I have been giving thought to how to instal a holding tank to Calista. Partly driven by requirements for the next trip to the Med (scheduled for 2013 now), and partly to make life easier when dried out on beaches.



So the issues: Most Mediteranean countries require visiting yachts to have and use holding tanks. Discharging efluent or “black water” in the jargon being punishable by heavy fines. It is also difficult when we visit any of those nice places where we dry out, Rock, Solva, Sandyhaven and Tenby being specific places where we dry out on sand, and people can walk around the boat when she is aground. There is also the issue of trailing long distances, when we use Calista as  a caravan.


Potential solutions: I have talked to a number of other owners about fitting a chemical toilet into the bows in place of the nowmal marine toilet. I have also fitted holding tanks to a previous boat, of the same size.


Pros and cons: The chemical toilet seems the easiest way to solve the problem. It is particularly useful when on the trailer because there is flushing water as well as somewhere to store the “black water”. However it means lifting the upper section off, and releasing the tank, to carry it through the boat and ashore. There is always the danger of dropping it either inside or on the way down to the ground. I am particularly concerned about how difficult it might be when afloat. I rarely use marinas, and am likely to want to empty it while at sea. That might be difficult to actually do while being thrown around. Having explored the different models available it seems that one with a 10 litre flushing tank and 10 litre holding tank would physically go into the space available. The problem is that to fit it I would have to remove and fill the holes left by the sea cocks. I would have prefered to be able to try it for a year and be able to replace the “Lavac” if I wanted to.


At first sight the holding tank seems more difficult. However there is room for one of at least 45 litres in the bows above and forward of the toilet. That has the advantage that it is above the waterline and should empty by gravity. That removes the need for a second pump. In the last boat I added a second tank low down for flushing water. It filled when the boat was afloat and provided water to flush the toilet when aground. It just needed the inlet hose to enter at the top of the tank and the outlet to the toilet at the bottom so that it stayed full when dried out. It did work. There is room for a reasonably sized tank in front of the toilet, and below the holding tank. This would solve the problems when using drying anchorages, and allow discharge at sea. With a pump out facility it would also be OK on inland waterways. I think that motor homes discharge tanks into a sort of concrete funnel which they drive over. If that is correct then that would work as well – just open the sea cock when over the “funnel” and use a hose to wash it all down afterwards!


I have made some tentative measurements and am starting to plan to make special tanks to fit made from epoxy resin and glass cloth. I was thinking of making the spiggots from the same materials formed around cardboard formers. I think bought tanks wont really fit, and will be expensive. I have some ideas to minimise the plumbing and mount the pump onto the lid of the tank. I thought of incorporating mounting flanges and other details as well. The tanks I made before were made from woven rovings and polyester resin. So I know that lould work.

So the idea would be that the sea water went from the inlet seacock to the inlet tank, then from that tank to the inlet to the toilet. The outlet pipe from the toilet would pass through the holding tank to an elbow on the top where it would enter the pump, and then from the pump into the top of the holding tank. An outlet would go from the bottom of the tank to the outlet seacock. There would be a pump out fitting in the deck  with a pipe to the top of the holding tank with an extended spigot ending near the bottom of the tank, over the outlet spigot (so it can be rodded through if needed). A vent would go from the top of the holding tank to the top of the hull side near the bows.


Comments and ideas invited – preferably before I make any decisions!

8 thoughts on “Holding Tanks

  1. The installation of the holding tank is now completed.



    It has all been painted, the toilet is fitted on a new bracket, and all the pipework is in place. I used new piping because the old pipes were not made from a vapour proof material – and were therefor not suitable for use on a holding tank system. I have not fitted a tank for the inlet water – that can wait. Perhaps its a job for next winter.



    You can see the pipework in detail above. The pump itself  is well out of the way and the pump out connection is on the left – going up under the deck. The filter is installed through the shelf with the breather outlet going up under the deck as well. The only diffeence in use is that the handle for the pump cant be left in place all the time – you can just see it mounted on brackets on the front of the locker on the right.



    Finally the foredeck. Calista is fitted with an anchor winch, and the fairleads to direct the chain to it are those brackets on the left of the picture. Above the forwardmost of these fairleads you can see the pump out fitting. I have added a screw in the lid – so that I will be able to tie the lid to the boat. It seems inevitable that it will go over the side at some time! The breather pipe ends in a plastic skin fitting under the ventilator. I  could have fitted it through the topsides, but that would have been another unsightly skin fitting visible  in the hull. Putting it through the deck, and using a surplus ventilator to stop water going down it seemed like a good idea. It is a bit cluttered up there now – but I never stand up there anyway – it is much easier to stand on the cover over the toilet and work safely supported at waist level. I am not really convinced that a pulpit is needed on these boats. I suppose it is something to hold the ends of the guardrail wires if nothing else!


