My vent line I believe is 3/4" and comes out under my walkway. I never notice odor outside my boat but I keep my ~25 gal tank fairly empty most of the time. I'm not sure if the line is designed for waste products or just a standard thick walled tubing. Be sure to install a charcoal filter and replace regularly. OHB has an article somewhere linked to this site about how to change the charcoal vs buying a new filter.
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1973 Chris Craft Aquahome, 34' Lake Lanier, GA