown at an alarming rate. People want to haul those big boats distances now that 20 years ago were unheard of. But missing amongst this new statistic is a level of information on preparing new owners of these boats, on the proper precautions and the dangers to be aware of. As a normal cycle of boat owners, many have upgraded from their 1500kg 5.5m boats, and due to their purchase of a new family large 4WD, or dual cab ute, now look towards the larger 7m to 8m boats that typically have a GVM of 2.5-3.5 tonnes. Rigs over 2000kg require override breaks, and bigger rigs of over 2500 – 3000 kgs must have more sophisticated braking systems. Once you get into the real big rigs of around 2500-3500 kgs, then breakaway emergency braking systems and in-cab controlled braking systems are a must. Usually, they brake both axles. Once the new 4WD and cruiser are hooked up, many think that driving these rigs is the same as their previous boats. The safety provided by these new braking systems in many cases provides a false sense of security, as large masses are involved. There are some issues that can impact on your towing safety.

Our Isuzu D Max LS 4WD towed the 3500kg Dickey 800 with ease.


Tow ratings have become pretty serious marketing fodder among SUV and pickup manufacturers. Superlatives abound about ‘best in class’ capabilities, but while the maximum rating is good to know, there is another value that is probably more important. Towball download. This is one of the most overlooked and misunderstood things about towing. Where the maximum rating deals with the mass of the towed trailer load, the towball mass looks at the weight or force, the trailer exerts directly down onto the towball itself. There is a common misconception that a general rule of thumb exists that the towball download rating is 7-10 percent of the maximum tow rating. This is simply not true, and end users must always abide by the vehicle manufacturers tow ratings, inclusive of towball down load. Users must be clear that some these ratings are as published by the vehicle manufacturer (on their website or within the motor vehicles owner handbook) and NOT the rating on a towbar that may be fitted to the vehicle (unless of course the towbar is a genuine towbar, approved by the vehicle manufacturer or more correctly the brand importer and distributors in New Zealand). Non genuine towbars sometime have labels applied to “meet a customer’s needs or a market perception / rule of thumb”, these non-genuine ratings do not align to the vehicle manufacturers tow ratings and if loads greater than those endorsed by the vehicle manufacturer are towed they are beyond the vehicles capability as determined by the vehicle manufacturer and may create an unsafe and unstable towing combination. In the case of PPB magazines own 2017 Isuzu DMax LS 4WD, it has a 3500kg tow rating and a 350kg tow ball mass rating, as recorded in the vehicles owners handbook and labelled on the genuine / approved towbar, manufactured and tested by Best Bars Ltd for Isuzu UTE’s NZ the brand importer. When you have too much or too little weight on the tow ball, the whole geometry of the vehicle and trailer setup changes, which makes it harder to control and more susceptible to swaying at speed – aside from overloading the hitch in the first place. Stick to the vehicle manufacture’s advised tow ratings and towball down load to ensure the vehicle stance, towing geometry and handling characteristics are not altered from the vehicle safety designers intentions. Users should also play close attention to the GVM (Gross Vehicle Mass) of their vehicle and the GCM (Gross Combination Mass – combination of GVM and trailered load). Users must never exceed either rating. In practice this means care must be taken, when trailering a load rated at the vehicles maximum tow rating, not to additionly overload the towing vehicle with people and other items. Many people seem to believe that as their new vehicle is legally able to tow 3500kg, and it will do that job as easily as taking the vehicle shopping. To tow these large boats, the vehicle is operating at the limits of its capacity in handling and braking – there simply is no room for error!

