Exhaust gas leaves a diesel far too hot for a rubber hose. The mixing elbow, bolted to the exhaust manifold, sprays the engine's raw cooling water into the gas, and the mix runs out through the exhaust hose and the water lock.
That's a harsh place for metal: hot gas on one side, salt water on the other, and scale building where they meet. SAIL's Peter Nielsen calls the elbow a consumable and puts its average life at about five years or 700 hours. On a petrol inboard the same job is done by the exhaust riser.
What a surveyor checks
The elbow and its hose joint with the engine stopped and running: rust bleeding from the casting, and weeps or soot at the flange. The water flow at the exhaust outlet, where a weak stream points to a blocked elbow. The bilge under the engine for unexplained water. The service records for when it was last changed.
A leak is serious. It can let exhaust gas into the boat, with the carbon monoxide risk, or let seawater run back into the engine.
In the report
"Mixing elbow, rust bleeding at the casting, weep at the flange gasket, no record of replacement. Replace before extended running." See exhaust elbow corrosion.