To reduce the possibility of an exhaust hose fire, the exhaust system must be
cooled effectively. For the same reason it's also advisable to cool your boat's
engine cylinders wherever possible.

In air-cooled engines, or where water is not passed through the exhaust
system, the exhaust pipe silencer and flanges must be well lagged or shielded.
This will reduce the risk of a fuel source being ignited by contact with a hot
surface and will also protect individuals from injuring themselves by touching a
hot metal exhaust. [2.23]