Loading, stability and buoyancy

Find out about loading and buoyancy here, including the types of flotation, Australian Builders Plate, overloading and tips to ensure stability.


Buoyancy is determined by the amount and placement of flotation. Almost all trailer boats have flotation, sealed air chambers or foam, to give support if the boat is swamped. How much flotation and where it is placed determine how effective the buoyancy will be.

  Types of flotation

Basic flotation

This is enough flotation to prevent the vessel and its maximum load from sinking when swamped. It does not necessarily support its passengers safely, it may float at any attitude and just give them something to cling to.

Level flotation

When swamped, a vessel with this flotation will float upright and level (unless it has been capsized), and support its maximum load and its designed complement of occupants. This allows the vessel to be bailed or pumped dry, and vastly improves the prospects of survival.

  Australian Builders Plate

Boats built after mid-2006 must carry the Australian Builders Plate (ABP), giving vital safety information to their owners, or to help prospective owners make buying decisions.

One item sometimes overlooked is motor weight. Makers are now quoting maximum outboard weight as well as maximum power. Other information on the ABP include:

  • Maximum number of people to be carried.
  • Maximum engine power and weight.
  • The boats buoyancy performance.

The ABP must be clearly visible on the vessel.

Find out more information about the Australian Builder's Plate.


If your boat does not have a plate or handbook recommending a maximum complement of people, use this table as a guide.

Maximum number of people to prevent overloading

Vessel length Maximum number of people
Less than 3.0 metres 2 people
3.0 metres to less than 3.5 metres 3 people
3.5 metres to less than 4.5 metres 4 people
4.5 metres to less than 5.0 metres 5 people
5.0 metres to less than 5.5 metres 6 people
5.5 metres to less than 6.0 metres 7 people


Stability is the measure of:

  • A vessels eagerness to return to the upright after being heeled.
  • How far it can safely heel.

It is affected by:

  • Total load.
  • How the load is distributed.
  • How securely stowed the load is.

Passengers are part of the load, and their movement can affect stability, especially in smaller boats.

Stability tips

  • Ensure that total load, including the number of people on board, is within the specifications of the boat.
  • Heavy items must be stowed low, and all items must be distributed so as not to cause a change in trim of the boat (not dip the stern or the bow).
  • No items may be stowed where they can shift with the vessel's motion. Scuba cylinders for instance, which are heavy and likely to move if unrestrained, have caused stability problems and outright damage.
  • Any gear it is not possible to stow securely must be restrained by straps or rope lashings.
  • Water in the vessel can endanger stability, both through increasing the total load on board and through a phenomenon called free surface effect. Water free to move around the vessel has an effect on stability out of all proportion to its quantity.

Image of boat from the water line

Page last updated: Fri Jan 31 2020 7:40:54 AM