Any idea what this is? Seems to be a breather. Connected to the fuel tank. Should it have a cap? What is the contemporary equivalent?

On a larger vessel the fuel tank breather is a goosenecked pipe with a gravity flap valve to keep it closed and not let t (too much ) water in.
The breather exits on deck. This is because when the tanks are being filled (fast) a lot of fuel is going in and a lot of pressure/fumes are being vented.

Your photo shows a fitting next to some kind of deck hatch so I am guessing that it is on deck or in the cockpit.
I can not imagine that this is a fuel vent/ breather. Could it be the water tank or grey water breather?
Either way it should have some hood covering it to prevent water ingress.
Old, worn out and needs to be replaced , whatever it is.
Something like this.


gary

Thanks Gary. I bought this and will see how it goes.