New Zealand has two main tow ball sizes, 1 7/8” and 50mm and that is believed to be a large contributor to towing accidents. Over 20 years ago the Government transport officials sanctioned a new 50mm tow ball size as New Zealand embraced the metric age and it was supposed to have replaced the old imperial 1 7/8” size. But trailers have continued to be made to fit 1 7/8” couplings and they still make up around 60% of the market, and about 40% of trailers have 50mm couplings. The problem is, an 1 7/8” tow ball is equivalent to 47.6mm, which means that although it slots into a 50mm coupling, the fitting is lose and therefore not secure. Stephen de Kriek, CEO of Best Bars Ltd says, “The potential risk of someone mistakenly hitching a trailer fitted with the larger 50mm coupling to the smaller imperial 47.6mm tow ball is real. We have seen 50mm couplings come away from imperial towballs quite easily, especially if the towball or the coupling is worn or poorly adjusted. Even if the trailer remains connected with a safety chain, there is still significant potential for an accident to happen.” Best Bars offer a simple solution with Convert-A-Ball. The “Convert-A-Ball” replaces a standard towball and comes with two clearly marked interchangeable balls – 50mm and 1 7/8”. They can be swapped over in seconds simply by removing a central pin, which can be pushed out with the tip of a key or pen, while the shank remains firmly attached to the towbar. Mr de Kriek says, “The Convert-A-Ball is easy to use and the spare towball can be carried safely secured in the spare tyre well of the vehicle to be swapped over at any time. It is a constant reminder to the driver to be aware that there are different sizes.” If you are towing any trailer, particularily heavy loads such as large boats, it is critical to ensure your coupling and towball are matched in size, with a preference to a heavy shanked 50mm towball where possible. Equally important is to make sure your towing vehicle is rated by the vehicle manufacturer to tow the weight you have on the back (and in the vehicle), and that your towbar is rate to support these loads. CM Trailers have come up with a different system they call the Multifit Master Dual Auto-Select Coupling unit. The unique design of the Multifit Master Dual Auto- Select Coupling, designed for a 2500 kg capacity trailer, eliminates any risk of mismatching a vehicle’s towball with a trailer coupling, resulting in safe and reliable connection everytime. Simply drop onto the towball and it automatically engages with 1 7/8 or 50mm balls. In addition the unit comes with a built in noise dampening device that eliminates any coupling rattle. Another plus is The Multifit Master Dual Auto-Select Coupling has been designed with an overall height of 131mm to avoid coming into contact with the tail door of some utility vehicles when opened.

Ford’s Ranger is powered by a grunty 3.2 TDCi diesel engine with 470Nm of torque and 147kW of power.


Understanding the different terminologies can be quite a head spinner if you are not technically minded. Do you know the difference between gross combination mass and gross vehicle mass or tare and kerb weights? And then there’s also your vehicle’s payload capacity? Tare is the weight when the vehicle is virtually empty, and Kerb is the weight when full of fuel. Of course, there are variables, such as the weight of a driver and so essentially both weights mostly mean when the vehicle isn’t loaded with all the extras. Technically, tare weight is a bit less than kerb weight, but there’s no universal standard. The total allowed weight of the vehicle is called Gross Vehicle Mass (GVM). This is for all the passengers, all the fluids, all the equipment. Most manufacturers list their GVM without all the extras such as a towbar, canopies, hard lids, bull bars and roll bars. It’s not difficult to add at least another 200 kgs to the vehicle’s overall weight, even before you add your family, plus load the back up with fishing and camping gear. Gross Combined Mass (GCM) is as it reads, the combination of the entire load of the boat, trailer and vehicle. If all that exceeds 6000 kgs then technically you should have an HT licence. It’s quite common now for manufacturers to offer alloy trailers for exceptionally large trailer boats, which can decrease the total weight of the BMT package, although only marginally. The weight saving is generally only in the trailer chassis as all the running gear will still remain the same. Most large 4WDs and SUVs are rated to 3500 kgs with a braked trailer, (750 kgs unbraked) but if you are looking for something bigger, then there are plenty of trucks available from the US such as the Dodge Ram, Chev Silverado, GMC Sierras and Ford F-Series. Some of these are rated to over 6000 kgs!

