I think there are two issues here. The original post said that everyone on deck knows you have used the head. This happens because gas is forced out of the above deck tank vent as the liquid enters the tank. You can install a filter in the vent line to capture the odor. Search the Internet for a Sealand SaniGard Holding Tank Vent Filter.
The rotten egg smell coming from the head itself during the first few flushes is likely from the intake seawater. When you don't use the head for a while the microorganisms in the seawater breakdown anaerobically in the intake hose which creates the sulfur smell. When you flush you pull the stinky water in the bowl. There are few solutions. Add a Y valve (and a check valve) so you can pull water from you tanks for flushing. You only need to use the fresh water for the last flush or two before you leave the boat. Add the valve as close to the seawater intake as you can. Or you can by a small inline treatment item that adds a chemical to the intake water as water flows past it. Or you can open a hatch when you arrive at the boat and flush the head a few times. As long as you use the head twice a day or there shouldn't be time for the anaerobic decay to to happen.
Last edited by SteveInMD; 07-30-2008 at 10:58 AM.