June 1996 Buoyancy of small craft
Accidents involving small craft continue to claim lives. Analysis of accident reports has shown that people are unaware of the speed with which a small vessel can be swamped and sunk. Vessels have either sunk stern first immediately or have rolled over to float upside down, either situation providing no protection or flotation support for the crew.
Safe practice tips
Once the hull has lost the buoyancy provided by the surrounding water, the vessel sinks. Maritime New Zealand recommends fitting vessels with permanent buoyant material which will enable the hull to float even when it is full of water.
When the placement of buoyant material is calculated to balance the weight of the hull and engine, then the vessel will float upright, and the crew will have support and protection until rescue arrives. See the diagram below.
Owners of small craft should be aware that if buoyant material is fitted under the floor or under the seats of their vessel, this will help the hull to float, but will encourage it to capsize as the hull fills with water. A capsized hull offers no support to people in the water and no protection from hypothermia if they are in the water for an extended period.
Many vessels have been built with "buoyancy chambers" which are air spaces fitted with a bung for drainage. This arrangement has caused a number of deaths. When water leaks into these buoyancy chambers, it is not apparent to people onboard the craft, and can lead to the vessel sinking very quickly. Prudent boat owners should have these air spaces filled with expanding foam polystyrene, as well as blocks of buoyant material placed to support the craft upright if swamped.
New Zealand Standard NZS 5829 "Code of Practice for the Safe Design, Construction and use of Motor Powered Pleasure Boats" also addresses buoyancy issues and is a useful reference.
The flotation necessary to support the rated persons capacity must be installed symmetrically about the passenger carrying area as far from the centreline and as close to the gunwales as possible.
Original source content - Boat Notice 091996, June: Buoyancy of small craft.