Unknown part

[mowa]

Hello, I am wondering for what is the little pipe connecting to the exhaust hose (marked on the picture). It looks very rusty.Must it be changed?

[Odysseus]

Exhaust sensed, will go to an alarm some place.

Odysseus

[IslandAlchemy]

Looks like you have a bit of seepage from it.

[Salty]

Yes, it's your exhaust temperature sensor, I used some black sealant around mine when it was installed, and also a couple of cable ties to secure it in place such that it doesn't leak.
Take care with this item because if it leaks sea water resulting in the salt crystals around the probe, it might also be leaking exhaust fumes as well, albeit at a fairly slow rate. But if it does leak fumes, the fumes will contain carbon monoxide, and that can kill you.

[mowa]

Good advise. Many thanks!!

[Spirit of Mary]

There is a plastic hose connected to it. On my boat BAV 38 2003 there is not an exaust temperature sensor installed. The hose connected to this nipple is leading to the anti syphoning valve located high in the engine room and installed in the seawater cooling circuit, downstream the seawater pump.