superstructures, in rotationally moulded polyethylene. The buoy is then (usually) filled with foam. A steel mould is required and the rotational moulding machine must be quite large but such machines are now commonly used to manufacture consumer products (dustbins, traffic cones, etc).
The polyethylene material may be linear or crosslinked. The linear material has the advantage that it can be melted and hence repaired by hot air welding.
Small buoys are produced at low cost but their life in all but the most sheltered waters will be limited. The problem of achieving a reliable mooring attachment is considerable and the resistance to high levels of impact is poor. These buoys are useful in sheltered river or marina situations or where a limited life is anticipated due to the buoys being regularly dragged from station by floating debris etc.
The polyethylene skin is difficult to repair and provides poor adhesion for conventional paints but specialised hot plastic spraying processes are available. When colour is moulded in pigments must be of the hlghest quality suitable for marine use.
The rotomoulding process is also used to manufacture buoyancy modules which are attached to a structural core which may be in the form of a buoy or a resilient beacon.
The manufacturing process has been developed in recent years to allow the production of large floats (3m diameter) with wall thickness in the order of l0mm which are now being used as the buoyancy element of substantial buoys. These floats maybe manufactured in four segments to simplify handling and transport and allow a reserve of buoyancy should one segment be damaged.
Disposal
A particular advantage of linear thermoplastics is that they can be recycled by melting, however it must be possible to separate any metal components from the plastic and remove any internal foam.
SMALL BUOYS
General
There is extensive and well established use of small plastic buoys by authorities who mark rivers and sheltered estuaries. Small would be around 1.2m diameter with the superstructure generally integral with the body and usually unlighted.
Construction
Basic unlighted buoys may be constructed with a moulded-in mooring eye while at the top of this size range a central steel member will be incorporated within the buoy to carry the mooring eye and a lifting eye. The buoy will be manufactured from self coloured material and maintenance will be limited to cleaning. It was noted by several authorities who took part in the survey that the colour retention of some plastics was not as good as anticipated and