As loads above 3500kg cannot be towed with a regular tow ball, a pintle hook system is required. These combine a pintle hook, which has a latch over the 50mm ball. This means you can still tow the garden trailer without having to change the ball size. Rigs of this size will require a Class 2 endorsement and be run under a COF, this may add complication and compliance costs that may not be desirable to the average boater looking for the largest trailerable boat on a Class 1 license, which according to the law, However, if you want to get all legal about it, you need to consider what you are towing, if you only have a regular Class 1 car licence. According to the law, the gross combined mass (GCM) of the trailer and tow vehicle must not exceed 6000 tonnes on a Class 1. So, what you can tow depends on the total weight of your vehicle and the weight of the rig on that back. For example, PPB’s Isuzu 4 WD D-Max LS, has a GVW of around 1940 kgs and a GVM of 3500 kg. If you subtract the weight of the D-Max (1940 kgs) from the GCM of 6000 kg, you are left with 4060 kgs as the maximum the Isuzu is legally allowed to tow. Isuzu, like most manufacturers of similar sized dual cab utes, places a tow limit of 3500 kgs on the vehicle, so you are well within the manufacturer’s tolerances to tow, and add a maximum of 560kg of people/gear to top out to 6 tonnes.

The Jeep Grand Cherokee V8 hauled this McLay 701 Premier HT with a trailerable weight of 2500kg, over the Crown Range, Wanaka with ease. However, it is rated to 3500 kg.


If most boat owners were asked, many would be unaware of the effect all those beautiful side covers and fill-in clears have on the towing ability of the boat package. Although very evident on smaller vessels, the effect on a larger rig that has high sides plus a raft of covers presents itself as a potential disaster on the road. The covers, when fitted, add dramatically to the side and height dimensions so that the effect of wind and drag is magnified on the road. It is always a good idea to tow with these covers removed! However, as most of today’s 7.5m plus boats have fixed hardtops you have no option, so just be aware of the huge mass you are towing and drive with care, especially in high winds. Drive to the conditions, and remember not to exceed 90 kph when towing and of course decrease speed well below this if the condition warrant.


There currently are a number of options available, including electric brakes, vacuum brakes, electro- or mechanical-hydraulic brakes and hydraulic over-ride brakes. Hydraulic override (or surge) brakes have been used for many years but are not legal in New Zealand on trailers with a gross laden mass over 2500kg. They are, however, legal on trailers under this weight and are effective if properly maintained and the tow ball weight is not excessive. This indirect brake system works as an action by the driver applying the towing vehicles brakes, in turn causing the trailer to push the towing vehicle and this force indirectly controls the trailer brakes. Electro-hydraulic brakes (for example the HydraStar) are a direct braking system using an electrically driven hydraulic pump to generate pressure in the trailer’s hydraulic braking system. These combine the features of both electric and hydraulic braking and are the ideal system to use in all situations, particularly in marine applications. These units typically have an adjustable controller located in the vehicle to regulated the desired braking performance. Electro-hydraulic braking is currently the primary braking system being installed on New Zealand boat trailers over 2500kg and has the potential to offer boat users a reliable and safe braking system. In contrast, the air-hydraulic brake IQ7 (Sens-a-Brake) provides the brakes with a fluid supply via an air driven master cylinder fitted to the trailer. A small compressor driven by an electric motor charges an air reservoir which in turn supplies a regulated pressure to the master cylinder. When the driver presses the brake pedal, a Sens-a-Brake pad fitted to the pedal sends a signal to the master control unit which calculates the desired brake pressure via the on-trailer unit. Breakaway brakes are compulsory for trailers with a gross laden weight between 2500kg and 3500kg. Breakaway brakes are those that will apply themselves automatically if the trailer is accidentally disconnected from the vehicle. They are a legal requirement once a trailer exceeds 2500kg. In this instance, no safety chains are fitted as it becomes difficult to restrain via safety chains should it become disconnected from the tow vehicle. All electro-hydraulic and air-hydraulic systems have this as a standard feature. Trailers of between 2000kg and 2500kg must be fitted with a pair of safety chains that cross over unless equipped with breakaway brakes.

The VW Amarok V6 TDI has an unladen weight of 2070kg and 3500kg towing capacity.


