The apparent complication of marine toilet installations today can have you reexamining the merits of a cedar bucket. Here is some help.
The least complicated configuration has the head's inlet connected to a 3/4-inch through-hull and the outlet to a 1 1/2-inch through-hull. But direct discharge of untreated sewage into the coastal and inland waters of the United States has been illegal since 1972. If your head discharge hose is connected directly and only to a through-hull fitting, you are long overdue for corrective measures.
A holding tank is the most common means of compliance with discharge laws. A holding tank has the advantage of being the only sewage handling method universally acceptable to all governing authorities, and it adds the least complication to sewage handling.
A holding tank does not require any change to the head's inlet connection. In fact, the inlet connection is the same for all head configurations detailed here. Use heavy-duty suction hose between the inlet fitting on the head and the through-hull; this need not be sanitation hose since sewage does not pass through it. Secure both connections with dual hose clamps.
If possible, the inlet through-hull should be on the opposite side of the keel from the discharge. If they must be on the same side, the inlet should be forward of and as far from the discharge as practical. A head inlet should not share a through-hull with any other inlet connection, but it can be teed off a discharge line — a sink or cockpit drain, for example — provided they are forward or on the opposite side of the keel from the head discharge.
If your boat never leaves inland or coastal waters, connecting the head directly to the tank is your only legal option. The tank itself will require two additional connections, one to a pump-out fitting on deck and the other to an outside vent fitting to prevent a build-up of explosive gas inside the tank.
All three of these hoses must be sanitation hose to resist gas permeation. Using the wrong hose on the discharge side of the toilet is the primary cause of smelly head compartments. Check existing hoses by wiping them with a clean, damp rag, then sniffing the rag. If it stinks, replace the hose.
Beyond the three mile limit, direct discharge is legal, and unless an offshore boat is equipped with an onboard treatment system, it is also unavoidable. Happily, the oceans are more than capable of handling the excrement of blue-water sailors without ill effect.
If you use your boat offshore, you will want the ability to pump sewage overboard rather than retaining it. This is most easily accomplished with a Y-valve in the head discharge line. Sewage can be directed into the holding tank or overboard via a through-hull fitting. Inside the three-mile limit this Y-valve must be locked in the holding-tank position or you will be subject to a substantial fine.
If your head is mounted below the waterline, or if it moves below when the boat heels, you must have a vented loop in a discharge line that connects to a through-hull fitting. Otherwise, if the head's internal valves are held open by debris — an inevitable occurrence — water will siphon back into the boat. Poor head installations sink boats every year.
Mount the vented loop so it will remain above the waterline at all heel angles. Clean the anti-siphon valve regularly to keep it functioning.
A vented loop in the inlet line can interfere with the proper functioning of the head, and its omission poses less risk because of the positive-action valve on the inlet side of the head. But if you leave the loop out, you must keep the inlet valve in good working order. A screen filter to exclude grass and other debris is highly recommended. If you want a vented loop on the inlet side, install it in the hose between the pump and the bowl.
Putting the Y-valve after the tank instead of before it allows you to empty the tank offshore when a pump-out station is unavailable. It also necessitates an additional pump, one designed to handle sewage.
Before or After
It is possible to combine these last two systems into one that provides total flexibility for the varying circumstances you encounter. With a Y-valve before the tank and one after, you can direct head discharge into the holding tank or directly overboard. And you can empty the tank at a pump-out station or offshore. A simple Y-connector allows both overboard discharge hoses to share the same through-hull fitting.
A treatment system offers nearly the same plumbing simplicity as direct discharge. Connect the head's discharge hose to the inlet side of the treatment unit, and connect the outlet side to the discharge through-hull.
Some onboard treatment systems do a better job than municipal sewage plants, but despite this a number of state governments are moving toward declaring all state waters a No-Discharge Zone. It has already happened in Rhode Island. So if your boating takes you into an NDZ, you will still need a holding tank. Or a cedar bucket.