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Black Water Tank Replacement

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Category: Hints & Tips
Forum Name: 385/388
Forum Description: 385/388 Hints, Tips and News
Printed Date: 01 June 2023 at 02:21
Software Version: Web Wiz Forums 12.06 -

Topic: Black Water Tank Replacement
Posted By: KNick
Subject: Black Water Tank Replacement
Date Posted: 22 September 2022 at 10:41
Hello to everyone,

I have a HANSE 385 2014 model.

Just received a new black water tank for the toilet from HANSE BOATOON website. Planning to replace it next week.

Previous one needs replacement as it blocks quite often.

There is a metal ring attached around the tank placed by the shipyard.

So my questions to anyone who might have replaced his are:


B) If this metal ring is also attached on the inside part of the hull, how the tank is removed? Cause it does not seem to have enough space to lift the tank further up from the rink perimeter and remove it before placing the new one.

Any thoughts / piece of advice would be highly appreciated!

Best regards,


Posted By: Jonw
Date Posted: 20 February 2023 at 16:13
Hi Nikos - I also have a 385, with a v slow draining holding tank.

The boat yard said on many other types of boats there is usually a 2inch / 50mm outlet from the holding tank, and out through the hull, and that mine has a 1.5" pipe from the tank, reducing to a 1.25" pipe through the hull, and that therefore it is susceptible to blockages / slow draining.

He also said it may be a tricky job to replace the holding tank, because of the covering over it.

May I ask - how did you / are you getting on with replacing your holding tank? Any tips / tricks? Would you do it again knowing what you know now..?

Many thanks


Posted By: is46664
Date Posted: 12 March 2023 at 23:25
The holding tank on my 385 has a long history of blockages. 

I noticed the holding tank as fitted slopes slightly and therefore ‘drains’ right to left. However the hose connected to the ‘drain’ side was coming from the toilet pump, whereas I expected  it to connect to the seacock outlet. What is happening is that after pumping sludge is draining back and blocking the tank. 

So I checked the manual and it shows the ‘drain’ side should go to the seacock. It looks to me like when it was built the hoses where fitted the wrong way around. 

I’d suggest checking the manual diagram of the hose routing against what is fitted. 

Ian Stevenson

Posted By: Wild
Date Posted: 13 March 2023 at 09:54
Maybe this can help you.

Wild and Wet

Posted By: is46664
Date Posted: 13 March 2023 at 14:38

Ian Stevenson

Posted By: is46664
Date Posted: 13 March 2023 at 14:48

In this diagram from the manual I have indicated the layout of the black tank hoses as shown in the manual with a YELLOW dashed line. I have shown the layout of how they were fitted at new by a solid BLUE line. The RED line illustrates the direction of slope of the tank.  It’s easy to see how if the hoses are not fitted correctly it will cause the tank to ‘drain’ towards the inlet. 

The other diagram in this thread is a different tank design (Wurth) - this had a vertical riser which would prevent sludge drainage back to the inlet. This kind of design is I think the far better design. 

Ian Stevenson

Posted By: Martin&Rene
Date Posted: 13 March 2023 at 15:57
I thought all holding tanks were designed with a riser as shown in the diagram of the Wurth holding tank, otherwise, when the joker valve leaked (as it will do at some time) then all the contents of the holding tank would discharge back into the toilet and overflow.  Not a pretty thought.  In the standard layout, when the joker valve leaks, only the contents  of the pipe come back to the toilet.

One simple test.  Pump water into the holding tank.  If the water enters at the bottom, then there will be little noise.  If it enters through a riser, then you may hear the water fall from the top of the riser pipe. 

Martin&Rene Hanse 341 Dipper Wheel steering, 3 cabin layout, normally based in Scotland

Posted By: is46664
Date Posted: 13 March 2023 at 18:14
At first I thought there was no riser but you are correct the tank has a riser

Ian Stevenson

Posted By: Martin&Rene
Date Posted: 15 March 2023 at 17:42
As you say, it certainly does not look like  the best design of tank.  Just looked at one of my catalogues and all their plastic holding tanks have a top inlet rather than using an internal riser pipe.

I do not know whether a procedure change may help the situation.  If the tank is full, I try to delay emptying it until we are in choppy water.  I pump the toilet at least 15 times, then open the seacock to empty the tank and keep on pumping the toilet until I think the tank is empty.  The aim is to stir up the contents of the tank as much as possible, so that there is minimal sludge left in the tank at the end.

