a corrosion - resistant , thermally insulative riser elbow for a marine engine . the riser elbow has an exhaust gas conduit made of fiber reinforced ceramic matrix composite material formed from a polymer - derived ceramic resin in its ceramic state and fibers . employing a frcmc material results in a low - cost , light - weight , corrosion - resistant exhaust gas conduit not available with existing riser elbows . in addition , a frcmc exhaust gas conduit is thermally insulative and so more of the heat of the exhaust is retained rather than being transferred to the conduit . this allows the cooling requirements for the riser elbow to be lowered , while still maintaining a touch temperature below prescribed levels . frcmc can also be formed into practically any shape and size desired and can be very ductile thereby making the exhaust gas conduit fracture resistant and capable of withstanding thermally - induced strains typical of the environment of a marine engine &# 39 ; s exhaust system . the riser elbow also includes a water jacket sleeve formed integrally with and surrounding the exhaust gas conduit so as to define a cooling water channel between the outer wall of the exhaust gas conduit and the inner wall of the water jacket sleeve . preferably , a plastic material capable of withstanding long - term exposure to at least 250 degree fahrenheit cooling water , which can be salt water , without structural degradation is employed to form the water jacket sleeve . in addition , the plastic material should have a sufficient ductility to withstand handling and thermally - induced strains associated with the riser elbow of a marine engine .