The BoatVent II can be added to any custom or semi custom marine cover. Made with the latest UV inhibited polymers you can be sure it will withstand the elements. This vent helps prolong the life of your boats upholstery by venting the enclosed space of the warm, moist air. BoatVent II helps prevents mold, mildew, and bacteria from growing under your cover, and causing irreversible damage to your boat's interior.