It’s generally believed that portholes first appeared in the vessel Henri-Grace-a-Dieu in 1515. Portholes were invented by a Frenchman and first put to use by the English during the realm of Henry VI (in the late fifteenth century). The invention of portholes gave the power of adding a second tier of guns and accordingly, the Henri-Grace-à-Dieu appears with two whole battery-decks, besides additional short decks, or platforms, both ahead and astern. It also allowed for larger sized cannon.
Although the portholes were used to increase the cannon population on a ship especially on the forecastle and aft castle, it’s possible they were first used to accommodate rows of oars. Other than an opening to allow for cannons, portholes are also use to allow light into the lower decks of a ship as well as allow for air circulation.
Before 1515 cannon were only mounted All Round Shipping on the deck, which limited the number of guns a warship could carry. If you just had holes in the side of the ship, too much water would get in during heavy weather; it could fill up the ship with water, and also rust the iron cannons. Gun ports, were fitted with a storm cover for heavy weather and when the cannon were not in use.
Portholes later came to mean any windows, round by custom, in the side of a ship. Portholes are also known as airports or side scuttles. Putting a round hole in something actually makes it stronger, while a square hole makes it weaker. The design of the porthole is such that it achieves its unassuming 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 purpose of maintaining hull strength and pressure of storm waves crashing against it.
A porthole consists of at least three structural elements. In modern ships, the porthole is a circular glass disk encased in a metal frame, usually bronze or brass, that is bolted securely into the side of a ship’s hull with a hinged storm cover. Sometimes the glass disk of a porthole is encased in a separate frame which is hinged onto the base frame so that it can be opened and closed. For model ship building, glass can be represented by clear plastic or by an opaque glaze.
In earlier ships the portholes were square or rectangular consisting of a frame, hinged storm cover and sometimes an inner wooden shutter.
The wood or metal storm covers can be securely fastened against the window when necessary. The main purpose of the hinged storm cover is to protect the opening from heavy seas. Older portholes can be identified by the protruding collar of their base plate which may be up to several inches deep, thus accommodating the thickness of a wooden hull. Always ensure that there is a storm cover on the portholes