Greig, here are a couple photos of the Y-Valve on my Mk II. Your boat ought to be similar in arrangement in this underseat compartment, I would think. This is the forwardmost portside compartment in the salon.

Just remember, when you're in US waters not only do the Y-Valve thruhull and the macerator thruhull need to be closed, they also need to be "secured" via a suitable device. I use large plastic tie-wraps for this purpose (not seen on the photos below). I have a Canadian friend who was busted by the USCG for not having secured the thruhulls, even though they were closed.

Greig,

My antisyphoning loop is of course on the port side inside that little compartment behind the cabinet. Can you not locate yours?? If it is missing it could be precarious!

The anti siphon valve on my 1989 is located on the port side along the inside wall of the hull- you have to open the cabinet door and lift up a panel that is designed for that purpose. You will also see the hose that is used to pump out.

Greig, there is no antisiphon in the sewage outflow hose, the large hose. That hose connects straight from the head to the Y-Valve.

I stongly recommend that, while you're doing surgery in the head system, you replace the black rubber hose with a higher quality white plastic hose. The black hose can/will develop microscopic 'weeps' that will be an incurable source of odor. My photos were taken before I learned this lesson the hard way, and the entire boat smells so much fresher now that I have changed the head output rubber hose to plastic hose.

Well, there is certainly a risk.

More of a risk if the Y-Valve connects the head directly to the outside. Certainly enough risk that you would want to monitor the head water level underway, perhaps once a watch, etc.

Less of a risk if the Y-Valve connects the head to the holding tank, and from the holding tank, through the macerator, to the macerator thru-hull. This is because the macerator thru-hull (on the Mk II...not sure about your boat) is opened only for the half minute or so it takes to macerate, and then is immediately closed. If you don't close the macerator thru-hull, the outside sea water will back-fill a large portion of the holding tank, thus destroying the potential to hold further effluent. The holding tank is located below sea level. I installed a macerator on/off switch near the macerator thru-hull to allow me to perform this action in a timely manner.

The joker valve should be replaced every few years as it prevents backflow from the holding tank and lines into the toilet. At this point, the only way to get a joker valve for Wilcox/Thetford HeadMate toilet is to order the rebuild kit. I have found out the the red Raritan C253 joker valve is a viable substitute and can be acquired separately.

[QUOTE=LCBrandt;4791]Less of a risk if the Y-Valve connects the head to the holding tank, and from the holding tank, through the macerator, to the macerator thru-hull. This is because the macerator thru-hull (on the Mk II...not sure about your boat) is opened only for the half minute or so it takes to macerate, and then is immediately closed. If you don't close the macerator thru-hull, the outside sea water will back-fill a large portion of the holding tank, thus destroying the potential to hold further effluent. The holding tank is located below sea level. I installed a macerator on/off switch near the macerator thru-hull to allow me to perform this action in a timely manner.[/QUOTE]

Larry's right.

There are two basic design philosophies about overboard discharge when you already have a holding tank. Your options are:

1. Everything goes through the holding tank. This precludes going directly overboard when offshore. It is the "preferred method" from WM Advisors, here: [url]http://www.westmarine.com/webapp/wcs/stores/servlet/WestAdvisorView?lang...

2. Go to EITHER to the HT as in #1, or directly overboard. This is what I prefer, since if you're gonna be "allowed" to do so, why go through the HT first?

Either way, a Y valve or judicious plumbing and piping, or with piping/hoses to a replaceable gate valve (Oh, NO, did I say that?!?) BALL VALVE, would work just as well.

PS - We have a gate valve that the PO installed, and have been "forced" to replace the "gate" in the valve a few times by un-screwing the bonnet and installing a new gate from the hardware store. Not so hard. I tried to install a plastic ball valve (1 1/2") a few years ago, but it wouldn't fit in our space - three hours of pure joy!) :)

Look at YOUR sanitation SYSTEM and decide what's right for you.