Wednesday, August 24, 2011

The important Blackwater Tank

Some time ago we decided to take the pumpout option when it came to the toilet system.  As storage capacity on the boat will be one of the three critical criteria I opted for a vacuum macerating toilet.  The idea  behind this choice was it would reduce the amount of water used to flush the system. 

After reading some of the technical literature on the subject I became slightly concerned about the distance between the toilet and the holding tank.  The bathroom is near the bow and the tank is in the back cabin.  In the end I reversed the bathroom layout which had the effect of moving the toilet three feet closer to the tank making them slightly over 20ft apart.  Additionally, the tank will be under the floor and my thought was that at least the toilet pump would be moving the waste with the assistance of gravity.

The tank will be only six inches high but cover almost the entire floor space of the back cabin.  We will need to leave some room either side of the tank for bilge drainage.  My rough dimensions are 6’x5’x6” giving a capacity of just over 400 litres.  I’ve calculated 5ft for the width to allow for the start of the curve in the swim.

My idea is the inlet pipe and capacity gauge will be in the forward port corner of the tank and the outlet plus rinse pipe will be in the starboard stern corner.  The trim of the boat should be bow high which means the tank should pumpout to an almost empty status.  I’m not sure at this time whether we will need a baffle plate to reduce the amount of liquid movement when we are underway.  The tank isn’t all that high and a baffle plate may just create an obstacle allowing the build-up of solids in the tank.

The weakest structural part of the tank is probably the top.  My idea is to either weld vertical plate or box steel to it thereby providing some rigidity.  These can also be used as supports for the floor bearers.

I need to talk all this through with the builder.

7 comments:

Peter and Margaret said...

KL is fitted with a Tecma of the vacuum macerating type, and also has the benefit of being fitted with a fresh water mixer tap which also makes it a bidet. It is fitted back to back with the large holding tank. Items to note in relation to its use. 1. Some boatyards try and charge double for one pumpout, as one standard one doesn't empty the very large tank. The card operated BW pumpouts always require two operations. 2. Even though we only use tissue which dissolves in water, available from the chandler, sometimes, visitors, who are not used to such sensitive marine matters, and have probably used too much tissue, block it up. Luckily, and because the tissue dissolves in a short time, the blockages have so far cleared themselves! 3. The tank, which is fitted to the stbd side of the boat does make a difference to the trim as it fills up, but as the Tecma is only fitted with a green / red LED to indicate when the tank is full, I use the trim issue to inform me when we should be thinking of visiting the pumpout station, using a hanging wall thermometer to indicate the appropriate time. What a subject! But full of important considerations to take into account. Regards, Peter.

John/Waimaru said...

Tom,
I just wonder if it would not be better for your rinse pipe entry to be at the forward end if at all possible. That way you will get a rinse through the tank from one end to the other. Only wish we had a separate rinse pipe which would be a devil to after fit. We sometimes remove the mattress from the bed in order to give a good rinse through the tank inspection hatch

Tom and Jan said...

John,
Yes, I have already thought of that. It could be positioned to clean the tank gauge adjacent to the inlet pipe.
The problem is there is an inline bed above the tank on that side and I don't know if the rinse pipe running to the gunwale would obstruct the bed.
Another thing to talk to the builder about :-()

Bruce in Sanity said...

Remember too that you'll never extract the last inch of waste, so your shallow tank will only be ⅚ of its technical capacity.

I personally think that vacuum toilets are a bad idea, too complex for boating use, but you're not a man to be deterred by complexity, I know!

All the best

Bruce

Tom and Jan said...

Bruce,

I'm not against listening to advice - it's accepting it that sometimes gives me a problem :-)

Your point about emptying the tank has also caused me some concern. The other option is to mount the tank vertically across the stern immediately in front of the engine compartment.

I haven't read of any particular issues with a vacuum system?

Bruce in Sanity said...

LeeSan have stopped doing their version for lack of demand and excess after sales calls, as I understand it.

One recent Braidbar owner now regrets going for the Vacuflush vacuum cassette, I know, because of recurrent problems with the pressure switches.

Probably the Tecma is better built in view of Peter's satisfaction with it.

The usual guideline is to have the tank as close as possible to the toilet, to avoid probs with the content of the pipework seizing up, not a happy thought. But, as you say, I'm sure your builder will have the best advice for your particular design

ATB

Bruce

Tom and Jan said...

Now you have me worried! I'm not the plumber in the family. If it's not a vacuum system then I assume from the technical literature more water is used?