    So Calista is now ready to go into the water – and in good time for the “Easter Cruise”



  2. Another update on progress:


    After lots of fiddly bits which had to be left to cure before the next bit could be added, the tank is ready to fit.


    Here are all the pipe spiggots epoxied into place, and flanges added to fix the tank into position. Ply pads had yet to be added to reinforce the top and bottom, and to take the pump when the photo above was taken. Then it was ready to fit permanently:


    You can see the flange on top, with a cuttout to go round the knee supporting the shelf, and a flange on each side. You can also see the mounts for the pump, with the studs sticking up. There is a spiggot on the bottom to empty the tank through the existing skin fitting on the left, and on the right a pipe passes right through the tank, the bottom end connects to the toilet, the top to the pump inlet on the right hand side of the pump. The spiggot on the left of the pump conects the pump outlet to the tank. On top of the tank on the extreme right is the connection for the breather, and on the extreme left the connection for the deck pumpout fitting.


    One of the reinforcing strips is also fitted to the front of the tank. It also is a mounting for the hinge which supports the front of the fold up lid over the toilet. For anyone not familiar with an Atalanta this lid allows you to stand in the forehatch to anchor or adjust sails etc.


    This last photo shows the lid fitted and folded up, and the pump mounted. New brackets have also been fitted to take the platform on which the toilet sits. We are making progress!


  3. A progress report on the holding tank:


    The focsl with the toilet removed and ready to paint:

    The tank will fit up against the stem, and just touch the bracket supporting the shelf.

    Mould made from offcuts of foam insulation to fit in the sapce in the very bows:


    Laid up with 6 layers of 200 g/sqm glass cloth. I have found that it works better with some filler powder in the epoxy to prevent the resin running out of the glass. Just enough to make it “gloopy”


    I made a lid seperately on another flat offcut of foam. A fillet was added around the edge of the tank to form a landing for the lid to sit onto before dismantling the mould. The lid bonded on with filled epoxy:


    The lid was cleaned up and the joint reinforced with a couple of layers of glass tape. The tank was airtight at this stage – I know because the lid was moving like a bellows as the workshop cooled down and warmed up. I made a small hole in it to equalise the pressure! It is upright with the pump perched on top to show where it will fit, and allow me to mark where the various holes have to go in the top. Note that the “lid” was actually the front. There will be a number of reinforcing strips accross the front, top and bottom, which will stiffen the panels – particularly the front:


    You can see that the tank is not a regular shape – it really is made to fit into the space!


    Todays job was to make up the “spiggots” which will allow pipes to be connected to the tank. Two long pipes to pass right through it, and two short ones. Made from strips of glass cloth wound round and round plastic pipe. 1 1/4 inch waste pipe to be exact – until they are a good fit on the 1 1/2 inch pipes.


    I have also ordered the carbon filter for the breather, and the deck pumpout fitting. By the way the breather has to be the same size as the outlet pipes. 






  4. Well I have made a decision! I almost went for the chemical toilet, but decided to fit the holding tank in the end. So I have ordered and recieved the materials for making the tanks, and have made a mould for the first tank.

    While I was making my mind up I have removed the toilet and all the fittings, and sanded down the woodwork behind where the tank was ready  to paint it all. The Lavac (as fitted to Calista) only has one pump, on the outlet and sucks flushing water in by creating a vacuum when the seat is down. The pump is just a bilge pump. So I have stripped and cleaned the pump and checked the rubber diaphram and valves. Surprisingly they were all in good condition so I have just reasembled it all.

    I will start to make the holding tank tomorow.

  5. I asuppose I will have to get a quote for a welded plastics tank. I have to admit that I was not looking forward to making the tanks – it is a messy horrible job.


    Another issue I have to consider is weight. It is all going to add up – and a portable chemical toilet might be much lighter.

  6. The idea for the flushing tank and holding tank sounds good Trevor. I was looking at a boat yesterday that had a composting toilet!! wouldnt really work on your boat I dont think! (it was on a 50′ trawler conversion) Re making the tanks out of FRP have you checked out welded plastic tanks?. A friend in Devon had a couple of custom tanks made and the cost was very reasonable. He made some mock ups of the tanks in cardboard to the shape of the space  (in this case it was in the bilges) and sent these in to the plastic welders to use as templates. It would be less messy than making FRP ones anyway!  

  7. When we went up the Thames after removing the Simpson Lawrence, I placed plastic tubes over the outlet and inlet with bungs in the ends, each secured with Jubilee Clips, then I made a small removeable platform and stood the Chemical Toilet (Thetford 365) on top.  For use at sea it would need further holding down but the removal of a full tank has no danger of spilling as a seal slides over the access hole and the discharge pipe has a screw cap on it.  For your use you would require a larger holding tank and your ideas appear to solve that problem.  Look forward to further news.  Colin

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.