Thoughts on twinning a holding tank?

[nmuir]

Hello. I need to do some work on my holding tank(s) and am looking for feedback and insights to an idea.

First the background: I want to increase the forward holding tank capacity from the existing 20 gal plastic tank. Room is limited due to stringers and floor supports so I cannot simply remove the existing and install a larger one in a similar location. The only location for a single larger tank is at the other end of the boat - 25 feet away and quite a bit higher up, requiring a lot of plumbing, new deck ports, etc.

I am therefore considering 'twinning' the existing tank by installing another tank nearby and joining the two with a good sanitary hose connection. I am thinking (hoping?) that if the connection is at the bottom of the tanks, with them installed at the same level, and with good venting in both, that everything would work and they would balance out. I am also thinking (hoping?) that with the existing deck pumpout and overboard pump unchanged they would - in effect - service both tanks. A bit like a single tank with a really large baffle in it.

The downside is that the new tank will have to be abeam of the other one, so boat roll may be an issue. (There is nowhere fore/aft of the current tank for an installation).

Is there anything I am missing here, and are there any thoughts and comments (positive or negative) on such a plan?


Thanks!

[Shrew]

Originally Posted by nmuir

[FONT=Arial]The downside is that the new tank will have to be abeam of the other one, so boat roll may be an issue. !

You could plumb a fitting on the bottom of the forward tank, run that to a discharge pump. Run the outlet of the discharge pump to a fitting on the top of the tank in the stern. Then use the discharge pump like a transfer pump.

You'd need a second pump out/discharge fitting for the aft tank.

Two tanks abeam with a passive transfer connection could be problematic on a sustained heal. I'd hate to have both tanks be 2/3 - 3/4 full then have one 'slack tank' into the other. You'll start dumping out your vent line (clogging any inline vent filter) and praying your joker valve on the head is in good shape. Just my .02.

[Shrew]

I supposed you could put in a Y valve on the holding tank discharge line and simply divert the head to an alternate tank in some other location just as easily.