What material forms the core of most traditional modern polyurethane surfboards?

Polyurethane foam forms the core of most traditional modern polyurethane surfboards.

A conventional polyurethane surfboard has a shaped foam blank covered with fiberglass cloth and polyester resin. The foam provides a lightweight, buoyant structure, while the fiberglass and resin create the hard outer shell that gives the board strength and a smooth riding surface.

Shapers usually add a wooden stringer along the center of the blank. The stringer helps reinforce the board and gives the shaper a reference line for symmetry. This construction became especially influential during the shortboard revolution of the 1960s.

Expanded polystyrene is another major surfboard material, normally paired with epoxy resin. EPS boards are not the same construction as traditional polyurethane-and-polyester boards, although both types can produce high-performance surfboards.