Towing a Cat Cruiser by the Numbers: ATM, Towball Download and GCM

By Jason Rieck
Co-Founder & Lead Designer, Cat Cruisers

Most towing mistakes aren't made on the highway. They're made at the kitchen table, when someone reads one number off a brochure, sees it's under their vehicle's towing capacity, and stops there. There are four legal limits, and all of them have to be satisfied at once. The one that catches people out is rarely the one on the brochure.

This is the technical version: the definitions, our weights, how the braking rules apply, and a worked example. For a quick answer for your own vehicle, our free Can You Tow It? tool runs the same checks.

The terms that matter

• ATM (Aggregate Trailer Mass): the total mass of the loaded trailer and boat, as if it were standing on its own.
• Towball download: the share of that mass carried by the tow vehicle's towball.
• GTM (Gross Trailer Mass): the mass carried by the trailer's own axles when it's coupled up. In simple terms, ATM minus towball download.
• GVM (Gross Vehicle Mass): the most the tow vehicle may weigh when loaded, including the towball download.
• GCM (Gross Combination Mass): the most the vehicle and trailer may weigh together.

Your vehicle's manufacturer sets a maximum braked towing capacity and a maximum towball download, alongside the GVM and GCM. All four have to be met at the same time. RACQ's towing guidance is a good plain-English summary.

Our weights and dimensions

Specifications are subject to final engineering.

• 650: about 1,750 kg on the trailer, the lightest of the range. Check your vehicle's braked towing rating and plan for a towball download of around 10 per cent.
• 750: heavier than the 650. Minimum tow vehicle: 3,500 kg braked.
• 870: the largest model, and a long, tall rig on its trailer, so check its dimensions against your carport or shed. Minimum tow vehicle: 3,500 kg braked.

Published weights are a starting point. Fresh water, gear, food and people's bags all add up, so the loaded weight on the road will be higher. Plan around the loaded figure, not the dry one. The only way to know your true numbers is to weigh the rig, loaded for a trip, at a public weighbridge.

For stable towing, the long-standing rule of thumb is a towball download of about 10 per cent of the trailer's weight. The 650 and 750 figures above sit close to that. Too little download and a trailer is prone to sway. Too much and you unload the tow vehicle's front axle and overload the rear. Ten per cent is a guide, not a legal limit. The legal limit is whatever your vehicle and towbar manufacturers specify.

How the trailer braking rules apply

Trailer braking requirements come from the Australian Design Rules and are summarised in the federal Vehicle Standards Bulletin 1 (VSB1). They're based on GTM:

• 750 kg GTM or less: brakes aren't required.
• Over 750 kg GTM: an efficient service braking system is required. Up to 2,000 kg GTM, brakes may be fitted to only one axle, and over-run (mechanical) brakes are permitted.
• Over 2,000 kg GTM: brakes must operate on all wheels, and they must be activated by a control signal from the tow vehicle, which in practice means an electric brake controller. A breakaway system must also apply the brakes automatically if the trailer becomes disconnected, and keep them applied for at least 15 minutes.

A 650 at about 1,750 kg is well over 750 kg, so it needs brakes. The 750 and 870 are heavier again, and once a trailer's GTM passes 2,000 kg it's in the top category: brakes on all wheels, an electric brake controller in the tow vehicle, and a breakaway system.

A worked example: the 870

Take a hypothetical boat and trailer at 2,490 kg empty (an illustrative figure, not a published spec) and add a realistic trip load: 180 kg of water and 120 kg of gear and provisions, for a loaded ATM of about 2,790 kg. Assume a towball download of 280 kg, which is 10 per cent. Hitch it to a hypothetical large SUV with these plate figures: 3,500 kg braked towing capacity, 350 kg maximum towball download, 3,300 kg GVM, 5,700 kg GCM and a 2,450 kg tare.

• Towing capacity: 2,790 kg against 3,500 kg. Pass.
• Towball download: 280 kg against 350 kg. Pass.
• GVM: 2,450 kg tare, plus two adults at 85 kg each, plus 100 kg of gear in the vehicle, plus the 280 kg download, comes to 3,000 kg against 3,300 kg. Pass.
• GCM: the loaded SUV at 3,000 kg plus the 2,510 kg carried on the trailer's axles comes to 5,510 kg against 5,700 kg. Pass, with 190 kg to spare.

Now add two more passengers and another 100 kg of gear in the vehicle. The SUV alone is at 3,270 kg, still just inside its GVM. But the combination is now 5,780 kg, 80 kg over the GCM. Every individual number still looks fine, and the combination is illegal.

GCM is the limit that catches people. It's why a vehicle with a 3,500 kg towing rating isn't automatically a 3,500 kg tow vehicle once the family and the gear are aboard. Check your vehicle's GCM, not just the headline towing figure.

Width

The general width limit for a light trailer and its load is 2.5 metres, including anything that sticks out the side. Check your boat and trailer against that limit. Loads wider than the limit fall under oversize rules, which differ from state to state.

At the ramp

The less water a boat needs to float, the less of the trailer (hubs, brakes and wiring) has to go under to launch it. The 870's draft is 0.2 m. A few habits keep a braked trailer healthy and a ramp session smooth:

• Do your preparation in the rigging area, not on the ramp: bow and stern lines on, tie-downs off, lights unplugged.
• Test the brake controller and check the breakaway battery's charge before every trip.
• After a saltwater launch, flush the brakes and hubs with fresh water, and check bearings regularly.
• With electric drive there's no engine to warm up and no fuel to check, and you get precise control at low speed as soon as the boat floats.

For tow vehicle choices and state speed and width rules, see our catamaran towing guide. For registration, see the state-by-state registration and licensing guide.

Before you buy

1. Find your vehicle's four plate figures: braked towing capacity, maximum towball download, GVM and GCM.
2. Estimate your loaded trip weight, including water, passengers and gear.
3. Run the numbers through the Can You Tow It? tool.
4. Once you own the rig, weigh it loaded at a weighbridge and confirm.

If the answer is tight, it's better to know now. Get in touch and we'll work through it with you.