@Nashville - Yes, I know they're vacuum-operated; I've been on several cruises. My 45 Hatteras had a Vacuflush head. It's advantage over the "traditional" water-carried flush is that fresh water consumption is MUCH lower (just enough for the bowl and to carry solid waste) and in addition there is little or remaining waste in the pipe between uses plus the entire system is under negative pressure until the vacuum pump. Many "recreational" vessels that don't have those systems use sea water for flush because fresh water is precious on board, but that's got its own set of problems including odor, which the vacuum system avoids entirely since it's sealed except when being flushed and it all goes one way. Never mind that the vacuflush system is MUCH less likely to get clogged and trust me on this -- unclogging a marine head system is a ****ty job, literally.

The vent on a recreational vessel's tank is typically hullside and, since there are only a few people on board, the "blast of ****-odor" is rare. I did not know they vented the cruise ships up the stack, but that does make sense.

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