Buoyancy

A prime function of a boat is to provide buoyancy to its occupants. A boat should float on the surface, even if flooded, swamped or capsized. The information on this page is provided to help you understand and calculate your buoyancy requirements.

A vessel is not fit for purpose if materials or items comprising part of the vessel's reserve buoyancy are missing or damaged. 

Ideally, the vessel should have level flotation – this means that it will stay upright even if full of water, and will support its own weight plus the motors and occupants.

The occupants of a swamped or flooded vessel will have a greatly improved chance of survival if it remains floating, upright and level, this being achieved by correctly sized and fitted internal buoyancy by:

  • Minimising the occupants' immersion in water, avoiding drowning and hypothermia
  • Providing access to the vessel's safety equipment, such as flares, radios, EPIRB or torch
  • Providing a larger target for searchers to see (a vessel is easier to see than people in the water)
  • Providing some shelter from wind and weather.

You can also download the information on this page as a handy PDF factsheet:

How to check the buoyancy of your boat

If your boat does not have an Australian Builder's Plate (ABP), or if the plate does not state that the boat has 'level flotation', it may not have sufficient buoyancy.

Level flotation means the boat will stay upright and support its own weight plus the motors and occupants, even if full of water.

Why is buoyancy important? 

Correctly sized and fitted internal buoyancy will keep the boat floating upright. This will help to reduce the occupants immersion in water and their chances of drowning or hypothermia provide the occupants with an opportunity to access their safety equipment, such as flares, radios, EPIRB, torch make it easier for searchers to see the boat than people in the water. 

It is also required by law for vessels to satisfy minimum buoyancy requirements. See Schedule 3 of the Marine Safety Regulations for more information..

Formulae for fitting buoyancy

If your boat has insufficient, damaged, or no reserve buoyancy, you should consider retro-fitting buoyancy. To determine how much additional buoyancy is required, you need to determine how much buoyancy you currently have. Below is a simple calculation to work out the minimum buoyancy your vessel will require. 

M = hull and deck mass

K = alum 0.62, GRP 0.375, Steel 0.87

F = mass of machinery and fittings

D = density of buoyancy material (foam approx 35kg/cubic metre).

Formula for an aluminium

GRP or steel vessel 1.2 x ((M x K) + F) divided by 1000 minus D

Formula for a timber vessel

1.2 x F divided by 1000 minus D

Example for an aluminium vessel

M = 425kg (hull and deck mass)K = alum 0.62F = 135kg (machinery/motor)D = 35kg/cubic metre 1.2 x (425 x 0.62 +135) divided by 1000 minus 35 

Amount of buoyancy required equals 0.496 cubic metres

In the case of boats with foam buoyancy, you need to measure the dimensions of the individual pieces of foam and multiply the length x width x height. 

Example for an aluminium vessel with foam buoyancy

Our aluminium vessel has a piece of foam measuring 750mm x 400mm x 350mm : 0.75 x 0.40 x 0.35 = 0.105 cubic metres.

Subtracting the current buoyancy from the required amount of buoyancy (0.496 cubic metres in our example) gives the amount of buoyancy you need to add, in this case 0.391 cubic metres. 

Important: If your vessel's hull has been constructed with a foam or balsa core you should not assume that this provides sufficient buoyancy. The core is fitted to provide internal strength not buoyancy. The material and its distribution may not contribute significantly to the vessels flotation and should be ignored for the purpose of this calculation.

Placement of buoyancy

Do not place all of the buoyancy in the bottom of the vessel. At least 50 percent of the foam should be placed in the sides of a boat (gunwales) and high in the hull so the foam crosses the water line and the boat remains stable if flooded.

Buoyancy should be distributed along the length of the boat in proportion to the distribution of weight in the vessel. For most small boats with outboard motors this will translate into these proportions: 50 percent aft (towards the stern or rear of the vessel), 25 percent midway, 25 percent in the bow.

Choice of material

The National Register of Compliant Equipment on the Australian Maritime Safety Authority (AMSA) website lists a range of accepted buoyancy products by brand and supplier. These include:

  • block foam - the most convenient way to fit buoyancy in a small boat
  • closed-cell foams
  • polyethylene and polyurethane foams. 

Non-marine foams are unsuitable because they:

  • react with metal hulls and cause corrosion
  • absorb water over time
  • may be flammable, or soluble in petrol. 

Foam blocks can be fixed:

  • under the gunwales and bench seats
  • to the underside of decking
  • against the face of the transom (the surface that forms the stern of a vessel). 

Foam should be firmly secured to prevent movement and stop it rubbing against the hull. This can create particles that block valves and pumps. If the boat is flooded, the buoyancy may also come loose. Wrap foam blocks in strong plastic sheeting to protect them and keep them dry. 

'Two pot' foam

This can be used to fill awkward spaces and provide excellent buoyancy. However, it should be installed by a qualified person. Be aware that:

  • pouring foam into a space you are unable to access makes it difficult to inspect or replace
  • it is important to accurately calculate quantities as, if used incorrectly, the foam can stress the hull and rupture joints. This occurs even if there are 'escape holes' as some foams expand in all directions. 

Foam buoyancy

Air cavities can admit water if welds give way or the structure becomes non-watertight. Cavities are best filled with closed-cell foam that is designed for marine use. Air-filled bladders are also an option.

Inspection of buoyant materials

You should inspect the buoyant materials every twelve months and replace any degraded materials.The following standards provide guidance about buoyancy for commercial vessels.

  • ISO 12217-1:2002 Stability and buoyancy assessment and categorisation Part 1: Non sailing boats exceeding 6m in length
  • ISO 12217-2:2002 Stability and buoyancy assessment and categorisation Part 2: Sailing boats exceeding 6m in length
  • ISO 12217-3:2002 Stability and buoyancy assessment and categorisation Part 3: Boats not exceeding 6m in length
  • Australian Standard AS1799: 2009 General requirements for small craft
  • American Yachting and Boating Federation standard

More information about the standards is available on the Australian Maritime Safety Authority (AMSA) website.