We put an Airhead composting toilet on our R-25 classic. Since we bought new, we never used the holding tank, so we were able to use it for back-up water supply using a Y-valve to switch the source. Swapped the waste cap for a second water cap.
The composting head could get through the summer before we had to empty it and we bought a second urine tank to stretch to about 5-6 days in between emptying (2-3 days per urine tank for 2 people.) We prefer a composting head over pumping out. It worked really well and we never had pump out anxiety. All paper went in the trash. Between uses, a strong vinegar solution in a spray bottle helps disinfects the bowl. (Can’t use cleaning solutions because they interfere with composting.) Never had any odor issues over the 10 years we owned the boat.
A hose vent ran to a fan installed on the stern gunwale, which we wired directly to one of our batteries. The fan needs to be powered 24/7 to keep flies out.
The boat had been built with a marine toilet installed. Since the raw water line running from the thru-hull/seacock to the marine toilet was still there, but unused, an engineer friend suggested modifying it to be able to flush the engine with fresh water in the raw water cooling system instead of sea water.
A Y-valve sits inline on the marine toilet supply line. One leg still connects (via the seacock) to the sea water thru-hull. The other leg goes to a hose barb/connector with a standard garden hose. By closing the seacock and opening the Y-valve to the hose side, fresh water from a dockside hose feeds into the engine's raw water intake. Make sure the engine is cool and off before flushing and that water is flowing when the engine is running.
