Boat trailer weights explained: ATM, GTM, tare and ball weight

Every year people buy a boat, hitch up the boat trailer, and discover on the drive home that their car was never rated to tow it. The information needed to avoid that is printed on two metal plates and takes about five minutes to check — but only if you know which four numbers matter.

The four numbers on a boat trailer

Tare weight is what the boat trailer and boat weigh empty. No fuel, no water, no gear, no batteries, no esky. It is the least useful number of the four, because nobody tows a boat empty, yet it is the one sellers quote most often.

ATM — Aggregate Trailer Mass is the maximum the boat trailer and everything on it may weigh, fully loaded, when it is *not* coupled to the car. This is the number that matters most. It is stamped on the trailer's compliance plate and it is what you compare against your vehicle's towing capacity — the braked figure if the trailer has its own brakes, the much lower unbraked figure if it doesn't. See "Braked versus unbraked" below, because for a small tinnie that difference decides the answer.

GTM — Gross Trailer Mass is the weight carried by the trailer's own axles when it *is* coupled. GTM is always lower than ATM, because once you hitch up, some weight transfers onto the towball.

Ball weight (or towball download) is that transferred portion — the downward force the coupling puts on your towbar. Roughly, ATM minus GTM. As a rule of thumb it lands around 10% of the loaded trailer weight, though where the boat sits on the trailer changes it significantly.

Where to find them

• Boat trailer compliance plate — usually riveted to the drawbar near the coupling. Carries ATM and GTM.
• Your vehicle's compliance plate and owner's manual — carries braked towing capacity and maximum towball download.
• Not the seller's memory. Plenty of honest sellers quote the tare weight when asked what their boat weighs, because that is the number they were told when they bought it.

If you are looking at an ad rather than standing next to the boat, ask the seller for a photo of the trailer plate. It is a reasonable request, it takes them ten seconds, and it saves you a wasted drive.

The mistake almost everyone makes

They compare their car's towing capacity against the boat's tare weight, not the boat trailer's ATM.

A 4.8 m aluminium runabout might have a tare of 900 kg. Add a full 100 L fuel tank (around 75 kg), two batteries, an anchor and chain, safety gear, rods, an esky and a couple of fenders, and you are comfortably past 1,100 kg. The boat trailer's ATM might be rated at 1,300 kg — and *that* is what you must stay under, and what your vehicle must be rated for.

The gap between "what the boat weighs" and "what you are actually towing" is routinely 200–400 kg on a small trailer boat, and more on anything with a cabin.

Braked versus unbraked

In Australia, any trailer over 750 kg loaded must have its own brakes. That is why vehicle specifications quote two towing figures — an unbraked capacity (often 750 kg) and a braked capacity, which is much higher. For any boat beyond a small tinnie, the braked figure is the one that applies.

Two things follow. First, when you read a towing capacity figure, check which one it is. Second, if you are buying a boat trailer near that 750 kg line, confirm whether it actually has brakes fitted and working — not just whether it is rated for them.

Towball download is a separate limit

This is the one that catches people who have done everything else right. Your car can have plenty of towing capacity and still be over its towball download limit, because that limit is about how much weight is pressing down on the back of the vehicle, not how much it is pulling.

A boat sitting too far forward on its trailer raises ball weight. Too far back reduces it — and a ball weight that is too *low* causes trailer sway, which is considerably more dangerous than being a few kilos heavy. Both limits matter, and they are checked independently.

Your towbar has its own rating too, and it is not always the same as the vehicle's. A towbar rated below the car's capacity becomes the binding limit.

Towing rules vary by state in Australia

The weight definitions above are national, but the road rules around towing are set state by state, and they differ in ways that matter:

• Speed limits while towing are lower than the posted limit in some jurisdictions and not others.
• Trailer registration is a state matter — a trailer registered interstate needs transferring when you move it, and requirements differ.
• Mirror requirements apply once the trailer is wider than the tow vehicle, which most boat trailers are.
• Safety chain and coupling standards are broadly consistent but inspection regimes are not.

Check your own state's transport authority before a long trip, particularly if you are crossing borders. A rig that is legal at home is not automatically legal everywhere in Australia.

Leave yourself margin

Being at 98% of your rated capacity is legal. It is also a rig with nothing left for a full fuel tank, a passenger in the back seat, or the gear that accumulates in a boat over a season.

That is why the compatibility indicator on listings here flags anything at 90% or more of your capacity as amber rather than green. It is not telling you the rig is illegal — it is telling you that you have used up your buffer before you have loaded anything.

Save your tow vehicle once and every listing shows green, amber or red against these numbers automatically. Add your tow vehicle, or use Trailer Match to see every boat you can tow.

Before you buy

1. Get the ATM and ball weight from the boat trailer's compliance plate — a photo of the plate is the ideal thing to ask a seller for.
2. Get your vehicle's braked towing capacity and maximum towball download from its manual, not from a forum.
3. Compare ATM against braked capacity, and ball weight against the download limit. Both must pass, and so must your towbar's own rating.
4. If any of them is within about 10%, treat it as a no unless you plan to tow a very light load.

These figures are a guide based on manufacturer ratings. Loading, tyre condition, towbar rating and state regulations all matter, and none of them are visible on a compliance plate. If you are close to a limit, get the rig weighed.

Next

• Buying a used boat privately in Australia: the complete checklist
• What a PPSR check on a boat actually tells you