This ventilation grille is used to cover openings into enclosed spaces, for example to provide ventilation to a locker to prevent the build-up of mould or to allow vapours to disperse safely. 

This grille is used by boat builders including Quicksilver, Beneteau, Jeanneau etc. 

In particular, this is used as a vent on various Quicksilver boats including Pilothouse, Cruiser and Weekend models. The Quicksilver part number is 875391NR.