Every boat has a slight bit of moisture in them even if your bilges are dry and clean. Moisture in the air create mold and mildew smells.
The only way to keep those from building up is to have a positive flow of air moving through the bilges and through stale air areas, thus keeping everything clean smelling.
I have installed four systems in this 100 footer, one in every water tight compartment. The more air moving through the boat, the better.
– Captain Douglas Grey, 82′ Hargrave, Ft Lauderdale, Florida