Circulates fresh air in boat cabin… protects against mold, mildew and musty air. Now with even more power, the Nicro Day/Night Plus moves over 1000 cubic feet of air in an hour or 24,000 cubic feet of air a day (4” model). With direct to deck installation it easily replaces any existing 4” NICRO vent product. The Day/Night Plus has complete air and green water shut-off from below, as well as a built-in on/off fan switch.