Keeps boat dry all year. Draft free. Allows light below. Powered by slight breeze, will automatically exhaust up to 350 cu. ft. of stale, hot or damp air from inside a closed boat. Eliminates mildew and odors. Low profile, 2-1/2" H, reducing deck clutter. Rain cannot enter even when open. Ideal for heads, galleys, forepeaks and preventing damage to interior wood cabinet work. Easily installed. Requires 4" hole. Comes with 8" protective cover.