Used to seal deck openings of tankers, barges, and cargo vessels. Simple and reliable cover which does not require the drilling of holes and the insertion of studs to install. Made of bronze or stainless steel to eliminate sparking. Fits to any standard 318 mm deck opening and will fit to a deck thickness of up to 76 mm. Unlike other deck covers, the quick action cover is perfectly flat. There are no protruding parts to trip over. Removed with a special speed wrench which provides security against theft and gives rapid action for opening and closing. Frequently used as tank ventilators in place of ventilator gratings.