Dehler 41DS holding tank
Some notes from John Walker on Syfruga
Not a nice problem to have!
It depends a bit on the vintage of the boat, is it an early one, with the toilet facing in board and washbasin aft, or a newer one when they re-arranged the heads swapping the position of the toilet and the basin? They did this, allegedly, to make servicing of the holding tank easier.
I have had a leak on the holding tank. This was self induced, when I introduced some bent wire to clear a blockage lying over the exit, and on withdrawing it punched a small hole in the bottom. I managed to fix this by cleaning the area, and applying epoxy and fibreglass ribbon over the spot. It has held for 7 years!
I have researched what would be necessary on an early model such as Fuga, and have a drawing of the tank if it would help. The tank is bolted to the panel forming the back of the saloon seating, in two places, and also to the shelf on which it is supported. This was done during build with no thought for subsequent service access. Basically, extracting the tank would require cutting away the panel (aft, in the saloon) extensively, from around the level of the battens on which the seat backs are supported, to well below the level of the seats. The seating panels would also need to be cut away towards the aft by about 30cms, and outboard to the edge of the tank, under the shelving at the back of the outboard seating. Access to the fixing forward is via a hole drilled in the aft end of the heads locker. Not to be undertaken lightly, but better than dismantling the entire heads area! (I have cut access holes from the under seat locker in the saloon into the bottom of the heads locker, to gain access to the holding tank pipe clips.)
We introduced a Y-valve on the exit from the toilet, diverting optionally to a conventional syphon loop up under the deck head, and then down to merge with the exit from the holding tank. The exit from the holding tank has a separate shut off valve to retain the contents. Therefore the holding tank can be bypassed completely, or used only on those occasions when it is necessary to hold on to the sewage. In this way the effective capacity (interval between dumping) is extended. This is not easy to achieve because of the very limited access, but might be at least a temporary solution if the holding tank facility is not required.