Vessel loading, stability and buoyancy

How to improve stability and prevent overloading your vessel.

A vessel’s stability is affected by:

  • Total load.
  • How the load is distributed.
  • How the load is stowed.
  • Type of buoyancy and floatation.

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

Overloading your vessel can be dangerous and reduce its stability.

You can find the maximum number of passengers, load and engine power on your vessel’s Australian Builders Plate.

  Overloading guidelines

If your boat does not have an Australian Builders Plate or handbook that identifies the maximum number 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


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.


Marine and Safety Tasmania: Buoyancy in boats video

Video updated: Wednesday, 3 May 2023 10:11 AM

  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.

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 intended number of passengers. This allows the vessel to be bailed or pumped dry, and vastly improves the prospects of survival.

  How to improve stability

  • 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 to reduce any change in trim of the boat (not dip the stern or the bow).
  • Do not stow items where they can shift with the vessel's motion. For example, scuba cylinders are likely to move if unrestrained and can cause stability problems and damage.
  • Any items that cannot be stowed securely must be restrained by straps or rope lashings.
  • Minimise the amount of water in a vessel. Water in the vessel can endanger stability, both through increasing the total load on board and through a phenomenon called free surface effect.
Page last updated: Wed May 3 2023 10:12:04 AM