Porthole
From Wikipedia, the free encyclopedia
A porthole is a small, generally circular, window used on the hull of ships to admit light and air. Porthole is actually an abbreviated term for "porthole window". Though the term is of obvious maritime origin, it is also used to describe round windows on armored vehicles, aircraft, automobiles (the Ford Thunderbird a notable example), and even spacecraft.
On a ship, the function of a porthole, when open, is to permit light and fresh air to enter the dark and often damp below-deck quarters of the vessel. It also affords below-deck occupants a limited, but often much needed view to the outside world. When closed, the porthole provides a strong water-tight, weather-tight, and sometimes light-tight barrier.
[edit] Structure
A porthole consists of at least two structural components, and in its simplest form is, in design and purpose, similar to any other type of window; the porthole is primarily a circular glass disk encased in a metal base frame that is bolted securely into the ship's hull. Sometimes the glass disk of a porthole is encased in a separate frame, hinged onto the base frame so that it can be opened and closed, thus permitting air exchange and access for cleaning. Many portholes also have storm covers, made of solid metal and also hinged to the base frame, that can be securely fastened against the window when necessary. The main purpose of the storm cover is, as its name implies, to protect the ship from surging storm waters. It is also used to block light from entering lower berths when darkness is preferred. In warfare, storm covers were used to prevent light from escaping the ship's lower berths, and to provide protection against hostile artillery fire should the porthole glass suffer a direct hit. Hinged porthole windows and storm covers are accessible from inside the ship's hull, and are typically fastened to their closed positions by hand tightening several pivoting, threaded devices, commonly referred to as "dogs." Older portholes can be identified by the protruding collar of their base plate, typically several inches deep, which in times past accommodated the thick hulls of wooden ships.
Portholes range in diameter from several inches to more than two feet, and weigh from several pounds to over one hundred pounds. Much of the porthole's weight comes from its glass, which can be as much as two inches thick. Metal components of a porthole are also typically very heavy; they are usually sand-cast and made of bronze, brass, steel, iron, or aluminium. Bronze and brass are most commonly used, favoured for their resistance to saltwater corrosion. The design of the porthole is such that it achieves its humble purposes without sacrificing the integrity of the ship's hull. The porthole's thick glass and rugged construction, tightly spaced fasteners, indeed even its round shape, all contribute to its aim of maintaining hull strength.
