Definition
A mixing elbow is the fitting where raw cooling water is injected into the exhaust gas stream downstream of the engine, cooling the exhaust so it can be carried in rubber hose and silenced in water.
The mixing elbow does the job that makes a wet exhaust possible. Hot dry gas leaves the engine, cold seawater is injected, and what continues down the line is cool enough for rubber hose.
Why it is a consumable
It lives where hot exhaust gas, salt water and carbon meet — the harshest environment on the engine. Over years, the internal passage narrows with carbon and corrosion, raising back pressure, and the wall eventually thins and perforates.
Experienced owners of older engines treat elbow replacement as scheduled maintenance, not a repair. The failure modes are expensive: restriction that strangles the engine, or a perforation that lets water where it must not go.
What failure looks like
Gradual loss of maximum rpm, black smoke, rising temperature, rusty staining at the joint, or in the worst case water reaching a cylinder. In the tropics the clock runs faster — see Darwin and Cairns.