How to select the correct trailer length and ATM.

Select the trailer's correct length, width & ATM

Length & Width;

Generally a boat trailer needs to be approximately 700mm to 1,000 plus longer than the boat.

Yet, it may be possible to put a 7 metre boat on a 7 metre trailer

This depends on the boat's design.

Boats are ofter named as a model such as an  XYZ - 6.5 which indicates the boat's size is 6.5 metres long.

It is not the overall boat length that determines what length of trailer your boat needs.

It is the distance in a straight, horizontal  line from the bow eye to the lowest part of the transom and indicated in the image above as " A "

The width of a boat  at the gunwale " B " is important as is the width at the chines " C " as these measurments  will determine the required trailer width and also if your trailer needs side lights.

The chine to keel height " D " may be important if you boat has a very deep V hull.

If you plan to contact a Spitfire outlet for a quote, it will be very necessary  for you to be able to provide this information.

Aggregate Trailer Mass (ATM)

ATM is the total mass ( Weight ) of the laden trailer when carrying the maximum load recommended by the manufacturer. This includes any mass imposed onto the drawing vehicle when the combination vehicle is resting on a horizontal supporting plane.

In addition to the fuel and water a boat may carry on a trailer there is often, depending on the size of the boat, an additional 200kg  to 300 kg or more of weight and this could include accessories that may have been added to the hull, but are not not normally in the manufacturer's weight calculation, such a toilet, duck board, swim ladder, hard top etc.

The image above will be a helpful guide in calculating your vessels weight.

Braked or Un-braked ?

Trailers in Australia do not need their own brakes if the ATM is under 750 kg.

Trailers with an ATM between 750 kg and 2,000 kg need brakes on one axle.

Every trailer over 2 tonnes GTM must have a brake system which will cause immediate application of the trailer brakes in the event of the trailer becoming detached from the towing vehicle (breakaway brake). Under these circumstances the brakes must remain applied for at least 15 minutes.

Trailers over 2,000 kg need brakes on 4 axles and a system that can be operated from the vehicle and this system must be able to apply and hold the brakes on in the case of accidental or unintentional separation of the trailer from the towing vehicle.


It is very important that a boat is fitted to a trailer correctly.

An unbalanced load or a boat  incorrectly fitted to a trailer can have deadly consequences.

Please consider a Spitfire fitting.

You can check the procedure your outlet will follow.