Blocked discharge pipe/ hull fitting from holding tank

[BelleSerene]

My holding tank is half full, despite the tap on the hull fitting being set to 'open'. It's a plastic tap, so I'm led to believe it's unlikely that the valve inside has separated from the tap (is this right?). I know there has been some calcium in the pipe from the heads to the tank inlet, so it's quite possible that this, or less pleasant substances, have blocked the pipe from the tank to the hull. It should be fed by gravity, I understand.

Sensible actions would seem to be to stuff a freshwater hose up the skin outlet (either in a dry suit in the water or else on mooring piles) or to hoist the whole boat out (time-consuming and expensive). It might be sensible first to pump out the holding tank with a vacuum pump at a major marina before attempting any of these, to avoid ending up in a pile of s**t so to speak.

Can anyone advise me?

 

[jonjo]

Can anyone advise me?

Yes.

The plastic seacock concerns me but otherwise you need to first siphon the current tank level down if it is more than (guess) one third full when you tackle the problem. I used an electric dinghy pump in reverse to start the siphon.

Then I was able to reach under the waterline and insert a large bore flexible pipe into the exit seacock. With the dinghy pump I used air pressure to blast the tank blockage away.

There is a more complete description of the process posted by me in the PBO forum. It might take a few blast/flush/blast cycles to completely clear the blockage.

 

[Networks]

Your idea of using air pressure to blast the blockage away will work fine if the holding tank outlet does not have a non-return valve in-line. With holding tanks that are not above the waterline they are likely to have a non-return valve to prevent back filling of the holding tank by sea water. In this case you cant "blast the tank blockage away".

 

[AndyPandy]

I pump out the tank until the level is below that of the inspection hatch. Then undo the hatch and find the exit pipe from the inside. Feed an old smelly hose pipe I keep for this job as far down the exit pipe as possible, then turn on the tap, bingo.

You should see loads of bits of bog roll, calcium deposits etc shooting out from under the boat.

I recon leaving the seacock open is a bad move as things find their way up from the outside and live in your pipes, ugh!

Hope this helps,

Andy

 

[snowleopard]

It's a plastic tap

My advice would be to swap it for a proper bronze seacock at the earliest opportunity. There have been cases of boats sinking as a result of a plastic skin fitting receiving a quite modest blow and breaking off. IMO plastic skin fittings have no place below or close to the waterline.

 

[mocruising]

unblocking

I have this problem on my aft holding tank sometimes it was made worse when we used the quilted loo paper. I swim under the boat with an old length of semi rigid hose and push it up the skin fitting. You have time to get clear and climb out sharpish.