On my boat, I also have a bottle of the toilet chemicals that the caravan owners use to liquify the waste in their cassettes." rel="nofollow -

So far I have never needed to use it.  It could be something you could try using on a regular basis to stop the blocking.

Final point, are you sure everybody abides by the toilet rules.  I have very strong (smelly) memories of being in Turkey with the temperature over 40deg C and having to take the plumbing totally apart on a Halberg Rassy, because the owner's son had forgot to tell his girl-friend not to put sanitary products down the loo. 

Jon   I would send KNick a private message.

Martin&Rene Hanse 341 Dipper Wheel steering, 3 cabin layout, normally based in Scotland

Posted By: Jonw
Date Posted: 16 March 2023 at 17:16
Thanks, this is all really useful.

For info, I've checked from outside the hull that there is no blockage, I setting an implement which went in easily.

And I've checked there's no vacuum (there is not)

So I think my next step is to remove and replace the pipe from the tank to the sea cock.

I'll also check which side this comes out from!

Thank you for all of your help


Posted By: Jonw
Date Posted: 04 May 2023 at 13:45
Hi All - a short update on this (for info)

(My 385's problem was the holding tank not draining / draining v v v slowly)

I disconnected the 'exit pipe' which runs from the holding tank to the seacock, from the seacock end.
(I couldn't get access to the jubilee clips on the end at the actual holding tank to undo the other end)

The pipe itself was clear, no scale, no blockages, I rodded it to make sure.

I then wondered if there was scale (calcium carbonate) inside the holding tank.

So I poured in some hydrochloric acid (patio cleaner) and it 'fizzed' incredibly, indicating it was having a strong reaction with something in the tank, probably calcium carbonate. 

I attach a photo.

The level of sea water in the tank is marked by the (low down) blue dots.

The 'fizzing' is marked by the higher up black dots. At times the fizzing reached the metal band and above.

I repeated this process a number of times, until there was no more fizzing when I poured in the hydrochloric acid.

At that point, the tank would drain incredibly fast, and (so far!) reliably!

We've also just had a long weekend on the boat, with multiple people on board utilising the facilities, and all has so far behaved very well, and the tank continues to drain v v fast when set to empty.

Also (for info) when 'rodding' the pipe, it appears that there may be an 'elbow' just inside the holding tank, like a 90 degree turn. (my conclusion based on rodding)

I hope this info may help others in time.

In other news, we had a very lovely few days sailing around the Solent, in light airs, which were just perfect for a shakedown sail / first outing of the season.

Wishing you all happy sailing.


Posted By: Ian2019
Date Posted: 06 May 2023 at 04:27
There is a small wire mesh filter in the side hull vent to the holding tank and the filter gets blocked, therefore not allowing air into the tank when draining so a slow drain. I took mine out ages ago and have had no problems.

H385 #317 2014 Tivat Montenegro

Posted By: Keakaka
Date Posted: 06 May 2023 at 09:15
I found it is useful to pour 5 litres of water into the deck connection when dumping the tank contents. The extra pressure pushes some of the blockage material out through the seacock. Guests always use too much paper and not enough water. 😥

This is not a rehearsal. Go for it. Kiakaha

Posted By: Wild
Date Posted: 07 May 2023 at 07:57
After to much issues with Wurth blackwater tanks I made a manhole controller hole just on top of the riser tube so I can clean the tank easy and make a SS tool (you can see it in front of te tank) to clean the rising tube from above. The rising tube is the most critical of the tank. There’s always black water in this tube (tank empty our not) And in a warm environment and salty waters this tube get easily blocked with salt- and urinecristals.
The filter in the ventilation cap is stupid. Throw it away.

Wild and Wet

Posted By: Mark&Catherine
Date Posted: 27 May 2023 at 08:54
I can’t empty my tank now (385), and I can’t see how to change the pipe, I can’t reach the jubilee clips on the hose at the bottom of the tank. Has anyone managed to change the pipe ? 

385 ubulukutu sail number GBR 3350L in Turkey and Greece with Mark and Catherine

Posted By: MasterKeil
Date Posted: 27 May 2023 at 17:28
Thats it Ian2019. When my waste tank was depleting slowly, I found this wire cage blocked by bugs and debris. After cleaning the mesh the depleting rate is back to normal.

Hanse 370#640 "Nemesis"

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