ABS - Acrylonitrile Butadiene Styrene

It's a common thermoplastic used to make light, rigid, molded products such as piping, musical instruments (most notably recorders and plastic clarinets), golf club heads (used for its good shock absorbance), automotive body parts, wheel covers, enclosures, protective head gear, vballs [reusable paintballs], and toys including LEGO bricks.

It was the original Hobie rudder, unfortunately, they tend to get brittle over time and break off right below the rudder casting.