Bulkheads play a greater part in a plastic hull than they do in wood or metel boats. Apart from separating compartments, they are essential in providing transverse rigidity necessary for maintaining from in the comparatively flexible FRP hull. A collision bulkheads should be installed forward and machinery space bulkheads should be watertight. Other bulkheads may replace strongbeams and be required to support masts or deck loads. Exterior grade plywood is usually used for bulkheads and should be fitted while the hull is still in the mould. The plywood should have asmall boats the thickness of the plywood is normally adequate for stiffness but some pillaring may need to be added in larger boats. All openings in bulkheads may be used but these will be more expensive. Bulkheads can be buit as part of an interior moulding to gain a moulded gelcoat finish or resin painted (flowcoat) if the visible face shows the reinforcement such as when the core is made from plywood sheets.