Toilets, holding tanks and associated matters

These diagrams show the various valve positions that occur during the typical cycle of a sea toilet. There are many variations in design but the principle remains very similar to this. The Lavac is the exception, in which the pump is external to the toilet.
The seawater valves may be at the top of the piston in modern designs such as the RM69 Sealock and the Jabsco Twist’n lock. In other designs they are at the base of the piston. The foul water discharge valve is known as the ‘joker’ valve. Its design allows a column of water to be supported in the loop above it. If the toilet bowl gradually fills with foul water over a period of time it is likely that the joker valve has failed.

Toilet smells

Seawater samples For many years I have seen it suggested that the cause of smelly toilets is the degradation of seawater itself. That seawater, or the organisms within it, can break down sufficiently to produce serious hydrogen sulphide (H2S) smells seemed to me to be unlikely. Coupled with the knowledge that some toilets, especially Lavac, apparently never smell despite being flushed with seawater, it seemed to me that some investigation was required.

I saved four large drinks bottles for the research, two green and two clear. I filled one of each almost completely with seawater and the other two half full. The purpose of this was to assess the suggestion that anaerobic bacteria only develop after the air has been consumed by their aerobic counterparts. All bottles were then screwed tightly closed. The seawater source in each case was the South Ionian Sea, well away from land. I left the green bottles in a dark locker beneath the galley refrigerator and the two clear ones in the heads where they received a reasonable amount of daylight. The reason for the double test was another suggestion: that H2S is produced in clear tubing but not in black or opaque.

For the purposes of research, I left the bottles sealed for periods of 24, 48 and 96 hours. All bottles were shaken before opening. Assessment was done by nose, by myself and my wife Jill, in the saloon to avoid any possibility that the H2S could be dispersed by the wind outside.  At the end of each test the bottles were rinsed and the seawater was replaced. These were the results.

  • 24 hours – No smell from any bottle
  • 48 hours – The half full bottle kept in darkness smelt fishy, whereas no others smelt at all.
  • 96 hours – No smell of H2S from any bottle. Slight fishy smell from all.

I also filled another green bottle fully and left it in the engine bilge, where it heated to between 30 and 40 degrees C whenever the engine was run, every couple of days on average. After two weeks there was no detectable smell of H2S.

I conclude from this research that hydrogen sulphide in sea toilets does not occur by degradation of seawater. It seems far more likely to be due to contamination by human waste. The second part of my investigation looked at this.


RM69 toilet and wrap-around holding tank

rm 69 tank  RM69 toilet with wrap-around holding tank, installed in a Sadler 34. The heads compartment on this boat is quite small, making any toilet a fairly tight fit. Using the tapped holes in the toilet plinth it was not possible to install the tank. A shelf of 19 mm marine plywood was installed to carry the toilet and tank, epoxy coated to protect from water. It is just visible lower front, left and right.

Although the tank was acceptable for weekend and summer cruise use it succumbed to use during a full summer of nearly six months. The check valve in the 1.5 inch hose between the toilet and tank is poorly designed, using a rubber disc that distorted in service. The diverter valve is a very simple spectacle type sealed by an O-ring. Movement of the valve plate sometimes dragged the O-ring across the ports, requiring the valve to be dismantled to refit it. Pump-out using the toilet pump was tedious and led to high wear of the piston rod seal.

The wooden seat is not standard, cost almost as much as the toilet but had a high comfort value.


Custom made holding tank in Sadler 34

 A custom made tank by Tek-tanks replaced the RM69 wrap-around tank. It was possible to fit the tank into the roughly triangular space behind the toilet due to the plywood floor previously installed on top of the moulded plinth. This enabled the toilet to be moved about 10 cm forward. The tank is of gravity design, with all toilet discharge passing through it. Input to the tank is via a dip tube that reaches to about 1 inch above the tank bottom. In use the seacock is normally left open, closed in harbours and anchorages.

In the photograph the inspection cover can be seen top left and the discharge seacock lower left. The fitting top right is the vent, 3/4 inch bore.

Top hoses
Top hoses
 Hoses in the original locker above the toilet. The discharge from the toilet enters the sink cupboard and rises to the right of photograph, then falls to the tank entry left. The vent hose goes to a plastic skin fitting top right.
Outlet hose  The outlet from the tank connects to the Blakes seacock via a short length of 1.5 inch hose. The tank is supported by a plywood shelf resting on battens at each side of the compartment. The tank rests on the lower plywood shelf that was epoxied to protect it from the inevitable water spills. It was necessary to cut a slot for the valve handle.

The plastic skin fitting that forms the tank outlet is not standard but was installed to correct a mistake on the designer’s part (!)

Tank with cover  The tank hidden from view. The front plastic sheet drops into hardwood guides that were slotted using a router. The top cover with fiddle makes a useful shelf and is removable to access the tank.
Tank level gauge  Although the polyethylene material used for tank construction is opaque it is not possible to see the tank contents through the walls. Level gauges for these tanks are very expensive and may be unreliable in some cases. A simple and cheap alternative is to shine a torch through the top, enabling the contents to be seen readily.


Salt deposits

 After a period of use, particularly with the seacock closed and with liquid in the holding tank, salt deposits accumulate in the tank, hoses and seacock. Hydrochloric acid will react with the deposits, which are largely composed of magnesium carbonate, but a large volume would be required and the time taken to completely remove all deposits would be excessive. Mechanical methods are preferable. Cleaning the tank is relatively easy via the inspection hatch, using water at mains pressure and scraping with a piece of timber.  Removal of the deposits shown in the photograph requires a hammer and small chisel.
 Salts that accumulate in the discharge hose result from reaction between urine and seawater. This shows a region at the top of the loop, shown in the photo above inside the locker above the toilet. The salts are softer than those that gather long-term in the seacock and some owners are successful in removing them by rodding through with something flexible such as an old rigging wire. If the hose can be removed from the boat the best way to empty it is to flog the hose against a wall although some owners have had success hammering and squeezing the hose in situ. In my case I found it necessary to remove the hose, a difficult job on this boat.  Fortunately the local chandler stocked similar hose and I replaced it.