As previously discussed, the braking systems used on these trailers, are controlled in-cab by way of a brake controller. The purpose of these controllers is to adjust the level of power applied to the brakes, making it adjustable for differences in weight, and the level of braking effort applied when the foot brake is pressed. PPB recently fitted an in-cab controller system to the editor, Barry Thompson’s Isuzu D-Max ute. Used as the test vehicle for a recent video on towing big rigs, we had a 3500kg Dickey 800 on the back, and the Isuzu towed it with ease. The HydraStar system fitted to the trailer provided positive and worry free braking. Fitting the in-cab controller was a simple job (we used an auto electrician) and the only change was from a 7 pin to a 12 pin plug. While in this case the controller was hard wired in the D-Max, there is available on another model of HydraStar braking system, the opportunity of a plug into the cigarette lighter socket controller. When you swap tow vehicles, you simply unplug the commander from one vehicle and swap to another. The controller also has features where in cases of emergency, a button can be applied to power the trailer brakes without the car brakes being applied, which can prevent an accident simply by “straightening out” a swaying trailer. I am sure some readers towing those big boats are not aware of the correct adjustment and operations of all the controller options. It would be advisable therefore in every purchase situation, to test, check and get used to these braking systems and their controllers – well before any long trip. Make a few short trips, in all conditions, and don’t be afraid to ask your dealer for advice on what you should and shouldn’t be doing. While we are on brakes and wheels, you should also keep an eye on your tyre pressures. Run the tyres at their maximum recommended pressure. They’ll run cooler, and you’ll consume less gas.



Many of the near-misses that occur with these large boat packages are due to sheer ignorance – the new owners were not being aware of the extra weight and length being towed – compared to their previous rig. It seems there is a definite opening for an advanced driving course to be made available to these new owners. Perhaps even go so far as making it mandatory for vehicles that tow over a certain weight, to have a licensed driver (to that level) at the controls – someone who has been trained and familiarised appropriately. The system as it stands now is that anyone, can hook up a 3500kg rig and jaunt off down the road – the very same road our kids go to school on. On a longer trip, we all know how we get complacent with the speed we are travelling and despite the massive weight being towed many of us get ‘into a groove’. We can be more concerned about what CD is playing, than being aware of the implications of what can happen in an emergency, or with changing road conditions. Pay attention and concentrate at all times, for it only takes a split second for a disaster to unfold. It’s also important to keep your speed down, as if things go wrong and your boat starts swaying at 100 km/h, you can be in for hairy ride till it settles….or doesn’t. The New Zealand speed limit for towing is 90 km/h and that’s plenty if you have a 3500 kg rig behind you. In fact this would be considered by many experienced tower as an absolute maximum when towing near or at the vehicles limits, many airing on the side of caution and decreasing spead 10-15% to allow an extra margin of safety. In short, it would be common sense that we all build up our experience in towing larger craft, to a level where we are aware of the responsibilities of towing such large boats. Ironically, and indeed sadly, it takes an experience such as the accident that introduced this article, to wake us all up to our skill levels, responsibilities, and set-up that is required to tow these larger packages.

Looking for something massive pulling power, then you might like to consider something like this Dodge Ram 3500.


Probably the best towing vehicle is either a full-size 4×4 ute or SUV with a turbodiesel engine and automatic transmission. With kerb weights between 2-3 tonnes and wheelbases of around 3000mm, they provide the heavy, sure-footed anchorage needed for high towing capacity with good control. Their high torque diesel is also tailor-made for this role with excellent pulling power and better fuel economy than petrol engines, combined with intelligent automatic transmissions that save you a lot of stick-and-pedal work and offer the convenience of sequential selfshifting when required. The influx of USA imports and alloy hardtops has seen vast quantities of those big boats increasing every season. Many boaters who traditionally would be buying 5.5m to 6.5m boats (where trailer towing skills are typically learnt) are now buying significantly larger packages. Many dealers can quote examples of newcomers to trailer boating starting out with a big 4WD and a 3500kg rig, with no towing experience whatever! This can and does happen without any dealer training or preparation for the roads ahead. Makes you think?

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