Towing by numbers: A quick guide to boat towing capacity

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Getting your pride and joy to and from the water safely relies on your understanding of boat towing capacity – including the various weights and masses prescribed by the manufacturers of your boat trailer and tow vehicle. Knowing your weights and boat towing capacity is essential to safe towing. So let’s talk weights:

Tare mass (vehicle)

This is the actual mass of the tow vehicle, or how much your car or truck weighs at manufacturer specification (including liquids like fuel and coolant) and with no payload.

Tare mass (boat trailer)

This is the mass of your boat trailer, as specified by the manufacturer, with no payload.

ATM (aggregate trailer mass)

This is the tare mass of your boat trailer plus its maximum payload when unhitched from the tow vehicle. It’s the maximum your boat trailer is allowed to weigh in transit.

ATM = tare mass (boat trailer) + max payload

GVM (gross vehicle mass)

This is the tare mass of your vehicle combined with its maximum allowable payload. It’s the maximum your vehicle is allowed to weigh while in transit.

GVM = vehicle tare mass + maximum payload


Payload is any mass you add to the vehicle’s tare mass. Payload can have a big impact on your boat towing capacity.

For tow vehicles, this includes passengers, luggage, aftermarket bull bars and the tow ball mass. Payload does not include standard specifications set by the manufacturer such as fuel and coolant.

Maximum payload (tow vehicle) = GVM – tare mass

For boat trailers this includes the boat, any fuels, other fluids, luggage, food, accessories and non-standard equipment contained inside the boat. It also includes any non-factory standard tools, equipment or spares [On the road with our boat trailer: spares] stowed in storage compartments on the trailer itself, but check your manufacturer documentation for clarification.

Maximum payload (boat trailer) = ATM – tare mass (boat trailer) – tow ball mass

TBM (Tow ball mass)

This is the mass exerted on the tow ball by your fully laden boat trailer (this includes the boat and any other payload). When your boat trailer is hitched on the tow ball, this mass is included in your tow vehicle’s payload.

GTM (gross trailer mass)

This is the maximum allowable mass placed over the boat trailer’s axle(s) while the boat trailer is hitched. It includes the maximum allowable payload but does not include the tow ball mass.

GTM = ATM – tow ball mass

GCM (gross combined mass)

The gross combined mass is a combination of the maximum allowed mass of the boat trailer (including boat) and the maximum allowed mass of the vehicle. It’s rare that you’ll need to consider this one.


You now understand a bit more about boat towing capacity. You’ll find the tare, ATM, TBM and GTM stamped on the vehicle identification number (VIN) plates. In the interest of safety, it is important that you understand these figures.

Once you’ve towed your boat, you’ll need to ensure you’re launching it correctly.


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