I'm installing a marine sanitation device type III (holding tank, then Y-valve, then (a) macerator pump to thru-hull, or (b) hose to pump-out deck plate) on my 1978 Catalina 30. The diagram from Catalina shows the hose going directly from the macerator pump to the thru-hull. Other diagrams that I've seen (such as the one on the macerator pump instructions) indicate the need for a vented loop above the water line, between the pump and thru-hull. Is the vented loop really necessary, if I don't open the thru-hull valve except when I'm pumping through it? (Besides, the "vented loop" sounds like a good way to get undesirable odors inside the cabin.)