1) A natural vent on a typical sailboat size holding tank will keep the contents sufficiently aerobic to control odor as long has a fairly straight run with a minimum of elbows and doesn’t angle up more than 45 degrees. 1” hose is the minimum size for good performance.
2) The nasty odors are heavier than air, thus vertical runs in the vent and a long distance from the tank to the through hull will trap them and prevent oxygen from reaching the contents.
3) A Sweet Tank system which pumps air into the bottom of the tank to both stir up the contents and provide oxygen is not necessary on tanks below about 30 gal capacity. It needs to run 24/7 so the electrical consumption is impractical for my boat anyway.
Peggie, have I got this right?
My boat was built before any of these things were known so I have berths, bulkheads, and joinerwork in the way. The only feasible vent run is tall and vertical with a lot of elbows. I’m using 1” hose but could only fit in a 3/4” through hull. It seems like there is a good chance I won’t get good air exchange.
(I’m using PVC for the parts of the vent and pump out runs I am going to enclose and hope to never see again. I’m keeping the hose to where I can replace it easily.)
Here’s what I would like to know now:
The amount of air exchange through the ideal vent seems like it would have to be much less than the Sweet Tank system and bottom stirring and bubbling is evidently not necessary on the 18 gallon tank I am installing.
Why would a small and much lower electrical draw air pump that simply injects into the top of the tank and keeps the air exchange going not improve the situation? It would basically just be overcoming the unavoidable deficiencies and restrictions in my vent system by adding a small amount of positive pressure to overcome the elbow restriction and “pooling” of the heavier gasses.
2) The nasty odors are heavier than air, thus vertical runs in the vent and a long distance from the tank to the through hull will trap them and prevent oxygen from reaching the contents.
3) A Sweet Tank system which pumps air into the bottom of the tank to both stir up the contents and provide oxygen is not necessary on tanks below about 30 gal capacity. It needs to run 24/7 so the electrical consumption is impractical for my boat anyway.
Peggie, have I got this right?
My boat was built before any of these things were known so I have berths, bulkheads, and joinerwork in the way. The only feasible vent run is tall and vertical with a lot of elbows. I’m using 1” hose but could only fit in a 3/4” through hull. It seems like there is a good chance I won’t get good air exchange.

(I’m using PVC for the parts of the vent and pump out runs I am going to enclose and hope to never see again. I’m keeping the hose to where I can replace it easily.)
Here’s what I would like to know now:
The amount of air exchange through the ideal vent seems like it would have to be much less than the Sweet Tank system and bottom stirring and bubbling is evidently not necessary on the 18 gallon tank I am installing.
Why would a small and much lower electrical draw air pump that simply injects into the top of the tank and keeps the air exchange going not improve the situation? It would basically just be overcoming the unavoidable deficiencies and restrictions in my vent system by adding a small amount of positive pressure to overcome the elbow restriction and “pooling” of the heavier gasses.