Got a big boat and new to all this, need help figuring out what this tank leading to engine is for

Hello, hope you guys can help me because I can't find much information
I'm confused by this tank, you can fill it on top of the boat similar to how you'd fill fuel, the yellowing clear hose leads out to a small hole where I assume whatever fluid could go out, the big black hose says oil resistant on it, it leads to the engine, I checked inside the hose and there seemed to be water but it could have been rain water that leaked into the tank.

I don't see what the tank is for, there is a pump to bring in seawater in the bottom for cooling., so what purpose could this tank have? Contain oil? But I thought I'd fill that in the engine block. Hmm I dont know

Also does anyone know what engine model it is? The owner or people I bought the boat from have said "volvo d96" and "Volvo penta 175hp" but I can't really find one that looks exactly the same online yet.

Volvo penta
https://imgur.com/gallery/xc8HXmi

Thanks for any help would appreciate it

no clue as to the purpose of your tank...maybe try following the hose(s) to get a better idea what its for...my guess on the clear hose its for air purging.

May also be of benefit to post this to the volvo diesel area vs the gasoline powered section.

If it is a volvo (& the red paint suggests its possible) then it should have an ID plate on it...should also be a plate on the gear.

may be a KMD96A engine...made in the late 50's....parts are NLA from Volvo, so that could explain the custom exhaust manifold and the other 'hacks' seen in the pictures...