Holding Tank below the Loo

[Wideocean7]

Hi All

Some time ago I removed the corroded holding tank, at the time thinking of just replacing it. But that space, beneath the for’d berth would be extremely useful for storage, which is in short supply on my yacht. Trying to preserve that space has been a key driver in my search for other loo options. So after much research, I have narrowed my options to two. Option 1 is keeping the status quo (and “losing” that storage). Option 2 is refitting the head with a holding tank beneath the loo. I welcome any constructive feedback from those who might have experience of Option 2. All I ask is that any comments are kept to these options. I have been through other options extensively including composting and gravity and don’t want to enter a debate on those. I have had the fortune to have discussed my options widely with Peggy Hall who has been an incredible help as has been her book.

So, Option 2. I’ve attached a photo of the head where I would install a holding tank beneath the drop-loo (apologies it's on its' side as I don't know how to rotate it). Doing so would need comprehensive surgery to the fit-out but I’m happy to do that as the existing fit-out is pretty dated. To avoid raising the toilet too high, a tank of only around 40L could be accommodated. However, from what I have learned, such a loo discharges less than half the volume of a standard holding tank, primarily because the latter uses more flushing water. Once framed in and boarded over, a drop-toilet with flush mechanism would sit on top. The tank would need to be fabricated (I am leaning towards poly) and fitted with ports for discharge, pump-out and vent, much the same as a ‘normal” holding tank arrangement.

I guess I should add what boat the system will be needed for. Mine is a 37ft sailboat which is exposed to long periods of rough water sailing. Especially being for’d, the tank contents would be exposed to a lot of sloshing around. A main concern I have of Option 2 is that the effluent slops out of the toilet gate when someone is flushing – not very nice! And on starboard tack, the volume might slosh to the shallower side of the tank away from the discharge port.

I’d really like to hear of anyone’s experience if they’ve adopted something along the lines of Option 2. I might also have missed other important factors, so those with experience – please chip in!

Cheers

[Don C L]

I have not used one of these but I have always thought it looked like a good option. Maybe it would work in your situation?
https://www.defender.com/product3.js...4286&id=105225

[Wideocean7]

We're on the same page done! That model (Dometic 711) is a Dometic 511 WC which comes with its' own tank. I'm thinking of the 511 but with a tank tailored to fit and maximise the volume.

[Don C L]

Yeah I understand. Still, 9.5 gallons isn't bad considering you won't be flushing a lot of water.

[Wideocean7]

May still be an option but I have to get my head around the possible slopping/spillage issue which could apply to the 711 tank or one I get fabricated.

[Don C L]

Is your head up in the bow facing aft? If you are sailing or motoring upwind in a steep chop you may have to slow down and fall off while someone is using the head otherwise I don't think there will be a problem unless it is absolutely full... but I have not tried this yet I confess!

[Rumrace]

It’s may take some hunting and surgery but I’ve seen hull saddle tanks designed to go between strings in the wall beside the toilet
I don’t know if you could fit a long linear one under floor boards.

[Wideocean7]

Originally Posted by Don C L

Is your head up in the bow facing aft? If you are sailing or motoring upwind in a steep chop you may have to slow down and fall off while someone is using the head otherwise I don't think there will be a problem unless it is absolutely full... but I have not tried this yet I confess!

Thanks Don. I get what you’re saying but the head is athwart facing. You got me thinking if I could change the orientation to make it aft/ahead facing but I’m pretty sure it won’t fit together with the hand basin. And I’d hate to rely on my remembering to luff up or bear off so someone can have a dry poo! As you say, maybe the answer is being diligent about keeping the level down when at sea. I need to give this some more thought.
Cheers Andy

[Wideocean7]

Originally Posted by Rumrace

It’s may take some hunting and surgery but I’ve seen hull saddle tanks designed to go between strings in the wall beside the toilet
I don’t know if you could fit a long linear one under floor boards.

That’s another good idea. Thanks Rumrace. I’ll take some measurements over the next few days to see if a tank might fit behind the head. Obviously that means moving away from the drop-loo but that’s absolutely ok if it works. And all this so I can preserve my previous storage up for’d!