RE: perkins cooling system
I had a W30 Nomad w/Perkins 4-107 awhile back that had that arrangement. It had a gate valve in the line that went directly overboard, and I think the idea was you could close off the gate valve just enough until you had adequate cooling in the exhaust line without excessive backpressure.
Fast forward several boats later, I now have another W30 Nomad w/Perkins 4-107, but the overboard line is not there. There is a thru-hull where it used to go, but now all the water goes into the exhaust. The riser is now a simpler Vetus unit, so maybe that makes a difference, but I can't see that there is any problem with all the water going through the exhuast hose.