what are these pipes?

[Scare_Rab]

I'm delivering a 33' yacht ... and have, with difficultly, crawled around the stern to find these pipes. The two white pipes heading down and away are off to the mid-ship head. The big clear hose drops into the exhaust. There is a valve on the big clear hose, currently open.
Question, what is these arrangement / stack of pipes for / called?
When would I close that valve on the big clear pipe?
there is a currently closed valve , at the very top ...what is for?

Poor pics I know, sorry.

[Blue Stocking]

Looks like a manifold of anti-syphon loops.The big valve to prevent seas from being driven back up the exhaust hose.