How do I install a marine toilet and holding tank on my boat?
I want to fit a proper sea toilet and holding tank on my 9-metre cruising yacht, which currently has a portable toilet. What size tank and hoses should I use, and how should I arrange the seacocks, tank vent and deck pump-out?
Answer
Mount the toilet on a solid base and pipe its discharge into a securely mounted, vented holding tank with a deck pump-out connection. Establish the loaded and heeled waterlines first, as these determine the siphon protection needed to stop seawater entering through the toilet plumbing.
Plan the layout
Check access beneath the toilet platform, space for the tank and the condition of existing seacocks. Draw every hose run, including the tank vent and pump-out line. Leave access to the toilet pump, joker valve, tank inspection opening and hose clips.
A straightforward arrangement runs from toilet discharge to tank, with a tank suction connection to the deck pump-out and a separate tank vent to the outside. This arrangement needs no overboard discharge connection, avoiding another underwater opening.
- Toilet: a manual model such as a Raritan PHII avoids electrical installation; an electric model needs a dedicated protected circuit.
- Tank: a purpose-made polyethylene sewage tank is a common choice. Around 40 to 60 litres is a starting point for a small cruiser, but calculate capacity from crew numbers, flush volume and days between pump-outs.
- Location: keep the tank close to the toilet where practical, with short hose runs and room to remove fittings.
Check the requirements for your cruising area before finalising the system. Holding-tank obligations can depend on boat dimensions and build year. Discharge restrictions vary between countries and waters.
Mount the toilet and tank
Position the toilet temporarily. Check seat clearance, comfort and room to operate a manual pump. The mounting surface must be flat and rigid; reinforce a thin moulded platform before drilling.
Raritan PHII and SeaEra installation instructions specify four mounting bolts or screws of at least 6 mm diameter. Use through-bolts with substantial washers or a backing plate where access permits. Seal exposed core around holes in a cored platform and tighten evenly to the manufacturer's instructions. There is no universal torque for different toilet bases.
Support the tank as its manufacturer specifies, usually on a smooth platform with restraints against movement in every direction. A full 60-litre tank holds roughly 60 kg of liquid, plus the weight of the tank itself. Keep straps clear of fittings and leave the inspection opening accessible.
Connect waste and vent hoses
Use low-permeation sanitation hose, such as Raritan SaniFlex, for sewage runs. Ordinary clear PVC hose soon causes odour problems.
- Connect the toilet discharge to the tank inlet. A manual PHII uses 38 mm internal-diameter discharge hose; some electric toilets accept 25 mm or 38 mm. Follow the instructions for your model.
- Connect the tank's bottom-reaching suction pipe or designated outlet to the deck waste fitting, commonly using 38 mm hose. Check that the fitting accepts the local pump-out equipment.
- Run a dedicated vent hose from the tank's top connection to an external vent fitting, clear of opening windows and air intakes. Use the bore specified by the tank manufacturer; do not reduce it to fit a smaller fitting.