A guide on how to properly install a boat tank

Level indication

It is important to fit some sort of level indication as an overfilled waste tank can be very messy indeed. The last thing you want is an exploding waste tank or waste coming out of the vent!

A high level alarm will bring a light on when the waste comes to about 70mm from the top of the tank. This will allow a few more toilet flushes before the tank needs emptying.

The next level of indication is a display with several lights or an analogue gauge. These two options give far more warning and indicate whether the tank is ¼, ½ or ¾ full.

If an electric toilet is fitted then it is possible to interlock the toilet control to the tank. When the tank is full the toilet will be disabled until the tank has been emptied.


Getting the waste out

Sea going boats will have two options. Either they can pump out overboard through the existing toilet seacock or they can have the tank pumped out by a shore-side pumpout station if a deck pumpout fitting has been installed. If a diverter/Y valve has been fitted then the toilet can be discharged to sea directly.

If the tank is above the waterline and is set up for gravity discharge, then it is a simple job to just open the seacock and let the waste out the tank.
A below waterline installation will have to be pumped out either with a manual 1 ½” pump or an electric diaphragm/macerator pump.

River boats will not have the option to discharge through a seacock as this is prohibited on the Inland Waterways. A visit to the local pumpout station is required.


How big should my tank be?

The industry rule of thumb is 2 gallons per person per day using a manual toilet.

An electric toilet will produce far less and a figure of ½ a gallon per person would be quite realistic. The space available for the tank will ultimately denote the size of tank you can fit.

If you have loads of space then fit a tank larger than what you have estimated as you will probably find you will need it. Bear in mind that if you have to pay for a pump out it will cost between £12 and £18 for which you get about 8 minutes at 25 litres/min.

If you can discharge to sea frequently then size of tank is not so much of an issue.


Fitting a holding tank

Where: A holding tank needs to be situated as close to the toilet as possible to avoid any long pipe runs. The longer the run and the more bends in it the less efficient the pump will be. Every 1 ½” elbow installed in the system reduces the pump discharge performance by approximately 900mm. Avoid pipe runs near heated areas, hot water systems or engine bays.


Above or below the waterline
Below waterline considerations:

– If the tank is on the waterline then treat it as below.
– Below waterline is slightly more complex and involves more pipe because of the extra vented loop.
– Hiding the pipe is often an issue.
– Tanks tend to be complex in shape and hence more expensive.
– Vented loops need to be 250mm (10″) above the waterline when the boat is heeled.
– Double clip all connections below the waterline.

Above waterline considerations:

Above waterline is more straight forward but does require a 2″ seacock to work properly. 1 ½” hose has a cross sectional area of 754mm², a 2″ diameter hose has a cross sectional area o 1385mm², almost double the area.

1 ½” can be used but be prepared to fit a pump at a later date.

Deck pumpouts should be positioned above the outlet so that the tank can be rodded through from the deck in the event of a blockage.