# Towing a Boat: Tow Ball Weight, Trailer Setup and Your Vehicle's Limits **By Sawley's Auto and Marine** · 2026-09-23 The school holidays put a lot of boats on the road that have not moved since Easter, behind cars that have never towed them further than the ramp. Most of those trips are fine. The ones that are not are almost always a weight problem, a tyre problem or a strap problem, and all three are decided in the driveway before the trip starts. ## Do you know what the rig weighs? Every boat trailer carries a compliance plate with two numbers on it, and every tow vehicle has a set of limits in its handbook, and the trip is legal and safe only if the first set fits inside the second. The numbers: - **ATM** — aggregate trailer mass. The maximum the trailer is allowed to weigh, loaded, with the boat, the fuel and everything in it, measured with the trailer standing on its own jockey wheel. - **GTM** — gross trailer mass. The same load, but measured on the trailer's wheels only, with the coupling on the tow ball. The difference between ATM and GTM is the ball weight. - **Towing capacity** — the vehicle's maximum for a braked trailer, and a much lower figure for an unbraked one. - **Tow ball download limit** — the maximum weight the vehicle and the tow bar are rated to carry on the ball. The tow bar has its own plate; the lower of the two figures rules. - **GCM** — gross combination mass. Car plus trailer plus everything in both. It is possible to be under the towing capacity and still over the GCM once the car is full of people and gear, and this is the limit most people have never checked. The honest way to learn what a loaded boat and trailer weighs is a public weighbridge, once. Most owners who do it are surprised. ### Brakes, and the thresholds that decide them Australian rules require brakes on any trailer over 750 kilograms GTM, and over 2000 kilograms they must act on all wheels and include a breakaway system that applies them if the trailer parts from the car. A lot of five-metre fibreglass boats on a tandem trailer are closer to the second threshold than their owners think. Our guide to [boat trailer brakes](/blogs/guides/boat-trailer-brakes-explained) covers the override, hydraulic and electric types and what each needs to keep working after a season of salt water. ### Loading the boat The single biggest cause of a trailer swaying at speed is too little weight on the ball. The rule of thumb is **about ten per cent of the loaded trailer weight on the tow ball**, within the vehicle's download limit. Too little and the trailer wags the car; too much and the car's front wheels go light and the steering with them. Ball weight is set by where the boat sits on the trailer, so the winch post and the bow stop need to be right for that hull, and the boat has to be winched fully home against the stop every time. Fuel and gear go low and central, not in the stern. A full tank at the transom of a boat that already sits back on its rollers can take a rig from fine to unstable. ### Tyres Trailer tyres are asked to carry a lot of weight and are left in the sun for months between trips, and they fail on age rather than tread. Check the date code on the sidewall; five or six years is the point at which many makers say replace regardless of how they look. Pressures on a boat trailer are usually well above car pressures, printed on the tyre placard or the sidewall, and a soft trailer tyre runs hot and lets go on the highway. Light-truck rated trailer tyres for the common rim sizes are in [trailer tyres and wheels](/collections/trailer-tyres-boat-trailer-tyres-wheels), and a spare that is actually inflated belongs on the trailer. Bearings share the hub with the tyres and they are the other thing that fails on a long tow. If they have not been looked at this year, read our guide to [boat trailer bearings](/blogs/guides/boat-trailer-bearings-roadside-failure) before the trip, not after. ### Tying it down The winch strap holds the boat forward. It does not hold it down, and on its own it does not hold it on. Three things do: - **A bow safety chain** from the winch post to the bow eye, in addition to the strap. - **Transom tie-downs** on both sides, tight, so the stern cannot bounce on the rollers. A boat that bounces on a rough road hammers its own hull and works the trailer. - **Safety chains** from the trailer to the tow bar, crossed under the coupling so a dropped coupling lands in the chains rather than on the road, with rated shackles rather than whatever was in the toolbox. Then the coupling itself: on the ball, latched, and the latch pin in. It takes four seconds to check and it is the check that matters most. Straps, chains, shackles, winches and the rest are in [boat trailer parts and accessories](/collections/boat-trailer-parts-accessories-rollers-winches-lights-hardware), with dedicated ranges for [Dunbier](/collections/dunbier-boat-trailers-marine-trailer-parts-accessories) trailers. ### On the road Mirrors that let you see past the boat. Extra following distance, because the rig weighs twice what the car does alone. Slow into corners and gentle on the brakes, especially on the dirt approaches to the peninsula ramps. And at the first fuel stop, walk around it: feel the hubs, look at the tyres, check the straps and the chains. Most things that go wrong on a long tow announce themselves in the first fifty kilometres. If the trailer has not been over a hoist since last summer, a pre-trip check through our [trailer and brake repair service](/pages/trailer-and-brake-repairs) at Kadina covers bearings, brakes, lights, tyres and the coupling in one visit. Ring [0412 532 984](tel:+61412532984) before the holidays, while there is still time to fix what it finds. **Tags:** Automotive, Guides, Trailers --- > Source: [Sawley's Auto and Marine](https://www.sawleysautoandmarine.com.au/blogs/guides/towing-a-boat-tow-ball-weight-trailer-setup)