On a boat, porthole, allows the entry of fresh air and light into dark areas under or on deck. It also allows the occupants of the lower cabins to have a view of the exterior of the boat. When closed, the porthole protects against water, bad weather and sometimes light when equipped with a shutter.
A porthole is a small, generally circular window pierced in the hull of a boat to allow air and light to enter.
Rectangular porthole is a large window, rectangular, located at the footbridge (wheelhouse) of a boat, to have an external vision and to ensure the conduct of the ship.