Its a vented loop
The hose running up the bulkhead is a vented loop and it should be between the toilet and the y-valve to the holding tank or thru hull. The vented loop is a safety feature not only keeping the holding tank from coming back into the head but also keeping the ocean (or lake or river) from coming back into the head. My understanding is that although they are unsightly, vented loops are pretty much the standard with any sea head. If you think of it, next time you are at the boat, check to see if the vent at the top of the loop is in fact venting. On my C27, the loop was vented but clogged shut so the vaccuum was never broken and the hose remained full all the time, more or less negating the purpose of the vented loop. Also, if you have a head that rinses with sea water, that hose should have a vented loop too between the pump and the toilet bowl. --Greg
