We decided to have a blackwater tank fitted this time last year and ASCAR yard in Cartagena did the necessary for us . Incidentally I have had some great work done by this yard, and they were alwaysgood at comunicating with us at the planning stage and during and after work done.
This has to be about the simplest way to do it as it does not require any more holes below the water line.
We have a Swan 43 ketch, which is fortunate enough to have plenty of storage, including a massive cockpit locker which is where we decided that the blackwater tank should go.
Measuring up we found we could conveniently fit a tank of 120 liters which although a bit on the small side was easily replaced with a bigger one if problems arose. The locker was just the other side of the bulkhead from the head so we were on a winner there.
the pipe from the head pump discharge was routed in to the top of the tank, and a pipe then routed back to the seacock where the discharge used to go.
A fitting designed for pumping out (ie it had waste water written on it!) was fitted in to the deck and plumbed in to the bottom of the tank, then a 1/2″ stainless vent was fitted in through the side of the hull, and a charcoal filter fitted between it and the top of the tank.
If you are at sea just leave the seacock open and the waste will vent through the tank and out of the seacock.
If you dont want to discharge the waste just shut the main seacock and the tank will fill. I just check every now and then to see the level through the inspection hatch, or by tapping on the tank. You can get level gauges but they are expensive.
When pumping out is taking place always flush through fresh water by pumping the head while the tank is being pumped out.
Fitted it all cost about €500 from what I remember.
I hate pumping out in anchorages and in town quays where there are no facilities, and having the tank has been great for that. We had no questions asked or problems in Greece, but might have had issues in Turkey if the tank was not installed.
We checked in to Turkey in Bozburun, just south of Marmaris and the agent sent us off to the fishermens co-op, who run the harbour , to get a bluecard. this cost very little and is a swipe card that holds the record of your pumping, we pumped out a couple of times during our sailing (we did the DADD Rally with about 40 other boats) one marina charged us all when we checked in and swiped our card, and we pumped out in Fethiye, where the boat is now before we left her for the winter.