Blocked Holding Tank

Rodding from below can work, although I don't envy anyone in UK water temperatures. On some boats it's possible to reach the outlet seacock from a tender alongside. If diving, and you prefer not to be dumped on (who wouldn't?), take down a tapered bung (pre-soaked so it doesn't expand to much) and shove it in the seacock once the blockage has cleared. Then discharge the rest at sea.

An alternative used in these parts on charter boats is to 'burp' the discharge hose: shove an inflatable pump into the discharge skin fitting, making a seal with a sponge wrapped round it, then pump: it'll usually disturb the blockage.
 
I used to have the same problem on a Bav30. Always solved by wrapping some tape around the end of my dinghy foot pump to make a tight fit when stuffed in the deck pump out fitting then with the sea cock open (of course) and someone blocking the breather vent on the side of the hull with a cloth, pressurise the system by pumping the foot pump. If it doesn't empty with a mighty whoosh! then try working the seacock open and closed a few times and if you have access to any pipe work try squeezing it a bit by hand.

I never leave the tank with any contents in it for any longer than necessary and when leaving the boat ie when going home, I always pour some environmentally friendly caravan holding tank fluid into the tank and leave the seacock closed.
 
What is it about murphy's law that means a 38mm diameter sanitary pipe will completely block - and withstand pressure, blocked only by a mixture of poo and paper applied wet. Yet the smallest undesired leak proves near impossible to block?
 
When I had a holding tank blockage last year the real culprit was limescale narrowing the outlet hose so that it blocked easily, and the final cure was to replace the hose.
 
I have had this problem on my B42. Always solved, as one post said by, rapping tape and a cloth round the dinghy pump hose, shoving it up the skin fitting and pumping. I did in fact use the tip of a fuel funnel taped to the hose. Fits better. Seals better.
Since the skin fitting is well under the boat you need to be in the water. HUM!! Not good in the Med with warm water just not acceptable in the UK. So my method now is to tape/tie the dinghy pump hose to one of the oars from the tender. Hop in the tender, jam the thing up the skin fitting and hold it there by using the dinghy tope as a fulcrum. This way you are in the dinghy when the blockage clears and you can have another go if it re-blocks.
I certainly would not try and pump the thing out by applying more pressure. This will/can put too much pressure on the tank. The inspection cap in the side will leak at best.
My experience has shown that blockages on my boat has mostly been the result of scale from the tank walls falling off partially blocking the outlet and then the final blockage as a result of what goes into the tank. All this is aggravated by the lower discharge hose scaling up. Always full of sea water and P***. The dam thing blocked 3 times last year so on the slip I replaced the hose. It was 50% blocked!! No trouble since. The other hose was clear. Nothing sits in it! Well only sea water after flushing.
 
Mine has blocked from time to time. I have a piece of dowel and after taking off the lid, poke the outlet. This always clears it at once with no fuss. The blockage is often caused, as already reported, by lime scale bridging the outlet. This lime scale is released when acid/vinegar is used to clear the pipe from the loo. It collects as a sediment in the bottom of the holding tank. The dowel is then cleaned in the wake!
 
It is easy to unblock a holding tank. Open the sea clock and use a common ol garden sink plunger from outside. It will clear after a few pumps and the presume is so low it will not cause any damage. All charter boats tanks are cleared regularly this way. Hence. Don't swim in Mediterranean harbours!
 
lenseman said:
So that's Turkey and also the San Blas Islands off your cruising grounds list then ?




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There are actually very few places where fitting a holding tank is compulsory. Legislation, if there is any, is usually written in terms of prohibition of discharge of waste in specific locations. There are alternate ways of complying with this requirement apart from fitting holding tanks. There are also very few locations where such restrictions apply to leisure vessels so it is not surprising that few people fit them, particularly given the cost and difficulty of retrofitting a tank on a small boat.
 
Thanks for all the replies.

In the end we connected a hose from a tap, and ran water through. We then put the open end down in the tank and quickly removed the hose from the tap end and put that end into the sea. The contents of the tank syphoned out slowly - although the hose got blocked a few times and we had to redo! Once the level was below the inspection plate on the holding tank, we opened the plate on the tank and then we could get inside (by reaching with an outstretched arm!) and poke with a sawn-off broom handle. A few pokes cleared the blockage at the bottom of the tank. Nasty work but job done with no mess inside the boat.

Next time I'll try to a plunger or dinghy pump from outside as has been suggested - but there was no way I would have gone in the water where we were on this occasion - very dirty water in a French marina.
 
Now you have it clear, give it a good wash out. Close the valve, add a generous dose of holding tank fluid and half fill it with fresh water. Let it soak for a while, then dump it. I hose ours out every few weeks - generally at the same time as filling the fresh water tanks - and it seems to stay free running.
 
Sorry to come into this late but I have had no end of "learning opportunities" with this. Some suggestions I would have made:
- I have made up a fitting that screws into the deck fitting that connects either an air or water supply to blow/flush the system - BE EXTREMELY CAREFUL TO NOT BLOW UP THE TANK / SKIN FITTINGS - mine is 12mm moulded polyester, 60 litres, and is quite robust;
- I have inserted, between the tank and the carbon filter on the vent, a Y valve and hose that I can either use to force air or water into the tank or poke down the self draining outlet to drain into the sea (when 3 miles out of the harbour of course) - SEE ABOVE WARNING. It is not a syphon but it does prevent overflowing into the carbon filter or into the bilge;
- if the tank and its various Y valves are not used frequently then plugs of dried solid form within the valves & pipe, in addition to the calcification. Hence exercise the valves/seacock(s) and empty the tank often. Always then add a fair bit of water to the tank, especially if being left for a while, so that the valves are filled with water and not left dry. I use at least a bucket of water, via a funnel into the deck fitting, after I've emptied it;
- I've had to empty a tank on the hard! A short length of hose of the right diameter, possibly with a little insulation tape wrapped around it to make it a tight fit, will fit nicely into the skin fitting - one person inside turning the seacock on and off, one outside transporting each load of material to the nearest WC - (it's not too bad really);
- the same "hose up the skin fitting" idea could be used to force air or water into the system when afloat (mine is only100mm below the water line);
- my greatest problem was a seacock that was never closing fully. In addition to slowly dripping polution where I didn't want to, it meant that I thought I could never empty the tank and spent hours devising ways to empty it (when in fact it was always empty without me knowing it). I now have a level minitor which indicates 1/3, 2/3, and (nearly) full. Nice in theory but only the last two levels are reliable.
Cheers,

Andrew
 
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