Porta-potty on our '77 h27
The porta-potty on our '77 h27 is attached to the floor in our head, on the stbd side facing the centerline.
It is connected to the outside in 2 ways. The pump-out line leads from the bottom of the tank to the standard pump-out fitting on the deck by the toerail behind and above the toilet. An inadequate air fitting leads from a fixture below the toe rail on the outer hull to an intake at the top of the tank.
Although the air line is too small, weekly doses of Odorlos supply plenty of oxygen to eliminate odor. The tank of the porta-potty (around 3 gallons on our unit) will hold a surprising amount of waste between pump-outs because there is little or no flushing water used. A holding tank system expands the waste by 2 or 3 times for the water to flush the toilet, and move the waste into the holding tank. Flush water from outside the hull also brings in some pretty potent bacteria, particularly from salt water.
We have used our porta-potty for 10 years, for cruises of one to four weeks. It will easily hold 3-4 days of waste from the Admiral and me. We have never had an issue with it, except a little odor when returning to the boat after several weeks away - a dose of Odorlos quickly solves that issue.
I recommend the porta-potty with a pump-out for smaller boats like ours. The system is simpler and more fool-proof than a holding tank, and has similar capacity to the 10 gallon tanks that most install.